Fertility in Angus Cross Beef Cows Following 5d CO-Synch + CIDR or 7d CO-Synch + CIDR Estrus Synchronization and Timed Artificial Insemination

The present study determined whether a 5d CO-Synch + CIDR protocol with a two doses of prostaglandin F2α (PGF) would improve timed AI pregnancy rate compared to 7d CO-Synch + CIDR protocol in beef cows. Angus cross beef cows (N = 1817) at 12 locations were randomly assigned to 5d CO-Synch + CIDR or 7d CO-Synch + CIDR groups. All cows received 100 μg of gonadorelin diacetate tetrahydrate (GnRH) and a controlled internal drug release (CIDR) insert on Day 0. Cows (n=911) in the 5d 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 h later, and 100 μg GnRH on Day 8 and were inseminated concurrently, 72 h after CIDR removal. Cows (n=906) in 7d 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 h 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 prior to AI (P<0.0001) and body condition score (P<0.01), cows in the 5d CO-Synch + CIDR group had greater AI pregnancy rate compared to cows in the 7d CO-Synch + CIDR groups (58.1% vs. 55.1%; P = 0.04). More cows that exhibited estrus at or prior to AI became pregnant compared to 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 to 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 5d COSynch + CIDR protocol had greater AI pregnancy rate than those that received the 7d CO-Synch + CIDR protocol.