Effect of Calf Separation During the 12-Hour Interval Between 2 Prostaglandin F2α Injections When Using the 5-Day CO-Synch + Controlled Internal Drug-Release Synchronization Protocol on Pregnancy Rate in Beef Cows

The objective of this study was to determine the effect of calf separation during the 12-h interval between 2 prostaglandin F2α (PG) injections on fixedtime AI (TAI) pregnancy rate in beef cows synchronized with a 5-d CO-Synch + controlled internal drug-release device (CIDR) protocol. This study was conducted at 2 locations using 319 cows of Angus, Limousin, and Hereford breeding at ranch 1, and 108 Angus-based cows at ranch 2. All cows were synchronized with gonadotropin-releasing hormone and CIDR on d − 5, with 25 mg of PG at 0 and 12 h after removal of CIDR on d 0, and with gonadotropin-releasing hormone with TAI at 72 h. At each ranch, half the calves remained separated from cows during the 12-h interval between PG injections and the other half of the calves were returned to their dams during this time. Body condition score was greater (P < 0.01) for cows with calves returned to their dams at ranch 2 (BCS 5.3) than for the other 3 treatment groups (average BCS 5.0). Interval from calving to breeding did not differ (P > 0.68) between ranches or calf treatments, averaging 80 d. Timed AI pregnancy rate and early-season pregnancy rate were greater (P < 0.01) at ranch 2 than ranch 1 (69.6 vs. 53.4% and 87.9 vs. 67.2%, respectively). Timed AI pregnancy rate and early-season pregnancy rate did not differ (P > 0.48) between the half of calves returned to their dams during this time and the other half of calves separated from cows, averaging 61.5 and 77.5%, respectively. Calf separation during the 12-h interval between PG injections had no effect on TAI pregnancy rate, but would simplify the application of this synchronization protocol.