Effect of the physiological stage of the ewe on the number of follicles ovulating following hCG injection

Abstract

The ovarian response to the injection of 750 IU of human chorionic gonadotrophin (hCG) was measured in Greek breeds of sheep (Chios and Kimis) before puberty, at specific times of the oestrous cycle and at two times during post partum anoestrus. There were no luteinising hormone (LH) sensitive follicles found at 6 weeks of age in either breed. The appearance of these follicles was noted at 10 weeks with the frequency of hCG-induced ovulations and the number of ovulated follicles increasing further at 14 weeks. There was no relationship between post natal follicle stimulating hormone (FSH) concentrations and hCG-induced ovulation rate at 10 weeks. At 14 weeks of age, in Chios but not in Kimis ewes, there was a negative correlation ( r=−0.52) between FSH concentrations at 3 weeks of age and hCG-induced ovulation rate. Adult ovulation rate was related to FSH concentrations at 7 weeks of age ( r=0.46) in Chios ewes. Prolificacy was related to hCG-induced ovulation rate in Kimis ewes ( r=0.61 and r=0.67 for 10 weeks and 14 weeks, respectively). There were significant day effects on ovulation rate induced by hCG given at Days 2, 5, 8, 11 and 14 of the cycle to Chios ewes with minimal values at Days 2 and 11 and high values at Days 5, 8 and 14. Administration of hCG to Chios ewes during the non-breeding season at 30 (before weaning the lambs) and 60 days post partum (after initiation of machine milking) induced ovulation in a high proportion of ewes at 30 days (88%) with a reduction observed at 60 days (59%). At both stages, the ovulation rate per ovulating ewe was typical of the breed (3.53 and 3.1 at 30 days and 60 days, respectively). Hence, with the exception of very young lambs, LH sensitive follicles are always present. Their number, however, varies with the physiological stage of the ewe.