Decreasing the interval between GnRH and PGF2α from 7 to 5 days and lengthening proestrus increases timed-AI pregnancy rates in beef cows

Four experiments were conducted in postpartum beef cows to evaluate the influence of reducing the interval from GnRH to PGF2α from 7 to 5 d in a Select-Synch + CIDR or CO-Synch + CIDR estrous synchronization program. In Expt 1, cows (n = 156) were treated with either a 7 or 5 d Select-Synch + CIDR program. A second PGF2α treatment was given to all cows in all experiments at 12 h after the initial PGF2α (to ensure that luteolysis occurred with the 5 d program). Estrous response, interval to estrus, conception rate, and first service AI pregnancy rates were similar between treatments. In Expt 2, cows (n = 223) were treated with either a 7 or 5 d CO-Synch + CIDR program, with timed-AI concomitant with GnRH at 60 h after PGF2α. Timed-AI pregnancy rates were similar between treatments. In Expt 3 (n = 223) and 4 (n = 400) cows were treated with either a 7 or 5 d CO-Synch + CIDR program with timed-AI concurrent with GnRH at either 60 h (7 d) or 72 h (5 d) after CIDR withdrawal. Timed-AI pregnancy rates were 13.3% (P < 0.05; Expt 3) and 9.1% (P < 0.05; Expt 4) greater for the 5 than 7 d program. In conclusion, timed-AI pregnancy rates were improved with a 5 d CO-Synch + CIDR program with timed-AI at 72 h after CIDR withdrawal, compared to a 7 d CO-Synch + CIDR program with timed-AI at 60 h after CIDR withdrawal.