Effect of timing of prostaglandin administration, controlled internal drug release removal and gonadotropin releasing hormone administration on pregnancy rate in fixed-time AI protocols in crossbred Angus cows

Two experiments were conducted to investigate the effects of timing of prostaglandin F2α (PGF2α) administration, controlled internal drug release device (CIDR) removal and second gonodotropin releasing hormone (GnRH) administration on the pregnancy outcome in CIDR-based synchronization protocols. In Experiment 1, suckled Angus crossbred beef cows (n = 580) were given 100 μg of GnRH + a CIDR on Day 0. Cows in Group 1 (modified Ovsynch-P) received 25 mg of dinoprost (PGF2α) and CIDR device removal on Day 8 (AM), 100 μg of GnRH 36 h later on Day 9 (p.m.), and fixed-time AI (FTAI) 16 h later on Day 10 (47.5 ± 1.1 h after PGF2α). Cows in Group 2 (Ovsynch-P) received 25 mg of PGF2α and CIDR device removal on Day 7 (p.m.), 100 μg of GnRH 48 h later on Day 9 and FTAI 16 h later on Day 10 (66.6 ± 1.2 h after PGF2α). Pregnancy rates were 56.5% (170/301) for Group 1 and 55.6% (155/279) for Group 2, respectively (P = 0.47). In Experiment 2, beef cows (n = 734) were synchronized with 100 μg of GnRH + CIDR on Day 0, 25 mg of PGF2α and CIDR device removal on Day 7 and either 100 μg of GnRH 48 h later on Day 9 (Ovsynch-P) and FTAI 16 h later on Day 10 (64.9 ± 3.3 h from PGF2α) or 100 μg of GnRH on Day 10 (CO-Synch-P) at the time of AI (63.2 ± 4.2 h from PGF2α). Pregnancy rates were 48.8% (180/369) for Ovsynch-P and 44.7% (163/365) for CO-synch-P groups, respectively (P = 0.11). In both experiments, there was a location × treatment interaction (P < 0.05); pregnancy rates between locations were different (P < 0.05) in the Ovsynch-P group. In conclusion, in a CIDR-based Ovsynch synchronization protocol, delaying administration of prostaglandin and CIDR removal by 12 h, or timing of the second GnRH by 16 h, did not affect pregnancy rates to FTAI. Therefore, there may be an opportunity to make changes in synchronization protocols with out adversely affecting FTAI pregnancy rates.