[Objective] To investigate the effects of interval between calving and first ovulation on ovarian cycles postpartum and conception rate in high-producing dairy cows. [Methods] Days of first ovulation postpartum and subsequent ovarian cycles until cows diagnosed pregnant were monitored by milk progesterone profiles in a commercial dairy herd with 60 Holstein-Friesian cows with average milk yield per cow per lactation of 10,200kg. Incidence of abnormal ovarian cycles was calculated as percentage of abnormal cycles in total cycles during an observation period in each cow. [Results] Of 53 cows examined, percentages of cows which ovulated 1-14 d, 15-20 d, 21-35 d, 36-60 d, and >60 d postpartum were 28.3%, 26.4%, 30.2%, 13.2 %, and 1.9%, respectively. Incidences of abnormal ovarian cycles during pre-service and service periods in 45 cows with early (1-35d) first ovulation and 8 cows with late (U>/U36d) first ovulation were 33.9% and 36.2%, respectively. Percentages of ovarian cycles with prolonged luteal phase were 22.3% in cows with early first ovulation and 13.8% in those with late first ovulation. First AI conception rate and pregnancy rate within 100 d postpartum in cows with early first ovulation were higher than cows with late first ovulation (26.5% vs 0% and 28.9% vs 0%, P<0.01). [Conclusion] Interval between parturition and first ovulation postpartum did not adversely affect subsequent ovarian cycles. However, cows with early first ovulation tended to have a higher incidence prolonged luteal phase than those with late first ovulation. Delayed first ovulation postpartum adversely affected reproductive performance.
