Last update: December 6, 2010 01:58:22 PM E-mail Print

 

IS SELECTION FOR DECREASED FIBRE DIAMETER IN A MERINO

FLOCK WITH OVERSTRONG WOOL VIABLE?

 

W.J. Olivier


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

E-mail: Willem Olivier

 


INTRODUCTION

During the 1980’s and early 1990’s there was an increased demand for finer wool (<20 µm) that resulted in price premiums being paid for the finer wool types during that period. This shift in the demand for finer wool led to more emphasis being placed towards the production of fine wool, rather than simply the amount of wool, which was the main selection objective since the 1950’s.

 

Consequently, the number of flocks in South Africa where selection for decreased fibre diameter was practiced increased markedly. In other words, selection for decreased fibre diameter was an important selection objective for many producers. In some instances it was the only objective, regardless of the effect on the other production traits. The price premium that was paid for the finer wool types was in some cases the end goal of producers, as it would lead to increased profitability.

 

One of the biggest constraints to wool producers in South Africa at that time was the fibre diameter of the ewe flocks, which was strong to overstrong (>23 µm). Therefore, the only quick fix with regard to the selection for decreased fibre diameter for wool farmers was to purchase or import fine and superfine wool Merino rams. According to Olivier et al. (2000) fibre diameter can normally, within flock selection, be reduced with 1 – 2 µm in 10 years’ time. This time can be shortened by using superfine wool rams as breeding sires. However, the progress with regard to reducing fibre diameter is largely dependant on the mean fibre diameter of the ewe flock.

 

Concurrently with the establishment of the genetic fine wool Merino stud at Cradock (Olivier et al., 2006), a strong x fine wool line was also established. This line (Strong x Fine) was established to simulate the upgrading of overstrong wool Merino ewes with fine or superfine wool Merino rams. The aim of this study was therefore to quantify the effect of upgrading of strong wool ewes with genetic fine wool rams on production and subjective traits of the progeny and the fibre diameter of the adult ewe flock.

 

MATERIALS AND METHODS

Data

Data collected on 5820 ram and ewe lambs (15-month age) born from 1988 to 2003 in the Cradock fine wool Merino stud and 6978 fibre diameter measurements of 2262 adult ewes shorn annually from 1988 to 2003 were used in this study. The Cradock Fine Wool Merino Stud (Fine line) was established in 1988 by buying ewes from Merino farmers with the finest clips throughout South Africa and importing four fine wool rams from Australia. The strong x fine (SF) wool line consisted of a group of 50 overstrong wool (±29 µm) Merino ewes of the original Cradock Merino flock. The ewes from both lines were run together as one group and were inseminated with semen of the same fine wool rams every year. The main selection objectives from 1988 to 1996 were to increase body weight, maintain or increase clean fleece weight and maintain or decrease fibre diameter. Since 1997, the objectives were changed to decreasing fibre diameter, maintain or increase body weight and clean fleece weight and to increase staple length. The average fibre diameter of the four rams that were imported from Australia was 20.6 µm.

 

Detailed descriptions of the management and selection procedures followed in this stud have been reported by Olivier et al. (2006). The fibre diameter of the adult ewe flock, as well as the 15-month body weight, clean fleece weight, mean fibre diameter and staple length of the lambs were analysed. The subjective traits assessed on a linear scale from 1 to 50 were wool quality, evenness of fleece and staple formation (Olivier et al., 1987).

 

Statistical analyses

The significance levels for the fixed effects were obtained with the PDIFF option under the PROC GLM procedure of SAS (Littell et al., 2002). Fixed effects for year of birth, sex, rearing status, line (Strong x Fine and Fine) and age of dam in years, as well as the 15-month age as a linear regression were tested for significance. Only effects and interactions which had a significant effect (P<0.01) on a specific trait were included in the final model.

 

The estimation of the genetic parameters and breeding values were done with ASREML (Gilmour et al., 2002). Log likelihood ratio tests were done to determine the most suitable model for the estimation of (co)variance components for each trait. The most suitable model for body weight, clean fleece weight, evenness of fleece and staple formation included both the direct additive genetic variance and the maternal additive genetic variance. The covariance between the direct and maternal additive variances was also included in the final model for clean fleece weight. The direct additive genetic variance and the maternal permanent environmental variance were included in the model for fibre diameter. Only the direct additive genetic variance was included in the models for staple length and wool quality. The genetic trends for the respective traits were obtained from univariate analyses.

 

RESULTS AND DISCUSSION

The average fibre diameter of the adult ewes and body weight, wool traits and subjectively assessed wool traits of the progeny over the 15 year period (± s.e.) are summarised in Table 1. The phenotypic trends of fibre diameter of the ewe flock and the progeny at 15 months of age are illustrated in Figures 1 and 2 respectively. It is evident from Table 1 and Figure 1 that the wool of the adult ewes of the Strong x Fine (SF) line was stronger (P<0.01) than the wool of the Fine (F) line. The fibre diameter of the SF and F line at the commencement of the project were 29.87±0.24 µm and 19.30±0.08 µm. There was an increase in the fibre diameter of the adult ewes of the F line to 21.32±0.09 µm within the first year. This can mainly be ascribed to the improvement of their feeding conditions, whereas the SF ewes were already adapted to the feeding conditions, as they originated from a flock that was run on pastures. From 1988 the fibre diameter of the adult ewes decreased at a rate of 0.65 µm and 0.11 µm for the SF and F lines respectively. It is evident from this graph that since 2002 there were either no or only small differences between the fibre diameter of the adult ewes of the two lines.

 

Table 1. The average fibre diameter of the adult ewes and the 15-month production and subjective traits of the progeny of the Strong x Fine and Fine lines over the 15 year period

 

Strong x Fine

Fine

Adult ewes

Fibre diameter (µm)

22.21±0.06a

19.87±0.03a

Progeny

Body weight (kg)

62.48±0.26a

61.62±0.14a

Clean fleece weight (kg)

4.84±0.03a

4.35±0.02a

Fibre diameter (µm)

20.02±0.28a

19.23±0.27a

Staple length (mm)

107.23±3.06a

105.47±3.03a

Number of crimps per 25 mm (n)

12.64±0.55a

14.10±0.55a

Duerden

99.38±2.06

100.29±2.04

Coefficient of variation of fibre diameter (%)

17.37±0.49a

16.69±0.49a

Wool quality

31.48±2.08

31.84±2.06

Evenness of the fleece

29.13±1.87a

31.99±1.86a

Staple formation

32.16±1.53a

30.08±1.52a

a - Values with the same superscript differed significantly (P < 0.01)

 

It is evident from Table 1 that the Strong x Fine (SF) progeny was heavier (P<0.01) than their F line counterparts. The SF progeny also produced significantly more wool, with longer (P<0.01) staples that were stronger (P<0.01) with a higher (P<0.01) coefficient of variation of fibre diameter. The fibre diameter (Figure 2) of the SF progeny decreased from 22.36±0.04 µm to 18.06±0.34 µm at a rate of 0.30 µm per year from 1988 to 2003. The comparable values of the F line are 20.62±0.33 µm to 17.77±0.28 µm at a rate of 0.18 µm per year. The fibre diameter of the progeny of both lines was on Duerden standard, which is a function of the fibre diameter and number of crimps per 25 mm. With regard to the subjective traits, there was no difference in wool quality between the lines. However, the fleeces of the Fine line were more (P<0.01) even and had poorer (P<0.01) staple formation compared to the SF line.

Figure 1. The annual mean fibre diameter of the adult ewes of the two lines

Figure 2. The phenotypic trends for the fibre diameter of the progeny of the two lines

The genetic trends for body weight and fibre diameter are illustrated in Figures 3 and 4 respectively, while the gradient, intercept and R2 values of the genetic trends of clean fleece weight, staple length, wool quality, variation over the fleece and staple formation are presented in Table 2. It is evident from Figure 3 that the genetic trend of body weight of the progeny of both lines increased with ±0.55 kg each year. Subsequently, the body weight of the SF and F lines progeny increased genetically with 7.98 kg and 8.49 kg respectively over the 15 year period.

Figure 3. The genetic trends for 15-month body weight of the two lines

Figure 4. The genetic trends for fibre diameter of the progeny of the two lines

 

It is evident from Figure 4 that the fibre diameter of the SF progeny decreased genetically with 0.15 µm per year compared to the 0.10 µm of the F line progeny. Subsequently, the fibre diameter of the SF line progeny decreased with 2.15 µm over the 15 year period compared to the 1.46 µm of the F line progeny. Despite the fact that the fibre diameter of the SF progeny decreased more rapidly, the F line progeny was still 0.35 µm finer than the SF progeny at the end of the study.

 

Table 2. The gradient, intercept and R2 values for the genetic trends of clean fleece weight, staple length, wool quality, variation over the fleece and staple formation of the two lines

 

Strong x Fine

Fine

 

Gradient

Intercept

R2

Gradient

Intercept

R2

Clean fleece weight

0.00

0.15

0.00

0.05

-0.48

0.81

Staple length

0.60

-8.79

0.78

0.71

-10.06

0.84

Wool quality

0.63

-3.89

0.91

0.60

-3.58

0.95

Variation over the fleece

0.31

-0.98

0.81

0.14

1.49

0.49

Staple formation

-0.11

0.86

0.59

-0.01

-0.57

0.02

 

The clean fleece weight (Table 2) of the SF progeny did not decrease over the 15 year period despite the rapid decrease in fibre diameter, while the clean fleece weight of the F line progeny increased with 0.05 kg annually. This can be ascribed to the fact that the selection objective with regard to clean fleece weight was to improve or maintain this trait. The SF progeny produced 0.61 kg more wool per animal at the start of the project. Staple length and wool quality score (Table 2) of both lines were improved at approximately the same rate per year. It is evident from Table 2 that the variation over the fleece score of the SF progeny was improved at a higher rate. This can be ascribed to the fact that the SF progeny had much more variation over the fleece compared the F line progeny at the start of the study. The staple formation score of the SF progeny decreased slightly over the 15 year period, while the staple formation score of the F line progeny was maintained.

 

Conclusions

It is evident from the results of this study that there was no difference in the fibre diameter of the SF and F progeny after 15 years of mating overstrong wool Merino ewes and their ewe progeny with fine wool Merino rams. Furthermore, despite the rapid decrease in fibre diameter there was a marked increase in the body weight of the SF progeny, while clean fleece weight was maintained and staple length improved slightly. It can therefore be concluded that the fibre diameter of a flock of overstrong wool Merino ewes can be decreased with approximately 8 µm over a period of 15 years without having a detrimental effect on the body weight and wool production of the progeny.

 

References

Gilmour, A.R., Gogel, B.J., Cullis, B.R., Welham, S.J. & Thompson, R., 2002. ASREML User’s Guide Release 1.0. VSN International Ltd, Hemel, Hempstead, HP11es, UK.

Littell, R.C., Freud, R.J. & Struop, W.W., 2002. SAS-system for linear models, 4th Ed. SAS Institute. Inc. Cary, N.C., USA.

Olivier, J.J., Bezuidenhout, A.G., Greyling, A.C. & Cloete, S.W.P., 2000. Evaluation of genetic fine and fine x strong wool Merinos on irrigated pastures. Proc. 38th SASAS Congress, 71-72.

Olivier, J.J., Delport, G.J., Erasmus, G.J. & Eksteen, T.J., 1987. Linear type scoring in Merino sheep. Karoo Agric. 3, 1–4.

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 World Congr. Gen. Appl. Livest. Prod., Belo Horizonte, 13-18 August, 84.

 

Published

Grootfontein Agric 9 (1)