The coefficient of variation of fibre diameter (CVFD) within the mid-side fleece sample is currently used to predict staple strength (SS) in Merino sheep (4.5 year old ewes). CVFD measures fibre diameter variation both between fibres and along wool fibres. It has been suggested that selection to improve staple strength should concentrate on reducing fibre diameter ariation along the staple, rather than CVFD. Our results indicate that measurements of fibre diameter variability along the staple had low heritabilities to moderate (0.01 to 0.20) and a low to moderate (0.15 to -0.43) phenotypic correlation with staple strength. In comparison, CVFD was highly heritable (0.78) and had a moderate (-0.44) phenotypic correlation with S. This suggests that there would be no advantage in using measures of fibre diameter variability along the staple as an indirect selection criterion for SS compared with the information provided by CVFD measured in a mid-side fleece sample.
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