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AN INVESTIGATION INTO POSSIBLE EARLY SELECTION MEASURES

OF LIFETIME REPRODUCTION IN AFRINO SHEEP

 

MA Snyman & J.J.Olivier

Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (E.C), 5900, South Africa

 

1. Introduction

 

The Afrino is the first South African sheep breed in which the separation of ram and ewe selection objectives was investigated 5. 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, is aimed at increasing lifetime reproductive and productive efficiency in the current flock, as well as the genetic merit of future generations. In almost any South African sheep enterprise, reproductive performance is of the utmost economic importance in determining the efficiency of sheep production. This is even more so for mutton and dual purpose sheep breeds.

 

In practice, most selection emphasis is normally placed on rams and little or no attention is given to ewe selection. As discussed by Snyman 5, 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, much is known about a young ewe's growth and wool production and accurate selection based on these traits is therefore possible. These traits, however, contribute much less to total lifetime productivity than the ewe's reproductive performance. At this stage little information on the young ewe's reproductive ability is known. The aim must therefore be to predict from the available information at selection age which ewes will be the highest producers, in order to increase gains in the current flock, as well as in future generations. The objective of this study was to investigate which early selection measures give the best indication of lifetime reproduction in Afrino sheep.

 

2. Material and methods

 

2.1 Data

Data collected from 1972 to 1994 on the Carnarvon Afrino flock - kept on natural pasture 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 - were used for this study. A detailed description of the Afrino breed, management and selection procedures followed in this specific flock is given by Snyman 3.

2.1.1 Prediction of current lifetime reproduction

Total weight of lamb weaned over three parities, as calculated by Snyman 7, was taken as measure of lifetime reproduction. Available information at selection age of the young ewe which could be used to predict her own lifetime reproductive performance, included data on the performance of the young ewe herself, data on her dam's performance, as well as data on her sire's performance.

 

These available information were used to estimate breeding values for the following traits :

All EBV’s, with the exception of those for FERT, LIT and WEAN, were obtained as back solutions of the DFREML-analyses performed to estimate (co)variance components for the various traits 4,7. Threshold sire model breeding values for FERT, LIT and WEAN have been obtained by means of a GFCAT set of programmes 1,6.

 

2.1.2 Prediction of lifetime reproductive performance of future generations

As ewe selection has a dual purpose, the prediction of lifetime reproductive performance of future generations should also receive some attention. For this analysis, the relationship between a ewe's lifetime reproductive performance and several body weight and fertility traits of her dam, was calculated. Breeding values available for the dam included a maternal breeding value for 100-day weaning weight (MWWD), breeding values for 100-day weaning weight (WWD), 9-month body weight (W9D) and 18-month body weight (W18D), as well as a breeding value for total weight of lamb weaned over the dam's lifetime in the flock (TWWD).

 

2.2 Statistical analysis

Prediction of both current and future lifetime reproduction from available EBV’s for several body weight and reproductive traits at selection age, was done by means of simple and stepwise regression methods using SAS 2. Firstly, simple linear regressions were fitted by the PROC REG procedure of SAS to determine which of the traits had a significant relationship with lifetime reproduction. Subsequently, those traits which showed a significant simple linear relationship with lifetime reproduction were included in a stepwise regression analysis in order to determine which trait or combination of traits give the most accurate prediction of the young ewe's as well as her progeny's lifetime reproduction performance.

 

3. Results and discussion

 

Relationships of lifetime reproduction of current and future generations, measured as total weight of lamb weaned over three parities, with several body weight and fertility traits are presented in Table 1. From Table 1 it is evident that body weight at weaning, nine- and 18-months of age and total weight of lamb weaned by the young ewe’s dam have a significant influence on current as well as future lifetime reproduction.

 

These results indicate that maternal breeding value for weaning weight of the sire has a low positive relationship with lifetime reproduction of his daughters, while a sire’s breeding values for fertility and litter size at birth and weaning have no significant relationship with lifetime reproduction of his daughters.

 

Table 1. Relationship of current lifetime reproduction and lifetime reproduction of future generations with several body weight and fertility traits in Afrino sheep

 

Trait

Intercept

b

R2

P

r

Current lifetime reproduction

MWW

111.574

8.199

0.019

0.0004

0.139

WW

108.675

6.103

0.051

0.0001

0.226

W9

108.019

2.944

0.048

0.0001

0.219

W18

107.277

2.402

0.056

0.0001

0.236

TWW18

108.981

0.555

0.004

0.1596

0.066

TWWD

108.177

1.848

0.075

0.0001

0.274

TWW1

106.409

6.992

0.132

0.0001

0.363

FERT

113.128

-9.543

0.003

0.1652

-0.060

LIT

113.241

1.188

0.000

0.9001

0.037

WEAN

113.230

2.475

0.000

0.8300

0.002

MWWS

108.886

5.577

0.014

0.0210

0.119

Lifetime reproduction of future generations

MWWD

111.244

4.579

0.017

0.0011

0.129

WWD

111.658

2.992

0.010

0.0140

0.097

W9D

111.492

1.504

0.010

0.0127

0.099

W18D

111.272

1.156

0.010

0.0109

0.101

TWWD

108.177

1.848

0.075

0.0001

0.274

 

Results of the stepwise regression analysis, summarised in Table 2, show the traits most accurately predicting the lifetime reproduction of a young ewe when selection is taking place at an early age when the young ewe did not yet have a lambing opportunity, as well as when selection is taking place after the young ewe's first parity. Results of the stepwise regression analysis predicting the lifetime reproduction of future generations are also presented.

 

Table 2. Results of stepwise regression analysis, indicating the most accurate predictors of current lifetime reproduction if selection is carried out at two different stages and of lifetime reproduction of future generations

 

Current lifetime reproduction

Selection at an early age

Selection after first parity

Trait

b

R2

P

Trait

b

R2

P

TWWD

1.449

0.075

0.0001

TWW1

6.326

0.133

0.0001

W18

1.623

0.022

0.0001

WW

3.881

0.019

0.0002

Intercept

105.211

Intercept

104.107

Model-R2

0.097

Model-R2

0.152

Model-P

0.0001

Model-P

0.0001

 

Lifetime reproduction of future generations

 

b

R2

P

TWWD

1.848

0.075

0.0001

Intercept

108.178

Model-R2

0.075

Model-P

0.0001

 

 

At selection age, total weight of lamb weaned by the young ewe’s dam is the best predictor of lifetime reproduction of the young ewe. After the first parity, total weight of lamb weaned by the young ewe herself, gives the best indication of her lifetime reproduction in the current flock. Total weight of lamb weaned by a ewe is also the best predictor of lifetime reproduction of that ewe’s daughters. The generally low heritability and repeatability of reproduction traits are reflected in these results, as is evident from the low R2 -values.

 

Indirect selection based on body weight would lead to a correlated genetic increase in total weight of lamb weaned 8 . However, the low phenotypic correlations estimated between TWW and WW (0.15) and between TWW and W9 (0.24) 8 would not guarantee that the highest producers be selected for the current flock. This is supported by the results of the present study, indicating that total weight of lamb weaned is the best predictor of lifetime reproduction in the current flock, as well as in future generations.

 

4. Conclusions

 

These results support the idea that ewe selection should be based on reproduction, while ram selection should concentrate on growth and wool traits.

 

5. References

 

1 KONSTANTINOV, K.V., 1995. GFCAT : User's guide. University of the Orange Free State, Bloemfontein

2 LITTELL, R.C., FREUD, R.J. & SPECTOR, P.C., 1991. SAS-system for linear models, 3rd Ed. SAS Institute. Inc. Cary, NC.

3 SNYMAN, M.A., ERASMUS, G.J. & VAN WYK, J.B., 1995. Non-genetic factors influencing growth and fleece traits in Afrino sheep. S. Afr. J. Anim. Sci. 25(3) : 70-74

4 SNYMAN, M.A., ERASMUS, G.J., VAN WYK, J.B., & OLIVIER, J.J., 1995. Direct and maternal (co)variance components and heritability estimates for body weight at different ages and fleece traits in Afrino sheep. Livest. Prod. Sci. 44 : 229-235

5 SNYMAN, M.A., 1996. An investigation into selection for production and reproduction in Afrino sheep. Ph.D-dissertation, University of the Orange Free State

6 SNYMAN, M.A., ERASMUS, G.J. & VAN WYK, J.B., 1997. An investigation into the possible genetic improvement of reproduction and survival rate in Afrino sheep using a threshold model. S. Afr. J. Anim. Sci. (in press)

7 SNYMAN, M.A., ERASMUS, G.J., VAN WYK, J.B., & OLIVIER, J.J., 1997. Genetic parameter estimates for total weight of lamb weaned in Afrino and Merino sheep. Livest. Prod. Sci. (in press)

8 SNYMAN, M.A., ERASMUS, G.J., VAN WYK, J.B., & OLIVIER, J.J., 1997. Genetic and phenotypic correlations among production and reproduction traits in Afrino sheep. S. Afr. J. Anim. Sci. (submitted)

 

 

Published

Proceedings 35th SASAS congress, Nelspruit, 1-3 July