Level of nutrition modulates the dynamics of oestradiol feedback on plasma FSH in ovariectomized ewes

The frequency of multiple ovulations in mature, cyclic ewes is strongly influenced by the level of nutrition, but it is difficult to demonstrate concurrent changes in plasma concentrations of gonadotropins. The failure to do so might be a consequence of rapid compensation by the homeostatic feedback mechanism linking secretion by the hypothalamus/pituitary gland and ovarian hormones. Most experimental models have examined the components of the homeostatic feedback system after steady state relationships had been established. We hypothesised that the effects of nutrition might be observed more readily if the system were disrupted and then examined while equilibrium was being re-established. This hypothesis was tested in three experiments in Merino ewes by allowing gonadotropin secretion to escape feedback for 5 days after ovariectomy and then replacing ovarian hormones and examining effects of feeding regimen on the return of plasma concentrations of FSH to baseline values. In all three experiments, oestrogen replacement caused plasma concentrations of FSH to decline more rapidly (P < 0.05) in ewes fed at 0.5 × maintenance, than in ewes fed at 1.4 × maintenance, with groups fed at maintenance being intermediate. No effect of diet was observed on plasma FSH concentrations in the absence of oestradiol, and neither progesterone nor charcoal-treated bovine follicular fluid influenced the effect of nutrition. Plasma concentrations of oestradiol were 9.8% lower on average (NS) in ewes fed above maintenance than in the sheep fed below maintenance over the three experiments, suggesting that there may have been a reduced clearance of oestradiol which contributed to the result. We conclude that feeding regimen affects the secretion or clearance of gonadotropins in mature ewes, as in the mature ram, and that this is one mechanism by which ovulation rate may be affected.