Select Synch and Co-Synch protocols using a CIDR yield similar pregnancy rates after a fixed-time insemination in suckled Bos indicus × Bos taurus cows.

Primiparous and multiparous suckled Bos indicus × Bos taurus cows were used to compare a Select Synch + controlled internal drug release (CIDR) and timed artificial insemination (AI) protocol (SSC) to a modified CO-Synch + CIDR protocol (COS). Five separate groups of cows were utilized (n = 659) and within a group, cows were allotted to two treatments on the basis of body condition score (BCS) taken 10 days before and days postpartum at CIDR insertion (Day 0). Blood samples were collected on Day -10 and 0 for assessment of circulating concentrations of progesterone to determine estrous cycling status. Cows in both treatments received intramuscular (im) GnRH (100 μg) and a CIDR inserted on Day 0. On Day 7, the SSC treatment had CIDR removed and received im PGF2α (25 mg), whereas COS treatment had CIDR removed and received im PGF2α (25 mg) on Day 7.5. In the SSC treatment, estrus was detected daily at 0700 and 1700 hours for 72 hours after PGF2α administration, and cows were artificially inseminated 8 to 12 hours after a detected estrus. Cows failing to exhibit estrus were subjected to timed-AI and received im GnRH (100 μg) 76 ± 4 hours after PGF2α administration. In the COS treatment, cows received im GnRH (100 μg) and were fixed-time artificially inseminated at 64 ± 4 hours after PGF2α administration. Although group differences (P < 0.05) existed for synchronized AI pregnancy rates, there were no significant interactions of group with any variable analyzed; hence, groups were pooled for further evaluation. Within the SSC treatment, the 72-hour estrous response was 50.6% (168 of 332). Conception rate of cows in estrus was 66.1% (111 of 168), and timed-AI pregnancy rate for nonresponders was 32.3% (53 of 164). Overall synchronized AI pregnancy rates were similar (P > 0.05) for SSC (49.4%; n = 164 of 332) and COS (47.1%; n = 154 of 327). Estrous cycling status and days postpartum did not (P > 0.05) influence synchronized AI pregnancy rates but BCS did (P < 0.05). Cows with a BCS greater than or equal to 5 had synchronized AI pregnancy rates of 50% compared to 39% for cows with BCS lesser than 5.0. In summary, the modified COS protocol yielded similar synchronized AI pregnancy rates compared to the SSC protocol, and it appears that the modified COS protocol has the potential to be an effective timed-AI protocol in suckled Bos indicus × Bos taurus cows.