Effects of pre-synchronization using combinations PGF2α and (or) GnRH on pregnancy rates of Ovsynch- and Cosynch-treated lactating Holstein cows

In Experiment 1, the effects of two pre-synchronization treatments on synchronized AI pregnancy rates of lactating dairy cattle were compared. Lactating Holstein cows (n = 159) received 100 mug of GnRH (im) on day -7 and 25 mg of PGF(2alpha) (im) on day 0 and were observed once daily for signs of estrus from day -3 to day 3. Cows detected in standing estrus and those that had lost significant amounts of tail-chalk in the previous 24 It were immediately inseminated in a once-daily observation/AI program. Cows not detected in estrus by 72 h after PGF(2alpha) received fixed-time AI (TAI) and a concurrent 100 mug injection of GnRH (im). Cows were randomly assigned by parity and calving date to receive one of the following pre-synchronization treatments: (1) 25 mg of PGF(2alpha) (im) on day -35 and day -21 (PGF-PGF) or (2) 100 mug of GnRH (im) on day -14 (GnRH). Fewer (P < 0.05) GnRH- (49%, 41/84) than PGF-PGF-pretreated cows (65%, 49/75) were detected in estrus, however, overall pregnancy rates were not affected by pre-synchronization treatment (30 versus 32%, respectively). In Experiment 2, lactating Holstein cows received 100 mug of GnRH (im) on day -7, 25 mg of PGF(2alpha) (im) on day 0 and TAI at 60-64 h after PGF(2alpha). Cows were randomized by parity and postpartum interval into pre- and post-synchronization treatments in a 2 x 2 factorial design. Pre-synchronization treatments included: (1) 25 mg of PGF(2alpha) (im) on day -35 and on day -21 (PGF-PGF; n = 168) or (2) 25 mg of PGF(2alpha) (im) on day -21 and 100 mug of GnRH (im) on day -14 (PGF-GnRH; n = 180). Within each pre-synchronization treatment, cows were further allocated by parity and postpartum interval to receive as a post-synchronization treatment 100 mug of GnRH (im) at either 48 h (Ovsynch; n = 175) or 60-64 h (Cosynch; n = 173) after PGF(2alpha). Pregnancy rates at TAI were not affected by pre- (PGF-PGF = 26%, 44/168 versus PGF-GnRH = 24%, 44/180) or post-synchronization treatments (Ovsynch = 29%, 50/175 versus Cosynch = 22%, 38/173). However, the numeric shift towards reduced pregnancy rates in Cosynch-treated cows suggests the 12 h interval between GnRH and AI may be important to optimize conception rates in GnRH-PGF(2alpha)-based TAI protocols in dairy cattle. In conclusion, each of the pre-synchronization protocols evaluated in present study performed with comparable efficacy. Although the Cosynch protocol facilitates more efficient labor utilization, numeric trends toward reduced conception warrants further investigation. (C) 2003 Elsevier Science B.V. All rights reserved.