THE SECRETION OF GONADOTROPINS, INSULIN, AND INSULIN-LIKE GROWTH-FACTOR-1 BY MERINO RAMS SUPPLEMENTED WITH DIFFERENT LEGUME SEEDS

Supplementing mature rams with lupin seed (Lupinus angustifolius, a hi ghly digestible source of energy and protein) increases gonadotrophin secretion within 5-10 days. When sheep receive a post-ruminal suppleme nt of protein and energy equivalent to that in the lupin supplement, L H 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 cen tres that control gonadotrophin secretion. This hypothesis was tested by comparing the responses to isonitrogenous and isoenergetic suppleme nts of 3 legumes: lupins, cowpeas, and soybeans. Rams were fed the sup plements for 10 days and blood was sampled every 20 min for 24 h on da ys -1, 5, and 10 relative to the start of supplementation. The plasma was assayed for LH, FSH, and the metabolic hormones insulin and insuli n-like growth factor 1 (IGF-1). Both the cowpea and the lupin suppleme nts increased LH pulse frequency and the mean concentrations of insuli n and IGF-1 on days 5 and 10 compared with day -1 (P < 0.05). The soyb ean supplement did not affect LH pulse frequency or the concentrations of insulin or IGF-1. Only the cowpea supplement increased the mean co ncentration of FSH. The nutritional stimulation of the reproductive ce ntres of the brain appears not to be specific to lupins, because the d iet supplemented with cowpeas evoked similar responses. The lack of re sponse to soybean seed suggests that the effects of diet on secretion of metabolic and gonadotrophic hormones are not due simply to the tota l energy and protein content of the diets, but to subtle differences i n their chemical constituents.