Wool growth and fibre diameter changes in young Merino sheep genetically different in staple strength and fed different levels of nutrition

The wool growth response to nutrition by Merino weaners (liveweight 33· 2±0·58 kg) bred for high or low staple strength (SS) and fed to produce changes in liveweight was examined. The hypothesis tested was that genetic differences in SS are associated with differences in along-fibre variation in diameter. Sheep fed to maintain liveweight produced wool at a more constant rate with smaller and less rapid changes in fibre diameter than that produced by sheep which lost and then gained liveweight (P < 0·001). There were significant (P < 0·05) but relatively small differences in wool growth rate and fibre diameter between the SS selection flocks, and wool from sheep selected for high SS had less (P < 0 ·001) variation in diameter between individual fibres than wool from sheep selected for low SS Minimum fibre diameter was most closely associated with SS, accounting for 66% (P < 0·001) of the total variance in SS generated by selective breeding and nutrition. An increase in minimum fibre diameter of 1µm was associated with an increase in SS of about 5 N/ktex. Minimum fibre diameter and the rate of change in fibre diameter to the point of break along the staple collectively accounted for 72% (P < 0·001) of the total variance in SS. Addition of a term for between-fibre variation in diameter measured at the point of break removed an additional 8% (P < 0·001) of the variance in SS. We conclude that the mechanisms responsible for nutritionally induced and genetic differences in SS are not the same. Nutrition influences SS by affecting along-fibre diameter changes, whereas genetic differences in SS, at least as far as they are represented by the flocks used here, are largely attributable to between-fibre variations in diameter. The independence of nutritional and genetic effects on SS means that they should be exploited concurrently to reduce the incidence of tender wool production.