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Effects of administration of prostaglandin F2α (PGF) at initiation of the 7-day CO-Synch+CIDR estrus synchronization protocol for replacement beef heifers
We determined the effect of administration of PGF at CIDR insertion during the 7-day COSynch+CIDR ovulation synchronization protocol on subsequent pregnancy rates of replacement beef heifers. At 10 locations, heifers were synchronized with the 7-day CO-Synch+CIDR protocol (100 μg injection of GnRH at CIDR insertion [d -10] with 25 mg injection of PGF at CIDR removal [d -3], followed by an injection of GnRH and TAI 54 h later on d 0). Heifers were stratified by BCS before being assigned to one of two treatments: 1) CO-Synch+CIDR (n = 498); and 2) heifers received a 25 mg injection of PGF at CIDR insertion of the CO-Synch+CIDR protcol (PG-CO-Synch+CIDR; n = 501). Follicle dynamics and corpus luteum development were assessed on d -10 and -3 and pregnancy status determined on d 30-35. Blood was collected on d -20, -10, -3, and 0 to determine P4. Overall fixed-time artificial insemination (TAI) pregnancy rates (54.0 ± 2.9% and 50.7 ± 2.9%, for CO-Synch+CIDR and PG-CO-Synch+CIDR, respectively) did not differ (P = 0.428) between treatments. A location effect (P< 0.001) existed with pregnancy rates being the greatest at the FL1 location (75.0 ± 8.1%) and the poorest at the MS1 location (28.6 ± 9.3%). Of the 521 heifers in which cyclic status was assessed 81% had attained puberty. No treatment x cyclic status interaction existed; however, pregnancy rates of heifers that had attained puberty (55.0 ± 2.5%) tended (P = 0.080) to be greater than those heifers that were prepubertal (44.2 ± 5.2%). A treatment x concentrations of P4 interaction existed where concentrations of P4 were greater (P < 0.001) for CO-Synch+CIDR (6.31 ± 0.40 ng/mL) than PG-CO-Synch+CIDR (4.63 ± 0.40 ng/mL) on d -3. Similarly, corpus luteum (CL) volume did not differ between treatments on d -10, but CL volume tended (P = 0.059) to be greater for CO-Synch+CIDR (2.24 ± 1.14 cm3) than PG-CO-Synch+CIDR (1.47 ± 1.14 cm3) on d -3. Diameter of the largest follicle on d -10 (10.7 ± 1.4 mm) and -3 (11.6 ± 1.4 mm) did not differ between treatments. We concluded that administration of PGF at CIDR insertion of the CO-Synch+CIDR protocol failed to increase TAI pregnancy rates in replacement beef heifers, but decreased concentrations of P4 and tended to decrease CL volume at CIDR removal.
Keywords: Ovulation Synchronization, Artificial Insemination, Beef Heifer