Effect of GnRH pretreatment on reproductive performance of postpartum suckled beef cows following synchronization of estrus using GnRH and PGF(2 alpha)

The effect of GnRH pretreatment on estrus detection rate, precision of estr us, and reproductive performance of postpartum beef cows synchronized to es trus using GnRH and PGF(2 alpha) was evaluated. In Exp. 1, Angus cows (n = 87) were randomly assigned by parity, postpartum interval, and body conditi on score (BCS) to receive either 1) GnRH on d -7 and PGF(2 alpha), on d 0 ( GP) or 2) the GP treatment and an additional injection of GnRH on d -16 (GG P). Estrus detection and AI were conducted twice daily from d -3 to d 3. At 72 h after PGF(2 alpha), all animals not previously detected in estrus wer e bred by AI and received a concurrent injection of GnRH (TAI). Synchronize d pregnancy rates were numerically increased (P = 0.15) in cows treated wit h GGP (55%) compared with those on the GP treatment (44%). In Exp. 2, 1,276 spring-calving, suckled beef cows in nine herds were randomized to treatme nts as described for Exp. 1, except that the initial GnRH injection for the CTG;P treatment was administered on d -14. Herd affected all indicators of reproductive performance (P < 0.05). The percentage of animals detected in estrus prematurely (d -3 to d 0; 7%) was not affected by treatment. Estrus response rate was influenced by postpartum interval (< 60 vs greater than or equal to 60; 61 vs 73%; P < 0.01) and a three-way interaction of parity, BCS, and treatment (P < 0.01). Within animals with a BCS <greater than or equal to> 5.5, the GGP treatment tended to increase the detection of estrus in primiparous cows (GP vs GGP; 76 vs 91%; P = 0.11) and decrease detectio n in multiparous cows (GP vs GGP; 78 vs 72%; P < 0.10). However, because co nception rate to TAI in animals with a BCS greater than or equal to 5.5 was greater (P < 0.05) in the GGP than in the GP group (28 vs 8%, respectively ), this interaction was interpreted to represent a shift in interval to est rus induced by the GGP treatment, rather than a reduction in the synchroniz ation of ovarian function. Conception rates of animals inseminated to an ob served estrus did not differ among treatments (P = 0.15). Synchronized preg nancy rate tended (P = 0.06) to be greater in GGP- (53%) than in GP-treated animals (47%). In conclusion, pretreatment with GnRH tended to increase pr egnancy rates during a 6-d synchronization period, primarily through enhanc ed conception rates of cows bred by TAI. In contrast to our hypothesis, GnR H pretreatment did not increase the percentage of animals detected in estru s or the precision of estrus expression.