Two experiments in Nelore cows subjected to conventional protocol for timed AI (TAI) were designed to compare the influence of antral follicle count (AFC; 1) on the ovarian follicular dynamics and (2) conception rates. First, multiparous cows with high (G-high, ≥45 follicles; n = 43) and low AFC (G-low, ≤15 follicles; n = 32) were selected to receive a conventional oestradiol/progesterone-based TAI protocol for monitoring ovarian follicular dynamics. Second, AFC from 962 multiparous suckling cows (40 ± 15 days postpartum) subjected to a conventional protocol for TAI were classified as G-high (≥45 follicles; n = 194), G-intermediate (≥20 and ≤40 follicles, n = 397), and G-low AFC (≤ 15 follicles; n = 243). Pregnancy status was evaluated 30 to 35 days later. Data were analysed using ANOVA followed by the Tukey test and model of logistic regression (P ≤ 0.05). In the first study, the AFC were greater (P ≤ 0.05) in G-high than in G-low, but remained stable in both groups during the evaluations. The ovarian diameter, perimeter, and area were greater (P ≤ 0.05) in G-high than in G-low (28.3 ± 3.9 v. 20.5 ± 3.2 mm, 100.9 ± 13.2 v. 73.8 ± 13.9 mm, and 67.5 ± 16.4 v. 37.7 ± 11.7 mm2; respectively). Follicular growth rate, time of ovulation, corpus luteum diameter, and progesterone concentration were similar (P > 0.05) for cows with low or high AFC. Dominant follicles showed greater (P ≤ 0.05) diameter in G-low than G-high at D4 (7.3 ± 2.2 v. 6.2 ± 1.4 mm; P = 0.069, tendency), D8 (11.2 ± 1.8v. 9.5 ± 1.8 mm), D9 (12.3 ± 1.7 v. 10.6 ± 1.7 mm), and D10 (13.4 ± 1.3 v. 12.2 ± 1.8 mm), and the estimated diameter of the ovulatory follicle tended (P = 0.08) to be higher in G-low than in G-high (14.4 ± 1.5 v. 13.4 ± 2.1 mm). In the second study, AFC of G-low cows showed higher (P ≤ 0.05) conception rate (61.7%) than those of G-intermediate (52.9%) and G-high (49.5%) cows. In Nelore cows subjected to ovulation synchronization protocol, cows with high AFC showed higher ovarian measurements, but those with low AFC had large follicular diameters. Additionally, the cows with low AFC showed higher conception rate than groups with intermediate and high counts.
Fertilizability of oocytes derived from Holstein cows having different antral follicle counts in ovaries
