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

Abstract

AbstractThe 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 25mg 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 48h post-PGF2α with (48-TAI-G) or without (48-TAI) GnRH and the other one-half were inseminated at 72h 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 48h 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 72h after PGF2α plus the addition of GnRH at TAI improved pregnancy rates in suckled beef cows.