Development of a progestin-based estrus synchronization program: I. Reproductive response of cows fed melengestrol acetate for 20 days with an injection of progesterone.

We designed two experiments to determine the efficacy of an estrus control system in cows that combined long-term progestin exposure (20 d) with an acute increase in progesterone concentration. In Exp. 1, cows (n = 30) were fed either melengestrol acetate (MGA; .5 mg x cow(-1) x d(-1)) or ground ear corn (MGA carrier) for 20 d. On d -15 (last day of MGA feeding = d 0), cows were administered 25 mg of PGF2alpha to regress the corpus luteum (CL) and establish an environment conducive to the development of persistent follicles. To synchronously regress persistent follicles, cows fed MGA (n = 15) were injected with 200 mg of progesterone on d -2 (MGA-P), and the cows fed the MGA carrier were not treated (CONT; n = 15). Cows in the CONT group were artificially inseminated 12 h after detection of spontaneous estrus from d -20 to d 8. Estrus was observed, and all cows in the MGA-P group were artificially inseminated during the period of estrus synchronization (SYNC; d 1 to 8). No difference in conception rate was observed between treatments. In Exp. 2, postpartum cows (n = 113) received either the MGA-P (n = 56) or CONT (n = 57) treatment. More (P < .05) cows were observed in estrus during SYNC in the MGA-P (50%) than in the CONT (28%) group. Of the cows in the MGA-P group that were not observed in estrus during SYNC, 50% were in estrus for the first time 23 to 29 d after MGA withdrawal (SYNC2), suggesting that these cows ovulated without observable estrus during SYNC. Estrus was observed for the first time during SYNC2 in more (P < .05) cows in the MGA-P (25%) than in the CONT (7%) group. Conception rate at the synchronized estrus, pregnancy rate, and interval to first service and pregnancy were similar between treatments. We conclude that administration of MGA-P results in the synchronization and(or) induction of a fertile estrus in cows.