Effect of Service Numbers on Resynchronization Responses in Lactating Dairy Cows during Warm Season

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= ۷۵۰) in five groups of ۱۵۰ with different service numbers according to the last service (AI۱= First service, AI۲= ۲nd, AI۳= ۳rd, AI۴= ۴th, AI≥ ۵) were assigned to receive Ovsynch۵۶ starting on day ۲۸ after fixed time AI (TAI). Ultrasonography was done during Ovsynch۵۶ injections and also for confirmation of pregnancy in all cows at ۳۲ and ۶۰±۴ days after fixed timed AI. Results showed proportion of cows that ovulated in response to the first GnRH injection of Ovsynch was greatest (P= ۰.۰۱) 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 PGF۲α of Ovsynch (۹۱.۸ vs. ۷۴.۲%, respectively) and finally greater conception rate (CR) at ۳۲ d after AI (۳۲.۱ vs. ۲۴.۶%, respectively) than those that did not ovulate. Conception rate at ۳۲ and ۶۰ days after AI was similar up to the third service (۲۹.۱ and ۲۱.۸% at days ۳۲ and ۶۰, respectively), but decreased after that (۱۹.۷۵ and ۱۰.۴% at days ۳۲ and ۶۰, respectively, P= ۰.۰۲). 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 ۲۰%.