Cycles of wool follicle activity in some shetland sheep

Abstract

SUMMARY The wool growth cycle in three rams and three ewes was studied by taking monthly wool and skin samples for periods up to 3 years. The percentage of inactive follicles and of medullated fibres was recorded, fleece length was measured and change in fibre diameter and pigmentation between summer and winter was investigated. Casting of the fleece appeared to depend more on the thinning of fibres than on the proportion with brush ends, casting in some instances taking place with only 30% brush ends, while not in others with as much as 70% follicle inactivity. Follicles began to become inactive in both sexes in September following the autumnal equinox. The secondary follicles had a greater proportion of inactivity (mean of 45% compared with 20% in the primaries) but tended to regrow earlier, and regrowth in the rams tended to begin before that in the ewes. Regrowth as a whole began from January to March. Subsidiary cycles of inactivity of about 2 months' duration were evident. Maximum growth in length occurred between April and October. A difference in mean diameter of only a few microns could be detected between summer and winter. Maximum medullation occurred in July in the rams and August in the ewes. Decrease in pigmentation in winter was brought about by a lowering of the density of pigment granules within the fibres.