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

Abstract 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 F 2α ( PGF 2α ) on d 7. All calves were removed at time of PGF 2α injection and returned to nurse at TAI. One-half of the cows were inseminated at 48 h post-PGF 2α with ( 48-TAI-G ) or without ( 48-TAI ) GnRH and the other one-half were inseminated at 72 h post-PGF 2α 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 2α plus the addition of GnRH at TAI improved pregnancy rates in suckled beef cows.