Methods to Synchronize Estrous Cycles of Postpartum Beef Cows with Melengestrol Acetate

This review considers recently developed methods to control estrous cycles of postpartum beef cows with melengestrol acetate (MGA®, Pharmacia Animal Health, Kalamazoo, MI). Melengestrol acetate is an orally active progestin that will suppress estrus and prevent ovulation in cattle if consumed on a daily basis. The duration of feeding may vary among the various protocols that are available, but the level of feeding (0.5 mg/d per animal) is consistent and critical to success. Feeding MGA® for 14 d followed by injection of prostaglandin F_2α (PGF_2α) 17 to 19 d after MGA® withdrawal was developed as an effective method of estrous cycle control for heifers. Studies in postpartum beef cows identified significant improvements in specific reproductive endpoints among cows that received MGA® prior to the administration of PGF_2α compared with cows that received PGF_2α only, including increased estrous response and improved synchronized conception (CR) and pregnancy (PR) rates. Recently, an improvement in synchrony of estrus was reported, without compromising fertility, in postpartum beef cows that were pretreated, either short- or long-term, with MGA® prior to gonadotropin-releasing hormone (GnRH) and PGF_2α. We proposed the general hypothesis that progestin (MGA®) treatment prior to the GnRH-PGF_2α estrus synchronization protocol would successfully 1) induce ovulation in anestrous postpartum beef cows; 2) reduce the incidence of a short luteal phase among anestrous cows induced to ovulate; 3) increase estrous response, synchronized CR, and PR; and 4) increase the likelihood of successful fixedtime insemination. Protocols that utilize this sequential approach to control the estrous cycle include the MGA® Select and 7-11 Synch protocols. The flexibility in matching specific protocols with the particular management system involved is a major advantage in using MGA® to control estrous cycles in cows.