Evaluation of Gonadotropin-Releasing Hormone and Insemination Time Using the CO-Synch Protocol in Beef Cows

The objectives of this study were to evaluate optimal timing for timed AI (TAI) using a CO-Synch protocol with or without gonadotropin-releasing hormone (GnRH) at TAI and calf removal. Multiparous composite and Hereford suckling beef cows (n = 226) from two separate calving seasons (March and April or May and June) were synchronized for AI. Early- (ECC) and late-calving cows (LCC) were randomly assigned to one of four treatment groups for a 2 x 2 factorial arrangement of treatments. All cows were injected with 100 μg of GnRH on d 0 followed by 25 mg of prostaglandin F2α (PGF2α) on d 7. All calves were removed at time of PGF2α injection and returned to nurse at TAI. One-half of the cows were inseminated at 48 h post-PGF2α with (48-TAI-G) or without (48-TAI) GnRH and the other one-half were inseminated at 72 h post-PGF2α with (72-TAI-G) or without (72-TAI) GnRH. Pregnancy rates to TAI were not affected (P>0.10) by season; therefore, data from both ECC and LCC were combined. Body condition and postpartum interval did not influence fertility (P>0.10); however, TAI pregnancy rates increased (P<0.01) 20.5% for cows inseminated at 72 vs 48 h and increased 15.9% for cows receiving GnRH vs no GnRH (P<0.05) at TAI. Pregnancy rates to TAI differed (P<0.05) for cows inseminated to sire A (33.3%) and sires B and C (51.8 and 54.3%). We concluded that fertility levels were less from one of the three sires used. Delaying insemination 72 h after PGF2α plus the addition of GnRH at TAI improved pregnancy rates in suckled beef cows.