Resumption of ovarian activity in postpartum of Chinese-Holstein cows

Ovarian activity in 10 postpartum Chinese-Holstein cows was monitored by ultrasonography every other day, stared from day 3 postpartum and finished by the end of a normal ovarian cycle. Meanwhile, the concentrations of progesterone and 17β-oestradiol in peripheral plasma of the cows were measured by radioimmunoassay (RIA). The results showed that the first postpartum ovulation was found in 67% of the cows on the ovary opposed to the gravid horn. The first dominant follicles ovulated for the six of ten cows. The time intervals from calving to the first ovulation were significantly associated with the number of follicular waves (r = 0.83, P<0.01). No difference between the diameters of the first and the second ovulated follicles was found in the study, however, the growth rates of the first and the second ovulated follicles were significantly different (P<0.05), and the concentrations of 17β-oestradiol prior to the first ovulation were significantly lower than that before the second ovulation.