The study tested a breeding schedule in beef cows that used a GnRH hormone and a progesterone device early after giving birth. It found that this early‑resynchronization approach actually lowered pregnancy rates and delayed calving compared with letting cows breed naturally later on.

This study evaluated the impact of a GnRH and progesterone-based early resynchronization strategy using color Doppler ultrasonography (CD) on the reproductive performance of Bos taurus beef cows. Postpartum cows (n = 419) were randomly assigned to one of two treatments: 1) Natural Service (NS): Cows were exposed to fixed-time artificial insemination (FTAI1; day 0) followed by natural service breeding for the remainder of the breeding season, or 2) Early Resynchronization (ER): cows received an intravaginal progesterone device (CIDR) on day 15, which was removed on day 20. On the same day, CD was used to diagnose pregnancy based on luteal blood perfusion. Non-pregnant ER cows received prostaglandin F_2α and were artificially inseminated concurrently with a GnRH injection 60-66 h after CIDR removal as their second service (day 23). Natural service breeding occurred for the remainder of the breeding season. Pregnancy rates to FTAI1 were similar (P = 0.13) between treatments; however, pregnancy rates to the second service were greater in NS cows (P < 0.01). Consequently, ER decreased (P < 0.01) cumulative pregnancies within the first 23 days of the breeding season and delayed calving in the subsequent calving season (P < 0.01). Finally, cows that received a false positive based on CD calved later compared with cows that became pregnant to FTAI1 (P < 0.01) and cows that failed to become pregnant to FTAI1 (P < 0.01), regardless of treatment. In summary, the ER strategy utilized herein as a second service strategy decreased reproductive performance compared with natural service.