Reproductive efficiency of Bos indicus cows under different estradiol-free ovulation synchronization protocols

The objective was to evaluate the fertility of Bos indicus cows submitted to an ovulation synchronization protocol without estrogen (E2). In experiment 1, injectable P4 (P4i) was administered 10 days (D-10) before the TAI protocol. On D0, cows received one intravaginal P4 device and estradiol benzoate (EB; Control) or buserelin acetate (GnRH/EC and 2GnRH). On D8, devices were removed, and all cows received eCG and PGF2α. Additionally, cows received estradiol cypionate (EC) on D8 (Control and GnRH/EC) or buserelin acetate on D10 (2GnRH). In Experiment 2, cows in the Control group underwent a protocol similar to Experiment 1. Cows in the GnRH/EC, 2GnRH/48h, and 2GnRH/54h groups were pre-synchronized with an intravaginal P4 device (D-7) and received gonadorelin and an intravaginal P4 device on D0. P4 devices were removed on D8, and all cows received eCG and PGF2α. Furthermore, the GnRH/EC group received EC, and cows in the 2GnRH/48h and 2GnRH/54h groups were administered GnRH 48 h (h) after removal of the P4 device. TAI was performed 48 (2GnRH/48h) or 54h (2GnRH/54h) after removal of the P4 device. In all experiments, the follicular diameter on D8 was greater in cows that received GnRH on D0 (P ≤ 0.01) as well as a greater ovulation rate on D0 (P ≤ 0.01). Furthermore, estrus expression was greater in cows receiving EC (P = 0.001; Exp. 2). P/AI was greater in the Control group (P = 0.001; Exp. 2). In conclusion, removal of estradiol in ovulation synchronization protocols resulted in lower fertility relative to E2-based controls.