Synchronization of Estrus in Heifers

Abstract

Numerous procedures have been developed to manipulate the estrous cycle of heifers to maximize the number that are in estrus during a short, predefined period with normal fertility-synchronized estrus. Early procedures consisted of synthetic progestins alone [melengestrol acetate; MGA® (Pharmacia and Upjohn Company, Kalamazoo, MI)], prostaglandin F2α(PGF2α) alone, and synthetic progestins in combination with PGF2α(7 d of MGA® with PGF2αon the last d) or estradiol valerate [Syncro-Mate B® (Merial Ltd., Iselin, NJ)]. The immediate estrus following MGA® treatment had suppressed fertility, and all of these early procedures resulted in either variability in the interval to estrus, poor results, or inconsistent results. During the past decade a new estrous synchronization procedure consisting of the daily feeding of MGA® (0.5 mg/d per head) for 14 d and an injection of PGF2α19 d after the last d of MGA® was demonstrated to have pregnancy rates of 50 to 62%. This procedure hastens puberty and is robust; it works in most situations. The CIDR® (Pharmacia and Upjohn), an intravaginal insert containing progesterone, also synchronizes estrus and hastens puberty. Studies using the CIDR® insert for 7 d with an injection of PGF2αon d 6 or 7 to synchronize estrus have reported pregnancy rates similar to heifers synchronized with the MGA®/PGF2αprocedure; however, this procedure has not been as extensively studied as the MGA®/PGF protocol and might not be optimized. Both systems have utility in managing reproduction of beef and dairy heifers.