Estradiol esters are dispensable in progesterone-based TAI protocols for Bos taurus suckling beef cows

Timed artificial insemination (TAI) protocols in South America are mainly based on progesterone (P4) and estradiol (E2). Considering the restrictions on E2 use, the study aimed to evaluate its replacement with GnRH. Multiparous suckling Bos taurus cows (n = 455) were pre-synchronized and received a P4 (1 g) intravaginal device (IVD) on D0. Also, on D0, cows in the Control group received 2 mg of estradiol benzoate IM and cows in the GnRH/GnRH and GnRH/EC groups received 20 µg of buserelin acetate IM. At IVD removal (D7), all cows received 500 µg sodium cloprostenol IM, 300IU eCG IM, and tail paint. At this time, cows in the Control and GnRH/EC groups received 1 mg estradiol cypionate (EC) IM. TAI was performed on D9 (48 h after IVD removal) in all cows, and 10 µg of buserelin acetate IM was administered only in GnRH/GnRH group. No difference was observed among Control (n = 85), GnRH/EC (n = 82) and GnRH/GnRH (n = 84) regarding largest follicle (LF) diameter on D7 (7.3 ± 0.2, 7.6 ± 0.3 and 7.6 ± 0.3 mm, respectively; P = 0.75) and percentage of CL on D7 (87.1, 80.5 and 81.0 %. respectively; P = 0.37). Significant effects of D7 LF diameter (P < 0.0001; odds ratio 1.58) and D7 LF by group interaction on pregnancy rate were observed (P < 0.01). No difference was observed in the estrus rate before TAI (D9; 72.1, 62.4 and 67.9 %; P = 0.22) or in the pregnancy rate (56.9, 53.1 and 53.6 %; P = 0.77) for control (n = 153), GnRH/EC (n = 147) and GnRH/GnRH (n = 155), respectively. In conclusion, estradiol replacement by GnRH in TAI protocols does not affect LF diameter, estrus expression and pregnancy rate in suckling Bos taurus beef cows.