Effect of timing of Cosynch on fertility of lactating Holstein cows after first postpartum and Resynch timed-AI services

To compare two intervals from the PGF2α injection to the second GnRH injection + timed artificial insemination (TAI) of Ovsynch, lactating Holstein cows received their first postpartum TAI after Presynch + Ovsynch (n = 352) and second and greater postpartum TAI after resynchronization of ovulation using Ovsynch (Resynch; n = 458). Each week, cows housed in each of four breeding pens were randomized by breeding pen to receive the second GnRH injection of Presynch + Ovsynch or Resynch and TAI either 48 h (Cosynch 48; n = 382) or 72 h (Cosynch 72; n = 428) after the PGF2α injection of Ovsynch or Resynch. Overall, pregnancies per AI (P/AI) did not differ for cows receiving Cosynch 48 (29%) versus Cosynch 72 (33%). Furthermore, treatment did not affect P/AI for cows receiving first postpartum TAI after Presynch + Ovsynch, for cows receiving second and greater TAI after Resynch, or the proportion of female calves born. In conclusion, delaying the second GnRH injection and TAI from 48 to 72 h after the PGF2α injection of Ovsynch did not affect P/AI or calf sex ratio. The lack of a difference in fertility between these Cosynch protocols may offer more flexibility for implementing a systematic synchronization protocol when a Cosynch strategy is used.