Parity affects the relationship between the insemination-ovulation interval and the conception rate in lactating dairy cows.

The aim of this study was to investigate the relationship between parity and the optimal insemination-ovulation interval (IOI) in lactating dairy cows, as well as the relationship between the optimal timing of artificial insemination (AI) based on the onset of oestrus and parity. A total of 183 postpartum lactating Holstein cows participated in the study and underwent 522 AI procedures using unsorted frozen semen between 2011 and 2018. Transrectal ultrasound was used to detect ovulation times in 326 AI procedures, and a pedometer was used to detect oestrus onset times in 470 AI procedures. An analysis was also performed on data from 265 AI procedures in which all the times of oestrus onset, oestrus end and ovulation were detected. The results showed that ovulation occurring 6-12 h after AI was associated with the numerical highest conception rate in multiparous cows. Moreover, the conception rate was higher when ovulation occurred between 6 and 12 h after AI than when it occurred between 0 and 6 h or 18 and 24 h after AI. In contrast, the effect of IOI on the conception rate in primiparous cows was insignificant. There was no effect of AI timing based on the oestrus onset on the conception rate in either primiparous or multiparous cows. Furthermore, the interval from oestrus onset to ovulation was similar in primiparous and multiparous cows. The difference in optimal IOI between primiparous and multiparous cows may be due to the larger uterus size in multiparous cows. These findings suggest that the optimal timing for AI in multiparous cows is more restricted than in primiparous cows. Therefore, it is essential to carefully determine the timing of AI when it is performed in multiparous cows.