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Repeatability and heritability of objective and subjective fleece traits

and body weight in South African Angora goats

 

M. A. Snyman* & J. J. Olivier

Grootfontein Agricultural Development Institute, Private Bag X529,

Middelburg Cape, 5900, South Africa

 

email : Gretha Snyman

* Author to whom correspondence should be addressed

 


1.  Introduction

A need  for the implementation of scientific breeding methods has been expressed by South African Angora goat breeders. Subsequently, the Department of Agriculture was requested to design a scientific breeding plan for their specific needs. A performance testing scheme was therefore established by the Department. The purposes of this scheme was to collect data for the estimation of genetic parameters (Snyman & Olivier, 1996), and to provide the necessary infra-structure through which objective measurement of production traits can be facilitated.

Due to the fact that young rams in South Africa are sold at 14 to 16 months of age with their full third fleece, performance testing has to be done at 8 to 9 months of age at the second shearing. Performance testing of body weight, greasy fleece weight and mean fibre diameter is done. The question is asked whether testing at the relatively young age of 8 to 9 months is accurate enough to identify genetically superior animals at that stage.

Furthermore, during the selection process of both rams and ewes, a lot of emphasis is placed on traits such as softness of face and ears, fullness of face, neck and bellies & points, evenness of fleece, style and character. From observations in practice, it seems as though many of these traits have a low repeatability. Stud breeders also expressed the need to include these subjective traits in a selection index, together with the objective production traits. For this to be possible, it is essential that reliable genetic parameters for the traits in question be available.

A study was done to estimate the repeatability and heritability of body weight, greasy fleece weight and mean fibre diameter, as well as various subjective fleece traits of South African Angora goats in order to be able to make recommendations regarding their inclusion in selection indices. This study was done with the co-operation of some of the stud breeders participating in the Angora goat performance testing scheme, as well as on the experimental flocks at the Jansenville Angora Goat Experimental Station.

 

2.  Material and methods

Two data sets were used for analysis in this study. The first data set was collected on the 1991- and 1992-born ewes of eight stud breeders who participated in the Angora goat performance testing scheme (AGPTS). For practical reasons, only data from ewes were available for this data set. Data recorded at the second (9 to 10 months of age), third (15 to 16 months of age) and fourth shearing (21 to 22 months of age) of these ewes were used. Pedigree information was obtained from the Angora Goat Stud Breeders' Society of South Africa. The results from this data set can be taken as representative of the Angora stud goats in SA.

The second data set comprised data collected on the experimental flocks kept at the Jansenville Angora Goat Experimental Station (JEF). These goats can be considered as representative of the commercial flock Angora goats in SA. Subjective traits were assessed on the 1992- to the 1995-born ram and ewe kids at the second (10 months of age), third (14 months of age) and fourth shearings (18 months of age). Performance testing data on body weight, greasy fleece weight and fibre diameter of the 1988- to the 1995-born kids were also available for each shearing. Full pedigrees were available for all animals.

The following traits were assessed subjectively on a linear scale ranging from 1 to 50 at each shearing, namely fullness of face covering, fullness of the neck, fullness of belly and points, style, character, evenness of fleece (in terms of fibre diameter, length, style and character),  softness of face and ears, extent of pigmentation in the face and on the ears, and presence of kemp and medullated fibres. As the consistency of the assessor would be incorporated into the repeatability estimates,  all traits were scored by only one person in an attempt to minimze this factor. The traits, as well as the scale of assessment are summarized in Table 1.

Table 1. Linear scale for the assessment of subjective traits

                           Scalea

Trait

1

25

50

Head:

Softness

Very hard

Average

Very soft

Face cover

Bald face

Average

Very good cover

Pigmentation

Excessive

Average

No pigment

Neck cover

Bald neck

Average

Very full neck

Fleece:

Style

No style

Average

Excellent style

Character

Very straight

Ideal

Over-curly

Evenness

Much variation

Average

No variation

Kemp

Very kempy

Average

No kemp/medullation

Bellies & points

Poor

Average

Very good cover

1 - 10 = Poor

  11 - 20 = Below average

  21 - 30 = Average

  31 - 40 = Above average

  41 - 50 = Excellent

Furthermore, performance testing of body weight, greasy fleece weight and mean fibre diameter was done at each shearing. Fibre diameter of the AGPTS-data set was determined from midrib samples by means of a Fibre Diameter Analyzer at Cape Mohair and Wool. In the case of the JEF-data set, fibre diameter was determined on a midrib fleece sample by the air‑flow procedure using a WIRA‑fineness meter. Body weight and greasy fleece weight were taken just after shearing. Prior to data analysis, greasy fleece weight was corrected to 180 days' hair growth.

 

3.  Results

3.1  Mean and coefficient of variation

Overall means and coefficients of variation for each of the traits are given in Table 3.

Table 3. Overall means (first row) and coefficients of variation (%, second row) for body weight and fleece traits in South African Angora goats

 AGPTS JEF

 

Trait

Shearing

 

Shearing

2nd

3rd

4th

2nd

3rd

4th

Body weight

(kg)

21.31

(15.11)

24.40

(14.55)

32.44

(14.21)

 

23.03

(18.07)

28.48

(17.54)

32.25

(15.79)

Fleece weight (kg)

1.97

(18.91)

2.33

(13.72)

2.67

(12.31)

 

1.77

(16.82)

2.16

(16.21)

2.70

(14.53)

Fibre diameter (Fm)

28.81

(8.33)

32.62

(7.44)

35.39

(6.77)

 

26.94

(8.24)

29.42

(9.39)

33.63

(8.13)

Softness

44.21

(6.36)

44.65

(6.28)

44.29

(7.33)

 

41.34

(10.89)

38.61

(18.83)

37.79

(17.88)

Face cover

42.78

(10.37)

42.07

(13.73)

40.09

(17.04)

 

29.78

(36.96)

32.94

(31.52)

31.03

(28.12)

Pigmentation

40.98

(13.00)

39.04

(16.19)

40.43

(13.40)

 

36.21

(16.61)

34.00

(17.31)

33.63

(18.10)

Neck cover

36.20

(12.08)

34.75

(18.56)

33.03

(17.27)

 

25.52

(22.11)

26.98

(26.40)

30.35

(19.16)

Style

29.62

(26.94)

29.46

(23.76)

25.79

(22.30)

 

23.25

(27.29)

23.44

(31.74)

28.03

(20.08)

Character

23.93

(21.71)

22.45

(21.68)

24.57

(18.97)

 

20.63

(25.53)

19.21

(33.01)

24.47

(18.74)

Evenness

34.27

(15.22)

31.18

(22.95)

31.92

(18.43)

 

33.35

(11.33)

36.49

(15.00)

34.35

(17.19)

Kemp

44.11

(5.86)

44.01

(5.16)

44.53

(4.42)

 

41.65

(8.27)

39.47

(17.80)

38.69

(18.29)

Bellies & Points

38.27

(11.84)

36.50

(14.36)

34.44

(13.87)

 

26.74

(16.41)

30.16

(18.23)

30.43

(15.61)

From Table 3 it is evident that body weight, fleece weight and fibre diameter increased from the second to the fourth shearing in both data sets.

Body weight and fleece weight of the AGPTS-stud ewes compare well with those of the  JEF-kids. Fibre diameter of the JEF-kids were approximately two micron finer, which was expected, as half of these are fine hair goats. As far as the average scores for the subjective traits are concerned, the AGPTS-stud ewes had higher scores for all traits, except for evenness of fleece. The scores are, however, quite comparable and there were no large differences.

Coefficients of variation for softness and kemp for the AGPTS-stud ewes were low, which implies that little variation is present in these traits. Style, character and evenness of fleece, on the other hand, had the highest coefficients of variation, indicating large scopes of selection possibilities in the studs. For the JEF-kids, the highest coefficients of variation were estimated for fullness of face cover, neck cover, style and character.

 

3.2  Heritability and repeatability estimates

Heritability and repeatability estimates obtained for the various traits in the two data sets are summarized in Table 4.

Table 4. Repeatability (SE for P2-values) and heritability estimates (SE) for body weight and fleece traits in South African Angora goats

Repeatability Heritability

Trait

AGPTS

JEF

AGPTS

JEF

Body weight

0.53

(0.12)

0.63

(0.06)

0.35

(0.05)

0.47

(0.01)

Fleece weight

0.41

(0.13)

0.27

(0.04)

0.22

(0.04)

0.22

(0.04)

Fibre diameter

0.68

(0.14)

0.35

(0.03)

0.30

(0.05)

0.29

(0.05)

Softness

0.32

(0.14)

0.31

(0.06)

0.07

(0.15)

0.33

(0.07)

Face cover

0.37

(0.11)

0.60

(0.07)

0.33

(0.12)

0.66

(0.11)

Pigmentation

0.62

(0.13)

0.62

(0.08)

0.43

(0.14)

0.49

(0.10)

Neck cover

0.26

(0.06)

0.39

(0.06)

0.13

(0.06)

0.33

(0.07)

Style

0.24

(0.06)

0.17

(0.05)

0.13

(0.06)

0.23

(0.06)

Character

0.35

(0.08)

0.39

(0.07)

0.14

(0.08)

0.34

(0.09)

Evenness

0.23

(0.09)

0.13

(0.04)

0.26

(0.10)

0.16

(0.05)

Kemp

0.29

(0.05)

0.25

(0.06)

0.01

(0.04)

0.32

(0.08)

Bellies & Points

0.22

(0.09)

0.12

(0.04)

0.30

(0.10)

0.30

(0.04)

The heritability for body weight was higher in the JEF than in the Angora studs, while the same heritabilities were estimated for fleece weight and fibre diameter in both data sets. The repeatability of body weight was higher in the JEF, but it was lower for fleece weight and fibre diameter than in the studs.

Heritability estimates for the subjective traits in the JEF were higher than those obtained in the studs for all traits, except for evenness of fleece and bellies & points. These higher heritabilities could most probably be ascribed to the broader genetic basis of the JEF, compared to the Angora studs. Heritability and repeatability estimates for all subjectively assessed traits, with the exception of face cover in the JEF and pigmentation in both data sets, were moderate to low. Pigmentation had the highest estimates of the subjective traits analysed, which could be expected.

 

4.  Discussion

At the third shearing, body weight and fibre diameter can be determined without shearing of the ram, but to obtain fleece weight the ram has to be shorn. Fleece weight also had the lowest repeatability of these three production traits. It is therefore proposed that fleece weight be left out of the selection index (Snyman et al., 1996), as an increase in fleece weight per se is not desirable, due to indications of a negative relationship between fleece weight per unit of body weight and reproduction performance (Herselman et al., 1998). The positive genetic correlation of 0.67 estimated between fleece weight and body weight (Snyman & Olivier, 1996) will assure that fleece weight will not decrease if selection is aimed at increasing body weight. Fleece weight should, however, be monitored.

From the overall means presented in Table 3 it is evident that, according to the scale of assessment, softness of face and ears, face cover, pigmentation and kemp of the Angora stud goats are excellent. Character of these goats is ideal, while neck cover and bellies & points are above average. However, style and evenness of fleece could still be improved further. In the JEF-goats, softness of face and ears, pigmentation, kemp, character and evenness of fleece are above average, while the other traits could be improved further. Face cover, neck cover and bellies & points already showed an improvement since 1992.

The traits face cover, neck cover and bellies & points are generally regarded by stud breeders as an indication of pureness or a sound genetic background. It is further claimed that these traits are related to fleece weight, and as fleece weight was the primary selection criterion in the past, emphasis was placed on these traits. In spite of this, average scores for neck cover and bellies & points were below 40. The moderate to low repeatability and heritability of these traits most probably contributed to the low selection response. The genetic correlations of these traits with fleece weight should be estimated in order to determine their economic value before recommendations of their inclusion in a selection index could be made.

It is further believed that softness of face and ears gives an indication of the amount of kemp and medullated fibres in the fleece. Continuous selection against frost and kemp in the Angora studs resulted in stud goats virtually being kemp free. The low coefficients of variation and heritabilities estimated for kemp and softness is evidence of the low variation in these traits among stud Angoras in South Africa. From these results (Tables 3 and 4) it  seems as if selection for softness and against kemp was successful in the Angora studs.

The higher heritability estimated for kemp in the JEF could possibly be attributed to higher levels of kemp and the larger variation in kemp among the JEF-goats. Accordingly, the percentage medullated fibres of the goats analysed by Gifford et al. (1991), was also higher than for stud goats in South Africa. A heritability of 0.39 was estimated for percentage medullated fibres in the latter goats. When the amount of kemp in the fleece is relatively high, selection based on subjective kemp scores would be effective. At lower levels and where only medullated fibres and no true kemp fibres are present, subjective assessment would, however, not be effective and selection should be based on objective measurement of medullation.

Style and character are the subjective traits on which the most emphasis are placed during selection. These two traits are generally regarded as one combined trait by the mohair industry. The results of this study, however, indicated that the repeatability and heritability estimates for character were generally higher than those obtained for style. These findings are in accordance with results obtained in a trial where the effect of nutrition on subjective mohair traits was investigated (Badenhorst et al., 1992). In that study it was found that style and character were affected differently by the level of nutrition. Character of the hair was enhanced by higher levels of nutrition, but style was not affected. Therefore it is recommended that style and character be assessed as two separate traits, as was done in this study. Furthermore, character of stud and JEF-goats is already at an acceptable level, but style of the hair could still be improved. The possible inclusion of style in the selection index should therefore be investigated further.

As far as the other subjectively assessed traits are concerned, it is evident from the foregoing discussion that most of them are on an acceptable level in both the studs and JEF. Any selection for the further improvement of these traits should be based on independent culling levels and they should not be included in the selection index.

 

5.  Conclusion

The subjective fleece traits on which a lot of emphasis is placed during selection, namely softness of face and ears, fullness of face, neck and bellies & points, evenness of fleece, style and character all had low repeatabilities compared to the economically important production traits. The method of assessment of the subjective traits could possibly have contributed to the lower estimates, as the consistency of the assessor was also incorporated into the estimate. Fleece weight had the lowest repeatability of the production traits, and as mentioned earlier, could be left out of the selection index if performance testing is done at the third shearing.

The low repeatabilities estimated for the subjective fleece traits are a cause for concern, as stud breeders and ram buyers are placing relatively more emphasis on these traits than on the more economically important traits, such as body weight and fibre diameter. More accurate parameters should be estimated when more data become available. Those estimated in this study would have to suffice in the interim when recommendation for suitable breeding plans are made to breeders. A premium should also be placed on the estimation of genetic correlations between these subjective traits and body weight, fleece weight, fibre diameter and especially reproduction, before recommendations of their inclusion in a selection index could be made.

 

6.  References

BADENHORST, M.A., DIEDERICKS, J.C., SCHLEBUSH, P.A. & KRITZINGER, N.M., 1992. Effect of  nutrition on certain Mohair quality traits. Karoo Agric 4(4) : 7-8

GIFFORD, D.R., PONZONI, R.W., LAMPE, R.J. & BURR, J., 1991. Phenotypic and genetic parameters of fleece traits and live weight in South Australian Angora goats. Small Rum. Res. 4(3) : 293-302

HERSELMAN, M.J., OLIVIER, J.J. & SNYMAN, M.A., 1998. Studies on small ruminant breeds with inherent differences in fibre production and hardiness. I. Relationship between hardiness and wool production potential. S. Afr. J. Anim. Sci. 28(1)

SHELTON, M. & BASSETT, J.W., 1970. Estimate of certain genetic parameters relating to Angora goats. Texas Agricultural Station Research Report (PR-2750) : 38-41

SNYMAN, M.A. & OLIVIER, J.J., 1996. Genetic parameters for body weight, fleece weight and fibre diameter in South African Angora goats. Livest. Prod. Sci. 47(1) : 1-6

SNYMAN, M.A., OLIVIER, J.J. & WENTZEL, D., 1996. Breeding plans for South African Angora goats. Angora Goat and Mohair Journal, 38(1) : 23-31

YALCIN, B.C., 1982. Angora goat breeding. Proc. 3rd Int. Conf. Goat Prod. Disease, Tucson, Arizona

Scalea 

Trait

 

1

25

50

Head:

 

 

 

Softness

Very hard

Average

Very soft

Face cover

Bald face

Average

Very good cover

Pigmentation

Excessive

Average

No pigment

Neck cover

Bald neck

Average

Very full neck

Fleece:

 

 

 

Style

No style

Average

Excellent style

Character

Very straight

Ideal

Over-curly

Evenness

Much variation

Average

No variation

Kemp

Very kempy

Average

No kemp/medullation

Bellies & points

Poor

Average

Very good cover