Factors affecting follicular emergence at 10 days postpartum in lactating dairy cows.

Several factors influence postpartum ovarian activity in dairy cows. Our previous research confirmed a significant seasonal effect on the occurrence of early ovulation until 26 d postpartum, yet the relationships with earlier follicular development were not examined. This retrospective study aimed to describe the association between follicular development at 10 d postpartum and the presence of early ovulation in 536 lactations. Furthermore, the factors affecting the emergence of ≥15 mm (LL-size) of follicles at 10 d postpartum were assessed, including calving season, parity of dams, BW loss, period of years, number of calves, and uterine abnormality. Approximately 12% of cows lacked ≥10.0 mm follicles at 10 d postpartum and had a lower frequency of early ovulation (32.3%). Early ovulation was observed more in cows with at least one LL-size but no 10.0 to 14.9 mm follicles (69.7%). The odds ratio of LL-size follicle presence in the cows with high uterine abnormalities was significantly lower than in the cows with low uterine abnormalities (odds ratio: 0.20, 54.0% vs. 83.5%). Fewer <10.0 mm follicles were detected in the former than the latter. The cows that calved in summer had a higher frequency of LL-size follicles than those that calved in winter and spring. The effects of BW loss and dam parity on the presence of LL-size follicles approached significance. In conclusion, calving season, BW loss, and dam parity influenced the likelihood of LL-size follicle emergence. In particular, an abnormal uterine status 10 d after calving inhibited early follicular development, which might affect early ovulation.