Relationships between follicle-stimulating hormone, follicle growth and ovulation rate in sheep

The changes in follicle-stimulating hormone (FSH) concentration required to affect follicle growth and ovulation rate within individual ewes were examined. Relationships between peripheral FSH concentrations during the late-luteal and follicular phase and subsequent ovulation rates were investigated in 22 ewes from 4 breeds over 3 successive cycles (Experiment 1). Ewes were grouped as follows: Group 1 (n = 6), ewes exhibiting the same ovulation rate at each oestrous cycle: Group 2 (n = 5), ewes with three different ovulation rates at each oestrous cycle; and Group 3 (n = 11), ewes with the same ovulation rate at two oestrous cycles and a different ovulation rate on one occasion. Data from ewes in Group 1 and 3 provided estimates on the variation in FSH concentrations between cycles which were not large enough to alter ovulation rate (range, 0-67% variation in FSH concentration). In Group-2 ewes, there was no consistent association between increases in ovulation rate and the proportional increases in FSH concentrations. Differences in FSH concentrations were often less than those that did not alter ovulation rate in Group-I ewes. Furthermore, only 3 of 11 Group-3 ewes demonstrated high FSH concentrations associated with high ovulation rate (or low FSH concentrations and low ovulation rate) when compared with the concentrations found at the two cycles in which ovulation rate was similar. Hence, there was little evidence that FSH concentrations during the late-luteal and follicular phase are associated with changes in ovulation rate within individual ewes. In Experiment 2, follicles of similar size obtained from the same ewe (FecBFec+ and Romanov) showed markedly different responses in vitro to graded doses of FSH as measured by aromatase activity. It is concluded that, within a ewe, the large variability between gonadotrophin-dependent follicles in their requirement for FSH prevented the expression of any thresholds of ovarian response to FSH.