Timed Artificial Insemination in Postpartum Beef Cows Using Melengestrol Acetate, Prostaglandin F2α, and Gonadotropin-Releasing Hormone

This study evaluated the use of melengestrol acetate (MGA), prostaglandin F2α (PGF2α) and gonadotropin-releasing hormone (GnRH) in a timed AI (TAI) protocol in postpartum beef cows. One hundred twenty-one Angus-crossbred cows in two herds were ranked by parity, days postpartum, body condition score (BCS), and BW and then randomly assigned to one of two treatments. Cows in both treatments received 1.8 kg of supplement containing 0.5 mg/d of MGA for 14 d, then received supplement without MGA for 17 d. After MGA withdrawal, cows in both groups received an injection of PGF2α. Cows receiving the MGAPGF2α (MPG) treatment were subjected to estrus detection and AI for 8 d. Cows receiving the MGA-PGF2α-GnRH (MPGG) treatment were injected with GnRH 48 h after PGF2α, followed by TAI 16 to 18 h after PGF2α. There was a difference (P<0.01) between herds in conception rate (CR) and pregnancy rate to AI (PR) and overall pregnancy rate (OP). The CR was not different between treatments (P≥0.24), but PR was greater in MPGG vs MPG (P≤0.08). There was no difference in OP between MPG and MPGG treatments (P≥0.25). The TAI procedure described in this study yielded results that were equal to those obtained with cows in a standard 14- to 17-d MGA-PGF2α with estrus detection and AI protocol and similar to previously described TAI procedures.