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GENETIC RELATIONSHIPS AMONG FIBRE DIAMETER MEASURES AT DIFFERENT AGES IN A FINE WOOL MERINO STUD

 

W.J. Olivier#,1, S.W.P. Cloete2,3, J.B. van Wyk4 & A.R. Gilmour5

1Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900

2Institute for Animal Production: Elsenburg, Private Bag X1, Elsenburg, 7607

3Department of Animal Sciences, University of Stellenbosch, Matieland, 7602

4Department of Animal, Wildlife and Grassland Sciences, University of the Free State, Bloemfontein, 9300

5School of Computing and Mathematics, Charles Stuart University, Orange, Australia

#E-mail: Willem Olivier

 

 

INTRODUCTION

Fibre diameter is one of the most important factors influencing wool price (Erasmus & Delport, 1987). This fact and the subsequent price premium paid for fibre diameter in the late 1980s, led to the emphasis being shifted to selection for decreased fibre diameter in the wool industry of South Africa. In some flocks, it is the only selection objective regardless of the impact on the other traits. Selection for a reduced fibre diameter is exercised at performance testing age and in most flocks no or little attention is given to the fibre diameter of the adult ewe flock. This selection procedure is practiced despite the fact that a large portion of the total wool clip marketed by wool producers is produced by the adult ewes. Therefore, the aim of this study was to quantify the genetic relationship among fibre diameter measures at different ages in a fine wool Merino stud in South Africa.

 

MATERIALS AND METHODS

The Cradock Fine Wool Merino Stud was established in 1988 as described by Olivier et al. (2006). Ewes (fine wool line) were bought from Merino farmers with the finest clips throughout South Africa and four fine wool rams were imported from Australia. A second group of ewes (strong x fine wool line) originating from the strong wool Merino flock at Cradock was run together with the fine wool line and were also mated to the same sires (Olivier, 2009). Data collected on 1176 adult ewes from the Cradock Fine wool Merino stud from 1988 to 2003 were used for this analysis.

The means and standard deviations for the fibre diameters were obtained with the PROC MEANS-procedure of SAS, and significance levels for the fixed effects were obtained with the PDIFF-option under the PROC GLM-procedure of SAS (SAS, 2009). Fibre diameter was measured on 6 (FD6), 15 (FD15), 22 (FD22), 34 (FD34), 46 (FD46), 58 (FD58), 70 (FD70), 82 (FD82) and 94 (FD94) months of age, and only data from ewes which have records of the first four measurements were included in the analysis. Several fixed effects (year of birth, line (fine or strong x fine), rearing status, age of the dam in years and age at measurement in months as a linear regression) were tested and only effects which had a significant effect (P<0.01) were included in the final operational model. The estimation of the genetic parameters with repeatability and random regression models were done with ASReml (Gilmour et al., 2009).

 

RESULTS AND DISCUSSION

The number of records, mean, coefficients of variation, standard errors, minimum and maximum of the different fibre diameter measures are presented in Table 1. The mean fibre diameter ranged from 17.8 µm (FD6) to 20.3 µm (FD70). The high maximum fibre diameter measurements were recorded on the early progeny of the strong x fine line.

 

Table 1. The number of records (n), mean, coefficient of variation (CV), minimum and maximum of the different fibre diameter measures

Fibre diameter 

 

n 

Mean (µm) 

CV 

(%) 

Minimum 

(µm) 

Maximum 

(µm) 

6 months of age (FD6)

1176

17.77

6.71

14.50

22.30

15 months of age (FD15)

1176

18.98

7.15

14.50

24.70

22 months of age (FD22)

1177

19.48

7.01

14.60

25.10

34 months of age (FD34)

1177

19.58

7.42

15.90

28.90

46 months of age (FD46)

957

19.91

7.22

15.60

28.70

58 months of age (FD58)

646

19.98

7.16

16.20

27.70

70 months of age (FD70)

425

20.28

7.08

16.00

25.70

82 months of age (FD82)

252

20.03

6.94

16.20

24.50

94 months of age (FD94)

50

19.57

6.56

16.50

22.60

 

Estimates of the heritabilities of fibre diameter at the different ages are illustrated in Figure 1 and the genetic correlations among the age-specific fibre diameter measurements are presented in Table 2. The heritability estimates ranged from 0.37 at 6 months of age to 0.86 at 94 months of age and the heritability of fibre diameter in a repeatability model (fitting only the intercept) amounted to 0.50.

It is evident from Figure 1 that the heritability of fibre diameter increased with age. A possible explanation is that the partitioning of available nutrients in lambs and hoggets is more in favour of growth (body size) than wool yield. Later in life, given adequate nutrition, their genetic potential for
fibre diameter may be expressed better. It can also be ascribed to some extent to the increase in the available genetic information on an individual with age. Therefore, the influence of the genetic makeup on the phenotype of an animal increases with age. The heritability estimates (point estimates) reported in the literature for adult fibre diameter ranged from 0.62 to 0.76 (Coelli et al., 1998; Lee et al., 2002; Cloete et al., 2003). The estimates of this study for 22 to 70 months of age fall within the reported range, while the heritabilities at 82 and 94 months of age are higher than the reported values.

 

Figure 1. Heritability estimates for fibre diameter of Merino ewes at the different ages

 

Table 2. Estimated genetic correlations among the fibre diameter measures

Traits 

FD15 

FD22 

FD34 

FD46 

FD58 

FD70 

FD82 

FD94 

FD6 

0.98

0.95

0.90

0.85

0.81

0.78

0.75

0.73

FD15 

 

0.99

0.97

0.94

0.91

0.88

0.86

0.85

FD22 

 

 

0.99

0.97

0.95

0.93

0.92

0.90

FD34 

 

 

 

0.99

0.99

0.98

0.97

0.96

FD46 

 

 

 

 

0.99

0.99

0.99

0.98

FD58 

 

 

 

 

 

0.99

0.99

0.99

FD70 

 

 

 

 

 

 

0.99

0.99

FD82 

 

 

 

 

 

 

 

0.99

 

It is evident from Table 2 that the genetic correlations among the fibre diameters at different ages range from 0.73 (between FD6 and FD94) and unity (between FD82 and FD94). The genetic correlations among fibre diameters at different ages reported in the literature ranged from 0.82 to 0.96 (Hickson et al., 1994; Coelli et al., 1998). The values estimated in this study correspond with the reported values. Furthermore, it is evident from Table 2 that the genetic correlations between fibre diameters were inversely proportional to the time lapse between measurements.

 

CONCLUSIONS

It can be concluded from this study that the heritability estimates of fibre diameter increases with age, while the genetic relationships decrease in strength among fibre diameter measures with increasing age. The high genetic correlations among fibre diameter at 15 months of age, which is performance testing age for Merino sheep in South Africa, and adult fibre diameter measures, indicate that selection for decreased fibre diameter at 15 months will have a favourable effect on the fibre diameter of adult ewes. The current method used by wool producers to decrease fibre diameter, based on performance testing and subsequent selection, will therefore ensure that fibre diameter of the adult ewe flock will decrease genetically.

 

REFERENCES

Cloete, S.W.P., Gilmour, A.R., Olivier, J.J. & Van Wyk, J.B., 2003. Age trends in economically important traits of Merino ewes subjected to 10 years of divergent selection for multiple rearing ability. S. Afr. J. Anim. Sci. 33, 43-51.

Coelli, K.A., Gilmour, A.R.  & Atkins, K. D., 1998. Comparison of genetic covariance models for annual measurement of fleece weight and fibre diameter. Proc. 6th WCGALP, 24, 31-34.

Erasmus, G.J. & Delport, G.J., 1987. Factors influencing the price of greasy fleece wool in South Africa. S. Afr. J. Anim. Sci. 17, 111-115.

Gilmour, A.R., Gogel, B.J., Cullis, B.R. & Thompson, R., 2009. ASReml User Guide Release 3.0 VSN International Ltd, Hemel Hempstead, HPI 1ES, UK.

Hickson, J., Kinghorn, B., Swan, A. & Piper, L., 1994. The relationship between hogget and adult production traits in Merino sheep. Proc. 5th WCGALP, 18, 139-142.

Lee, G.J., Atkins, K.D. & Swan A.A., 2002. Pasture intake and digestibility by young and non-breeding adult sheep: the extent of genetic variation and relationships with productivity. Livest. Prod. Sci. 73, 185-198.

Olivier, W.J., 2009. Is selection for decreased fibre diameter in a Merino flock with overstrong wool viable? Grootfontein Agric. 9, 27-33.

Olivier, W.J., Olivier, J.J., Cloete, S.W.P. & Van Wyk, J.B., 2006. Genetic analysis of the Cradock fine wool Merino stud. Proc. 8th WCGALP, 599-602.

SAS Institute Inc., 2009. SAS OnlineDoc® 9.2. Cary, NC, SAS Institute Inc.

 

Published

Grootfontein Agric 12 (1) : 1