Seasonality in male Australian cashmere goats: Long term effects of castration and testosterone or oestradiol treatment on changes in LH, FSH and prolactin concentrations, and body growth

Abstract

Australian cashmere goat bucks exhibit marked seasonal changes in gonadotrophin secretion and growth. We investigated the mechanisms underlying these changes by testing the effects of castration and gonadal steroid replacement in grazing bucks at 29 °S, 153 °E over a period of 2 years. Three year old bucks were either left intact ( n = 6), castrated ( n = 6), or castrated and treated with testosterone propionate ( n = 5) or oestradiol-17β ( n = 5). Oestradiol- 17β (E 2) was provided by 2 subcutaneous Silastic ® implants while testosterone propionate (TP) was provided by implanting four 23.5 mg pellets subcutaneously every 12 weeks. Intact bucks exhibited marked circannual cycles of plasma LH, FSH, prolactin and testosterone concentration, body mass, paired testicular mass and male odour. Castration resulted in long term increases in LH and FSH concentration and abolition of the circannual pattern of change in these hormones without affecting prolactin concentrations. It also reduced the magnitude of seasonal changes in body mass. Treatment of castrates with TP, which produced peripheral concentrations of testosterone in the lower physiological range (mean 1.2 ± 0.08 μg L −1), had no effect on plasma concentrations of LH, FSH or prolactin. On the other hand treatment with E 2, which produced E 2 concentrations in the mid physiological range (mean 12.3 ± 1.08 ng L −1), re-imposed a circannual pattern in plasma LH and FSH, albeit significantly different from that of intact bucks. It had no effect on prolactin concentrations. Treatment with both steroids resulted in growth cycles of intermediate magnitude between those of intact bucks and castrates. We conclude that the circannual cycle of gonadotrophin concentrations in goat bucks is dependent upon the presence of the testis, and suggest that circulating metabolites of testosterone, including oestradiol, are more important than testosterone itself in the maintenance of this cycle. We also suggest that the seasonal growth cycle observed in these bucks is driven by factors other than prevailing nutrition, probably photoperiod, and is mediated by both steroid-dependent and independent mechanisms.