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A technique for radio-labelling short regions of growing wool fibres was modified by using intradermal injections of 3H-cystine and a rapid detection method. This technique allowed emergence times and dimensional changes of individual wool fibres to be rapidly and precisely monitored. Seasonal changes in fibre dimensions were determined for five sheep on maintenance nutritional levels following monthly injections over 12 months. Fibre diameter and length-growth varied seasonally, maximum and mimimum values occurring in summer and winter respectively. However, the amplitude and timing of each of these cycles differed between sheep and between fibres from individual sheep. As a consequence, the seasonal cycle in wool production (i.e., fibre volume) may be brought about by different combinations of fibre diameter and length-growth changes. Monthly diameter and length-growth measurements were often poorly correlated, suggesting that the cellular/physiological mechanisms determining these parameters in the follicle are complex.

Wool fibre tenacity and its relationship to staple strength was studied in a pen-feeding trial of 40 New Zealand Romney ewes. A method for estimating fibre tenacity is described based on normalizing fibre breaking load by cross-sectional area measured on fibre fracture-surfaces. The validity of this protocol was demonstrated by showing that (i) fibre diameter measured at the point of rupture was not significantly (P > 0.05) changed by breaking, and (ii) that the tenacity estimate was not significantly (P > 0.05) correlated with cross-sectional area variation along fibres, and (iii) showed lower fibre-to-fibre variation than other accepted measures of intrinsic material strength. The majority (83.5%) of the 1200 fibres broken in this study cleaved in smooth-planed fractures normal to the long axis of the fibre, the remaining fibres demonstrated axial-splitting following rupture. Tenacity did not differ significantly between the two break-types (P > 0.05). Tenacity demonstrated small negative correlations (P < 0.05) with fibre cross-sectional area and ellipticity. No change in tenacity (P > 0.05) occurred within fine, slow-growing 'wool-break' regions and tenacity did not vary significantly (P < 0.05) between body sites on sheep. Tenacity showed no significant variation (P > 0.05) with differential winter feeding, but significant variations (P < 0.01) of wool tenacity occurred between sheep. The sheepto-sheep differences in tenacity showed significant relationships with two indices of staple strength, work (P < 0.01) and peak-force (P < 0.01). Variability in fibre tenacity between sheep and its relationship to staple strength may provide some explanation of the origin of 'tender' and 'sound' wools.

Two methods of measuring New Zealand Romney and Corriedale wools for staple strength were compared. There was a good correlation (r = 0.92) between staple strength measurements of the same midside wools using either a simple manual tester or a commercial tensile tester. Staple strengths standardised by either the clean mass of wool broken or the more rapidly determined greasy linear density at the position of break were also highly correlated. The more rapid method could be used instead of conventional methods without major changes in the rankings of the wools, although the absolute staple strength values obtained may differ according to the method of standardisation used. A moderately close relationship was found between staple strength and barbe (fibre length after carding).

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