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

There is large variation in dominant follicle diameter at the time of GnRH-induced ovulation in the CO-Synch protocol [a first GnRH injection on d -9 (GnRH1), followed by PGF(2alpha) on d -2, and a second GnRH injection (GnRH2) with timed AI on d 0], and the reason for the presence of small dominant follicles at GnRH2 is not known. Our hypothesis was that ovulatory response to GnRH1 and progesterone exposure [controlled intravaginal drug-releasing insert (CIDR; EAZI-Breed, Pfizer Animal Health, New York, NY)] would affect ovulatory follicle size at GnRH2 in anestrous cows. This study used a 2 x 2 factorial arrangement of treatments in which anestrous suckled beef cows (n = 55) either ovulated (Ov1+) or failed to ovulate (Ov1-) after GnRH1 and either received (CIDR+) or did not receive (CIDR-) a 7-d CIDR treatment (from GnRH1 to PGF(2alpha)), resulting in the following treatment groups: Ov1+CIDR+, Ov1-CIDR+, Ov1+CIDR-, and Ov1-CIDR- (n = 9, 17, 11, and 18, respectively). The Ov1+ cows had larger follicles at GnRH2 (12.3 vs. 11.0 mm; P = 0.04), a decreased proportion of small follicles within cows that ovulated to GnRH2 (2/16 vs. 14/23; P = 0.003), and a similar growth rate of the ovulatory follicle from d -5 to 0 (d 0 = GnRH2; 1.1 +/- 0.06 vs. 1.1 +/- 0.07 mm/d; P = 0.99) compared with Ov1- cows. Administration of a CIDR had no effect on follicle diameter at GnRH2 (11.8 vs. 11.2 mm; P = 0.3), proportion of small ovulatory follicles at GnRH2 (7/19 vs. 9/20; P = 0.6), and follicular growth rate from d -5 to 0 (d 0 = GnRH2; 1.2 +/- 0.07 vs. 1.1 +/- 0.07 mm/d; P = 0.76). Administration of a CIDR, but not ovulation to GnRH1, increased follicle growth from d -2 to 0 (d 0 = GnRH2; P = 0.03 and 0.9, respectively). Large follicles (>11 mm) had a similar growth rate from d -5 to 0 (d 0 = GnRH2; P = 0.44) compared with small follicles (1.1 +/- 0.07 vs. 1.2 +/- 0.07 mm/d), but the large ovulatory follicles were larger at d -5 compared with small ovulatory follicles (P < 0.001). Follicle diameter was positively correlated with serum concentrations of estradiol at GnRH2 (r = 0.622; P < 0.0001). In summary, ovulation to GnRH1, but not CIDR administration, resulted in increased dominant follicle diameter at GnRH2 in anestrous suckled beef cows. Large follicles were already larger 5 d before GnRH2 but grew at a rate similar to small follicles; follicle size was positively correlated with serum concentrations of estradiol at the time of GnRH-induced ovulation.