The secretion of gonadotrophins, insulin and insulin-like growth factor 1 by Merino rams supplemented with different legume seeds

Abstract

Supplementing mature rams with lupin seed (Lupinus angustifolius, a highly digestible source of energy and protein) increases gonadotrophin secretion within 5-10 days. When sheep receive a post-ruminal supplement of protein and energy equivalent to that in the lupin supplement, LH secretion is increased but not to the same extent as with lupin seed itself. This suggests that lupin seed contains a specific mixture of nutrients or an unknown component that exerts effects on the brain centres that control gonadotrophin secretion. This hypothesis was tested by comparing the responses to isonitrogenous and isoenergetic supplements of 3 legumes: lupins, cowpeas, and soybeans. Rams were fed the supplements for 10 days and blood was sampled every 20 min for 24 h on days -1, 5, and 10 relative to the start of supplementation. The plasma was assayed for LH, FSH, and the metabolic hormones insulin and insulin-like growth factor 1 (IGF-1). Both the cowpea and the lupin supplements increased LH pulse frequency and the mean concentrations of insulin and IGF-1 on days 5 and 10 compared with day -1 (P < 0.05). The soybean supplement did not affect LH pulse frequency or the concentrations of insulin or IGF-1. Only the cowpea supplement increased the mean concentration of FSH. The nutritional stimulation of the reproductive centres of the brain appears not to be specific to lupins, because the diet supplemented with cowpeas evoked similar responses. The lack of response to soybean seed suggests that the effects of diet on secretion of metabolic and gonadotrophic hormones are not due simply to the total energy and protein content of the diets, but to subtle differences in their chemical constituents.