NUTRITIONAL INFLUENCES ON REPRODUCTION IN MATURE MALE SHEEP AND GOATS

Changes in the nutrition of mature rams and goat bucks lead to profoun d responses in testicular size and therefore the rate of production of spermatozoa. These effects are largely due to changes in the size of the seminiferous tubules and in the efficiency of spermatogenesis. Wit h the exception of severe undernutrition, the effects on spermatogenic function are not accompanied by similar changes in endocrine function of the testes, as measured by the production of testosterone or inhib in. In rams, moderate changes in nutrition affect gonadotrophin secret ion for only a few weeks, whereas testicular growth is affected for se veral months. In mature male goats during the non-breeding season, nut rition-induced testicular growth does not seem to be associated with a gonadotrophin response. Such observations have led us to develop the hypothesis that nutrition-driven testicular growth is at least partly independent of changes in gonadotrophin secretion. The energetic compo nents of the diet, rather than the protein content, seem to be respons ible for affecting gonadotrophin secretion in rams. The volatile fatty acids, and not glucose, are the active factors, although intracerebra l insulin may also play a role. Where these substrates act and whether they are also involved in the ,gonadotrophin-independent pathways req uires testing. In conclusion, nutritional signals exert powerful effec ts on the reproductive system of mature male ruminants, and the respon ses-are partly independent of changes in gonadotrophin secretion. In t he gonads, the gametogenic tissue responds rapidly to changes in nutri tion, but the endocrine compartments are less affected. Variations in the expression of the nutritional responses among sexes, breeds and sp ecies probably reflect variations in the role of this environmental fa ctor as a modulator of reproductive function.