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AN INVESTIGATION INTO SELECTION FOR PRODUCTION AND REPRODUCTION IN AFRINO SHEEP

by

MARGARETHA ANNA SNYMAN

 

Dissertation submitted to the Faculty of Agriculture,

Department of Animal Science,

University of the Orange Free State,

in partial fulfilment of the requirements for the degree

PHILOSOPHIAE DOCTOR

 

Promoter : Professor J.B. van Wyk

Co-promoter : Professor G.J. Erasmus

 

Bloemfontein, November 1996

 


TABLE OF CONTENTS

PREFACE

1. INTRODUCTION

 

2. HISTORY OF THE CARNARVON AFRINO FLOCK

2.1 Management of the stud

2.2 Selection procedures

2.3 Description of the environment

 

3. NON-GENETIC FACTORS INFLUENCING GROWTH AND FLEECE TRAITS

3.1 Introduction

3.2 Material and methods

3.2.1 Data

3.2.2 Statistical analysis

3.3 Results and discussion

3.3.1 Model specification

3.3.2 Two-way interactions

3.3.3 Least-squares means

3.3.4 Age of dam and age of lamb at recording

3.4 Conclusions

 

4. DIRECT AND MATERNAL (CO)VARIANCE COMPONENTS AND HERITABILITY ESTIMATES FOR FLEECE TRAITS AND BODY WEIGHT AT DIFFERENT AGES

4.1 Introduction

4.2 Material and methods

4.2.1 Data

4.2.2 Variance component and genetic parameter estimation

4.3 Results and discussion

4.3.1 Direct and maternal heritability estimates

4.3.2 Permanent environmental effect of the dam

4.3.3 Genetic correlation between direct and maternal effects

4.4 Conclusions

 

5. AN INVESTIGATION INTO THE POSSIBLE GENETIC IMPROVEMENT OF REPRODUCTION AND SURVIVAL RATE USING A THRESHOLD MODEL

5.1 Introduction

5.2 Material and methods

5.2.1 Data

5.2.2 Statistical analysis

5.3 Results and discussion

5.4 Conclusions

 

6. GENETIC PARAMETER ESTIMATES FOR TOTAL WEIGHT OF LAMB WEANED IN AFRINO AND MERINO SHEEP

6.1 Introduction

6.2 Material and methods

6.2.1 Data

6.2.2 Variance component and genetic parameter estimation

6.3 Results and discussion

6.4 Conclusions

 

7. GENETIC AND PHENOTYPIC CORRELATIONS AMONG PRODUCTION AND REPRODUCTION TRAITS

7.1 Introduction

7.2 Material and methods

7.2.1 Data

7.2.2 Covariance component and genetic parameter estimation

7.3 Results

7.4 Discussion

7.5 Conclusions

 

8. CONCLUSIONS AND RECOMMENDATIONS

8.1 Ram selection

8.2 Ewe selection

8.3 Record keeping

8.4 Conclusions

 

ABSTRACT

OPSOMMING

 

BIBLIOGRAPHY

 

PREFACE

This dissertation is presented in the form of five separate scientific publications (Snyman et al., 1995a; Snyman et al., 1995b; Snyman et al., 1996a; Snyman et al., 1996b; Snyman et al., 1996c) augmented by a general introduction, a chapter on the history of the Carnarvon Afrino flock and general conclusions and recommendations in an effort to eventually create a single unit.

The author wishes to express her sincere appreciation and gratitude to the following persons and institutions :

The Director, Grootfontein Agricultural Development Institute, for kind permission to use the data.

Prof. Japie van Wyk, who acted as promoter, for his valuable guidance and assistance with the statistical analysis of the data and for his encouragement during the study.

Prof. Gert Erasmus, who acted as co-promoter, for his advice, constructive criticism and stimulating discussions as well as assistance in preparing the manuscript.

Dr. Buks Olivier for his support, valuable advice and able guidance throughout the course of this study.

Prof. Bill Hill for some very useful suggestions.

Mr. Arnold Cloete for the outstanding way in which the data were collected and recorded.

Other personnel of the Carnarvon Experimental Station who contributed to the experiment.

My colleagues from the Animal Science Section for their support.

My family for their support and encouragement.

My Creator for His unfailing kindness and blessings.

 

Table of contents

 

CHAPTER 1

INTRODUCTION

The ultimate aim of sheep breeding research is to provide estimates of the parameters required to construct a genetic improvement plan leading to improved viability, productivity and profitability.

The Afrino is a white woolled breed developed at the Carnarvon Experimental Station for wool and slaughter lamb production under extensive conditions (see Chapter 2). In a dual purpose sheep breed such as the Afrino, the aim of the breeding programme should be to increase the efficiency of both slaughter lamb and wool production. Simultaneous selection for more than one trait therefore seems necessary to increase overall productivity in the flock.

As approximately 80% of the income from Afrino sheep is generated through mutton production (Snyman, 1995; Unpublished), the primary selection objective should be to increase the lifetime weight of lamb produced per ewe. Fleece traits such as clean fleece weight and fibre diameter should also receive attention during selection. With regard to wool production, the quality of the fleece, in terms of particularly fibre diameter, could be considered more important than the amount of wool produced, especially in an arid environment where quantity, but not quality, could pose a serious restriction.

The Afrino is the first South African sheep breed in which the separation of ram and ewe selection objectives was investigated (Badenhorst, 1989). Ram selection is aimed at increasing the genetic merit of the next generation, contributing very little to the economic optimilization of the current flock. Ewe selection, however, has a dual purpose. In the first place it is aimed at increasing income from the current flock through own productivity and secondly, to contribute genetically to possible superior future generations.

In practice, most selection emphasis is normally placed on rams and little or no attention is given to ewe selection. As discussed by Badenhorst (1989), ram selection could concentrate on the economically important growth and wool traits, while ewe selection should be done mainly on reproductive performance. At the selection stage (two tooth age), much is known about a young ewe's growth and wool production. Accurate selection based on these traits is therefore possible. These traits, however, contribute much less to total productivity than the ewe's lifetime reproductive performance. At this stage little information on the young ewe's reproductive ability is known. The aim must therefore be to try and predict from the available information at selection age, which ewes will be the best performers over their lifetime in the flock.

After the selection objective has been defined, it is necessary to identify suitable selection criteria. There are several factors which should be kept in mind when deciding on the most suitable selection criteria. The most important of these are the heritability, genetic correlations with other traits and the measurability of the trait being considered as selection criterion. These factors must therefore be known before any attempt at constructing a viable breeding plan can be made.

The use of mixed model methodology (Henderson, 1984), leading to best linear unbiased prediction (BLUP) of breeding values has become an important tool in selection programmes. Olivier et al. (1995) demonstrated that selection response in the Grootfontein Merino stud was increased substantially when selection was based on BLUP of breeding values. In order to obtain the most accurate estimation of genetic parameters, it is essential that the most suitable model of analysis be fitted to the data. It is therefore important to identify the different sources contributing to the phenotypic variance of each trait. Possible contributing sources of variation include non-genetic effects, direct additive genetic effects, maternal additive genetic effects, maternal permanent environmental effects, as well as dominance and epistatic effects.

The purpose of the first part of this study was to quantify the effect of non-genetic factors such as year of birth, sex and birth status of the lamb and age of dam on various growth and fleece traits of Afrino sheep, in order to contribute to the construction of the first part of an operational model for the accurate estimation of genetic parameters.

To optimise genetic gain, information on direct as well as maternal breeding values, where applicable, for the traits under selection should be utilised. The availability of modern statistical software has simplified the partitioning of variance into components resulting from either direct or maternal effects. These components must be known, at least to proportionality, for use in mixed model equations to obtain BLUP of breeding values. Reports on maternal variance and heritability estimates for growth and fleece traits in sheep are, however, not in abundance (Khaldi & Boichard, 1991; Burfening & Kress, 1993; Maria et al., 1993; van Wyk et al., 1993b; Mortimer & Atkins, 1994; Olivier et al., 1994; Swan & Hickson, 1994; Tosh & Kemp, 1994; Hickson et al., 1995; Mortimer & Atkins, 1995; Vaez Torshizi et al., 1995).

Selection progress can sometimes be inaccurately predicted from direct heritability estimates alone where there is a strong maternal component. This could be as the result of a negative correlation between direct and maternal influences, which in turn results in a lower total heritability. Negative relationships between direct and maternal effects for birth and weaning weights in sheep have been reported by Burfening & Kress (1993), Maria et al. (1993), van Wyk et al. (1993b) and Tosh & Kemp (1994). The sign and magnitude of this relationship for Afrino sheep is, however, still unknown.

The occurrence of frequent multiple births in sheep gives rise to the question of the influence of a permanent environmental effect of the dam on traits such as birth and weaning weight. These effects have not yet been quantified for sheep.

The objectives of the second part of this study were, firstly, to determine the most suitable random effects to include in a restricted maximum likelihood (REML) animal model to analyze body weight at different ages, clean fleece weight and mean fibre diameter measured at 16 months of age; and secondly, to estimate (co)variance components and genetic parameters for each of these traits.

In South African sheep enterprises, reproductive performance is of utmost importance in the efficiency of sheep production. This is even more so in the case of mutton and dual purpose sheep breeds. In these breeds, total weight of lamb weaned per year is normally regarded as the best single measure of a flock's productivity.

Numerous studies involving the component traits of reproduction, i.e. fertility, litter size, lamb survival rate and number of lambs born and weaned per ewe joined, have been done to obtain heritability estimates for these individual traits (Fogarty, 1995). Most of these estimates were obtained from paternal half-sib analyses, treating the traits as continuous variables and assuming they follow a normal distribution. However, in these traits the phenotype is expressed in two or more distinct, mutually exclusive and exhaustive categories. If a polygenic mode of inheritance is assumed for these traits, it is evident that the threshold concept (Wright, 1934) as discussed inter alia by Dempster & Lerner (1950), Bulmer (1980) and Gianola (1982) should apply.

In this case any linear model, even BLUP, does not yield the maximum likelihood estimate of the best predictor since the data and genetic values to be predicted do not follow joint normal distributions. These variables are not normally distributed and a single heritability for the trait does not exist on the discontinuous scale. Gianola (1982) states that the main theoretical reason for not using BLUP with categorical data is that breeding values and residuals are not independent of each other and their marginal distributions are difficult to specify.

Heritabilities and breeding values on the underlying scale for different reproductive traits and survival rate in Afrino sheep, fitting a threshold model, were therefore estimated to ascertain whether these traits can be improved by selection.

In contrast with the component traits of reproduction, the composite trait, total weight of lamb weaned per ewe joined, has received much less attention (Fogarty, 1995). Lambing percentages in excess of 150% under harsh, extensive conditions lead to the production of a high quantity of lambs, but the quality of these lambs are in many instances not acceptable. In view of the limited natural resources, an increase in number of lambs per ewe joined is not the answer to generate higher income from a specific farming enterprise. Selection for increased reproductive performance in such flocks should be aimed at increasing the quality and monetary value of the product in terms of weight and carcass quality. The aim with slaughter lamb production under extensive conditions is to produce slaughter lambs that can be marketed as soon as possible after weaning, without the need for supplementary feeding. Selection for litter size, without taking the weaning weight of the individual lambs into consideration, would be short sighted.

Furthermore, litter size is directly related to ovulation rate, which in turn is influenced by only a few hormones (Hafez et al., 1980). Total weight of lamb weaned, however, is determined by litter size as well as several other factors, such as mothering ability, milk production of the ewe and growth potential of the lamb. The genes influencing these different traits would all have an effect on total weight of lamb weaned. Selection for reproduction should ideally be based on some measure closely resembling the true breeding objective, which is the total weight of lamb weaned per ewe joined. If selection is aimed solely at increasing litter size, the frequency of genes affecting total weight of lamb weaned would perhaps not be sufficiently influenced.

Almost all the heritability estimates for weight of lamb produced cited in the literature were obtained from paternal-halfsib or regression methods (Basuthakur et al., 1973; More O'Farrell, 1976; Martin et al., 1981; Fogarty et al., 1985; Owen et al., 1986; Abdulkhaliq et al., 1989; Long et al., 1989; Boujenane et al., 1991b). However, with an analysis under an animal model, the full relationship matrix is exploited and any genetic variance due to the dam would also be accounted for in the estimation of heritability.

Heritability estimates for total weight of lamb weaned in Afrino sheep were obtained by applying restricted maximum likelihood procedures (REML) under an animal model. Genetic improvement of lifetime reproductive performance is not practical by direct selection, but is dependent upon selection for correlated traits. Therefore the genetic correlation between total weight of lamb weaned at the first and subsequent parities was also estimated to ascertain whether selection cannot be performed earlier.

In wool producing breeds which have to produce and reproduce under adverse or sub-optimum conditions, hardiness and adaptability are of paramount importance. In studies regarding hardiness in small stock, a negative relationship between reproduction performance and fibre production relative to body weight has been indicated (Herselman et al., 1996). Ewes which produced less wool relative to their body weight, weaned more kilogram of lamb over three parities than their flock mates which produced more wool relative to their body weight. This negative relationship was obtained irrespective of breed or environment. Negative correlations of -0.77, -0.93 and -0.70 (P=0.00) were obtained for Carnarvon Afrino, Carnarvon Merino and Grootfontein Merino sheep respectively (Herselman et al., 1996). Before a viable breeding plan can be formulated for Afrino sheep, it is important to know the relationships between body weight, fleece weight and lifetime reproduction performance.

Numerous genetic and phenotypic correlations for different breeds have been published (Fogarty, 1995). Some of the genetic correlations reported in these studies are highly variable, especially those estimated between fleece weight and body weight at different ages and the genetic correlations of the reproduction traits with the different body weight and fleece traits. No correlation estimates between these economically important traits are available for Afrino sheep. Therefore, covariance components and genetic and phenotypic correlations were estimated among weaning weight, nine- and 18-month body weight, clean fleece weight, fibre diameter and reproduction traits in the Carnarvon Afrino flock.

The purpose of this study was to use the results to construct a viable, practical breeding plan which could be implemented by Afrino stud breeders and commercial producers in order to increase the productivity of, and income from, their flocks. Information generated by this study could also provide other sheep breeds with additional information where suitable data sets for the estimation of (co)variance components do not exist, and hopefully provide the impetus to conduct similar studies.

 

Table of contents

CHAPTER 2

HISTORY OF THE CARNARVON AFRINO FLOCK

Due to the relatively low wool prices and higher meat prices in the late sixties, a tendency developed among Merino farmers to crossbreed Merino ewes with mutton producing breeds, in order to take advantage of the higher mutton prices. All these breeds, however, had either a kemp and/or colour problem, which in turn lead to the contamination of the South African wool clip. This resulted in a request to the Department of Agriculture by the wool industry, via the South African Agricultural Union, to develop a white woolled breed for use as a terminal sire in crosses with Merino ewes in the extensive sheep grazing areas.

This breed :

* had to be free of kemp and coloured fibres;

* had to be hardy and well adapted to the environment of the extensive grazing areas;

* had to produce a good slaughter lamb at an early age in crosses with Merino ewes;

* had to have a good reproductive ability.

Consequently, a breeding project was initiated at the Carnarvon Experimental Station in the Karoo Region in 1969. This project involved eight different crosses between Merino ewes and several white woolled and white haired mutton breeds (Olivier et al., 1984). Thereby it was attempted to develop a white woolled mutton sheep, which combined the quality wool of the Merino with the reproductive performance and mutton producing abilities of the mutton breeds.

In 1976 it was evident that the cross consisting of 25% Merino, 25% Ronderib Afrikaner and 50% SA Mutton Merino best fulfilled the requirements set for the new breed. It was decided to retain only this cross for further upgrading and development of the breed that is today known as the Afrino.

On 5 February 1980, the Afrino Sheep Breeders' Society was established at a meeting on the Carnarvon Experimental Station and breed standards for this new white woolled mutton breed were drawn up. Although in subsequent trials it was shown that the Afrino gave satisfactory results as a terminal sire in crosses with Merino ewes - which was the original objective - the Afrino on that day became a breed in its own right.

 

2.1 Management of the stud

The researchers who initiated this project were the late Dr Wouter Hugo, Dr Hans Nel, Dr Quinton Campbell and Mr Arnold Cloete. Mr Cloete is still responsible for the management of the flock and collection of all the data at the Carnarvon Experimental Station. The researchers responsible for the flock were Dr Nel (1969-1976), Dr Marais (1976-1980), Dr Olivier (1980-1989) and the author since 1989.

Initially, all the ewes were mated during October of each year, but the resulting lambing percentages were unsatisfactory with the spring mating. From 1974 to 1976, ewes that failed to lamb were again mated in the following autumn. In 1979 it was decided to switch to an autumn breeding season.

Rams were randomly allocated and hand-mated to the ewes. Until 1981, most rams were used for up to four or five years. Since 1982, most rams were replaced annually. However, some of the rams - one or two per year - were used for two consecutive years. The number of ewes mated, sires used and lambs born from 1972 to 1994 are summarized in Table 2.1.

Table 2.1 Number of ewes mated, sires used and lambs born in the Carnarvon Afrino flock from 1972 to 1994

Year-seasona No. of ewes No. of sires  No. of lambs
1972-1 48 5 32
1973-1 50 5 14
1974-1 68 7 51
1974-2 7 4 8
1975-1 72 10 77
1975-2 17 8 26
1976-1 100 7 39
1976-2 45 7 57
1977-2 53 8 45
1978-2 130 11 129
1979-1 135 16 137
1979-2 57 10 73
1980-2 205 12 294
1981-2 168 9 211
1982-2 195 14 299
1983-2 177 12 150
1984-2 191 11 262
1985-2 190 9 305
1986-2 184 9 295
1987-2 166 8 275
1988-2 199 8 339
1989-2 222 12 334
1990-2 232 9 327
1991-2 222 12 310
1992-2 213 10 251
1993-2 198 10 261
1994-2 199 11 293

a -1 = Autumn lambing season; -2 = spring lambing season

All lambs were identified and tagged at birth and birth weight as well as sex, birth status and pedigree information were recorded. Lambs were weaned at approximately 100 days of age. All ram and ewe lambs were retained until the age of 18 months. Monthly body weight from five to twelve months, as well as at 18 months of age were taken for all lambs. Clean fleece weight (CFW) and mean fibre diameter (MFD) measurements were recorded at 16 months of age on a mid-rib fleece sample under the National Woolled Sheep Performance and Progeny Testing Scheme as follows :

 

2.2 Selection procedures

Selection during the early stages of the experiment was aimed mainly at growth rate of lambs, reproductive performance of ewes and absence of kemp and coloured fibres. Subjective selection at 18 months of age eliminated all animals with poor conformation or an excessive amount of kemp or coloured fibres. Rams were then selected on the basis of post-weaning growth rate. Initially, ewes were not selected for growth performance in order to allow for a rapid increase in the breeding flock.

During March 1981, the ewe flock was classed according to breed standards for the first time, which explains why only 168 ewes were mated in that year. All young ewes with an 18 month body weight of more than 10% below average were culled. Since 1985, however, it was decided to mate all available young ewes, with the exception of those with conformation or wool faults, or those having a fibre diameter of more than 5% above average. Final ewe selection was then done on the basis of total weight of lamb weaned after the first parity.

As the fibre diameter in the stud was increasing genetically, it was decided in 1985 to cull all rams with a fibre diameter of more than two micron above average. Since 1987, however, only rams with above average indices for pre- and post weaning growth traits, as well as below average fibre diameter, were used in the stud. Much emphasis was also placed on wool quality and evenness of fleece, while the amount of wool produced received little attention. Since 1991, all selection done in the flock has been based on BLUP of breeding values for the specific traits under selection.

 

2.3 Description of the environment

The Afrino flock is kept under natural conditions at the Departmental Experimental Station near Carnarvon (30E 59'S, 22E 9'E) in the North-western Karoo region of the Republic of South Africa.

The experimental station is representative of the extensive low potential areas of the Karoo region. The natural pasture varies from mixed grass and shrub veld to karoo shrub veld and is described by Acocks (1988) as arid karoo. The official grazing capacity norm is estimated at 5.5 ha per small stock unit. The climate is characterised by severe winters and hot summers.

The average annual rainfall is 209mm and occurs mainly during the autumn months. The average annual rainfall and monthly distribution for Carnarvon from 1974 to 1994 are illustrated in Figures 2.1 and 2.2 respectively. The only two periods when supplementary feeding in the form of alkali-ionophore-treated grain was given, was from 1982 to 1984 for ewes and young animals and during 1992 for pregnant and lactating ewes. These were the only times when a year(s) with below average rainfall followed a period of average rainfall.

 

Figure 2.1 Average annual rainfall recorded on the Carnarvon Experimental Station from 1974 to 1994.

 

Figure 2.2 Monthly distribution of rainfall on the Carnarvon Experimental Station

 

Table of contents

 

CHAPTER 3

NON-GENETIC FACTORS INFLUENCING GROWTH AND FLEECE TRAITS

3.1 Introduction

The use of best linear unbiased prediction (BLUP) of breeding values has become an important tool in selection programmes. Since selection in Afrino sheep is partially based on body weights recorded at an early age, not only direct additive breeding values, but also maternal breeding values are of importance. In order to obtain the most accurate estimate of an animal's true breeding value, important non-genetic sources of variation must be identified and statistically eliminated.

The purpose of this study was to quantify the effect of non-genetic factors such as year of birth, sex and birth status of the lamb and age of dam on various growth and fleece traits in Afrino sheep, in order to contribute to the construction of an operational model for the accurate estimation of genetic parameters and breeding values.

 

3.2 Material and methods

3.2.1 Data

Data analyzed in this study were obtained from the Carnarvon Afrino flock. In order to eliminate inclusion of first cross progeny, only data collected from 1975 to 1992 on the Afrino flock were used in this study. Before editing, the data consisted of 4235 individual lamb records, the progeny of 146 sires and 946 dams. For each of these lambs, full pedigrees were available. Data on the following traits were analyzed, namely birth weight (BW), weaning weight (WW), monthly body weight from five to 12 months of age (W5 to W12) and 18 month body weight (W18), clean fleece weight (CFW) and mean fibre diameter (MFD).

Data were edited to force sires nested within year-season. For sires used in more than one year-season, the data of the year-season in which each sire had the most progeny records, were retained. Furthermore, all records from sires with less than five progeny were deleted. The following data sets were used for the various analyses, namely 3340 records for birth weight, 3122 for weaning weight, 2958 for monthly body weight from five to 12 months of age and 2996 for 18 month body weight, clean fleece weight and mean fibre diameter.

 

3.2.2 Statistical analysis

In an analysis of variance, using mixed model least-squares procedures (Harvey, 1990), the following general model was fitted for each trait :

Yijklm = µ + ri + ysj + sk + wsl + (yss)jk + (ysws)jl + (sws)kl + b1AD + b2AD2 + b3AL + eijklm

where

Yijklm = trait of the m'th animal of the l'th birth/rearing status of the k'th sex of the j'th birth year-season of the i'th sire,

µ = overall mean,

ri = random effect of the i'th sire with zero mean and variance IF2r,

ysj = fixed effect of the j'th birth year-season,

sk = fixed effect of the k'th sex,

wsl = fixed effect of the l'th rearing status (birth status in the case of BW),

(yss)jk = effect of the interaction between the j'th birth year-season and the k'th sex,

(ysws)jl = effect of the interaction between the j'th birth year-season and the l'th rearing status,

(sws)kl = effect of the interaction between the k'th sex and the l'th rearing status,

b1,b2 = linear and quadratic regression of the appropriate deviation from the mean of age of dam (AD),

b3 = linear regression coefficient of the appropriate deviation from the mean of age at recording (AL; except for BW) and

eijklm = random error with zero mean and variance IF2e

 

3.3 Results and discussion

3.3.1 Model specification

Significance levels obtained after fitting the model for each trait, are summarized in Table 3.1. From Table 3.1 it is evident that year-season of birth, sex and birth/rearing status of the lamb had a significant (P<0.001) influence on body weight at all ages as well as on CFW. MFD was significantly influenced by year-season of birth and sex of lamb. Age of dam had a significant quadratic effect on all traits, with the exception of MFD, while age at recording significantly influenced all traits for which it was included.

Table 3.1 Model specification for BW, WW, W5 - W12, W18, CFW and MFD  

Effect

BW

WW

W5 - W12

  W18 CFW MFD
  df P df P df P df P
YS 18 *** 17 *** 18 *** 16 *** *** ***
Sex 1 *** 1 *** 1 *** 1 *** *** ***
BS 1 ***                
WS     2 *** 2 *** 2 *** *** ns
YS*sex 18 ns 17 ** 18 *** 16 *** *** **
YS*WS 18 ns 31 *** 31 *** 30 * ns ns
Sex*WS 1 ns 2 ns 2 ns 2 ns ns ns
Regr                    
Age (L)     1 *** 1 *** 1 *** *** ***
AD (L) 1 *** 1 ** 1 *** 1 ** ** ns
AD (Q) 1 *** 1 *** 1 *** 1 * *** ns
Error 3168   2936   2771   2810      

 

df = Degrees of freedom

YS = Year-season of birth

BS = Birth status WS = Rearing status

AD = Age of dam

* = P<0.05; ** = P<0.01; *** = P<0.001; ns = P>0.05

 

3.3.2 Two-way interactions

For all traits, the interaction sex*WS was non-significant, and can therefore be ignored in all cases. Significant two-way interactions between year-season of birth and sex for WW, W5 to W12, W18, CFW and MFD, and between year-season of birth and rearing status for WW, W5 to W12 and W18 were found.

The significant two-way interaction of YS*sex for body weight can most probably be explained as follows : Ram lambs were consistently heavier than ewe lambs. However, during favourable years this difference was much more accentuated. It seems reasonable to assume that under good feeding conditions, the full growth potential of the rams could be expressed. The significance (P<0.001) of the YS*sex interaction pertaining to CFW, was probably due to the fact that in some years rams had a higher and in other years a lower CFW than ewes, although in most years there was no significant difference in CFW between the two sexes. The same applies to MFD, where ewes generally had a higher MFD when compared to rams.

The interaction YS*WS was significant for all body weights (except birth weight). Single born and raised lambs (S/S) were consistently heavier than multiple born and raised lambs (M/M). The multiple born single raised lambs (M/S) were heavier than S/S lambs in some years; some years closer to S/S lambs in body weight, while closer to M/M lambs in other years. M/S lamb groups for the various years differed probably as a result of variation in the time they were single. Some M/S lambs were raised as single lambs from birth, some from within a week after birth, while others have become single approaching weaning.

 

3.3.3 Least-squares means

Least-squares means, coefficients of variation (CV%) and least-squares effects for sex and birth/rearing status of the lamb are presented in Table 3.2. The effects are expressed as deviations from the least-squares mean. The effect of year-season of birth is obvious and well documented and will therefore not be discussed here.

Ram lambs were heavier than ewe lambs at birth and remained heavier throughout their lives. The 0.27 kg difference recorded at birth had already increased to 2.30 kg at weaning. Thereafter it increased progressively to a difference of 11.87 kg in body weight at 18 months of age. Similar differences in body weight between ram and ewe lambs were recorded for several other South African sheep breeds (Heydenrych, 1975; Fourie & Heydenrych, 1982; Cloete & de Villiers, 1987; Badenhorst, 1989; van Wyk et al., 1993a).

Multiple born lambs were 17% lighter at birth than single born lambs (Table 3.2). At weaning, S/S lambs were 4.8% and 26.4% heavier than M/S and M/M lambs respectively, while M/S lambs had a 20.6% advantage over their M/M counterparts. These differences decreased gradually to 1.2%, 6.9% and 5.6% respectively at 18 months of age. Similar to the foregoing, significant effects of birth/rearing status on body weight at various ages are well documented for different sheep breeds (Shelton & Campbell, 1962; Bichard & Cooper, 1966; Wiener & Hayter, 1974; Olson et al., 1976; Fourie & Heydenrych, 1982; Shrestha & Vesely, 1986; Cloete & de Villiers, 1987; Mavrogenis, 1988; Boujenane et al., 1991a; van Wyk et al., 1993a).

Table 3.2 Least-squares means (SE), coefficients of variation (CV%) and least-squares effects for BW, WW, W5 - W12, W18, CFW and MFD by sex and birth/rearing statusa

Trait LS (SE) CV% Rams Ewes Single Multiple
BW 4.84 (0.04) 16.22 0.135 -0.135 0.415 -0.415

 

Trait LS (SE) CV% Rams Ewes S/S M/S M/M
WW 29.55 (0.31) 17.34 1.149 -1.149 2.738 1.263 -4.001
W5 30.95 (0.36) 15.79 1.492 -1.492 2.315 1.237 -3.552
W6 33.38 (0.35) 14.97 1.916 -1.916 1.958 1.395 -3.353
W7 36.57 (0.38) 13.53 2.120 -2.120 1.837 1.249 -3.086
W8 39.27 (0.41) 13.17 2.197 -2.197 1.739 1.249 -2.988
W9 42.35 (0.44) 12.33 2.291 -2.291 1.519 1.305 -2.824
W10 45.96 (0.47) 12.55 3.417 -3.417 1.461 1.140 -2.601
W11 48.81 (0.50) 12.93 4.138 -4.138 1.115 1.536 -2.651
W12 52.32 (0.52) 12.99 4.501 -4.501 1.529 1.149 -2.678
W18 54.13 (0.55) 14.86 5.935 -5.935 1.418 0.739 -2.157
CFW 2.01 (0.03) 17.26 0.030 -0.030 0.090 -0.015 -0.075
MFD 21.47 (0.09) 6.21 -0.214 0.214 -0.052 -0.001 0.053

a Multiple = Lambs born as twins or triplets; S/S = Single born, single raised lambs;

M/S = Multiple born, single raised lambs; M/M = Multiple born, multiple raised lambs

Rams produced fleeces which were 0.06 kg heavier and 0.21 micron finer than those produced by the ewes. S/S lambs produced fleeces which were 0.105 kg and 0.165 kg heavier than M/S and M/M lambs respectively. Fleeces of M/S lambs were 0.090 kg heavier than those of M/M lambs. Rearing status had no significant effect on mean fibre diameter in this study.

 

3.3.4 Age of dam and age of lamb at recording

Regression coefficients for age of dam (b1 and b2) and age of lamb at recording (b3) are summarized in Table 3.3.

Regression coefficients calculated for body weight on age of dam were lower than those reported by Olson et al. (1976). Performance of progeny from 2-year old and 7-year and older ewes generally differ significantly from that of 3- to 6-year old ewes (Shelton & Campbell, 1962; Bichard & Cooper, 1966; Fourie & Heydenrych, 1982; Shrestha & Vesely, 1986; Cloete & de Villiers, 1987; Mavrogenis, 1988; Schoeman, 1990; Boujenane et al., 1991a; van Wyk et al., 1993a). A possible explanation for the results obtained in this study, is that ewes had been mated at 18 months of age for the first time. At that stage, they had nearly reached mature weight, and consequently did not suffer more than the older ewes from the stress of pregnancy and lactation. Furthermore, due to the harsh grazing conditions at Carnarvon, teeth wear easily and no ewes older than 6 years were kept in the flock.

Age at recording had a significant influence on all traits analyzed. From Table 3.3 it is evident that this influence got smaller with age, but it never disappeared completely.

Table 3.3 Regression coefficientsa for age of dam and age of lamb for BW, WW, W5 - W12, W18, CFW and MFD

Trait b1 b2 b3
BW 0.224D-3 (0.027D-3) -0.026D-5 (0.005D-5)  
WW 0.427D-3 (0.159D-3) -0.151D-5 (0.027D-5) 0.211 (0.009)
W5 0.546D-3 (0.164D-3) -0.118D-5 (0.028D-5) 0.188 (0.009)
W6 0.511D-3 (0.168D-3) -0.114D-5 (0.029D-5) 0.184 (0.009)
W7 0.585D-3 (0.174D-3) -0.094D-5 (0.030D-5) 0.171 (0.009)
W8 0.591D-3 (0.179D-3) -0.134D-5 (0.030D-5) 0.163 (0.009)
W9 0.667D-3 (0.187D-3) -0.142D-5 (0.032D-5) 0.153 (0.009)
W10 0.670D-3 (0.201D-3) -0.126D-5 (0.034D-5) 0.140 (0.012)
W11 0.683D-3 (0.212D-3) -0.083D-5 (0.036D-5) 0.125 (0.013)
W12 0.693D-3 (0.226D-3) -0.107D-5 (0.039D-5) 0.117 (0.014)
W18 0.566D-3 (0.232D-3) -0.076D-5 (0.040D-5) 0.090 (0.014)
CFW 0.040D-3 (0.016D-3) -0.011D-5 (0.003D-5) 0.004 (0.001)
MFD     0.014 (0.004)

a b1 = Age of dam (linear); b2 = Age of dam (quadratic); b3 = Age of lamb (linear)


3.4 Conclusions

The results obtained in this study confirm the importance of non-genetic factors as sources of variation in body weight and fleece traits of Afrino sheep. Year-season of birth, sex and birth/rearing status of the lamb as well as age of dam and age of lamb were important sources of variation for CFW and body weight at all ages. MFD was significantly (P<0.001) influenced by year-season of birth, sex and age of lamb. Accordingly, these factors should thus be included in an operational model fitted for the estimation of genetic parameters or breeding values for Afrino sheep. Failure to do so could result in the impediment of selection progress due to the use of inaccurate genetic parameters and breeding values in the selection programme.

 

Table of contents

 

CHAPTER 4

DIRECT AND MATERNAL (CO)VARIANCE COMPONENTS AND HERITABILITY ESTIMATES FOR FLEECE TRAITS AND BODY WEIGHT AT DIFFERENT AGES

4.1 Introduction

To optimise genetic gain, information on direct as well as maternal breeding values, where applicable, for the traits under selection should be utilised. The availability of modern statistical software has simplified the partitioning of variance into components resulting from either direct or maternal effects. These components must be known, at least to proportionality, for use in mixed model equations to obtain BLUP of breeding values.

The objectives of this study were, firstly, to determine the most effective model of analysis, applying restricted maximum likelihood procedures (REML), for body weight at different ages, as well as clean fleece weight and mean fibre diameter measured at 16 months of age, and secondly, to estimate (co)variance components and genetic parameters for each of these traits.

 

4.2 Material and methods

4.2.1 Data

Data collected on the Carnarvon Afrino flock over the period 1975 to 1992 were used for this study. Traits analyzed were body weight at birth (BW), weaning (WW), monthly from five to 12 months of age (W5 to W12), and at 18 months of age (W18), as well as clean fleece weight (CFW) and mean fibre diameter (MFD).

The data sets for BW and W18 respectively included 4235 and 3748 animals with data records, 146 and 145 sires and 946 and 928 dams with progeny in the respective data sets. The adjusted mean and coefficient of variation for each trait are presented in Table 4.1.

Table 4.1 Description of the data set

Traita Mean CV%
BW 4.65 kg 16.43
WW 27.67 kg 19.69
W5 29.82 kg 19.62
W6 32.20 kg 19.12
W7 35.61 kg 17.02
W8 38.37 kg 17.57
W9 41.35 kg 17.09
W10 45.48 kg 17.88
W11 48.78 kg 18.67
W12 52.32 kg 18.33
W18 53.80 kg 18.98
CFW 2.01 kg 20.83
MFD 21.40 µm 7.68

 

a BW = birth weight, WW = weaning weight, W5 - W12 = five to twelve month body weight, W18 = eighteen month body weight, CFW = clean fleece weight, MFD = mean fibre diameter

 

4.2.2 Variance component and genetic parameter estimation

(Co)variance components were estimated using the DFREML programme of Meyer (1989, 1993). Single trait animal models were fitted for all traits. By ignoring or including maternal genetic or environmental effects, five different models of analysis were fitted for each trait :

                y = Xb + Z1a + e                             (1)

                y = Xb + Z1a + Z2m + e

                with cov(a,m) = 0                             (3)

                y = Xb + Z1a + Z2m + e

                with cov(a,m) = AFam                       (4)

                y = Xb + Z1a + Z2m + Z3c + e

                with cov(a,m) = 0                            (7)

                y = Xb + Z1a + Z2m + Z3c + e

                with cov(a,m) = AFam                       (8)

where y is a vector of observed traits of animals; b, a, m and c are vectors of fixed effects, direct additive genetic effects, maternal additive genetic effects and maternal permanent environmental effects respectively; X, Z1, Z2 and Z3 are the corresponding incidence matrices relating the effects to y; e is the vector of residuals; A is the numerator relationship matrix, and Fam is the covariance between direct additive genetic and maternal additive genetic effects.

It was assumed that :

V(a) = AF2a; V(m) = AF2m; V(c) = IF2c; V(e) = IF2e

where I is an identity matrix, F2a, F2m, F2c and F2e is the direct additive genetic variance, maternal additive genetic variance, maternal permanent environmental variance and residual variance respectively.

The fixed part of the model for BW included fixed effects for year-season of birth (YS), sex and birth status of the lamb, as well as a covariate for age of dam (linear and quadratic). For WW, W5 to W12 and W18, fixed effects for YS x sex and YS x rearing status, and covariates for age of dam (linear and quadratic) and age of the lamb at each body weight, were included. For CFW, fixed effects for YS x sex and rearing status were included, while only YS x sex was included for MFD. Furthermore, the covariate age of dam, was excluded for MFD. A detailed description of all fixed effects and covariates included in the models is given in Chapter 3.

Log likelihood ratio tests were carried out amongst all five models to determine the most suitable model for each trait. An effect was considered to have a significant influence when its inclusion caused a significant increase in log likelihood, compared to the model in which it was ignored. When minus two times the difference between the log likelihoods was greater than values of the chi-square distribution with one degree of freedom, the effect was considered to have a significant influence. A significance level of P<0.01 was used for all traits.

Approximate sampling errors were calculated for each trait for the model which best describes the respective data by fitting a quadratic function to the profile likelihood for each parameter involved (Meyer & Hill, 1992).

 

4.3 Results and discussion

In Table 4.2 the log likelihood values obtained under the five different models of analysis are summarized for each trait as deviations from the log likelihood value obtained under the most suitable model.

The best model for BW included a maternal genetic as well as permanent environmental effect (Model 7). The latter incorporated both similarities between twins and similarities between lambs born to the same ewe in different years.

For all other body weights, the choice whether the genetic covariance between direct and maternal effects had a significant influence or not, depended upon a very narrow margin. These differences could be ascribed solely to differences in the individual data sets. It was therefore decided, for the purpose of this study, to consider Model 3 as the most suitable model for WW, W5 to W12 and W18.

 

CFW and MFD were significantly influenced only by the direct additive genetic effect (Model 1).

Table 4.2 Log likelihood valuesa obtained for each trait under five different models of analysis

Trait Model 1 Model 3 Model 4 Model 7 Model 8
BW -93.829** -10.516** -9.700** 0 0.485
WW -48.980** 0 1.670 1.109 2.482
W5 -48.964** 0 3.372** 0.859 3.739**
W6 -38.937** 0 3.173 0.201 3.202
W7 -25.620** 0 -3.293 0.171 3.312
W8 -24.832** 0 -2.309 0.116 2.312
W9 -15.973** 0 3.485** 0.173 3.486**
W10 -12.058** 0 0.778 0.001 0.778
W11 -12.124** 0 2.161 0.076 2.161
W12 -6.580** 0 1.869 1.202 2.540
W18 -10.252** 0 2.653 0 2.652
CFW 0 2.566 0.019 3.254  3.342**
MFD 0 0.013 0.029 0.650 0.695

 

a as deviation from the most suitable model

** P<0.01

 

Estimates of (co)variance components and genetic parameters for growth and fleece traits, as estimated under the most suitable model for each trait, are presented in Tables 4.3 and 4.4 respectively. Published heritability estimates for body weight and CFW and MFD are summarized in Tables 4.5 and 4.6 respectively.

 

4.3.1 Direct and maternal heritability estimates

Direct heritability estimates obtained for body weights in this study (Table 4.4) were within the scope reported by various authors for several sheep breeds worldwide (Table 4.5). From Table 4.4 it is evident that direct heritability (h2a) of body weight increased up to eight months of age, and then it seemed to stabilise. Maternal heritability estimates (h2m) for body weight, on the other hand, increased from birth (0.09) to five months of age (0.17), whereafter it decreased gradually. Heritability of the total additive genetic component (h2T) displayed the same tendency as h2a; it increased up to eight months of age, and then stabilised. Similar results of increased heritability for body weight with age were obtained by Ercanbrack & Price (1972) for Rambouillet and Columbia sheep.

From Tables 4.3 and 4.4 it is evident that the maternal variance and h2m for birth weight were lower than the direct variance and h2a. These results are contradictory to those found in most other studies (Table 4.5). Meyer (1992) and Waldron et al. (1993) indicated that the relative values of h2m and h2a are influenced by the specific model used. Estimates of h2m could be biased upwards if a maternal permanent environmental effect exists and is not included in the model. Exclusion of the permanent environmental effect of the dam in this study (Model 3), for example caused h2m (0.25) for BW to be higher than h2a (0.18). Maternal variance, in this case, also included variance due to the permanent environmental effect of the dam. This could possibly explain the relatively higher h2m reported by van Wyk et al. (1993b), in whose study permanent environmental effects were not included in the model.

Direct heritability estimates obtained for CFW and MFD in the present study (Table 4.4), were higher than those reported for several South African sheep breeds (Table 4.6). These differences could partly be explained through the use of different models of analysis. It seemed that the more detailed animal model yielded higher heritability estimates than the more simple sire model. Both Mortimer & Atkins (1994) and Olivier et al. (1994), using an animal model, for example, reported a direct heritability of 0.62 for MFD in Merino sheep at 14 - 16 months of age. The relatively high heritability estimated for CFW in this study could further be ascribed to the large variation that exists in this flock in terms of CFW. This is due to the fact that no selection for CFW was carried out in the flock since it originated in 1969.

 

4.3.2 Permanent environmental effect of the dam

The recorded c2 for BW of 0.12 is in accordance with that of 0.10 obtained by Maria et al. (1993), lower than those of 0.27 to 0.37 reported by Tosh & Kemp (1994), but higher than that of 0.02 estimated by Swan & Hickson (1994). The relatively high c2 for BW could most probably be ascribed to the permanent environmental effect of the uterine environment, as well as the effect of multiple births.

Factors which permanently influence the milk production of the ewe, such as udder defects, largely contributed to the c2 value for WW. In this flock, however, all ewes with defective udders were culled. Consequently, this source of variation in c2 was eliminated and could possibly explain why c2 for WW was non-significant in this study.

Table 4.3 Estimates of variance componentsa for BW, WW, W5 to W12, W18, CFW (kg2) and MFD (µm2)

Trait F2a F2m F2c F2e F2p
BW 0.092 0.035 0.049 0.233 0.409
WW 4.283 2.253   6.641 13.178
W5 5.393 2.348   6.458 14.199
W6 6.993 2.042   5.971 15.006
W7 8.004 1.718   6.363 16.084
W8 10.413 1.740   5.642 17.795
W9 11.462 1.494   6.315 19.272
W10 13.032 1.235   7.540 21.806
W11 12.826 1.619   8.939 23.385
W12 15.299 1.331   9.667 26.297
W18 16.103 1.822   10.642 28.566
CFW 0.089     0.055 0.144
MFD 1.746     0.640 2.386

a F2a = direct additive genetic variance, F2m = maternal additive genetic variance, F2c = maternal permanent environmental variance, F2e = residual variance, F2p = phenotypic variance

 

Table 4.4 Estimates of genetic parametersa for BW, WW, W5 to W12, W18, CFW and MFD

Trait h2a h2m c2 h2T
BW 0.22 0.09 0.12 0.27
WW 0.33 0.17   0.41
W5 0.38 0.17   0.46
W6 0.47 0.14   0.53
W7 0.50 0.11   0.55
W8 0.59 0.10   0.63
W9 0.59 0.08   0.63
W10 0.60 0.06   0.63
W11 0.55 0.07   0.58
W12 0.58 0.05   0.61
W18 0.56 0.06   0.60
CFW 0.62     0.62
MFD 0.73     0.73

 

a h2a = direct heritability; h2m = maternal heritability; c2 = maternal permanent environmental variance as a proportion of phenotypic variance; h2T = total heritability; SE range for h2a = 0.03 - 0.06; h2m = 0.01 - 0.04; c2 = 0.03

 

Table 4.5 Summary of published heritability estimatesa (SE) for BW, WW and W12b

Birth weight
Breed PHS-h2 AM-h2a AM-h2m Reference
Bikaneri 0.10(0.03)     Chopra & Acharya (1971)
Dohne 0.21(0.14)     Fourie & Heydenrych (1982)
D'man 0.34(0.08)     Boujenane & Kerfal (1990)
Afrino 0.30(0.08)     Badenhorst et al. (1991)
Ra, T, Coc   0.20-0.34 0.30-0.65 Burfening & Kress (1993)
Romanov   0.04 0.22 Maria et al. (1993)
Dormer   0.16(0.03) 0.43(0.03) van Wyk et al. (1993b)
Hampshire   0.39 0.22 Tosh & Kemp (1994)
P Dorset   0.12 0.31 Tosh & Kemp (1994)
Merino   0.30(0.05) 0.29(0.05) Vaez Torshizi et al. (1996)
Weaning weight
Bikaneri 0.24(0.05)     Chopra & Acharya (1971)
Dohne 0.39(0.17)     Fourie & Heydenrych (1982)
D'man 0.52(0.10)     Boujenane & Kerfal (1990)
Afrino 0.21(0.07)     Badenhorst et al. (1991)
Ra, T, Co   0.09-0.22 0.07-0.48 Burfening & Kress (1993)
Romanov   0.09 0.01 Maria et al. (1993)
Dormer   0.13(0.03) 0.20(0.03) van Wyk et al. (1993b)
Merino   0.19(0.03) 0.16(0.02) Swan & Hickson (1994)
Hampshire   0.39 0.19 Tosh & Kemp (1994)
P Dorset   0.25 0.08 Tosh & Kemp (1994)
Romanov   0.14 0.02 Tosh & Kemp (1994)
Merino   0.27(0.03) 0.11(0.01) Mortimer & Atkins (1995)
Merino   0.27(0.05) 0.42(0.06) Vaez Torshizi et al. (1996)
12 month body weight
Bikaneri 0.48(0.07)     Chopra & Acharya (1971)
Dohne 0.37(0.17)     Fourie & Heydenrych (1982)
Merino 0.41(0.15)     van Wyk et al. (1985)
Afrino 0.22(0.07)     Badenhorst et al. (1991)
Merino   0.28(0.06) 0.14(0.03) Swan & Hickson (1994)
Merino   0.42(0.05) 0.07(0.02) Mortimer & Atkins (1994)

  

a PHS-h2 = Paternal half-sib heritability estimate; AM-h2a = Animal model direct heritability estimate; AM-h2m = Animal model maternal heritability estimate

b Published paternal half-sib heritability estimates of body weight at various ages is summarized by Shrestha & Heaney (1985)

c Ra = Rambouillet; T = Targhee; Co = Columbia

 

Table 4.6 Summary of published heritability estimatesa (SE) for CFW and MFD

Clean fleece weight
Breed PHS-h2 AM-h2a Reference
SAMMb 0.30(0.21)   Vosloo (1967)
Merino 0.31(0.07)   Heydenrych (1975)
Dohne 0.25(0.15)   Fourie & Heydenrych (1982)
Merino 0.23(0.12)   van Wyk et al. (1985)
Afrino 0.30(0.08)   Badenhorst et al. (1991)
Merino   0.36(0.03) Mortimer & Atkins (1994)
Merino   0.29(0.00) Olivier et al. (1994)
Merino   0.37 Vaez Torshizi et al. (1995)
Mean fibre diameter
Merino 0.22(0.06)   Heydenrych (1975)
Dohne 0.60(0.20)   Fourie & Heydenrych (1982)
Merino 0.23(0.12)   van Wyk et al. (1985)
Afrino 0.39(0.09)   Badenhorst et al. (1991)
Merino   0.62(0.04) Mortimer & Atkins (1994)
Merino   0.62(0.02) Olivier et al. (1994)
Merino   0.45(0.04) Swan & Hickson (1994)
Merino   0.67 Vaez Torshizi et al. (1995)

 

a PHS-h2 = Paternal half-sib heritability estimate; AM-h2a = Animal model direct heritability estimate

b South African Mutton Merino

 

4.3.3 Genetic correlation between direct and maternal effects

Although not significant according to log likelihood ratio tests, positive genetic correlations of 0.36 to 0.46 between direct and maternal effects were estimated for the different body weights in this study. Positive genetic correlations between direct and maternal effects (rGam) reported for body weight at 15 months (Mortimer & Atkins, 1994), 14 - 16 months (Olivier et al., 1994), 12 months (Swan & Hickson, 1994), 4, 6 and 18 months (Snyman et al., 1996) and 16 and 22 months of age (Vaez Torshizi et al. (1996) in Merino sheep are in accordance with those found in this study. These estimates are in contrast with other negative rGam values reported for sheep (Khaldi & Boichard, 1991; Burfening & Kress, 1993; Maria et al., 1993; van Wyk et al., 1993b; Tosh & Kemp, 1994; Vaez Torshizi et al. 1996).

Meyer (1992) and Waldron et al. (1993) also reported conflicting results for different beef breeds for 12 and 18 month weight. Waldron et al. (1993) provided a possible explanation for these contradictory results. They contemplated that as most of the variance components for beef cattle had been estimated with sire-maternal grandsire (S-MGS) models, the use of a more detailed animal model could have caused these differences. This, however, does not explain the different results obtained by van Wyk et al. (1993b) for Dormer sheep, Maria et al. (1993) for Romanov sheep, Olivier et al. (1994) and Snyman et al. (1996) for Merino sheep and Tosh & Kemp (1994) for Hampshire, Polled Dorset and Romanov sheep, all having used an animal model.

Apart from discrepancies caused by different models applied and real differences between populations analyzed, differences in data structure and size could play an important role. Breed differences, however, most probably do exist in terms of the magnitude and sign of the genetic correlation between direct and maternal effects.

 

4.4 Conclusions

This study proved the importance of implementing the correct model of analysis for the estimation of (co)variance components and genetic parameters. For example, ignoring all maternal effects if these effects have a significant influence, leads to the over-estimation of direct as well as total heritabilities. Furthermore, the exclusion of the maternal permanent environmental effect, when it has a significant influence, as for BW, could cause estimates of maternal heritabilities to be biased upwards. On the other hand, sampling errors are greatly increased when maternal effects are estimated and these effects must not be included if they have no significant influence on the trait in question.

The maternal influence never disappeared completely, because of a carry-over effect after weaning, and was significant for all body weights in the Carnarvon Afrino flock. Direct additive genetic effects were, however, more important than maternal genetic effects for all traits analyzed.

At this stage it seems reasonable to recommend that selection could be based on single breeding values, based on total heritability, for the traits considered important.

 

Table of contents

 

CHAPTER 5

AN INVESTIGATION INTO THE POSSIBLE GENETIC IMPROVEMENT OF REPRODUCTION AND SURVIVAL RATE USING A THRESHOLD MODEL

5.1 Introduction

In almost any sheep enterprise, reproductive performance is of utmost importance in the efficiency of sheep production. Numerous studies involving the component traits of reproduction, i.e. fertility, litter size, lamb survival rate and number of lambs born and weaned per ewe joined, have been done to obtain heritability estimates for these individual traits (Fogarty, 1995). Most of these estimates have been obtained from paternal half-sib analysis, treating the traits as continuous variables and assuming they follow a normal distribution. However, in these traits the phenotype is expressed in two or more distinct, mutually exclusive and exhaustive categories. If a polygenic mode of inheritance is assumed for these traits, it is evident that the threshold concept (Wright, 1934) as discussed inter alia by Dempster & Lerner (1950), Bulmer (1980) and Gianola (1982) should apply.

The objective of this study was to estimate heritability and breeding values on the underlying scale for different reproductive traits and survival rate in Afrino sheep to ascertain whether they can be improved by selection on estimated breeding values.

 

5.2 Material and methods

5.2.1 Data

Data collected on the Carnarvon Afrino flock over the period 1972 to 1994 were used for this study. The reproductive traits analyzed included fertility (whether a ewe lambed or not; 0 or 1), litter size (number of lambs born to a ewe mated; 0, 1, 2 or 3), fecundity (number of lambs born to a ewe that lambed; 1, 2 or 3), number weaned (number of lambs weaned to a ewe mated; 0, 1, 2 or 3) and survival rate from birth to weaning (whether a lamb born alive, was dead or alive at weaning; 0 or 1).

Data were edited to include only sires with more than 7 progeny. The number of data records, number of sires, categories and thresholds for each of the traits analyzed, are summarized in Table 5.1.

Table 5.1 Description of the data set

Trait No. of records No. of sires No. of categories No. of thresholds
Fertility 3580 113 2 1
Litter size 3580 113 4 3
Fecundity 3186 108 3 2
Number weaned 3580 113 4 3
Survival rate 4816 148 2 1

 

5.2.2 Statistical analysis

Data were analyzed by means of the GFCAT set of programmes, developed by Konstantinov (1995). GFCAT is a set of programmes for the analysis of mixed threshold models with support for REML-type variance components estimation based on the methods of Gianola & Foulley (1983). Under these models, the respective traits occur as a result of an underlying unobserved phenotype exceeding a given threshold (Konstantinov et al., 1994). The unobserved continuous phenotypes are assumed to be normally distributed. For each trait a vector, µ, of means corresponding to subpopulations determined by combinations of levels of fixed b and random s factors, is modelled as :

µ = Xb + Zs

where µ is a vector of underlying means, b is a vector associated with the effects of year-season (26), age of dam in years (5) and birth status (3; for survival rate only), s is a vector of sire effects and X and Z are design matrices. The s effects are assumed to be normally distributed, with E(s)=0 and Var(s)=AF2s, where A is a numerator relationship matrix. Solutions for thresholds, b and s were computed as described by Konstantinov et al. (1994). All traits were analyzed separately.

 

5.3 Results and discussion

Solutions for thresholds, age of dam and birth status are presented in Table 5.2. As the underlying scale is unknown, these solutions are expressed in units of residual standard deviations of the underlying variable.

Table 5.2 Thresholds and solutions for age of dam and birth status

  Fertility Litter size Fecundity Number weaned Survival rate
Thresholds
1 0.0000 0.0000 0.0000 0.0000 0.0000
2   1.6258 2.2994 1.4742  
3   3.7969   3.5387  
Age of dam
2 1.5079 0.9872 -0.4011 0.7357 0.0552
3 1.7746 1.4092 0.0686 1.1960 0.2264
4 1.8913 1.6067 0.3210 1.3769 0.1997
5 1.9096 1.6379 0.3327 1.3565 0.1313
6 1.7220 1.5268 0.2726 1.2606 0.0000
Birth status
1         1.4450
2         1.1137
3         0.8872

Fertility, litter size, fecundity and number weaned increased with an increase in age of dam from two to four years, after which it showed a slight decline. The effect of age of dam on survival rate showed no distinct pattern. As expected, survival rate decreased with increasing birth status of the lambs.

Estimates of sire variances and heritabilities for the different traits are supplied in Table 5.3.

Table 5.3 Estimates of sire variances and heritabilities on the underlying scale

Trait Sire variance Heritability
Fertility 0.05342 0.20
Litter size 0.07366 0.27
Fecundity 0.11832 0.42
Number weaned 0.04919 0.19
Survival rate 0.00515 0.02

These results, indicating that reproduction rate in Afrino sheep can be improved genetically by selection for multiple births, are in accordance with previous findings in the literature (Konstantinov et al., 1994; Fogarty, 1995). Virtually no sire direct (genetic) influence was exhibited for survival rate.

Table 5.4 shows features of the sire genetic evaluation for the different traits. It is interesting to note that the sires with the worst breeding values for littersize, fecundity and number weaned had more records than the sires with the best breeding values for the respective traits, which can easily happen in practice if the breeding values are unknown.

Table 5.4 Breeding value estimatesa for the best and the worst sire for each trait

Trait Best sire (n)b Worst sire (n)
Fertility 0.7097 (79) -0.2943 (28)
Litter size 0.5564 (32) -0.2947 (55)
Fecundity 0.5383 (15) -0.5933 (28)
Number weaned 0.4025 (7) -0.2216 (55)
Survival rate 0.0310 (48) -0.0440 (39)

 

a Expressed in units of Fe

b No. of records

 

5.4 Conclusions

The results suggest that reproduction rate, but not survival rate, can be increased in Afrino sheep by selection on breeding values estimated on the underlying scale applying a threshold model. These estimates could be especially useful in the final selection of sires to be used in AI programmes.

 

Table of contents

 

CHAPTER 6

GENETIC PARAMETER ESTIMATES FOR TOTAL WEIGHT OF LAMB WEANED IN AFRINO AND MERINO SHEEP


6.1 Introduction

In almost any sheep meat enterprise, total weight of lamb weaned per year is the best single measure of a flock's productivity. In contrast to the component traits of reproduction, the composite trait, total weight of lamb weaned per ewe joined, has received much less attention (Fogarty, 1995).

In breeds such as the Afrino, which has a weaning percentage of approximately 130 % under extensive conditions, an increase in litter size under these conditions is undesirable. In view of the limited natural resources, an increase in number of lambs is not the answer to generate higher income from a specific farming enterprise, but the aim should rather be to increase the quality and monetary value of the product in terms of weight and carcass quality.

Almost all the heritability estimates for weight of lamb produced cited in the literature were obtained with paternal-halfsib or regression methods (Basuthakur et al., 1973; More O'Farrell, 1976; Martin et al., 1981; Fogarty et al., 1985; Owen et al., 1986; Abdulkhaliq et al., 1989; Long et al., 1989; Boujenane et al., 1991b). However, with an analysis under an animal model, the full relationship matrix is exploited and any genetic variance due to the dam would also be accounted for in the estimation of heritability.

As virtually no animal model heritability estimates for total weight of lamb weaned could be found in the literature with which to compare those obtained after analysing the Afrino data, and since the available Afrino data set was relatively small, it was decided to include two larger data sets, obtained from the Carnarvon Merino flock and Grootfontein Merino stud respectively, in the analysis. In doing so it was hoped to obtain more reliable results on which recommendations regarding selection for total weight of lamb weaned could be based.

The objective of this study was therefore to obtain heritability estimates for total weight of lamb weaned in Afrino and Merino sheep by applying restricted maximum likelihood procedures (REML) under an animal model. Genetic improvement of lifetime reproductive performance is not practical by direct selection, but is dependent upon selection for correlated traits. Therefore the genetic correlation between total weight of lamb weaned at the first and subsequent parities was also estimated to ascertain whether selection cannot be performed earlier.

 

6.2 Material and methods

6.2.1 Data

Data collected on the Carnarvon Afrino flock (from 1972 to 1994), the Carnarvon Merino flock (from 1962 to 1983) and the Grootfontein Merino stud (from 1966 to 1993) were used for this study. Detailed descriptions of the management and selection procedures followed in these flocks are given in Chapter 2 for the Afrino flock, Erasmus et al. (1990) for the Carnarvon Merino flock and Olivier (1989) for the Grootfontein Merino stud.

The Carnarvon Afrino and Merino flocks were kept on natural pasture at the Departmental Experimental Station near Carnarvon in the North-western Karoo region (Chapter 2). The Grootfontein Merino stud is kept at Grootfontein Agricultural Development Institute near Middelburg (31E 28'S, 25E 1'E) in the North-eastern Karoo region. The stud is run under favourable nutritional conditions, which include irrigated pastures and supplementary feeding.

Young ewes in all three flocks were mated at 18 months of age for the first time. Average recorded two tooth body weights were 54.13 kg for Afrino sheep (Chapter 3), 35.18 kg for Carnarvon Merino sheep (Snyman et al., 1996) and 50.39 kg for Grootfontein Merino sheep (Olivier et al., 1994). The average number of lambs born per ewe lambing over the experimental period was 1.53 for Carnarvon Afrino, 1.14 for Carnarvon Merino and 1.56 for Grootfontein Merino sheep. The corresponding number of lambs weaned per ewe joined was 1.27, 0.70 and 1.09 respectively.

For all three flocks, the available data for each ewe for each lambing season included identity of the sire of the lamb/s, birth date, identity, birth weight, sex, birth status and weaning weight of each lamb.

From these data, the total weight of lamb weaned for each ewe joined for each lambing season (TWW/EJ) was calculated as follows :

Firstly, within each lambing season, weaning weight for all lambs was corrected to 120 days, followed by least-squares corrections for sex of the lamb. No corrections were made for birth status. Secondly, TWW/EJ was calculated by adding the corrected weaning weight of all the lambs weaned by each ewe in that specific lambing season.

Subsequently, total weight of lamb weaned by each ewe over n lambing opportunities (TWWn; n=1...7), was calculated. For example, total weight of lamb weaned over three lambing opportunities (TWW3) was calculated as the sum of TWW/EJ for the first, second and third lambing opportunities. For each ewe in the data set, total weights of lamb weaned over one (TWW1), two (TWW2), three (TWW3), four (TWW4), five (TWW5), six (TWW6) and seven (TWW7) lambing opportunities were calculated, depending upon the number of lambing opportunities each ewe had over her lifetime in the specific flock. The number of ewe records available for total weight of lamb weaned in the original data sets for the three flocks for each n number of lambing opportunities are summarized in Table 6.1. These include only records of ewes which had n consecutive number of lambing opportunities for each respective TWWn.

From Table 6.1 it is evident that between 50 and 75 % of the ewes in the three flocks had at least three to four lambing opportunities. For the purpose of this study, TWW3 and TWW4 were taken as an indication of lifetime reproductive performance for Afrino and Merino ewes respectively. Therefore, the traits TWW1, TWW2 and TWW3 were analyzed for the Carnarvon Afrino flock, and TWW1, TWW2, TWW3 and TWW4 were analyzed for the Carnarvon and Grootfontein Merino flocks.

Table 6.1 Number of ewe records for each n number of lambing opportunities

Traita No. of ewe records
  Carnarvon Afrino Carnarvon Merino Grootfontein Merino
TWW1 1025 2510 2570
TWW2 794 2237 2073
TWW3 640 1991 1616
TWW4 452 1891 1195
TWW5 243 1466 635
TWW6 0 746 232
TWW7 0 93 27

 

a TWW1 ... TWW7 = Total weight of lamb weaned by ewes which had at least one ... seven consecutive lambing opportunities respectively;


6.2.2 Variance component and genetic parameter estimation

(Co)variance components were estimated using the DFREML programme of Meyer (1989, 1991, 1993). Full pedigrees were available for all flocks. Single trait animal models, including direct additive genetic effects and a fixed effect for year-season of birth of the ewe, were fitted for the estimation of heritability for TWW1, TWW2, TWW3 and TWW4. Bivariate animal models were fitted for the estimation of genetic and phenotypic correlations among these traits. Approximate sampling errors were calculated by fitting a quadratic function to the profile likelihood for each parameter (Meyer & Hill, 1992).


6.3 Results and discussion

The number of records, mean, phenotypic standard deviation and coefficient of variation for TWWn, number of lambs born (NLB3) and weaned (NLW3) per ewe joined and weaning weight for individual lambs (WW) for the three flocks are summarized in Table 6.2.

As the mean TWWn included records from ewes that did not lamb or wean a lamb, as well as from ewes that weaned more than one lamb per lambing opportunity, relatively large standard deviations and coefficients of variation were recorded. For all breeds, the coefficients of variation for TWWn decreased from the first to the third and fourth joining.

From Table 6.2 it is evident that TWWn for the Carnarvon Merino flock was very low, compared to that of the Carnarvon Afrino and Grootfontein Merino flocks. This could be ascribed to the lower number of lambs born and weaned per ewe joined, as well as the lower weaning weight of the individual lambs. The main contributing factor was that 57% of the young ewes in the Carnarvon Merino flock did not wean a lamb at their first parity, compared to 20% of the Carnarvon Afrino and 14% of the Grootfontein Merino young ewes. This was most probably due to their lower body weight at first mating.

Table 6.2 Description of the data sets for the three flocks

Traita No. records Mean (kg) SD CV%
Carnarvon Afrino
TWW1 1025 31.59 18.24 57.73
TWW2 794 73.80 25.18 34.12
TWW3 640 116.80 32.53 27.85
NLB3 640 4.27 1.32 30.91
NLW3 640 3.94 1.35 34.01
WW 3978 27.67 3.63 13.12
Carnarvon Merino
TWW1 2510 8.77 10.03 114.42
TWW2 2237 22.58 16.38 72.56
TWW3 1991 37.77 21.12 55.91
TWW4 1891 54.38 26.26 48.28
NLB3 1991 2.22 1.00 45.05
NLW3 1991 1.88 1.03 54.79
WW 8480 20.75 3.16 15.23
Grootfontein Merino
TWW1 2570 48.47 28.20 58.17
TWW2 2073 55.34 26.27 47.47
TWW3 1616 90.20 33.43 37.06
TWW4 1195 121.97 41.27 33.84
NLB3 1616 4.01 1.28 31.92
NLW3 1616 3.34 1.34 40.12
WW 9657 26.22 3.99 15.22

 

a TWW1 ... TWW4 = Total weight of lamb weaned by ewes which had at least one ... four consecutive lambing opportunities respectively;

NLB3 = Number of lambs born per ewe joined over three lambing opportunities;

NLW3 = Number of lambs weaned per ewe joined over three lambing opportunities;

WW = Weaning weight of individual lambs

Estimates of variance components and heritability for TWWn are presented in Table 6.3, while the genetic and phenotypic correlations estimated between TWW1 and TWW2, TWW3 and TWW4 for the three flocks are given in Table 6.4.

Table 6.3 Variance components (kg2) and direct heritability estimatesa for TWW1, TWW2, TWW3 and TWW4 for the three flocks

Trait F2A F2E F2P h2A
Carnarvon Afrino
TWW1 20.125 312.493 332.618 0.06 (0.04)
TWW2 34.418 599.494 633.913 0.05 (0.05)
TWW3 179.107 878.914 1058.021 0.17 (0.07)
Carnarvon Merino
TWW1 9.014 91.607 100.621 0.09 (0.03)
TWW2 44.936 223.497 268.433 0.17 (0.04)
TWW3 99.927 345.938 445.865 0.22 (0.04)
TWW4 177.149 512.301 689.450 0.26 (0.05)
Grootfontein Merino
TWW1 66.998 728.073 795.070 0.08 (0.03)
TWW2 31.091 659.223 690.314 0.05 (0.03)
TWW3 147.178 970.431 1117.609 0.13 (0.05)
TWW4 170.377 1532.888 1703.265 0.10 (0.05)

 

 

a F2A = Direct additive genetic variance, F2E = Residual variance, F2P = Phenotypic variance, h2A = Direct heritability.

 

For Afrino sheep, heritability estimates of 0.06, 0.05 and 0.17 were obtained for TWW1, TWW2 and TWW3 respectively. Corresponding estimates for Carnarvon Merino sheep ranged from 0.09 for TWW1 to 0.26 for TWW4. Similar estimates of 0.08, 0.05, 0.13 and 0.10 were obtained for Grootfontein Merino sheep. High positive genetic (rG) and phenotypic (rP) correlations were estimated between TWW1 and lifetime reproductive performance in all three flocks. The unity genetic correlation estimates obtained for the Carnarvon Merino flock could most probably be ascribed to the fact that the DFREML-programme used for the analysis, forced the estimates within the parameter space. Therefore sampling errors could also not be obtained.

Table 6.4 Genetic and phenotypic correlations a between TWW1 and TWW2, TWW3 and TWW4 for the three flocks

  TWW2 TWW3 TWW4
Carnarvon Afrino
TWW1 rG 0.61 (0.37) 0.79 (0.21)  
             rP 0.74 (0.02) 0.61 (0.03)  
Carnarvon Merino
TWW1 rG 1.00 - 1.00 - 1.00 -
             rP 0.71 (0.01) 0.61 (0.01) 0.55 (0.02)
Grootfontein Merino
TWW1 rG 0.87 (0.13) 0.91 (0.10) 0.74 (0.27)
             rP 0.70 (0.01) 0.57 (0.02) 0.51 (0.02)

a rG = Genetic correlation, rP = Phenotypic correlation.

The heritability estimates obtained in this study are in accordance with the animal model estimate of 0.13 (0.06) reported by Fogarty et al. (1994) for total weight of lamb weaned. The present estimates also fall within the range of other reported paternal-halfsib estimates, namely 0.03 to 0.09 (Basuthakur et al., 1973), 0.25 and 0.30 (More O'Ferrall, 1976), 0.14 (0.10) (Martin et al., 1981), 0.06 to 0.09 (Fogarty et al., 1985), 0.13 to 0.28 (Abdulkhaliq et al., 1989), 0.10 (0.16) (Long et al., 1989), 0.08 (0.05) (Boujenane et al., 1991b) and 0.14 (0.10) (Hall et al., 1994).

The higher heritabilities estimated for total weight of lamb weaned in the Carnarvon Merino flock, compared to the Carnarvon Afrino and Grootfontein Merino flocks, indicated a larger proportional genetic variance in total weight of lamb weaned in this flock. This could possibly be explained through the fact that no selection for reproduction or weaning weight per se had been carried out during the experimental period in this flock. In the Carnarvon Afrino flock ewes were, however, selected on reproductive performance since the start of the experimental period. In later years, ewes in this flock were culled for low reproductive rate and below average total weight of lamb weaned after two parities. In the Grootfontein Merino flock, 10 % of the ewe and 50% of the ram lambs were culled at weaning due to low adjusted 120-day weaning weight from 1969 to 1985 (Olivier, 1989). From 1985 onwards, ewes in this flock that failed to lamb in two consecutive years were culled.

The highest heritability estimate within each flock for the traits analyzed, was obtained for TWW3 for Carnarvon Afrino and Grootfontein Merino sheep and for TWW4 for the Carnarvon Merino flock. Compared to the generally low heritability of reproductive traits, the estimates obtained in this study indicate that sufficient genetic variance in lifetime reproductive efficiency is present to warrant the use of total weight of lamb weaned as selection criterion. However, it would be impractical to select ewes only after three or four parities. The high genetic correlations estimated between TWW1 and TWW3/TWW4 indicate that these traits are influenced by the same genes. TWW1 could therefore be an effective selection criteria to increase lifetime reproductive performance in the current flock.

The composite trait total weight of lamb weaned, which should be the true breeding objective, incorporates the component traits fertility, prolificacy, lamb survival and lamb weaning weight. The heritabilities of these component traits (Fogarty, 1995) are of the same order as, or even lower than that estimated for lifetime total weight of lamb weaned. Therefore, if the breeding objective is to increase lifetime reproductive efficiency of the flock, the composite trait total weight of lamb weaned should be the selection criterion.

 

6.4 Conclusions

The results of this study indicate that there is a relatively large phenotypic variation in total weight of lamb weaned, regardless of the reproductive rate of the flock. This variation may have a genetic basis and could therefore be exploited to genetically increase lifetime reproductive efficiency in any flock.

Ewe selection is aimed at increasing lifetime reproductive and productive efficiency in the current flock, as well as the genetic merit of future generations. The high genetic and phenotypic correlations estimated between TWW1 and future performance indicate that selection based on TWW1 will ensure that the highest producers will be selected and therefore that gains in the current flock would be increased. The genetic variance exploited in this way, should also increase the genetic merit of these ewes' daughters in terms of lifetime reproductive efficiency.

 

Table of contents

 

CHAPTER 7

GENETIC AND PHENOTYPIC CORRELATIONS AMONG PRODUCTION AND REPRODUCTION TRAITS


7.1 Introduction

For the construction of a viable breeding plan for any breed, it is essential that the genetic and phenotypic parameter estimates between the economically important production and reproduction traits for the specific breed are known.

Numerous genetic and phenotypic correlations for different traits for various breeds have been published (Fogarty, 1995). Some of the genetic correlations reported in these studies are highly variable, especially those estimated between fleece weight and body weight at different ages and the genetic correlations between the reproduction traits and different body weight and fleece traits. No correlation estimates between these economically important traits are available for Afrino sheep.

The objective of this study was to estimate covariance components and genetic and phenotypic correlations among weaning weight (pre-weaning growth), nine month body weight (post-weaning growth), 18 month body weight, clean fleece weight, fibre diameter and reproduction traits in the Carnarvon Afrino flock.


7.2 Material and methods

7.2.1 Data

Data collected on the Carnarvon Afrino flock from 1972 to 1994 were used for this study. Production traits included in the analysis were weaning weight (WW), 9 month body weight (W9), 18 month body weight (W18), clean fleece weight (CFW) and mean fibre diameter (MFD). The reproduction traits analyzed were total weight of lamb weaned per ewe joined over three parities (TWW), number of lambs born per ewe joined over three parities (NLB) and number of lambs weaned per ewe joined over three parities (NLW).

Total weight of lamb weaned per ewe joined over three parities (TWW) was calculated as described in Chapter 6. The number of lambs born (NLB) and weaned (NLW) per ewe joined over three parities were calculated by adding the number of lambs born or weaned over the first, second and third parities.


7.2.2 Covariance component and genetic parameter estimation

Covariance components were estimated using the DFREML programme of Meyer (1991, 1993). Bivariate animal models, including only direct genetic effects, were fitted throughout. Although NLB and NLW are threshold traits, they were treated as continuous variables in this analysis in order to obtain genetic correlations between them and the production traits. The number of animals with records for bivariate analyses was 3748 for the production traits and 618 for the reproduction traits. Fixed effects and covariates, as evaluated by least squares procedures, included for each trait in the different models are presented in Table 7.1.

Table 7.1 Fixed effects and covariatesa included for each traitb

  YSSEX YSRS RS YSEWE AD(Q) AGE
WW * *     * *
W9 * *     * *
W18 * *     * *
CFW *   *   * *
MFD *          
TWW       *    
NLB       *    
NLW       *    

 

a YSSEX = Subclass for year-season of birth and sex of the lamb

YSRS = Subclass for year-season of birth and rearing status of the lamb

RS = Rearing status

YSEWE = Year-season of birth of the ewe

AD(Q) = Age of dam (Quadratic covariate)

Age = Age of lamb (Linear covariate)

b WW = Weaning weight

W9 = Nine month body weight

W18 = Eighteen month body weight

CFW = Clean fleece weight

MFD = Mean fibre diameter

TWW = Total weight of lamb weaned per ewe joined over three parities

NLB = Number of lambs born per ewe joined over three parities

NLW = Number of lambs weaned per ewe joined over three parities

It was assumed that :

V(a) = EA*A and V(e) = EE*I, where A is a numerator relationship matrix, I is an identity matrix, EA the q x q matrix of additive genetic covariances, EE the matrix of error covariances and * denotes the direct matrix operator.

Full pedigrees were available, but parents with only a single link to one offspring were treated as unknown, as they did not contribute any information and unnecessarily increased the number of effects in the analysis (Meyer, 1994).

Initially, partial maximization of the likelihood, with respect to the covariance components only, was done by employing the Simplex procedure. For this run, variance components were fixed to their univariate values. After convergence (variance of function values in the Simplex was less than 10-8), iterations were restarted, this time considering the complete parameter vector. Additional restarts were performed until no further improvement in log likelihood occurred. Approximate sampling errors were calculated by fitting a quadratic or cubic function to the profile likelihood for each parameter (Meyer & Hill, 1992). An estimate was considered significant when the sampling error was less than half that of the estimate.


7.3 Results

The mean and coefficient of variation (CV) for each trait are presented in Table 7.2. The CV for the reproduction traits is higher than for production traits (WW, W9, W18, CFW and MFD), which can be explained by the fact that some ewes produced 0 and others up to 8 lambs over three parities.

Table 7.2 Description of the data set

Trait Mean CV (%)
WW 27.67 kg 19.69
W9 41.35 kg 17.09
W18 53.80 kg 18.98
CFW 2.01 kg 20.83
MFD 21.40 µm 7.68
TWW 116.80 kg 27.85
NLB 4.27 30.87
NLW 3.94 34.26

 

Genetic and phenotypic correlations estimated between body weight, clean fleece weight, mean fibre diameter and the reproduction traits are presented in Table 7.3.

High genetic (0.90 to 0.96) and phenotypic (0.64 to 0.80) correlations were estimated among WW, W9 and W18. There were no significant genetic correlations between CFW and WW, W9 or W18, while the corresponding phenotypic correlations were 0.15, 0.14 and 0.10 respectively. Mean fibre diameter was neither genetically nor phenotypically significantly correlated with WW, W9 or W18, the estimated phenotypic correlations being close to zero. Low positive genetic (0.18) and phenotypic (0.16) correlations were estimated between CFW and MFD. High significant genetic (0.83 to 0.99) and phenotypic (0.79 to 0.92) correlations were obtained among the reproduction traits.

High positive genetic correlations of 0.75, 0.75 and 0.88 were estimated between TWW and WW, W9 and W18 respectively. The corresponding phenotypic correlations were low positive (0.15, 0.24 and 0.27). The genetic as well as phenotypic correlations of NLB and NLW with the three body weight traits increased with an increase in the age at which the body weight was recorded. None of these correlations were, however, significant.

The estimated genetic correlations of MFD and CFW with the reproduction traits were generally low to moderate negative. The negative genetic correlation of -0.52 estimated between CFW and TWW is of special interest. It should, however, be viewed with caution because of the relatively high standard error of the estimate. The phenotypic correlations estimated between the reproductive traits and CFW and MFD were also negative, but close to zero.


7.4 Discussion

With some exceptions, most of the correlations estimated in this study fall within the ranges reported in the literature (Fogarty, 1995). Some of the genetic correlations reported in various studies are highly variable, especially those estimated between CFW and the different body weights and the genetic correlations of the reproduction traits with the different body weight and fleece traits.

Table 7.3 Genetic (above diagonal) and phenotypic (below diagonal) correlations between body weight, clean fleece weight, fibre diameter and reproductive traits

  WW W9 W18 CFW MFD TWW NLB NLW
WW   0.96 0.90 0.04 -0.10 0.75 -0.01 0.11
    (0.01) (0.02) (0.06) (0.06) (0.19) (0.19) (0.26)
W9 0.78   0.96 -0.01 -0.05 0.75 0.23 0.29
  (0.01)   (0.01) (0.04) (0.03) (0.18) (0.18) (0.21)
W18 0.64 0.81   -0.09 -0.04 0.88 0.31 0.40
  (0.01) (0.01)   (0.06) (0.05) (0.18) (0.19) (0.21)
CFW 0.15 0.14 0.10   0.18 -0.52 -0.33 -0.39
  (0.02) (0.02) (0.02)   (0.05) (0.22) (0.17) (0.20)
MFD -0.02 0.02 0.04 0.16   -0.11 -0.17 -0.09
  (0.02) (0.02) (0.02) (0.02)   (0.22) (0.18) (0.20)
TWW 0.15 0.24 0.27 -0.06 -0.03   0.83 0.84
  (0.05) (0.05) (0.04) (0.05) (0.05)   (0.09) (0.07)
NLB 0.04 0.12 0.16 -0.02 -0.03 0.79   0.99
  (0.04) (0.05) (0.05) (0.04) (0.05) (0.02)   (0.00)
NLW 0.03 0.10 0.15 -0.02 -0.01 0.92 0.87  
  (0.04) (0.05) (0.05) (0.04) (0.03) (0.01) (0.01)  

Genetic correlation estimates are notorious for their inconsistency and large standard errors. Despite the small data set used for the present analysis, relatively low standard errors (SE) were obtained for the genetic correlations estimated among WW, W9, W18, CFW and MFD, as well as for the genetic correlations obtained among TWW, NLB and NLW. The SE obtained for the genetic correlation estimates of the reproduction traits with WW, W9, W18, CFW and MFD, were however, larger. This is obviously due to the relatively smaller data set available for the reproduction traits. All the SE obtained for the phenotypic correlation estimates were small and in accordance with those reported in the literature. Since the results of this study are of vital importance in the formulation of a selection strategy, they should be verified when more data become available. However, these are the best available and should therefore be used as others might be a long time coming.

The estimated genetic correlations which should have the most important influence on the formulation of a viable breeding plan for Afrino sheep are the high positive genetic correlations between TWW and body weight at all ages, as well as the negative correlations between CFW and the reproduction traits.

No correlation estimates between WW and TWW or W9 and TWW could be found in the literature, while only a few genetic correlation estimates between W18 and TWW are available. Most of these are lower than the estimate of 0.88 obtained for Afrino sheep in this study. Literature estimates of -0.16 and 0.62 (More O'Farrell, 1976), 0.58 (Martin et al., 1981) and 0.51 (Fogarty et al., 1994) are reported for various breeds. Snyman

(1996, Unpublished) also estimated genetic correlations of 0.67 (0.13) and 0.72 (0.07) between W18 and TWW for Merino sheep at Grootfontein and Carnarvon respectively.

The genetic correlation between CFW and TWW (-0.52) estimated in the present study is of special interest, as it is not in line with corresponding estimates reported in the literature (0.02 by Basuthakur et al., 1973; 0.53 by More O'Ferrall, 1976; 0.63 by Martin et al., 1981; 0.29 by Fogarty et al., 1994) and 0.26 and 0.06 estimated in the Grootfontein Merino stud and Carnarvon Merino flock respectively (Snyman 1996, Unpublished).

The small non-significant negative genetic correlation obtained between CFW and W18 (-0.09) in this study is also in contrast with the generally positive estimates, ranging from 0.04 (Gregory, 1982) to 0.63 (Blair, 1981), reported in the literature. The phenotypic correlation estimated in this study is also lower than other reported estimates.

According to the results of this study, selection for either TWW or W18 will decrease CFW, TWW more so than W18. Currently approximately 80 % of the income from Afrino sheep is generated through mutton production. Selection for higher CFW would adversely affect other economically important traits, therefore less emphasis should be placed on the quantity of the wool and more on the quality. In a wool or dual purpose breed CFW should, however, at least be maintained. MFD has low negative genetic correlations with all traits, except CFW. Negative selection pressure on MFD would therefore not adversely affect other economically important traits.

Selection for TWW would result in an increase in body weight at all ages, as well as in NLB and NLW. This would also be accompanied by a favourable decrease in MFD, but a substantial unwanted decrease in clean fleece weight. However, TWW is sex limited as well as a laborious and time consuming measurement. The results of this study imply that TWW can be improved by indirect selection for body weight at any age. Selection for W18 would be practical under certain circumstances, but not possible where selection takes place at an earlier age. In such instances, WW or W9 could be used as selection criterion. Preferably W9 would be a better choice, as it also includes a measure of post weaning growth, as opposed to WW, which is largely influenced by maternal effects. Furthermore W9 also has a higher heritability than WW (0.63 vs. 0.41).

Indirect selection will be more effective than direct selection if rAhY is greater than hX, where rAhY is the correlation between breeding values of the desired trait X and phenotypic values of the selected trait Y, while h is the accuracy of direct selection (Falconer & Mackay, 1996). As rAhY (0.432) is greater than hX (0.412), indirect selection for WW to improve TWW should be more effective than direct selection. In the case of indirect selection on W9, rAhY (0.578) is also greater than hX (0.412). A higher selection intensity with indirect selection on body weight is also possible because males can be selected, which should make it even more effective. In practise it is, however, difficult to quantify selection intensity as other traits are also considered.

 

7.5 Conclusions

Favourable genetic correlations estimated between TWW, body weight and MFD are in line with the selection objectives set for Afrino sheep. Although genetic progress in the desired direction is possible, the negative genetic correlation estimated between TWW and CFW necessitates that fleece weight be monitored if selection is aimed at increasing TWW. Results obtained in this study can be utilized in the construction of a viable breeding plan for Afrino sheep.

 

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CHAPTER 8  

CONCLUSIONS AND RECOMMENDATIONS

The most important result of this study is, in all probability, that indirect selection for reproductive performance, defined as total weight of lamb weaned (TWW), based on early recorded traits, which are not sex limited, could be more effective than direct selection.

In the recommendation of a breeding plan for Afrino sheep, the requirements of both stud breeders and commercial producers should be accommodated. Ram selection will naturally only be applicable to stud breeders, while ewe selection should play an important role in the breeding plan of stud breeders as well as commercial producers. Recommendations for ram, ewe, as well as overall selection will therefore be made.

In order to be able to make recommendations for a viable breeding plan for Afrino sheep, the results obtained in this study were used to construct various selection indices. The SELIND-programme of Cunningham & Mahon (1977) was used for this purpose. This programme is based on the principal that selection objectives, each with a relative economic value, are set, and that selection is based on certain traits (selection criteria) in order to achieve the set objectives.

The following multi-trait selection objective was set for Afrino sheep :

Economic values of R30 for total weight of lamb produced, R10 for own body weight, R170 for clean fleece weight and -R62 for fibre diameter were estimated and used. In order to achieve these objectives, the following traits were identified as possible selection criteria :

These traits were used in various combinations in order to obtain several possible selection indices for Afrino sheep. It was thought that including the highly correlated WW and W9 in the same index would make little sense.


8.1 Ram selection

All the information needed for the selection of potential sires are available at an early age. Selection can therefore be based on a selection index incorporating the relevant objective traits. Traits such as breed standards, conformation and wool quality could be assessed subjectively and rams which meet these requirements could then be selected on the basis of one of the four selection indices presented in Table 8.1. The genetic response obtained per generation in each selection objective with each of the recommended selection indices are summarized in Table 8.2.

Table 8.1 Selection indices for Afrino sheep

  WW W9 CFW MFD
Index 1 +4   +1 -3
Index 2   +3 +2 -2
Index 3 +1     -1
Index 4   +1   -1

 

It should be mentioned that it is the prerogative of each registered Afrino breeder or commercial producer to decide how much emphasis he would like to place on reproduction, growth performance, fleece weight and fibre diameter, as long as he abides to the general breed standards set for the Afrino breed. It is therefore essential that commercial producers buy rams from breeders who have the same selection objectives than they have.

Table 8.2 Genetic response obtained per generation in the selection objectives with each of the recommended selection indices

  W9 (kg) CFW (kg) MFD (�m) TWW (kg)
Index 1 2.19 0 -0.42 5.68
Index 2 2.68 0 -0.30 7.42
Index 3 2.10 -0.10 -0.56 5.70
Index 4 2.63 -0.02 -0.42 7.62

 

The objective of the Afrino is that it should be able to produce and reproduce under extensive conditions. As 80% of its income is generated through reproduction and growth (mutton production), and bearing in mind the negative genetic correlation estimated between CFW and TWW, selection for an increase in fleece weight under extensive conditions, given present price structures, would probably not be advisable. However, due to this negative genetic correlation it is advised that fleece weight should at least be monitored should selection be aimed at only increasing TWW and direct growth.

Taking all this information into account, the breeder should decide if he wants to include CFW in the selection index. If he wants to do so, Index 1 or Index 2 could be used. The difference between these two indices is that Index 1 places more emphasis on fibre diameter and less on reproduction and growth than Index 2, as is evident from Table 8.2.

Should the breeder choose to ignore fleece weight in his breeding plan, Index 3 or Index 4 are options. In this instance, Index 4 places more emphasis on reproduction and growth and less on fibre diameter than does Index 3. With both these indices a decline in CFW over the long run can be expected.


8.2 Ewe selection

Not all of the information required for the accurate identification of superior ewes is available at an early age. Adequate information on the productive performance (growth and wool traits) are available, but little is known about the young ewe's reproduction potential, barring that obtained from early recorded correlated traits.

The results of Chapter 7 indicate that selection for WW or W9 will lead to a correlated genetic increase in TWW. However, the low phenotypic correlations estimated between TWW and WW (0.153) and between TWW and W9 (0.242) would not guarantee that the highest producers be selected for the current flock. Snyman (1996, Unpublished) indicated that TWW1 is the most accurate predictor of current lifetime reproductive performance. It is therefore recommended that ewe selection should take place in two phases.

In the first phase, ewes could be assessed subjectively for breed standards and conformation or wool faults. Preliminary selection on the basis of WW or W9 could then be done. The correlated response in WW/W9, W18, CFW, MFD, TWW, NLB and NLW (expressed as percentage increase per 10% increase in the trait under selection), if selection is based on WW/W9, are summarized in Table 8.3.

Table 8.3 Percentage response in the correlated traits with a 10% increase in WW or W9

% Response in : 10 % Change in :
  WW  W9
WW   8.8
W9 10.5  
W18 9.0 8.8
CFW 0.7 -0.2
MFD -0.8 -0.4
TWW 11.5 10.5
NLB -0.2 4.0
NLW 2.0 4.8

 

If preferred, one of the selection indices recommended for Afrino ram selection could be used. Each breeder or commercial producer should decide which selection strategy is most suitable for his specific circumstances and requirements. As the production traits contribute much less to total productivity than the ewe's lifetime reproductive performance, more than the required number of young replacement ewes should be selected during the first phase. These ewes should then be mated and final selection (second phase) could be done after their first parity.

With the second phase of ewe selection, selection should be based solely on reproduction performance. Selection intensity at this phase is dependent upon several factors. The most important of these are the prevailing environmental conditions especially during mating, but also pregnancy and lactation, as well as the age at first mating. In extremely poor years it would be advisable to leave final selection of young ewes till after their second parity.

As discussed earlier, lifetime reproduction performance in Afrino sheep can best be defined as total weight of lamb weaned. The high genetic and especially phenotypic correlations estimated between total weight of lamb weaned after the first parity (TWW1) and future performance indicate that selection based on TWW1 will ensure that the highest producers will be selected and therefore that gains in the current flock would be increased. The commercial producer should concentrate his efforts on current flock gains, as genetic gains will entirely be due to the efforts of his ram supplier. Second phase selection could therefore be done by culling young ewes that fail to wean a lamb or produce below average TWW1.

For selection to be based on TWW1, records of ewes mated, dam-offspring identification and weaning weight of lambs should be kept. The recording of weaning weight is a prerequisite for Afrino breeders affiliated with the Afrino Breeders' Society. However, the keeping of records under extensive conditions is not an easy task and many commercial producers would not be able to do this, or be unwilling to make the extra inputs required for the keeping of the necessary records. Due to increasing input costs, breeders and commercial producers can not afford to keep unproductive ewes in the flock. It is therefore recommended that records of at least ewes that failed to lamb should be kept. By culling such ewes, genetic progress with regard to fertility, albeit slow, could be achieved in the flock.

 

8.3 Record keeping

The feasibility of any breeding plan depends upon the number of records the breeder or producer has to keep. The aim with the recommended breeding plan is to achieve maximum gains with the minimum record keeping.

Afrino stud breeders who are members of the Breed Society should be compelled to keep the following records :

For the purpose of the accumulation of a data basis for a between flock BLUP analysis, it is recommended that records of both ewes and rams for fleece weight and fibre diameter should be kept.

Commercial producers should not rely solely on the ram breeders to improve the productivity and profitability of their flocks. Through the selection of high producing ewes, profitability of the current flock could be increased substantially. This could, however, only be brought about by the keeping of records. The minimum records that a commercial producer should keep include a list of the ewes that were mated and the identification of ewes that failed to produce a marketable lamb.

 

8.4 Conclusions

The holistic approach of this study, where production and reproduction traits were analyzed simultaneously, provided useful information for aggregate genetic improvement and could be followed for other breeds. The problem with lifetime reproduction traits is that it normally takes a long time to obtain sufficient data for accurate analyses. Although results obtained in this study can be utilized in the interim, they should be verified when more data, especially on the reproductive traits, become available.

In wool producing breeds, much emphasis is traditionally placed on subjective wool and conformation traits such as wool quality, crimp frequency, variation, creeping belly, face covering, pigmentation and hocks. The influence of these traits on the economically important production and reproduction traits should also be studied and quantified.

 

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ABSTRACT

Data, consisting of 4325 lamb records, the progeny of 146 sires and 946 dams, collected on the Carnarvon Afrino flock over the period 1975 to 1992, were analyzed to investigate factors influencing birth weight, weaning weight, monthly body weight from five to 12 months, body weight at 18 months of age as well as clean fleece weight and mean fibre diameter at 16 months of age in Afrino sheep.

Year-season of birth, sex and birth/rearing status of the lamb as well as age of dam and age of lamb were significant (P<0.01) sources of variation for clean fleece weight and body weight at all ages. Mean fibre diameter was significantly (P<0.01) influenced by year-season of birth, sex and age of lamb. Significant (P<0.01) two-way interactions were found between year-season and sex and year-season and weaning status.

Least-squares means (kg) for body weight were 4.84±0.04 at birth, 29.55±0.31 at weaning, 39.27±0.41 at eight, 52.32±0.52 at twelve and 54.13±0.55 at eighteen months of age. For clean fleece weight and mean fibre diameter, least-squares means of 2.01±0.09 kg and 21.47±0.09 micron were recorded.

Variance components resulting from direct additive genetic effects, maternal additive genetic effects, maternal permanent environmental effects, as well as the relationship between direct and maternal effects for birth weight, monthly body weight from weaning at four months to 12 months of age, 18 month body weight, 16 month clean fleece weight and 16 month mean fibre diameter were estimated by REML procedures. By ignoring or including maternal genetic or environmental effects, five different models of analysis were fitted in order to determine the most effective model for each trait.

The direct heritability estimate for body weight increased from birth [0.22 (0.04)] up to eight months of age [0.59 (0.06)], where it seemed to stabilise. Maternal heritability estimates for body weight, on the other hand, increased from birth [0.09 (0.04)] to five months of age [0.17 (0.02)], whereafter it decreased gradually. The maternal permanent environmental effect was significant only for birth weight [0.12 (0.03)]. Direct heritability estimates of 0.62 (0.04) and 0.73 (0.03) were obtained for clean fleece weight and mean fibre diameter respectively. Maternal effects had no significant influence on clean fleece weight or mean fibre diameter.

Data collected on the Carnarvon Afrino flock over the period 1972 to 1994 were analyzed by means of a GFCAT set of programmes to estimate heritability and breeding values on the underlying scale for different reproductive traits and survival rate in Afrino sheep to ascertain whether they can be improved by selection on estimated breeding values obtained under a threshold model. The reproductive traits analyzed included fertility (whether a ewe lambed or not; 0 or 1), litter size (number of lambs born to a ewe mated; 0, 1, 2 or 3), fecundity (number of lambs born to a ewe that lambed; 1, 2 or 3), number weaned (number of lambs weaned to a ewe mated; 0, 1, 2 or 3) and survival rate from birth to weaning (whether a lamb born alive, was dead or alive at weaning; 0 or 1).

Heritabilities on the underlying scale of 0.20, 0.27, 0.42, 0.19 and 0.02 were estimated for the respective traits. The results suggest that reproduction rate, but not survival rate, can be increased in Afrino sheep by selection on breeding values estimated on the underlying scale applying a threshold model.

Data collected on the Carnarvon Afrino flock, the Carnarvon Merino flock and the Grootfontein Merino stud were used to estimate genetic parameters for total weight of lamb weaned in Afrino and Merino sheep.

For Afrino sheep, heritability estimates of 0.06 (0.04), 0.05 (0.06) and 0.17 (0.07) were obtained for total weight of lamb weaned after the first (TWW1), second (TWW2) and third (TWW3) parities respectively. Corresponding estimates for Carnarvon Merino sheep ranged from 0.09 (0.03) for TWW1 to 0.26 (0.05) for TWW4. Similar estimates of 0.08 (0.03), 0.05 (0.03), 0.13 (0.05) and 0.10 (0.05) were obtained for Grootfontein Merino sheep.

High positive genetic (rG) and phenotypic (rP) correlations were estimated between first parity and lifetime reproductive performance in all three flocks. For the Carnarvon Afrino flock, rG increased from 0.61 (0.37) between TWW1 and TWW2 to 0.79 (0.21) between TWW1 and TWW3. The corresponding rP, however, decreased from 0.74 (0.02) to 0.61 (0.03). A similar decrease in rP from 0.71 (0.01) and 0.70 (0.01) (between TWW1 and TWW2) to 0.55 (0.02) and 0.51 (0.02) (between TWW1 and TWW4) were observed for the Carnarvon and Grootfontein Merino flocks respectively. Unity rG estimates were obtained between TWW1 and TWW2, TWW3 and TWW4 for the Carnarvon Merino flock. Corresponding rG estimates of 0.87 (0.13), 0.91 and 0.74 (0.27) were obtained for the Grootfontein Merino flock.

The results of this study, obtained with two different breeds and in two different environments in flocks with a high and a low reproductive rate, indicate that selection for increased lifetime reproductive performance could be based on total weight of lamb weaned after the first parity.

Genetic and phenotypic correlations were estimated among weaning weight (WW), nine (W9) and 18 month body weight (W18), clean fleece weight (CFW), mean fibre diameter (MFD), total weight of lamb weaned over three parities (TWW), as well as number of lambs born (NLB) and weaned (NLW) over three parities in the Carnarvon Afrino flock. Covariance components and genetic correlations were estimated using DFREML-procedures.

High genetic (0.90 to 0.96) and phenotypic (0.64 to 0.80) correlations were estimated among WW, W9 and W18. There were no significant genetic correlations between CFW and WW, W9 or W18, while the corresponding phenotypic correlations were 0.15, 0.14 and 0.10 respectively. Mean fibre diameter was neither genetically nor phenotypically significantly correlated with WW, W9 or W18, the estimated phenotypic correlations being close to zero. Low positive genetic (0.18) and phenotypic (0.16) correlations were estimated between CFW and MFD. High significant genetic (0.83 to 0.99) and phenotypic (0.79 to 0.92) correlations were obtained among the reproduction traits. High positive genetic correlations of 0.75, 0.75 and 0.88 were estimated between TWW and WW, W9 and W18 respectively. The corresponding phenotypic correlations were low positive (0.15, 0.24 and 0.27). The estimated genetic correlations of MFD and CFW with the reproduction traits were generally low to moderate negative. A negative genetic correlation of -0.52 was estimated between CFW and TWW.

The results of this study imply that TWW, which is sex limited as well as a laborious and time consuming measurement, can be improved by indirect selection for body weight at any age. Possible breeding plans for Afrino stud breeders and commercial producers are recommended.

 

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OPSOMMING 

Altesaam 4325 lam rekords, die nageslag van 146 vaders en 946 moeders, wat vanaf die Afrinoskaapkudde te Carnarvon Proefstasie verkry is, is gebruik om faktore wat geboortegewig, speengewig, maandelikse liggaamsgewig vanaf vyf tot 12 maande ouderdom, 18 maande liggaamsgewig sowel as skoonvaggewig en gemiddelde veseldikte op 16 maande ouderdom beïnvloed, te ondersoek.

Jaar-seisoen van geboorte, geslag en geboorte/speenstatus van die lam sowel as moederouderdom was betekenisvolle (P<0.01) bronne van variasie vir skoonvaggewig en liggaamsgewig op alle ouderdomme. Gemiddelde veseldikte is betekenisvol (P<0.01) beïnvloed deur jaar-seisoen van geboorte, geslag en ouderdom van die lam. Betekenisvolle (P<0.01) tweerigting interaksies is gevind tussen jaar-seisoen en geslag en jaar-seisoen en speenstatus.

Kleinste-kwadrate gemiddeldes (kg) vir liggaamsgewig was 4.84±0.04 by geboorte, 29.55±0.31 by speen, 39.27±0.41 by agt, 52.32±0.52 by twaalf en 54.13±0.55 by agtien maande ouderdom. Vir skoonvaggewig en veseldikte is kleinste-kwadrate gemiddeldes van onderskeidelik 2.01±0.09 kg en 21.47±0.09 mikron verkry.

Variansie komponente vir direkte additiewe genetiese effekte, maternale additiewe genetiese effekte, permanente maternale omgewingseffekte en die verwantskap tussen direkte en maternale effekte vir geboortegewig, speengewig, maandelikse liggaamsgewig vanaf vyf tot 12 maande ouderdom, 18 maande liggaamsgewig sowel as skoonvaggewig en gemiddelde veseldikte is met behulp van REML-prosedures beraam. Deur maternale genetiese- of omgewingseffekte in te sluit of weg te laat, is vyf verskillende modelle gepas ten einde die mees geskikste model vir elke eienskap te bepaal.

Die direkte oorerflikheid vir liggaamsgewig het vanaf geboorte [0.22 (0.04]) tot agt maande ouderdom [0.59 (0.06)] toegeneem, waarna dit gestabiliseer het. Maternale oorerflikheidsberamings het vanaf geboorte [0.09 (0.04)] tot vyf maande ouderdom [0.17 (0.02)] toegeneem, waarna dit geleidelik afgeneem het. Die permanente maternale effek was slegs vir geboortegewig betekenisvol [0.12 (0.03)]. Direkte oorerflikhede van 0.62 (0.04) en 0.73 (0.03) is vir skoonvaggewig en gemiddelde veseldikte beraam. Maternale effekte het geen betekenisvolle invloed op skoonvaggewig en gemiddelde veseldikte gehad nie.

Data wat vanaf 1972 tot 1994 op die Carnarvonse Afrinokudde ingesamel is, is met behulp van 'n GFCAT-program ontleed om oorerflikhede en teelwaardes op die onderliggende skaal vir verskillende reproduksie eienskappe en lamoorlewingstempo in Afrinoskape met behulp van 'n drumpelwaarde model te beraam. Die volgende eienskappe is ontleed, naamlik vrugbaarheid (of 'n ooi gelam het of nie; 0 of 1), aantal lammers gebore (aantal lammers gebore per ooi gepaar; 0, 1, 2 of 3), fekunditeit (aantal lammers gebore per ooi gelam; 1, 2 of 3), aantal lammers gespeen (aantal lammers gespeen per ooi gepaar; 0, 1, 2 of 3) en lamoorlewingstempo (of 'n lam wat lewendig gebore was, dood of lewendig is met speen; 0 of 1).

Oorerflikhede op die onderliggende skaal van 0.20, 0.27, 0.42, 0.19 en 0.02 is vir die onderskeie eienskappe beraam. Die resultate dui daarop dat reproduksietempo, maar nie lamoorlewingstempo nie, verhoog kan word in Afrinoskape deur seleksie gebaseer op teelwaardes beraam op die onderliggende skaal met behulp van 'n drumpelwaarde model.

Data wat op die Carnarvonse Afrinokudde, die Carnarvonse Merinokudde en die Grootfonteinse Merinostoet ingesamel is, is gebruik vir die beraming van genetiese parameters vir totale massa lam gespeen in Afrino en Merinoskape.

Oorerflikhede van 0.06 (0.04), 0.05 (0.06) en 0.17 (0.07) is vir Afrinoskape beraam vir totale massa lam gespeen na die eerste (TWW1), tweede (TWW2) en derde (TWW3) lamkans onderskeidelik. Ooreenstemmende beramings vir Carnarvon Meirnoskape wissel van 0.09 (0.03) vir TWW1 tot 0.26 (0.05) vir TWW4. Soortgelyke beramings van 0.08 (0.03), 0.05 (0.03), 0.13 (0.05) en 0.10 (0.05) is vir Grootfontein Merinoskape verkry.

Hoë positiewe genetiese (rG) en fenotipeise (rP) korrelasies is tussen die eerste en leeftydsreproduksieprestasie in al drie kuddes beraam. In die Carnarvon Afrinokudde het rG toegeneem vanaf 0.61 (0.37) tussen TWW1 en TWW2, tot 0.79 (0.21) tussen TWW1 en TWW3. Die ooreenstemmende rP het egter afgeneem van 0.74 (0.01) tot 0.61 (0.03). Soortgelyke afnames is vir die Carnarvonse en Grootfonteinse Merino's verkry. Besondere hoë rG is tussen TWW1 en TWW2, TWW3 en TWW4 vir die Carnarvonse Merino's beraam. Die ooreenstemmende rG vir die Grootfonteinse Merino's was 0.87 (0.13), 0.91 en 0.74 (0.27).

Die resultate van hierdie studie, wat met twee rasse en in twee omgewings verkry is, dui daarop dat seleksie vir verhoogde leeftydsreproduksieprestasie op totale massa lam gespeen na die eerste lamkans, gebaseer kan word.

Genetiese en fenotipiese korrelasies tussen speengewig (WW), 9 maande gewig (W9), 18 maande gewig (W18), skoonvaggewig (CFW), veseldikte (MFD), totale massa lam gespeen oor drie lamkanse (TWW), sowel as aantal lammers gebore (NLB) en gespeen (NLW) oor drie lamkanse is vir die Carnarvonse Afrinokudde beraam. Kovariansie komponente en genetiese korrelasies is met behulp van DFREML-prosedures beraam.

Hoë genetiese (0.90 tot 0.96) en fenotipiese (0.64 tot 0.80) korrelasies is tussen WW, W9 en W18 beraam. Daar was geen betekenisvolle genetiese korrelasies tussen CFW en WW, W9 en W18 verkry nie, terwyl ooreenstemmende fenotipiese korrelasies van 0.15, 0.14 en 0.10 beraam is. MFD was nie geneties of fenotipies met WW, W9 of W18 gekorreleerd nie. Lae positiewe genetiese (0.18) en fenotipiese (0.16) korrelasies is tussen CFW en MFD beraam. Hoë betekenisvolle genetiese (0.83 tot 0.99) en fenotipiese (0.79 tot 0.92) korrelasies is tussen TWW, NLB en NLW beraam. Hoë positiewe genetiese korrelasies van 0.75, 0.75 en 0.88 is beraam tussen TWW en WW, W9 en W18 onderskeidelik. Die ooreenstemmende fenotipiese korrelasies was laag positief (0.15, 0.24 en 0.27). Die beraamde genetiese korrelasies van MFD en CFW met die reproduksie eienskappe was oor die algemeen laag negatief. 'n Negatiewe genetiese korrelasie (-0.52) is tussen CFW en TWW beraam.

Die resultate van hierdie studie impliseer dat TWW, wat 'n baie belangrike, maar geslagsbeperkte asook arbeidsintensiewe en tydrowende meting is, deur middel van indirekte seleksie vir liggaamsgewig op enige ouderdom, verbeter kan word. Teelplanne vir Afrinotelers en -kommersiële produsente word voorgestel. 

 

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PhD-thesis, University of Orange Free State