Melengestrol acetate, estradiol-17 beta and GnRH for synchronization of estrus and ovulation in beef cows

Three experiments were conducted with suckled crossbred beef cows to determ ine the efficacy of various short-term regimens for synchronizing estrus or ovulation and to determine whether feeding melengestrol acetate (MGA) afte r timed AI will increase pregnancy rate. In exp. I, all cows (n = 141) were given 0.5 mg MGA head(-1) d(-1) for 7 d (days 1 to 7), 1 or 5 mg E17 beta (and 100 mg progesterone) or 100 mu g GnRH on day 1, and 500 mu g cloproste nol on day 7. There were no significant differences among groups for estrou s or synchronized conception rates (average, 84.4 and 52.1%, respectively). With an identical protocol in exp. 2 (78 cows), by day ii, cows given 5 mg E17 beta had the highest estrous and synchronized pregnancy rates (65.4 an d 42.3%), compared to 1 mg E17 beta (46.2 and 15.4%) and GnRH (34.6 and 11. 5%). Cows not detected in estrus were timed-inseminated and given 100 mu g GnRH, 100 h after cloprostenol. There were no significant differences among groups for synchronized conception rate to timed AI (65.0%) or synchronize d pregnancy rate to all inseminations (56.4%). Overall, 5 mg E17 beta gave the most consistent results. In exp. 3, 84 cows were given 100 mu g GnRH on days 1 and 9, 500 mu g cloprostenol on day 7, and were timed-inseminated o n day 10. Half were fed MGA (0.5 mg head(-1) d(-1)) on days 16 to 22, but t he pregnancy rate was not different from that in the remaining cows (55.0 v ersus 47.8%, P > 0.5).