Abstract

Staple tenacity is an estimate of the peak force to break a staple of a certain cross-sectional area at the point of break. Staple strength is a measure of peak force to break a certain weight of staple. Using the density of wool and the length of staple broken, both estimates can be converted to force per unit linear density (N/ktex). The relationship between these two estimates was examined. Midside wool samples were collected at shearing from 164 ewe hoggets from lines of New Zealand Romneys that have been selected for or against staple tenacity, and a randomly selected control line. Five staples from each sample were measured for staple strength and staple tenacity. This was accomplished by defining the cross-sectional area at the break point, breaking the staple to estimate peak force and then cutting the portion of the staple from between the jaws and determining the weight of clean wool after scouring. The High staple tenacity line had a significantly higher staple strength (P<0.001) and staple tenacity (P<0.001). Although a similar weight of greasy wool was removed from between the jaws for each selection line (P=0.336), the weight of clean wool was significantly greater in the High staple tenacity line (P=0.003). Covariate analysis revealed that the fitted relationships between staple strength and staple tenacity for the three selection lines had similar slopes (P=0.822) but significantly different intercepts (P=0.029). Staple strength is thus biased relative to staple tenacity in these lines, because of the differences in weight of wool between the jaws, for staples which are all the same cross-sectional area at the point of break. Staple tenacity is therefore more useful for some purposes than staple strength and vice versa, and care should be taken when drawing comparisons and conclusions and when considering which to use.

GB, Nicoll

Proceedings of the New Zealand Society of Animal Production, Volume 55, , 1-4, 1995