The effects of anthelmintic treatment and season on the quantity and quality of wool grown by Merino sheep

Abstract

Groups of 30 Merino weaner sheep were grazed in East Gippsland, Vic., and subjected to one of four anthelmintic programs during the 12-month period from December 1977. They were treated with thiabendazole either weekly (W), nine times (H), three times (L) or once (S). These programs resulted in marked differences in the degree of parasitism between the groups. Over the 12 months, parasites had no significant effect on yield, staple length or crimp frequency. However, compared with the wool from sheep treated weekly, greasy wool production was depressed by 13, 22 and 20% respectively in sheep given nine, three or one anthelmintic dose. Fibre diameter was also reduced. Dyebands were used to delineate wool grown during summer, autumn, winter and spring. Where parasite burdens were minimized (W group), wool growth, fibre diameter and rate of staple growth increased each season throughout the year. There was little increase in these characters in the other three groups until the spring, and wool growth and fibre diameter were significantly less than that of the W sheep in all but the initial summer season. Wool growth:fibre volume ratios indicated that sheep continued to initiate new fibres at least until the end of summer, when they were 9 months old. The subsequent depression in the wool growth of sheep dosed at less than weekly intervals provides further evidence of the deleterious effects parasites may have on the production of young sheep, even where anthelmintic is administered fairly frequently.