ARS | Publication request: Factors affecting pre-ovulatory follicle diameter and ovulation rate following GnRH in postpartum beef cows Part II: Anestrous cows

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 5, 2010
Publication Date: March 26, 2010
Citation: Atkins, J.A., Smith, M.F., Wells, K.J., Geary, T.W. 2010. Factors affecting pre-ovulatory follicle diameter and ovulation rate following GnRH in postpartum beef cows. Part II: Anestrous cows. Journal of Animal Science. 88:2311-2320.

Interpretive Summary: There is large variation in dominant follicle diameter at the time of GnRH-induced ovulation in the CO-Synch protocol (GnRH on d -9 [GnRH1] followed by PGF*2a on d -2 and GnRH with timed AI on d 0 [GnRH2]) 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]) would affect ovulatory follicle size at GnRH2 in anestrous cows. This study used a two by two factorial design 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*2a) resulting in the following treatment groups: Ov1+CIDR+, Ov1-CIDR+, Ov1+CIDR-, and Ov1-CIDR- (n = 9, 17, 11, and 18, respectively). The Ov1+ had larger follicles at GnRH2, a decreased proportion of small follicles within cows that ovulated to GnRH2, and a similar growth rate of the ovulatory follicle from d -5 to d 0 (d 0 = GnRH2) compared to Ov1- cows (12.3 vs. 11.0 mm, 2/16 vs. 14/23, and 1.1 vs. 1.1 mm/d, respectively; P = 0.04, 0.003, and 0.99, respectively). Administration of a CIDR had no affect on follicle diameter at GnRH2, proportion of small ovulatory follicles at GnRH2, and follicular growth rate from d -5 to d 0 (d 0 = GnRH2; 11.8 vs. 11.2 mm, 7/19 vs. 9/20, and 1.2 vs. 1.1 mm/d for the CIDR+ vs. CIDR- cows, respectively; P = 0.3, 0.6, and 0.76, respectively). Administration of a CIDR, but not ovulation to GnRH1, increased follicle growth from d -2 to d 0 (d 0 = GnRH2; P = 0.03 and 0.9, respectively). Large follicles (> 11 mm) had a similar growth rate from d -5 to d 0 (d 0 = GnRH2; P = 0.44) compared to small follicles (1.1 vs. 1.2 mm/d) but the large ovulatory follicles were larger at d-5 compared to 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 five days before GnRH2 but grew at a similar rate to small follicles and follicle size was positively correlated with serum concentrations of estradiol at the time of GnRH-induced ovulation. Cows that responded to GnRH1 ovulated a new and rapidly growing large follicle that may yield a healthier oocyte and/or corpora lutea that improves embryonic development and pregnancy establishment.

Technical Abstract: The objectives of this study were to identify factors affecting ovulatory follicle size in anestrous suckled beef cows so that we could determine how ovulatory follicle size affects fertility. This study used a two by two factorial design 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*2a) resulting in the following treatment groups: Ov1+CIDR+, Ov1-CIDR+, Ov1+CIDR-, and Ov1-CIDR- (n = 9, 17, 11, and 18, respectively). The Ov1+ had larger follicles at GnRH2, a decreased proportion of small follicles within cows that ovulated to GnRH2, and a similar growth rate of the ovulatory follicle from d -5 to d 0 (d 0 = GnRH2) compared to Ov1- cows (12.3 vs. 11.0 mm, 2/16 vs. 14/23, and 1.1 vs. 1.1 mm/d, respectively; P = 0.04, 0.003, and 0.99, respectively). Administration of a CIDR had no affect on follicle diameter at GnRH2, proportion of small ovulatory follicles at GnRH2, and follicular growth rate from d -5 to d 0 (d 0 = GnRH2; 11.8 vs. 11.2 mm, 7/19 vs. 9/20, and 1.2 vs. 1.1 mm/d for the CIDR+ vs. CIDR- cows, respectively; P = 0.3, 0.6, and 0.76, respectively). Administration of a CIDR, but not ovulation to GnRH1, increased follicle growth from d -2 to d 0 (d 0 = GnRH2; P = 0.03 and 0.9, respectively). Large follicles (> 11 mm) had a similar growth rate from d -5 to d 0 (d 0 = GnRH2; P = 0.44) compared to small follicles (1.1 vs. 1.2 mm/d) but the large ovulatory follicles were larger at d-5 compared to 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 five days before GnRH2 but grew at a similar rate to small follicles and follicle size was positively correlated with serum concentrations of estradiol at the time of GnRH-induced ovulation.