High numbers of antral follicles are positively associated with in vitro embryo production but not the conception rate for FTAI in Nelore cattle

The objective was to compare the conception rates for FTAI and in vitro embryo production between Nelore cows with different antral follicle counts (AFC = number of follicles ≤3 mm in diameter in the ovaries). Nelore cows (n = 547) were subjected to ovulation synchronization. Randomly during the estrous cycle (D0), cows received an intravaginal device containing 1.9 g P4 (CIDR®) and 2 mg BE (Estrogin®), IM. When the device was removed (D8), the cows received 500 μg PGF2α (Ciosin®), 300 IU eCG (Novormon®) and 1 mg EC (ECP®), IM. All cows were inseminated 48 h after P4 device removal. Antral follicles ≥3 mm were counted using an intravaginal microconvex transducer (D0), and the cows were assigned to high (G-High, ≥25 follicles, n = 183), intermediate (G-Intermediate, 16–20 follicles, n = 183) or low AFC groups (G-Low, ≤10 follicles, n = 181). In another experiment, COCs were retrieved by OPU from Nelore cows (n = 66), which were assigned to groups according to oocyte production: G-High (n = 22, ≥40 oocytes), G-Intermediate (n = 25, 18–25 oocytes) or G-Low (n = 19, ≤7 oocytes). All COCs from the same cow were cultured individually (maximum of 25 COCs per drop) and then in vitro fertilized using thawed frozen sperm (2 × 108/dose) from a Nelore sire of known fertility. The data were analyzed using a Kruskal–Wallis and a Chi-square test (P ≤ 0.05). There was no difference in the conception rates after FTAI between Nelore cows with high, intermediate or low AFC (51.9 vs. 48.6 vs. 58.6%). The number of viable embryos was 18.4 ± 6.7 (G-High), 6.1 ± 3.6 (G-Intermediate) and 0.6 ± 0.7 (G-Low; P < 0.05). Therefore, AFC had no influence on the conception rates for FTAI; however, Nelore cows with high oocyte production exhibited better in vitro embryo production.