Fertility in Angus cross beef cows following 5-day CO-Synch + CIDR or 7-day CO-Synch + CIDR estrus synchronization and timed artificial insemination

The present study determined whether a 5-day CO-Synch + controlled internal drug release (CIDR) protocol with two doses of PGF2α would improve timed artificial insemination (AI) pregnancy rate compared with 7-day CO-Synch + CIDR protocol in beef cows. Angus cross beef cows (N = 1817) at 12 locations were randomly assigned to 5-day CO-Synch + CIDR or 7-day CO-Synch + CIDR groups. All cows received 100 μg of GnRH and a CIDR insert on Day 0. Cows (n = 911) in the 5-day CO-Synch + CIDR group received two doses of 25 mg PGF, the first dose given on Day 5 at CIDR removal and the second dose 6 hours later, and 100 μg GnRH on Day 8 and were inseminated concurrently, 72 hours after CIDR removal. Cows (n = 906) in 7-day CO-Synch + CIDR group received 25 mg of PGF at CIDR removal on Day 7, and 100 μg GnRH on Day 10 and were inseminated concurrently, 66 to 72 hours after CIDR removal. All cows were fitted with a heat detector aid at CIDR removal and were observed twice daily until insemination for estrus and heat detector aid status. Accounting for estrus expression at or before AI (P < 0.0001) and body condition score (P < 0.01), cows in the 5-day CO-Synch + CIDR group had greater AI pregnancy rate compared with cows in the 7-day CO-Synch + CIDR group (58.1% vs. 55.1%; P = 0.04). More cows that exhibited estrus at or before AI became pregnant compared with cows that did not [65.7% (681/1037) vs. 44.5% (347/780); P < 0.0001]. The AI pregnancy rate was lesser for cows with body condition ≤4 [≤4 − 49.3% (101/219), 5–6 − 57.9%; >6 − 55.8%]. The mean AI pregnancy rate difference between treatment groups and projected economic outcome varied among locations. In conclusion, cows synchronized with the 5-day CO-Synch + CIDR protocol had greater AI pregnancy rate than those that received the 7-day CO-Synch + CIDR protocol.