Preovulatory follicular status and diet affect the insulin and glucose content of follicles in high-yielding dairy cows

Insulin and glucose may be limiting factors for ovarian function in dairy c ows genetically selected for high milk yield. The effects of nutrition on t he intrafollicular content of insulin and glucose were investigated in Isra eli Holstein dairy cattle fed a basic total mixed ration and producing 34-3 9 kg of milk daily. In experiment 1, carried out in 11 oestrus-synchronised cows, little variation in insulin concentration was found in plasma sample d during the luteal phase, but high variation was found in plasma sampled d uring the follicular phase. Therefore, in order to prevent confounding the effects of diet and of phase in cycle in the following experiments, experim ental diets were fed during the luteal phase of synchronised oestrus cycles . In experiment 2, designed as Latin-Square, six cows received sequentially diets containing 17.1 (control) or 19.7% of crude protein, using two sourc es of supplementary protein, i.e. soyabean meal(SBM) and corn gluten meal ( CGM), differing in ruminal degradability and leucine content. When dry matt er intake was used as covariant, plasma insulin on day 16 was 29.5 and 26.4 % higher in cows fed diets containing SBM and CGM than in the control (P < 0.05), In experiment 3, 17 cows were individually fed the basic diet and th en switched to isoenergetic diets containing SBM (n = 5), CGM (n = 6) or co rn grain (CG, n = 6) given from day 10 to 16 of the synchronised oestrus cy cle. On the eve of day 16, and in the morning of day 17, they were administ ered PGF(2<alpha>) and the content of 26 largest follicles was aspirated by using the transvaginal ovum pick-up technique, Follicles were sorted into two classes (preovulatory and subordinate) according to oestradiol concentr ation and the progesterone:oestradiol ratio in follicular fluid (FF). Highe r concentrations of insulin (0.282 versus 0.127 ng/ml, P < 0.0001) and of g lucose (0.614 versus 0.386 g/l, P < 0.002), were found in FF from preovulat ory follicles. The insulin concentration in the FF of cows fed the CG diet was 26% higher than in their counterparts fed CGM (P < 0.04), SBM being int ermediate. Dietary effects did not reach significance in subordinate follic les. The finding that preovulatory follicular status is associated with increase d intrafollicular insulin and glucose suggests that insulin is involved in follicular maturation. The nutritional effect on intrafollicular glucose an d insulin may have practical implications to optimise feeding in dairy cows during phases of the oestrus cycle. (C) 2000 Elsevier Science B.V. All rig hts reserved.