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Our objective in this study was to consider the effect of service numbers on the percentage of lactating cows that responded to the resynchronization and fertility during warm season. Lactating dairy cows (n= 750) in five groups of 150 with different service numbers according to the last service (AI1= First service, AI2= 2nd, AI3= 3rd, AI4= 4th, AI≥ 5) were assigned to receive Ovsynch56 starting on day 28 after fixed time AI (TAI). Ultrasonography was done during Ovsynch56 injections and also for confirmation of pregnancy in all cows at 32 and 60±4 days after fixed timed AI. Results showed proportion of cows that ovulated in response to the first GnRH injection of Ovsynch was greatest (P= 0.01) in the first service cows compared with other services. In addition, cows that ovulated in response to the first GnRH of Ovsynch had greater response to PGF2α of Ovsynch (91.8 vs. 74.2%, respectively) and finally greater conception rate (CR) at 32 d after AI (32.1 vs. 24.6%, respectively) than those that did not ovulate. Conception rate at 32 and 60 days after AI was similar up to the third service (29.1 and 21.8% at days 32 and 60, respectively), but decreased after that (19.75 and 10.4% at days 32 and 60, respectively, P= 0.02). In conclusion, results of this study showed service numbersaffected the proportion of cows that responded to resynchronization protocol. Conception rate was reasonable up to the third service, but, after the third breeding, proportion of cows that responded to resynchronization decreased and CR dropped below 20%.

Keywords: Conception rate, Ovsynch, Pregnancy loss, Ultrasonography

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