Pregnancy Rates in Angus Cross Beef Cows Bred at Observed Oestrus With or Without Second GnRH Administration in Fixed-Time Progesterone-Supplemented Ovsynch and CO-Synch Protocols

Crossbred cows (n = 1073) from five locations had oestrous cycles synchronized with 100 μg of GnRH IM and insertion of controlled internal drug release device (CIDR) on Day 0 followed by 25 mg of PGF₂α IM and CIDR removal on Day 7. Kamar® patches were placed on all cows at CIDR removal. Cows were observed three times daily for oestrus after PGF₂α administration. In the Ovsynch-CIDR group, cows detected in oestrus (n = 193) within 48 h after PGF₂α were inseminated using the AM-PM rule. Among these cows, 80 received and 113 did not receive a second GnRH at 48 h after PGF₂α. Cows (n = 345) not detected in oestrus received a second GnRH at 48 h after PGF₂α on Day 9, and fixed-time AI 16 h after the GnRH on Day 10. In the CO-Synch-CIDR group, cows detected in oestrus (n = 224) within 48 h after PGF₂α were inseminated using the AM-PM rule. Among these cows, 79 received and 145 did not receive a second GnRH at 64 h after PGF₂α. Cows (n = 311) not detected in oestrus received a second GnRH on Day 10 at the time of AI, 64 h after PGF₂α. The AI pregnancy rates were not different between the Ovsynch-CIDR and CO-Synch-CIDR groups (p = 0.48). There were no differences in the AI pregnancy rates for cows inseminated at a fixed time (p = 0.26) or at detected oestrus (p = 0.79) between the treatment groups. Among cows inseminated in oestrus, there were no differences in the AI pregnancy rates between cows that received or did not receive the second GnRH (p = 0.47). In conclusion, acceptable AI pregnancy rates can be achieved with or without inclusion of oestrus detection in the Ovsynch-CIDR and CO-Synch-CIDR protocols. Among cows detected in oestrus, cows that received a second GnRH yielded similar pregnancy rates when compared with cows that did not receive the second GnRH.