Development of an estrus synchronization protocol for beef cattle with short-term feeding of melengestrol acetate: 7-11 Synch

An estrus synchronization protocol (7-11 Synch) was developed to synchroniz e the first follicular wave and timing of ovulation in postpartum beef cows . In Exp. 1, follicular development and timing of ovulation in response to the following protocol were evaluated. Beef heifers (n = 12) and cows (n = 6), at random stages of the estrous cycle, were fed melengestrol acetate (M GA; . mg.animal(-1).d(-1)) for 7 d and injected with PGF(2 alpha) (PG; 25 m g) on the last day of MGA. A second injection of PG was administered 11 d a fter cessation of MGA. After the second injection of PG, estrus was synchro nized in 6/12 heifers and 3/6 cows. The interval to estrus in heifers and c ows was 54 and 64 h, respectively (P >.10). All animals exhibiting estrus o vulated first-wave follicles. Animals that failed to respond to the second injection of PG were in estrus later than 6 d after cessation of MGA and ha d corpora lutea that were unresponsive to the injection of PG. Based on the variation in interval to estrus following the first PG injection on the la st day of MGA feeding in Exp. 1, an injection of GnRH (100 mu g) was added to the protocol 4 d after the cessation of MGA to ensure ovulation or lutei nization of dominant follicles and synchronization of first-wave follicular development. This revised protocol was termed "7-11 Synch." In Exp. 2, two estrus synchronization protocols were compared. Multiparous beef cows were stratified by breed and postpartum interval and randomly assigned to the 7 -11 Synch (n = 44) or Select Synch protocols (GnRH injection followed by PG injection 7 d later; n = 45). Timing of estrus after the last PG injection (0 h) ranged from 42 to 102 h in the 7-11 Synch group and -30 to 114 h in the Select Synch group. Eight cows (18%) in the Select Synch group exhibite d estrus 30 h before to 18 h after PG. Synchronized estrus peaked between 4 2 and 66 h after the last PG injection, and a maximum number of cows were i n estrus at 54 h for both treatment groups. Synchrony of estrus from 42 to 66 h was greater (P <.05) in 7-11 Synch (91%: 41/44) than in Select Synch c ows (69%: 31/45). Artificial insemination pregnancy rate from 42 to 66 h wa s greater (P <.05) in the 7-11 Synch group (66%: 29/44) than in the Select Synch group (40%: 18/45). In summary, the 7-11 Synch protocol improved sync hrony of estrus without reducing fertility. This protocol has potential fut ure application for fixed-time AI in beef cattle production systems.