The relationship between wool fibre intrinsic material strength variation and staple strength in a flock of New Zealand romney ewes

A study of wool fibre intrinsic material strength (IMS) variation in relation to staple strength was performed. Fleece wools of varying midside staple strength were generated in a pen-feeding trial in which four groups of non-pregnant Romney ewes were placed on differential nutritional levels over winter. Two principal sources of staple strength variation were identified in the trial; 1/ a variation in mean staple strength between the four nutritional groups and 2/ variation in mean staple strength between sheep within the groups. The variation in staple strength between nutritional groups was determined largely by the level of wool fibre cross-sectional area reduction undergone by the groups. No significant variation in wool fibre IMS occurred between groups. The between-sheep within-group differences in staple strength were found to be unrelated to fibre cross-sectional area change and also could not be wholly explained by variation in winter wool growth efficiency between sheep. However, large differences in IMS occurred between sheep. This variation in IMS was strongly correlated with the variation of midside staple strength between sheep within the groups. In a systematic analysis it was found that the sheep-to-sheep differences were the most important source of IMS variability within the trial flock. Only small variations in IMS were found between nutritional groups, between fleece body-sites within sheep, with fibre diameter, with fibre ellipticity, or with fibre break-type. No significant change in fibre IMS occurred within the "break" region of staples with low tensile strength. The between-sheep variation in IMS and its role in determining staple strength may resolve the longstanding controversy on the origin and nature of so-called "tender" and "sound" wools