Effects of the Persistent Dominant Follicle on the Ability of Follicle Stimulating Hormone to Induce Follicle Development and Ovulatory Responses

Three experiments were conducted to evaluate the effect of an induced first wave persistent dominant follicle on folliculogenesis and ovulatory responses induced by FSH. On d 6 of a synchronized estrous cycle (d 0 = estrus), cows were treated with a Syncromate-B implant and two injections of PGF2, (25 mg, 0700 h; 15 mg, 1900 h, i.m.). Cows in the control group retained a first-wave persistent dominant follicle, but in the aspirated group, the first-wave dominant follicle was removed via transvaginal aspiration on d 10 (d 0 = estrus). Beginning on d 12, cows received 32 mg of FSH-P i.m. in decreasing doses at 12-h intervals over a 4-d period. On d 15, the Syncromate-B implant was removed, and cows were ovariectomized (experiment 1, n = 8) or inseminated (experiment 2, n = 11) at 10 and 22 h after the onset of estrus. Cows in experiment 3 received a used controlled intravaginal drug releasing (CIDR) device and two injections of PGF2alpha (25 mg, 0700 h; 15 mg, 1900 h; i.m.) on d 6. On d 8, the first-wave dominant follicle was aspirated (n = 6) or left intact (n = 5), and FSH treatment was initiated (20 mg of Folltropin in decreasing doses at 12-h intervals over a 4-d period), and on d 10 the used CIDR device was removed from all cows. Ovarian follicle size and number were examined daily by ultrasonography from d 5 of the estrous cycle. The persistent dominant follicle increased in size from 10.7 mm on d 5 to 15.4 mm on d 10 (experiments 1 and 2), and from 9 mm on d 5 to 20.4 mm on d 11 (experiment 3). From d 11 to 14, the number of class 1 (2 to 5 mm) follicles was lower in the aspirated group than in the control group; the number of class 2 (6 to 9 mm) follicles was higher on d 12 and 13 for the aspirated group (experiments 1 and 2). The number of class 3 (> or =10 mm) follicles was higher in the aspirated group on d 14 to 16, but the same on d 17. Ovarian and embryo responses to superovulation did not differ between groups. In experiment 3, the numbers of class 1, 2, and 3 follicles, as well as ovarian and embryo responses following ovulation did not differ between groups. Initiation of exogenous FSH treatment appears to override any systemic inhibitory effect that a persistent dominant follicle may be exerting at the pituitary and possibly the ovary.