Evaluation of Gonadotropin-Releasing Hormone at Fixed-Time Artificial Insemination in Beef Heifers Synchronized Using a Modified CO-Synch Plus Controlled Internal Device Release Protocol

The objective of this study was to determine whether a second injection of gonadotropin-releasing hormone (GnRH) at 54-h timed AI (TAI) of a CO-Synch/ CIDR (controlled internal device release) protocol improves fertility in replacement beef heifers. Heifers (n = 375) at three locations (Colorado, Wyoming, and South Dakota) were stratified by BW within body condition score (BCS) and randomly allotted to one of two treatments. All heifers received 100 µg of GnRH and a CIDR insert (d-7), followed by CIDR removal and 25 mg of prostaglandin F2α (PG; d 0). On d 2, control and treatment heifers underwent TAI at 54 h post-PG, and treatment heifers received a second 100-µg injection of GnRH. Blood samples were collected from all heifers at Colorado and Wyoming (d-17 and -7) to determine pubertal status. Ultrasonography was used to determine ovulation rate after TAI from a subsample of heifers (Colorado, n = 19; Wyoming, n = 49). No treatment x location interaction (P>0.10) occurred and pooled TAI pregnancy rates were similar (P>0.01) for control (46%) vs treatment (55%) heifers. Pubertal rates were greater (P<0.01) for heifers at Colorado (97.4%) than for heifers at Wyoming (46.4%); however, TAI pregnancy rates were similar (P> 0.10) for pubertal and prepubertal heifers. Ovulation rates tended to be different (P=0.10) for treatment (81.3%) than for control (62.5%) heifers. We conclude that the second injection of GnRH at TAI in the CO-Synch/CIDR protocol does not increase pregnancy rates to TAI in beef heifers, but that it may be economically viable and may guard against reduced fertility.