Effect of synchronization protocols on reproductive indices, progesterone profile and fertility under subtropical environmental conditions in repeat breeder Holstein cows.

The objectives were to evaluate the reproductive indices and survival analysis of pregnancy outcomes in multiparous repeat breeder Holstein cows (n = 557). The cows were synchronized to ovulate by Ovsynch, new controlled internal drug release device (N-CIDRsynch), and once-used CIDR device (U-CIDRsynch). The pregnancy per AI at 28 days post-insemination (P/AI 28) in the N-CIDRsynch group (28.75%) was significantly (COR = 1.49; p = 0.011) greater than that reported in the Ovsynch (23.46%) and U-CIDRsynch (21.73%) groups. Furthermore, the pregnancy per AI at day 75 post-insemination (P/AI 75) in the N-CIDRsynch group was significantly greater than the Ovysync group (COR = 1.35; p = 0.050). The repeat breeder cows received a N-CIDR device had a significantly higher progesterone level on day 2 and day 4 of CIDR insertion (1.38 and 1.67 ng/ml, respectively) than those received a U-CIDR device or the control group (p = 0.012 and 0.001, respectively). The Cox regression model recorded significant associations for synchronization protocols, THI at the TAI and season of calving with the hazard of P/AI 28 and P/AI 75 (p = 0.044 and 0.046; 0.001 and 0.005; 0.003 and 0.001, respectively). Multiparous repeat breeder cows (>3) had a lower hazard ratio (HR) of P/AI 28 than that reported in the reference (2nd parity) group (HR = 0.74, p = 0.050). The repeat breeder cows inseminated at 76-80 and >80 temperature-humidity index (THI) had significantly lower HR of P/AI 28 than those inseminated at the baseline (<70) THI value (HR = 0.73 and 0.30, p = 0.036 and 0.001, respectively). The current results indicate that the use of N-CIDR synch protocol may achieve satisfactory pregnancy outcomes in repeat breeder cows.