A DOMINANT FOLLICLE DOES NOT AFFECT FOLLICULAR RECRUITMENT BY SUPEROVULATORY DOSES OF FSH IN CATTLE BUT CAN INHIBIT OVULATION

It has been suggested that superovulation in cattle is impaired if FSH injections are initiated in the presence of a dominant follicle, but the results of experiments to test this hypothesis have been contradic tory. However, previous experiments were conducted during mid-cycle, w hen the absence or presence of a dominant follicle is difficult to ass ess. We took a different approach by comparing the effects of initiati ng superovulatory injections of FSH (11 equal doses of FSH-P, every 12 h) on Day 1 of the bovine estrous cycle, when a dominant follicle cle arly is not present, vs initiation on Day 6, when a dominant follicle clearly is present and actively growing (n=17 heifers in a ''crossover '' design). In 8/17 heifers initiation of FSH injections in the presen ce of a dominant follicle (Day 6 group) caused ovulation of the domina nt follicle within 1 to 2 days and formation of a smaller than normal CL. These animals had higher than normal concentrations of plasma prog esterone around the time of expected estrus (P < 0.05) and failed to e xhibit estrus. Although the mean number and diameter of the follicles recruited in response to FSH injections in heifers that ovulated the d ominant follicle prematurely were not different from the other heifers in the Day 6 group, no ovulations were observed, and no embryos or ov a were recovered 6 d after insemination. Conversely, when FSH injectio ns were initiated on Day 1 in these 8 heifers, they exhibited estrus, and their plasma progesterone around the time of estrus, mean ovulatio n rate, and number of total and transferable embryos recovered did not differ from the responses observed in the remaining 9 heifers treated either on Day 1 or on Day 6. Taken together, these results indicate t hat a dominant follicle does not affect the ability of smaller follicl es to be recruited in response to exogenous FSH, but may impair their ovulation. These findings provide an explanation for previous reports of decreased superovulatory responses during times of the cycle when a dominant follicle would be expected to be present.