RELATIONSHIPS BETWEEN FOLLICLE-STIMULATING-HORMONE, FOLLICLE GROWTH AND OVULATION RATE IN SHEEP

The changes in follicle-stimulating hormone (FSH) concentration requir ed to affect follicle growth and ovulation rate within individual ewes were examined. Relationships between peripheral FSH concentrations du ring the late-luteal and follicular phase and subsequent ovulation rat es were investigated in 22 ewes from 4 breeds over 3 successive cycles (Experiment 1). Ewes were grouped as follows: Group 1 (n = 6), ewes e xhibiting 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 oestro us cycles and a different ovulation rate on one occasion. Data from ew es in Groups 1 and 3 provided estimates on the variation in FSH concen trations 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 rat e and the proportional increases in FSH concentrations. Differences in FSH concentrations were often less than those that did not alter ovul ation rate in Group-1 ewes. Furthermore, only 3 of 11 Group-3 ewes dem onstrated 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 dur ing the late-luteal and follicular phase are associated with changes i n ovulation rate within individual ewes. In Experiment 2, follicles of similar size obtained from the same ewe (Fec(B)Fec(+) and Romanov) sh owed markedly different responses in vitro to graded doses of FSH as m easured 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 ovar ian response to FSH.