Follicular dynamics and hormonal profile in Prepubertal nelore and Murrah heifers subjected to fixed-time artificial insemination (FTAI).

We aimed to evaluate puberty induction, follicular dynamics, and hormonal profiles of progesterone (P4) and estradiol (E2) in prepubertal Nelore and Murrah heifers subjected to synchronization for fixed-time artificial insemination (FTAI). Six Nelore heifers (14 to 17 months old; 320 to 350 kg) and six Murrah heifers (14 to 17 months old; 422 to 523 kg) confirmed as prepubertal by ultrasonography due to the absence of a corpus luteum were used. The protocol included an intravaginal P4 device (0.5 mg) on D0, along with the administration of estradiol benzoate (EB) and prostaglandin F2α. In Nelore, the device was removed on D7, followed by the administration of estradiol cypionate, prostaglandin F2α, and eCG, with insemination occurring 48-56 hours after intravaginal device removal (D9). In Murrah, the device was removed on D9, followed by prostaglandin F2α and Ecg administration, with insemination on D12 after GnRH administration (D11). Blood samples were collected (Nelore: D7 and D9; Murrah: D9 and D12) to measure hormone levels, and ultrasonography monitored follicular dynamics and ovulation. The ovulatory follicle diameter was 7.83 ± 2.25 mm (Nelore) and 7.7 ± 1.63 mm (Murrah), with no significant differences (P > 0.05). The dominant follicle diameter at P4 device removal was 6.5 ± 1.59 mm (Nelore) and 6.9 ± 4.97 mm (Murrah). The mean P4 levels were 0.76 ng/mL (Nelore) and 1.42 ng/mL (Murrah) at P4 device removal, and 1.00 ng/mL and 0.52 ng/mL at the moment of FTAI. The mean E2 levels were 9.28 pg/mL (Nelore) and 26.75 pg/mL (Murrah) at the P4 device removal, and 11.20 pg/mL and 11.59 pg/mL at the moment of FTAI. Ovulation rates were 100% in Nelore and 50% in Murrah. These results highlight the importance of tailoring FTAI protocols to the specific reproductive characteristics of each breed.