Effects of eCG Administration 4 Versus 2 Days Prior to Timed AI on Nellore Cows

The objective of this study was to evaluate growth of the dominant follicle (DF) in multiparous Nellore cows treated with equine chorionic gonadotropin (eCG) either 4 or 2 d prior to timed AI (TAI). We hypothesized that early eCG administration improves follicular growth and increase DF diameter at TAI. Cows (n=137) had their estrous cycle synchronized with an intravaginal device containing 1.9 g of progesterone (CIDR) and 2.0 mg of estradiol benzoate (d 0), 12.5 mg of PGF_2α (d 7), CIDR withdrawal and 0.3 mg of estradiol cypionate (d 9), and TAI 48 h later (d 11). On d 7, ovaries were evaluated by ultrasonography and cows were blocked based on DF diameter (< 9 mm vs. ≥ 9 mm). Within block, cows were randomly assigned to receive 300 IU of eCG either on d 7 (eCGD7; n=64) or on d 9 (eCGD9; n=73). Diameter of the DF was evaluated by transrectal ultrasonography on d 7, 9, and 11. Follicular growth from d 7 to 9, 7 to 11, and 9 to 11 was analyzed by ANOVA using the GLIMMIX procedure of SAS. Follicular diameter was analyzed by ANOVA for repeated measures and cow was considered a random effect. Follicular growth from d 7 to 11 tended to be greater (P = 0.08) for eCGD7 compared with eCGD9 (4.9 ± 0.2 vs. 4.4 ± 0.2 mm). The overall effect of treatment was explained mostly by DF growth from d 7 to 9 (eCGD7 = 2.6 ± 0.2 vs. eCGD9 = 2.0 ± 0.2 mm; P < 0.01) as growth from d 9 to 11 did not differ (P = 0.44) between treatments (eCGD7 = 2.3 ± 0.2 vs. eCGD9 = 2.4 ± 0.1 mm). Follicular diameter was affected (P < 0.01) by the interaction between treatment and day. Whereas no difference was observed on d 7 (eCGD7 = 9.7 ± 0.2 vs. eCGD9 = 9.6 ± 0.2 mm), DF in eCGD7 cows was larger (P < 0.01) on d 9 (12.3 ± 0.2 vs. 11.5 ± 0.2 mm). In addition, DF on d 11 did not differ (P = 0.12) between treatments (eCGD7 = 14.5 ± 0.3 vs. eCGD9 = 14.0. ± 0.2 mm). Treatment with 300 IU of eCG 4 d prior to TAI improved follicular growth but did not increase overall mean of DF diameter at TAI compared with eCG administration 2 d before TAI in Nellore cows.