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 μg of GnRH (im) on day −7 and 25 mg of PGF2α (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 h were immediately inseminated in a once-daily observation/AI program. Cows not detected in estrus by 72 h after PGF2α received fixed-time AI (TAI) and a concurrent 100 μg 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 PGF2α (im) on day −35 and day −21 (PGF–PGF) or (2) 100 μg 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 μg of GnRH (im) on day −7, 25 mg of PGF2α (im) on day 0 and TAI at 60–64 h after PGF2α. Cows were randomized by parity and postpartum interval into pre- and post-synchronization treatments in a 2×2 factorial design. Pre-synchronization treatments included: (1) 25 mg of PGF2α (im) on day −35 and on day −21 (PGF–PGF; n=168) or (2) 25 mg of PGF2α (im) on day −21 and 100 μg 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 μg of GnRH (im) at either 48 h (Ovsynch; n=175) or 60–64 h (Cosynch; n=173) after PGF2α. 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–PGF2α-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.