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 F 2α ( PGF 2α ) 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 PGF 2α . Cows receiving the MGAPGF 2α ( MPG ) treatment were subjected to estrus detection and AI for 8 d. Cows receiving the MGA-PGF 2α -GnRH ( MPGG ) treatment were injected with GnRH 48 h after PGF 2α , followed by TAI 16 to 18 h after PGF 2α . There was a difference (P 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-PGF 2α with estrus detection and AI protocol and similar to previously described TAI procedures.