Abstract
The ovaries of 10 Japanese black cows were examined by ultrasound twice weekly from Day 6 to Week 10 postpartum. The mean (± S.D.) number of days from calving to detection of the first dominant follicle was 10.7 ± 3.9, and the first postpartum dominant follicle ovulated in half of the cows (5/10). The interval from calving to first ovulation (25.8 ± 21.0 days) was characterized by the growth and regression of various dominant follicles (2.2 ±1.5; range 1-6). Estrous behavior was not detected at the first ovulation but was detected in 7/10 cows at the second ovulation. Following the first ovulation, the subsequent cycle length shortened (15.5 days) in 2/10 cows, remained normal (19.7 days) in 3/10 cows, and lengthened (46.2 days) in 5/10 cows. The growth rate and maximum diameter of ovulatory dominant follicles during follicular development were greater than in non-ovulatory atretic follicles prior to the first and third ovulations (P<0.01). These data show that in beef suckler cows, follicular development and dominant follicle formation occur soon after calving, but the incidence of ovulation of the first dominant follicle and occurrence of estrus at the first ovulation are low.
