Effect of ewe body condition on mean ovulation rate and associated hypothalamic activity and responsiveness to GnRH

Abstract

In many breeds of ewe, body condition at mating is a major determinant of ovulation rate and lambing rate. It has been shown that differences in body condition are associated with differences in the number of large, potentially ovulatory ovarian follicles (McNeilly, Jonassen and Rhind, 1987) and mean circulating FSH concentrations (Rhind and McNeilly, 1986). Differences in circulating gonadotrophin concentrations could be a function of parallel differences in either hypothalamic activity or pituitary sensitivity to GnRH; either or both of these factors could be affected by ovarian steroid feedback. The aim of this experiment was to investigate the effect of ewe body condition on these parameters in the absence of ovarian steroidal feedback.LH secretion was used as an indirect measure of the secretion of GnRH from the hypothalamus since LH secretion closely mirrors GnRH secretion (Clarke and Cummins, 1982). Pituitary sensitivity was assessed by measurement of LH release following injection of a GnRH bolus.At 2 and 7 days after bilateral ovariectomy, blood samples were collected from each of 2 groups of 12 mature Scottish Blackface ewes in condition scores of 2.75/3.00 (H) or 1.75/2.00 (L) and with mean(+ s.c.) liveweights of 53.4 ± 1.47 kg and 38.2 ± 1.80 kg respectively. Samples were collected every 15 mins for 8 h and then for a further 3 h after each ewe was injected (I.V.) with 10 μg GnRH. They were analysed for concentrations of FSH and LH.