Season effect on follicular dynamic and ovulation time were studied on 25 non-lactating Gir cows (10 cows in the winter and 15 in the summer) and 15 non-lactating Nelore cows (7cows in the winter and 8 in the summer) exhibiting normal estrus cycles. After estrus detection cows were submitted to ultrasound examination every six hours to determine the time of ovulation, and after that, the examinations were performed once-a-day throughout the estrus cycle for each season. No effect of breed and season on follicular characteristics was observed. The follicular dynamic was characterized by the predominance of three (68.0 and 66.7%) and two (24.0 and 26.7%) follicular waves for Gir and Nelore cows, respectively. The estrus-to-ovulation intervals were shorter for cycles with two follicular waves. Most estrus signs were detected in the morning (57.3%) and late afternoon (28.0%). On average, Gir and Nelore cows ovulated 23 hours after estrus onset.
cow; zebu; follicle; ovulation; season