Association of buserelin acetate with estradiol benzoate in early resynchronization of Bos taurus beef heifers improved conception rates.

The aim of this study was to evaluate the use of buserelin acetate, a GnRH analog, in combination with estradiol benzoate (EB) at the beginning of an early resynchronization protocol (R22) in Bos taurus beef heifers. A total of 656 heifers underwent a first fixed-time artificial insemination (FTAI; designated as D0) and received a reused intravaginal progesterone (P4; 1 g) device on D22. At that time, heifers were randomized into two treatment groups: EB (1 mg; n = 329) or GnRH + EB (1 mg of EB plus 10 μg of buserelin acetate; n = 327). On D30, the P4 device was removed, and a pregnancy diagnosis was performed. Non-pregnant heifers (EB = 152/329; GnRH + EB = 134/327) received 200 IU of equine chorionic gonadotropin, 1 mg of estradiol cypionate and 0.5 mg of cloprostenol sodium, followed by a second FTAI on D32. A subset of heifers underwent ovarian ultrasonography on D22 (n = 463), D30 (n = 193), and D32 (n = 165); a second pregnancy diagnosis was performed on D62. Heifers treated with EB alone had larger dominant follicles on both D30 and D32 (P = 0.0001) and a higher incidence of early ovulation before FTAI compared to GnRH + EB group (18.9 % vs. 8.8 %; P = 0.03). The GnRH + EB group achieved a superior pregnancy rate at R22 (EB = 40.8 % vs. GnRH + EB = 56.0 %; P = 0.008) and a higher cumulative pregnancy rate (EB = 68.4 % vs. GnRH + EB = 79.2 %; P = 0.01). In conclusion, the administration of 10 μg of buserelin acetate combined with 1 mg of EB at the beginning of the R22 protocol improved conception rates in Bos taurus beef heifers.