Effect of 2, 4, and 5-hour intervals between 2 prostaglandin F2α injections administered with 5-day CO-Synch + CIDR protocol on pregnancy rate in beef cows

The objective of this study was to compare fixed-time AI (TAI) pregnancy rates resulting from intervals of 2, 4, or 6 h between 2 prostaglandin (PG) F2α injections administered in a 5-d CO-Synch + controlled internal drug-release device (CIDR) estrus synchronization protocol. Angus and Angus-crossbred cows (n = 901) maintained on native pasture at 3 locations were randomly assigned to treatments after blocking for BCS and postpartum interval. All cows received gonadotropin-releasing hormone and a CIDR insert on d -5, PG and CIDR removal on d 0, and gonadotropin-releasing hormone with TAI at 72 h. Cows also received a second injection of PG either 2, 4, or 6 h after the first injection of PG. Pregnancy rates were determined by transrectal ultrasonography 40 ± 2 d following TAI. Body condition score and postpartum interval did not differ (P > 0.72) between estrus synchronization protocols, averaging 4.3 and 81 d, respectively. Fixed-time AI pregnancy rate was least (P ≤ 0.05) in cows receiving 2 PG injections separated by 2- and 4-h intervals (average 51%) compared with cows receiving 2 PG injections separated by a 6-h interval (59.7%). Reducing the interval between PG injections from 6 to 2 h resulted in a 9-percentage-point (15%) reduction in TAI pregnancy rates for cows synchronized with a 5-d COSynch + CIDR protocol.