The effect of zeranol and testosterone on Merino wethers exposed to highly oestrogenic subterranean clover pasture.

Groups of Merino wethers treated with 2 doses of zeranol (6 mg and 12 mg), or testosterone cyclopentyl propionate (150 mg) and untreated controls were grazed at 2 sites, one an oestrogenic subterranean clover (Trifolium subterraneum) pasture and the other a low oestrogen medic (Medicago truncatula) pasture. The influence of oestrogenic subterranean clover on these treatments was assessed by measuring changes in teat length, bulbourethral gland weight and pathology, bodyweight, carcase weight, dressing percentage and greasy and clean fleece weights. Teat lengths were increased by all treatments except 6 mg of zeranol where increases were not significant, and although increased by exposure to oestrogenic pasture this effect were not additive. Bulbourethral gland weights were increased by both of the zeranol treatments and by oestrogenic pasture, and these effects appeared to be additive. Differences observed histologically indicated that testosterone protected whereas zeranol exacerbated the influence of oestrogen. The bodyweights of all treated groups were heavier than the controls, but carcase weights were not significantly different. However an effect was seen in the group given the 6 mg dose of zeranol on the low oestrogen site, where the dressing percentage was significantly lower than in the control and testosterone treated groups. Differences in greasy and clean fleece weights were not significant except that the washing yield of the testosterone-treated group was significantly lower at the low oestrogen site.