Timed Insemination in Beef Heifers After Synchronization of Estrus with Controlled Intravaginal Drug Releasing Device and Melengestrol Acetate1

Two progestin estrous synchronization programs with timed AI (TAI) were used to compare effects on conception to TAI and overall pregnancy rates in beef heifers. Angus-cross heifers in two herds were sorted by age of heifers, body condition score (BCS), and BW then randomly assigned to a treatment. Treatments were 1) MPGG [melengestrol acetate (MGA®; Pfizer Animal Health, New York, NY) at 0.5 mg per head for 14 d followed by prostaglandin F2α (PGF2α) 17d after MGA® withdrawal and a gonadotropin-releasing hormone (GnRH) injection 48 h after PGF2α or 2) CIDR [a new controlled intravaginal drug-releasing (CIDR®) device inserted for 6 d with GnRH at CIDR insertion, PGF2α: at CIDR removal, and a second GnRH injection 48 h later. All heifers in both treatments were bred 18 h after the second GnRH injection.] Bulls were placed with heifers 10 d following TAI for approximately 50 d. Age of heifers, BCS, and BW in Herd 1, Herd 2, or the combined herds were not different (P≥0.25) between treatments. Across treatments, there was a difference between herds in BCS and BW (P<0.0001), but no difference in age of heifers (P=0.51). Conception rates (CR) and overall pregnancy rates (OP) were not different between herds, or between treatments in Herd 1, Herd 2, or in the combined groups (P≥0.40). The results in this study demonstrate similarities between MPGG and CIDR estrous synchronization protocols on conception rates to TAI and overall pregnancy rates in beef heifers.