Summary. The hypothesis was tested that greater growth of the dominant follicle of wave 1 (first follicular wave of an interovulatory interval), compared with that of subsequent anovulatory waves, is due to lower circulating concentrations of progesterone during the growing phase of the follicle. Control heifers (n = 6) were compared with heifers (n = 6) treated with a decreasing dose of progesterone from day 0 to day 5 (ovulation = day 0). Maximum diameter (12·7 ± 0·9 versus 15·3 ± 0·7 mm) and mean diameter of the dominant follicle of wave 1, averaged over days, were smaller (P < 0·05) in the progesterone-treated than in the control group. Progesterone treatment did not suppress circulating follicle-stimulating hormone (FSH); but the second FSH surge was earlier, resulting in earlier emergence of wave 2 as indicated by a tendency (P ≤ 0·1) for group × day interactions attributed to earlier detection of the dominant follicle and an earlier rise in the total number of follicles detected. The stated hypothesis was supported.
We also tested the hypothesis that exposure to low circulating concentrations of progesterone at the end of the growing phase of the anovulatory dominant follicle of wave 1 results in continued growth and prolonged maintenance of the dominant follicle. Heifers (n = 6 per group) were given a luteolytic dose of prostaglandin F2α (PGF2α) on day 6 and treated with a low (30 mg day−1), physiological (150 mg day−1), or high (300 mg day−1) dose of progesterone on days 6 to 20. Continued periodic emergence of anovulatory follicular waves occurred (2·1 ± 0·0 waves, 2·8 ± 0·2 waves, 3·8 ± 0·3 waves, respectively; P < 0·05) until treatment was stopped (interovulatory intervals: 26·2 ± 1·0, 30·8 ± 0·6 and 40·3 ± 1·7 days, respectively; P < 0·05). Compared with the physiological dose group, the growth of the dominant follicle was inhibited to a lesser degree in the low-dose group since it grew for longer (P < 0·05) and to a larger diameter (P < 0·05), and persisted for longer (P < 0·05). Prolonged dominance of this oversized (> 20 mm) follicle was associated with delayed emergence of wave 2. The hypothesis was supported. Results also showed that the high dose of progesterone suppressed the dominant follicle more than the physiological dose when given during the growing phase, but not when given after the growing phase. The suppressive effects of progesterone were not mediated by decreased circulating FSH, since progesterone treatment did not suppress FSH. Collectively, the results demonstrated that progesterone inhibited the dominant follicle in a dose-dependent manner when the follicle was exposed during its growing phase.