Effects of administration of prostaglandin F_2α at initiation of the seven-day CO-Synch+controlled internal drug release ovulation synchronization protocol for suckled beef cows and replacement beef heifers

Two experiments were conducted to determine the effect of administering PGF_2α at the initiation of the 7-d CO-Synch+controlled internal drug release (CIDR) fixed-timed AI (TAI) protocol on pregnancy rates of suckled beef cows and replacement heifers. Within location, cows were stratified by days postpartum (DPP), BCS, and parity (Exp. 1; n = 1,551) and heifers were stratified by BCS (Exp. 2; n = 999) and randomly assigned to 1 of 2 treatments: 1) CO-Synch+CIDR (100-μg injection of GnRH at CIDR insertion [d-10] with a 25-mg injection of PGF_2α at CIDR removal [d-3] followed by injection of GnRH and TAI on d 0) or 2) PG-COSynch+ CIDR (a 25-mg injection of PGF_2α on d-10 of the CO-Synch+CIDR protocol). Follicle diameter and corpus luteum (CL) development were assessed on d-10 and-3, and pregnancy status was determined on d 30 to 35. Blood was collected on d-20,-10,-3, and 0 relative to TAI to determine concentrations of progesterone (P4). In Exp. 1, TAI pregnancy rates did not differ (P = 0.667) between treatments and were affected by BCS (P = 0.003) and DPP (P = 0.006). Concentrations of P4 were greater (P < 0.0001) on d-3 for CO-Synch+CIDR than for PG-CO-Synch+CIDR (4.1 ± 0.2 and 3.4 ± 0.2 ng/mL, respectively). Follicle diameter on d-3 differed (P = 0.05) between PG-COSynch+ CIDR (13.4 ± 0.3 mm) and CO-Synch+CIDR (12.5 ± 0.3 mm) treatments. Cows with P4 > 2.5 ng/ mL on d-10 had greater (P = 0.024) pregnancy rate to TAI (56.5%) compared with cows with 2.5 ng/mL < P4 > 1 (43.0%), whereas cows with P4 < 1 ng/mL were intermediate (51.6%). Cows with a CL on d-10 had greater (P = 0.012) pregnancy rates to TAI than cows without a CL (66.3 vs. 39.4%, respectively). In Exp. 2, TAI pregnancy rates did not differ (P = 0.316) between treatments. Concentrations of P4 differed (P < 0.0001) on d-3 with greater concentrations of P4 for CO-Synch+CIDR than for PG-CO-Synch+CIDR (3.75 ± 0.20 ng/mL and 3.60 ± 0.21 ng/mL, respectively). Follicle diameter was similar (P = 0.749) between treatments on d-10 and-3. Regardless of treatment, cyclic status tended (P = 0.062) to improve pregnancy rates to TAI (55 vs. 45%, for cycling and noncycling heifers, respectively). We concluded that addition of PGF_2α to the 7-d CO-Synch+CIDR protocol decreased concentrations of P4 in cows and heifers and increased follicle diameter at CIDR removal in cows but failed to increase TAI pregnancy rates.