Ovarian and Estrous Response of Suckled Beef Cows to the Select Synch Estrous Synchronization Protocol

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This study evaluated ovarian and estrous responses as affected by day of the estrous cycle at initiation of treatment among cows synchronized with the Select Synch protocol. Fifty-six suckled Hereford x Red Angus multiparous cows were fitted with HeatWatch® electronic estrous detection monitors 22 d before initiation of treatment. Thirty-two percent of cows were not detected in estrus during this 22-d period, and were classified as anestrus. Treatment included an injection of Gonadotropin releasing hormone (GnRH; 100 μg) on d 7, followed by an injection of Prostaglandin F2α (PGF2α; 25 mg) on d 0. Ovaries of cows were scanned daily via ultrasound from d 12 until the time of insemination to evaluate follicular and luteal response to the GnRH and PGF2α injections. Cows were visually observed for signs of estrus twice daily from d 0 to d 5 for approximately 2 h to compare visual detection of estrus with electronic detection. There was no difference (P>0.5) in the percentage of cows detected in estrus between visual (85%) and electronic (88%) observation. Ovarian and estrous responses to the GnRH and PGF2α injections were dependent on day of the estrous cycle when the protocol was initiated. Ovulation resulting in formation of a new corpus luteum (CL) and follicular turnover occurred following the injection of GnRH in 66 and 70% of cows, respectively. Only 14% of cows that were between d 15 and 17 of their estrous cycle responded to either GnRH or PGF2α administration. Additionally, these cows exhibited estrus 11 ± 19 h (mean ± SD) before injection of PGF2α. Anestrous cows exhibited estrus earlier (P<0.05) than cyclic cows (40.2 ± 16 h and 56.3 ± 39 h, respectively). Synchronized pregnancy rate was 71% for the 5 d breeding period, and included a 61% pregnancy rate among anestrous cows. We conclude that the Select Synch protocol is capable of synchronizing estrus among the majority of cows in a herd, and we emphasize that accurate estrous detection beginning 24 to 30 h before the PGF2α injection is important.