Abstract
The potential of greasy wool colour subjective assessment Visual Colour Score (VCS) and the yellow predictive test (YPC) as indirect selection criteria for reduction of clean wool colour (CWC) in Corriedale sheep was examined. The heritability of these wool colour traits and the wool production traits, greasy (GFW) and clean fleece weights (CFW), and mean fibre diameter (MFD) and the phenotypic and genetic correlations among these traits were estimated from a Corriedale flock using AIREML procedures. A high genetic correlation between YPC and CWC was observed, indicating that YPC could be a suitable indirect selection criterium for CWC. However, direct selection against CWC was predicted to produce faster genetic improvements in CWC than that expected under indirect selection via YPC. Single trait selection based on VCS or YPC were expected to reduce the response in CWC to 51% and 49% of that estimated for direct selection. The positive genetic correlations of CWC, YPC and VCS with CFW and MFD would cause a reduction in both MFD and CFW to result from selection that reduces wool colour. The results showed that the most effective way to genetically improve CWC was through indirect selection to reduce MFD, CFW or GFW, followed by direct selection, but the premiums for CWC in the Corriedale breed may not be sufficient to justify the expected losses in CFW.
Highlights
Reductions in clean wool colour are sought by wool breeders because yellow wool is penalised in price due to a loss in versatility in dyeing
Studies on the genetics of wool yellowing have found that the heritability of clean wool colour (CWC) - measured in Y-Z units - varies from 0.04 ± 0.07 in Romney sheep in New Zealand (Hawker et al, 1988) to 0.42 ± 0.14 for Collinsville Merinos in Australia (James et al, 1990)
Mean fibre diameter (MFD), CWC, yellow predictive test (YPC) and Visual Colour Score (VCS) were determined from midside samples taken in August from both sexes in each year in accordance with AS/NZS (1999a)
Summary
Reductions in clean wool colour are sought by wool breeders because yellow wool is penalised in price due to a loss in versatility in dyeing. Studies on the genetics of wool yellowing have found that the heritability of clean wool colour (CWC) - measured in Y-Z units - varies from 0.04 ± 0.07 in Romney sheep in New Zealand (Hawker et al, 1988) to 0.42 ± 0.14 for Collinsville Merinos in Australia (James et al, 1990). The variation between these heritability estimates might have been due to environmental variation caused by differences in weather conditions between the locations of these studies. The finding of a suitable indirect selection criterion may be a better approach to selection against wool yellowing in low rainfall areas
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