Nutritional influences on the hormonal control of reproduction in female pigs

Effects of nutrition on age at puberty, return to oestrus after weaning, ovulation rate and embryo survival have been observed in the pig. Nutrition may influence reproduction at the three levels of the hypothalamic-pituitary-ovarian axis via neuroendocrine pathways and/or variation in metabolic clearance of reproductive hormones. For instance, undernutrition impairs the gonadotrophin-releasing hormone (GnRH) pulse generator while refeeding restores luteinizing hormone (LH) secretion. Nutrition influences follicular growth and maturation; for example, the percentage of small (1-3 mm) healthy follicles and the ovulation rate are decreased under feed restriction. Detection of hormone receptors in tissues and both in vitro and in vivo studies suggest that insulin, cortisol, thyroid hormones and hormones from the somatotrophic axis could be mediators of the effects of nutrition on reproduction. These hormones are able to alter folliculogenesis directly at the ovarian level as either hormones controlling nutrition of the cells, growth factors (insulin and insulin-like growth factor-I) or amplifiers of the action of the gonadotrophins. Insulin and cortisol, at least, may influence gonadotrophin secretion through action at the hypothalamic-pituitary level. Decreased metabolic clearance of progesterone resulting in increased plasma concentrations of this hormone could be involved in the inhibition of gonadotrophin release and reduction of the ovulation rate occurring in feed-restricted cyclic gilts. (C) 2000 Elsevier Science B.V. All rights reserved.