Use of Gonadotropin-Releasing Hormone at Fixed-Time Artificial Insemination at Eighty or Ninety-Seven Hours Post Prostaglandin F2α in Beef Cows Administered the Long-Term Melengestrol Acetate Select Synch

In a 2-yr trial, the pregnancy response of 549 Angus-based cows (BCS 5.1 ± 0.56, postpartum interval 69.7 ± 19.1 d) was evaluated when estrous cycles were synchronized by feeding melengestrol acetate for 14 d with gonadotropin-releasing hormone (GnRH) given on d 26 and prostaglandin F2α (PG) on d 33. Of all cows, those observed in estrus within 72 h after PG were inseminated 9.5 ± 4.01 h after estrus was first observed. The remaining cows were fixed-time inseminated (TAI) and randomly assigned to receive or not receive GnRH at TAI, and randomly assigned to be inseminated at 79.7 ± 2.26 h (early) or 96.6 ± 1.94 h (late) post-PG. There were no GnRH × year (P = 0.47) or GnRH × time of TAI (P = 0.96) interactions. Cows that received GnRH at TAI had a greater (P < 0.001) pregnancy rate than those receiving no GnRH. There was a tendency (P = 0.07) for a year × time of TAI interaction. In yr 1, late TAI cows had a greater (P = 0.01) pregnancy rate than early cows; however, in yr 2, pregnancy rate was not different (P > 0.10) between early and late cows. Results indicate that within the long-term melengestrol acetate Select Synch protocol, pregnancy rate of cows not observed in estrus within 72 h of PG could be increased by adding a second GnRH injection at TAI, whereas the effect of delaying time of TAI until 97 h post-PG on pregnancy rate was inconsistent.