Factors affecting preovulatory follicle diameter and ovulation rate after gonadotropin-releasing hormone in postpartum beef cows. Part I: Cycling cows.

Cows induced to ovulate small dominant follicles were reported to have reduced pregnancy rates compared with cows that ovulated large follicles. The reason for the presence of small dominant follicles at the time of GnRH-induced ovulation in timed AI protocols is unknown. The objectives of this experiment were to examine the role of day of the estrous cycle at initiation of treatment on ovulation after the first GnRH injection (GnRH1) and associated effects on growth rate and final size of the ovulatory follicle at the second GnRH injection (GnRH2), serum concentrations of estradiol at GnRH2, and subsequent luteal concentrations of progesterone in suckled beef cows. Estrous cycles of cows were manipulated to be at 1 of 5 specific days of the cycle (d 2, 5, 9, 13, and 18, d 0 = estrus; n = 12 per treatment group) at the beginning of the CO-Synch protocol (GnRH1 on d -9, PGF(2alpha) on d -2, and GnRH2 on d 0). Day of the estrous cycle at GnRH1 did not affect the size of the preovulatory follicle or the proportion of cows ovulating at GnRH2 (P = 0.65 and 0.21, respectively). When all cows were included in the analysis, cows that ovulated after GnRH1 had similar follicle size at GnRH2 compared with cows that did not ovulate after GnRH1 (11.4 and 10.4 mm, respectively; P = 0.23). When only cows that could ovulate after GnRH1 (excluding cows treated on d 2) were included in the analysis, cows that ovulated to GnRH1 had a larger follicle at GnRH2 than cows that did not ovulate after GnRH1 (11.4 and 9.5 mm, respectively; P = 0.04). Follicle growth from d -5 to 0 was similar between cows that ovulated after GnRH1 and cows that did not (1.01 vs. 0.89 mm/d, respectively; P = 0.75). There was a tendency for faster follicle growth rate in cows that ovulated a large follicle (>11 mm) compared with cows that ovulated a small follicle (< or = 11 mm; 1.01 vs. 0.86 mm/d, respectively; P = 0.07). Serum concentrations of estradiol at GnRH2 and progesterone after ovulation were reduced in cows that ovulated small follicles compared with cows that ovulated large follicles (P = 0.006 and 0.005, respectively). In summary, day of the estrous cycle at initiation of synchronization did not affect ovulatory follicle size, but follicle growth rates affected the size of the follicle at GnRH2. Cows that ovulated a small follicle had reduced serum concentrations of estradiol at GnRH2 and progesterone after ovulation.