Staple strength is the second most important wool quality trait after fibre diameter and can be improved by selection. Coefficient of variation of fibre diameter (CV D ) is highly correlated with staple strength and is a practical selection tool for breeders, whatever time of year they shear, according to new research by Johan Greeff from Agriculture Western Australia.

Agriculture Western Australia research has shown that rams with lower CVD (range of fibre diameters in the fleece) will breed progeny with higher staple strength than their peers. Because CV D results are provided routinely by wool testing laboratories when measuring fibre diameter, they provide a very economical selection tool for stud breeders compared with the more costly separate measurement of staple strength.

Until recently, most research involved spring-shorn sheep and there was concern about how widely such results could be applied to animals shorn at other times. An example would be stud breeders who generally shear in autumn and use the fleece testing results for selection purposes. Different environmental factors can evoke different gene actions, and it is known that tenderness in wool could be caused by different environmental events. For example, shortage of good quality grazing for a limited period may result in tender wool because of the decrease in fibre diameter, but other factors such as trauma may have the same effect. Hence the underlying genes affecting the same trait might be different.

Trials were conducted at the Great Southern Animal Research Institute near Katanning in which half of the male progeny of a group of 121 Merino sires were allocated randomly to be shorn in autumn or in spring, while a third of the female progeny were shorn in spring and the rest in autumn. Full pedigrees were collected on each animal. Analysis showed that clean fleece weight, fibre diameter, staple strength and CV D were all heritable traits (i.e. a proportion of the variation within a flock will be transferred to the next generation) but heritability varied between seasons. The genetic correlation between these traits in spring and autumn-shorn wool did not differ significantly from unity and are shown in Table 1. This indicated that the traits measured in the different seasons were genetically the same.

The phenotypic and genetic relationship between staple strength and CV D within seasons and across seasons varied from medium to strongly negative (see Table 2) and confirmed earlier results. This implies that rams with lower CV D than their contemporaries should sire progeny with sounder wool. However, the genetic and phenotypic correlation between staple strength and CV D in autumn-shorn wool was lower than in spring-shorn wool and may be related to the position of the break being near the end of the staple. This indicates that CV D in autumn-shorn wool is not as good a predictor of staple strength as in spring wool but it is still a very useful result.

The results confirm that staple strength and CV D are both heritable traits and can be improved by selection. They also show that CV D is a very useful indirect selection criterion to improve staple strength irrespective of when animals are shorn, but especially in spring-shorn wool.

The research on this new filament gene has shown that it is turned on (Figure 2) at a vital time during the formation of the fibre in the follicle and the protein it produces contributes to the hardness of the sheath. What is particularly interesting is that this protein is markedly different from any other so far described. It clearly plays a distinctive role in the formation of wool. The results have been submitted for publication.

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