The components of staple tenacity and the determinants of length after carding

A theoretical model for the tensile strength of staples of wool was used to propose variables that could be measured on thirty six fleece samples from Romney sheep genetically selected on the basis of wool staple tenacity. Other variables that have been noted elsewhere in the literature were also measured. Nearly 75 % of the variation in staple tenacity was described by average and minimum cross-sectional area of the fibres, and the relaxed length of the fibres held between the jaws of the instrument used to break the staple. The addition of staple length and variation in total length of the fibres in the staple improved this to almost 84%. The ultimate goal of the genetic selection was to increase the average length of fibres following carding (Hauteur and Barbe), which were also measured on the fleece samples, and the fibre, staple and sample measurements used in stepwise regression to reveal the determinants of length after carding. Variation in Hauteur was dependent on the total energy required to break the staple and the proportion of new tips present in the post-break region of the staple, yet as little as 52% of the variation was accounted for. Average cross-sectional area accounted for a further 3% of the variation in Hauteur but this was not statistically significant. Barbe was dependent on staple tenacity, which accounted for 48% of the variation. In hindsight, staple tenacity may not be an appropriate measure to genetically select sheep in order to improve fibre length after carding given the variables that affect these traits.