Seasonal changes in the morphology of wool follicles in Finewool and Strongwool Merino strains grazing at different stocking rates in southern Australia

Abstract

An experiment was conducted to determine the impact of stocking rate and Merino strain on follicle morphology before and after the break of the season in the highly seasonal Mediterranean environment of southern Australia. Groups of Finewool and Strongwool Merino sheep were allocated to 9 stocking rates on mixed legume-grass pastures, and skin biopsy samples were taken at monthly intervals from February to June. A scoring system, based on the morphology of follicles in transverse section, was used to characterise these samples. The proportion of follicles classified as ‘normal’ dropped markedly, and the proportion of follicles which contained no fibre correspondingly increased, after the break of the season in April. On average about 10% of the follicles became inactive but there was considerable variability (range 2-63%) between animals. The proportion of inactive follicles was significantly affected by stocking rate but there was little difference between Merino strains. Maximum follicle inactivity coincided with the period of minimum fibre diameter and minimum liveweight in May, approximately 1 month after the break of the season. The proportion of inactive follicles accounted for 27% and 28% of the variance in staple strength of the Finewool and Strongwool strains, respectively. Minimum fibre diameter accounted for 63% and 61% of the variance in staple strength, and coecient of variation in fibre diameter accounted for 49% and 58% of the staple strength variance, respectively, in the 2 strains. Together, minimum fibre diameter and coecient of variation in fibre diameter accounted for almost 75% of the variance in staple strength in both strains. Addition of a term for the proportion of inactive follicles did not remove any additional variance in staple strength. These results suggest that the follicles of Merino sheep in Mediterranean environments undergo significant morphological changes throughout the year. These changes differ from the normal sequence of events associated with the hair cycle and appear to be associated with the break of the season in autumn. The morphological changes which occur in the follicles are similar to those induced by epidermal growth factor or cortisol, and may reflect a stress response. Our results suggest that nutritional stress is at least partially responsible for the follicular pathology described. Management strategies aimed at reducing the decrease in fibre diameter which occurs in autumn, shearing sheep in autumn to coincide with the minimum fibre diameter, and selection of sheep which have a low coecient of variation of fibre diameter, are likely to be the most effective means of preventing low staple strength in sheep grazing in Mediterranean environments. Nevertheless, the impact of follicle shutdown and changes in follicle morphology on wool characteristics other than staple strength needs to be determined.