Indirect selection criteria against clean wool colour in Corriedale sheep and their effects on wool production traits

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.