Last update: November 22, 2010 03:51:58 PM E-mail Print

 

HOW MUCH EMPHASIS SHOULD BE PLACED ON SUBJECTIVELY ASSESSED

FLEECE TRAITS IN AFRINO SHEEP?

 

M. A. Snyman*, W. J. Olivier, P .J. Griessel & J. A. N. Cloete1


Grootfontein ADI, Private Bag X529, Middelburg (E.C.), 5900

1Carnarvon Experimental Station, Carnarvon, 8925

*email: Gretha Snyman

 


1.   Introduction

The selection objectives of the Afrino breed are to produce good quality slaughter lambs at a relatively early age from the veld, and to produce a reasonable amount of good quality fine wool. As with every woolled sheep breed, there are a lot of subjective fleece traits, not necessarily of economic importance, that are taken into account during selection of replacement sires and dams. In many instances, animals are culled on the basis of these traits, but this occurs at the expense of the economically important traits such as reproduction. Subjective fleece traits on which emphasis are placed, are softness of face covering, extent of pigmentation in the face and on the ears, crimp definition, density of fleece, evenness of fleece, creeping belly and various conformation traits.

For the purpose of this paper, the heritabilities of and genetic and phenotypic correlations amongst the subjective fleece traits, as well as pigmentation and softness of face, were estimated. Genetic and phenotypic correlations of these traits with body weight, objective fleece traits and reproduction were also estimated. This was done in an attempt to clarify the importance of these traits in the selection programme of Afrino sheep.

 

2.  Material and methods

Data used for this study were collected on the Carnarvon Afrino flock, and include data records of 3291 animals, the progeny of 127 sires and 772 dams, born from 1986 to 1998. At 16 months of age, 7 subjective traits were assessed on a linear scale ranging from 1 to 50 in all young rams and ewes. These traits were softness of face covering, extent of pigmentation in the face and on the ears, softness of fleece, crimp definition, density of fleece, evenness of fleece (in terms of fibre diameter, length, and crimp definition) and creeping belly (extent to which belly wool creeps up the sides). The scale of assessment for these traits is summarized in Table 1.

Table 1. Linear scale for assessment of subjective traits in Afrino sheep

 

 Scale of assessment a

  

 1

 25

  50

  Softness of face (FACE)

 Hard

 Average

 Very soft

 Pigmentation (PIGM)

 None

 Ideal

 Excessive

 Softness of fleece (SOFT)

 Hard

 Average

 Soft

 Crimp definition (CRIM)

 Poor

 Average

 Ideal

 Density of fleece (DENS)

 Poor

 Average

 Ide

 Evenness of fleece (EVEN)

 Uneven

 Average

 No variation

 Creeping belly (CBEL)

 High up the sides

 Average

 None

  a1 - 10 = Poor

  11 - 20 = Below average

  21 - 30 = Average

  31 - 40 = Above average

  41 - 50 = Excellent

 

Data records on weaning weight, body weight at 9- and 12-months of age, as well as clean fleece weight, fibre diameter and staple length measured at 16 months of age were also available for the same animals. Reproduction data of 686 ewes born from 1986 till 1997 used for the study, include lifetime number of lambs weaned and lifetime weight of lamb weaned.



3.  Results

3.1   Heritability of and correlations amongst subjective traits

Heritability estimates of and genetic and phenotypic correlations amongst the subjective traits are summarised in Table 2.

 

Table 2. Heritability estimates (SE) of and genetic and phenotypic correlations (SE among various subjective traitsa

 

 FACE

 PIGM

 SOFT

 CRIM

 DENS

 EVEN

 CBEL

 FACE

 0.23 (0.04)

 0.10 (0.09)

 0.20 (0.09)

 0.05 (0.10)

 -0.05 (0.11)

 0.18 (0.12)

 0.03 (0.09)

 PIGM

 0.05 (0.02)

 0.50 (0.04)

 -0.09 (0.07)

 -0.05 (0.07)

 -0.03 (0.09)

 0.07 (0.10)

 0.09 (0.07)

 SOFT

 0.11 (0.02)

 0.00 (0.02)

 0.51 (0.04)

 0.80 (0.03)

 -0.64 (0.06)

 0.81 (0.05)

 -0.52 (0.05)

 CRIM

 0.04 (0.02)

 0.03 (0.02)

 0.62 (0.01)

 0.47 (0.04)

 -0.61 (0.07)

 0.62 (0.07)

-0.67 (0.05)

 DENS

 -0.05 (0.02)

 -0.02 (0.02)

 -0.40 (0.02)

 -0.36 (0.02)

 0.26 (0.04)

 0.02 (0.12)

 0.51 (0.07)

 EVEN

 0.04 (0.02)

 0.01 (0.02)

 0.32 (0.02)

 0.40 (0.02)

 0.05 (0.02)

 0.28 (0.04)

 -0.27 (0.10)

 CBEL

 0.00 (0.02)

 0.00 (0.02)

 -0.35 (0.02)

 -0.45 (0.02)

 0.31 (0.02)

 -0.15 (0.03)

 0.37 (0.04)

 aHeritability on diagonal, genetic correlations above diagonal and phenotypic correlations below diagonal

From Table 2 it is evident that the heritability estimates for the various subjective traits ranged from moderate (FACE, DENS and VAR) to high (PIGM, SOFT, CRIM and CBEL). Genetic response to selection based on individual phenotype of the latter four traits, could therefore be achieved.

PIGM and FACE had no genetic correlation with any of the subjectively assessed fleece traits. High genetic correlations were estimated amongst the subjective fleece traits, with the exception of DENS and EVEN. Generally, animals with softer fleeces had better crimp definition, their fleeces were more even, less dense and had larger creeping bellies.

The phenotypic correlations amongst the subjective fleece traits were similar, but somewhat lower in magnitude, than the corresponding genetic correlations.

 

3.2   Correlations of subjective traits with body weight, fleece traits and reproduction

Genetic correlations of reproduction, body weight and fleece traits with the various subjectively assessed fleece traits are summarised in Table 3.

Table 3. Genetic correlations (SE) of reproduction, body weight and fleece traits* with various subjective traits

 

 TWW

 NLW

 WW

 W9

 W12

 CFW

 FD

 SL

 FACE

 0.49 (0.24)

 0.42 (0.21)

 -0.08 (0.13)

 0.02 (0.10)

 -0.06 (0.10)

 0.40 (0.08)

 -0.22 (0.08)

 0.28 (0.10)

 PIGM

 -0.17 (0.25)

 -0.15 (0.21)

 0.13 (0.10)

 0.12 (0.07)

 0.07 (0.07)

 -0.02 (0.06)

 0.08 (0.06)

 0.04 (0.08)

 SOFT

 0.01 (0.34)

 -0.13 (0.29)

 -0.22 (0.11)

 -0.07 (0.08)

 -0.09 (0.08)

 0.02 (0.07)

 -0.80 (0.03)

 0.00 (0.09)

 CRIM

 0.45 (0.36)

 0.38 (0.29)

 -0.08 (0.11)

 0.06 (0.08)

 0.09 (0.08)

 -0.02 (0.07)

 -0.42 (0.05)

 -0.09 (0.09)

 DENS

 -0.08 (0.53)

 -0.02 (0.47)

 0.11 (0.13)

 0.14 (0.10)

 0.11 (0.10)

 0.22 (0.08)

 0.48 (0.06)

 -0.38 (0.09)

 EVEN

 -0.50 (0.33)

 -0.32 (0.27)

 -0.08 (0.14)

 -0.04 (0.10)

 -0.03 (0.10)

 -0.02 (0.09)

 -0.57 (0.07)

 -0.19 (0.10)

 CBEL

 -0.33 (0.23)

 -0.25 (0.20)

 -0.26 (0.10)

 -0.31 (0.08)

 -0.35 (0.07)

 0.53 (0.05)

 0.41 (0.05)

 0.27 (0.08)

*TWW = Lifetime weight of lamb weaned; NLW = Lifetime number of lambs weaned; WW = 100 day weaning weight;

  W9 = 9-month body weight;  W12 = 12-month body weight; CFW = clean fleece weight; MFD = fibre diameter; SL = Staple length

Due to the limited number of reproductive records, standard errors for the genetic correlations were relatively high. These results should therefore be regarded merely as an indication whether positive or negative relationships exist between traits. High genetic correlations were estimated between the reproductive traits and FACE. No significant genetic correlations were estimated between reproduction and PIGM. Genetic correlations variable in sign and magnitude were estimated between the fleece traits and reproduction. EVEN and CBEL were negatively correlated with TWW and NLW, while CRIM was positively correlated with reproduction.

Of the subjective fleece traits, CBEL had the highest genetic correlation with body weight, ranging from -0.26 ± 0.10 for WW to -0.35 ± 0.07 for W12. The other fleece traits (SOFT, CRIM, DENS and EVEN), as well as FACE and PIGM, were not significantly correlated with body weight.

FACE had favourable relationships with CFW, FD and SL. Genetically, fleeces with lower fibre diameter were softer, had better crimp definition, were less dense and more even, and had more creeping bellies. Staple length was negatively correlated with DENS (-0.38 ± 0.09) and EVEN (-0.19 ± 0.10), but positively with CBEL (0.27 ± 0.08).

No significant phenotypic correlations were discernable between reproduction or body weight and any of the subjectively assessed traits. Of the objective fleece traits, only FD had some significant phenotypic correlations with the subjective fleece traits. These were similar in sign, but smaller in magnitude than the corresponding genetic correlations, where fleeces with lower fibre diameter were softer (-0.80), had better crimp definition (-0.42), were less dense (0.48) and more even (-0.57), and had more creeping bellies (0.41).

 

4.  Discussion

From the selection objectives for Afrino sheep, it follows that selection should be aimed at increasing total weight of lamb produced (TWW), increasing the animal's own or direct growth performance (WW, W9), decreasing or maintaining fibre diameter (FD) and at least maintaining fleece weight (CFW).

Softness of face cover and the extent of pigmentation on the face and ears are two of the subjective traits on which a lot of emphasis is placed during selection. The results of this study indicated a favourable relationship between softer faces and reproduction and fleece production. Selection for softer faces would therefore not adversely affect the economically important traits. The same applies for the extent of pigmentation on the face and ears, as no noteworthy genetic correlations between PIGM and any of the other traits analysed, were estimated.

Since 1988, selection for wool production in the Carnarvon Afrino flock focused on improving crimp definition and evenness of fleece, while decreasing fibre diameter. No selection was done for amount of wool produced. The results of this study indicate that fleeces with lower fibre diameter had better crimp definition, were less dense and more even, and had larger creeping bellies. The genetic correlations estimated amongst these traits imply a strong genetic relationship amongst them.

This is clearly illustrated through the genetic trends in fibre diameter, creeping belly and crimp definition, depicted in Figure 1. From these it is obvious that selection for decreased fibre diameter and improved crimp definition in the Carnarvon Afrino flock lead to the appearance of more creeping belly.

Fig. 1. Genetic trends in fibre diameter (MFD), crimp definition (CRIM) and creeping belly (CB) in the Carnarvon Afrino flock

Decreasing fibre diameter and improving crimp definition are two of the most important aspects in the breeding plan of Afrino breeders. At the same time, breeders discriminate strongly against creeping bellies, even to the extent of culling animals superior in terms of the economically important traits. This practice should be reviewed, as indiscriminate selection against creeping bellies could result in increased fibre diameter and decreased crimp definition.

Other unfavourable correlations, in light of the current practice of discriminating against creeping bellies, are the genetic correlations estimated between CBEL and reproduction and body weight at all ages. Animals with higher body weights and ewes with higher reproductive rates also had larger creeping bellies. The economic implications, in terms of loss of genetic gain in body weight and reproductive rate when selection is done against creeping bellies, are obvious. However, the implication on wool income if less selection emphasis is placed on creeping belly, should be investigated further. Most probably this would be far less detrimental than anticipated, owing to the fact that fleeces with creeping bellies are usually finer, and would therefore fetch higher prices. It is proposed that only animals with extreme creeping bellies be culled.

Other subjective fleece traits analysed would not be influenced to any major extent when selection is done for increased body weight or reproduction. The possible exception is evenness of fleece, which is negatively correlated with the reproduction traits.  A similar conclusion, albeit based on phenotypic correlations, was drawn by Olivier et al. (1997) for Merino sheep.

 

5.  Conclusion

There is a fine balance between the various production functions, which should be kept in mind when constructing and implementing a breeding plan for any sheep breed. This is especially important in dual-purpose sheep breeds where it is expected from the ewes to produce good quality slaughter lambs, as well as a suitable amount of good quality fine wool. It is therefore important that selection priorities be based on economic value of the traits, and that breeders should learn to place more emphasis on traits of economic importance, and less on fancy traits. With the exception of one or two, the subjectively assessed traits would not be influenced detrimentally when selection is based on the economically important production traits.

As far as creeping belly is concerned, more data, especially reproductive data, is needed to quantify the economic implications if less selection pressure is placed against creeping belly. In the meantime, it is proposed that only animals with extreme creeping bellies be culled.