TURNOVER OF DOMINANT FOLLICLES PRIOR TO 1ST OVULATION AND SUBSEQUENT FERTILITY IN POSTPARTUM DAIRY-COWS

Content: The ovaries of 64 multiparous Holstein cows were scanned twice weekly by ultrasonography from Day 6 postpartum until 60 days or return to normal estrous cycle, and turnover of dominant follicles in relation to subsequent fertility were determined. The mean (+/-SD) number of days from parturition to detection of the first postpartum dominant follicle (greater-than-or-equal-to 10 mm) was 9.0 +/- 0.5, which was predominantly located in the ovary contralateral to previous pregnancy in 54/64 cows. The first postpartum dominant follicle ovulated in 34 cows,failed to ovulate in 18, became cystic in 10, and only 2 were diagnosed as ovarian quiescent which delayed the detection of dominant follicles until days 25 and 30 postpartum. Number of dominant follicles in cows with anovulated first dominant follicle was 2.8 +/- 1.1 prior to first ovulation (range 2-5). Growth and regression of cystic follicles were observed 2-4 times in 6/10 cows, and a total of 20 cystic follicles persisted 32.7 +/- 9.0 days (range 18-58 days) except one which persisted for 124 days. Conception rate was 89.5% and postpartum interval to conception within 180 d after calving was 96.7 days in cows detected with less than 3 dominant follicles before first ovulation compared with those of 42.9% and 141 days in cows detected with more than 4 dominant follicles. It is concluded that the number of dominant follicles detected prior to first ovulation varied from 1 to 5, which positively correlated with ovulation intervals, while only those cows which first ovulated less than 3 dominant follicles resulted in reasonable reproductive performance.