Pregnancy rates in angus cross beef cows bred at observed oestrus with or without second GnRH administration in...

Crossbred cows (n = 1073) from five locations had oestrous cycles synchronized with 100 microg of GnRH IM and insertion of controlled internal drug release device (CIDR) on Day 0 followed by 25 mg of PGF(2alpha) 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 PGF(2alpha) administration. In the Ovsynch-CIDR group, cows detected in oestrus (n = 193) within 48 h after PGF(2alpha) were inseminated using the AM-PM rule. Among these cows, 80 received and 113 did not receive a second GnRH at 48 h after PGF(2alpha). Cows (n = 345) not detected in oestrus received a second GnRH at 48 h after PGF(2alpha) 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 PGF(2alpha) were inseminated using the AM-PM rule. Among these cows, 79 received and 145 did not receive a second GnRH at 64 h after PGF(2alpha). Cows (n = 311) not detected in oestrus received a second GnRH on Day 10 at the time of AI, 64 h after PGF(2alpha). 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.