The precise time of ovulation was determined by an ultrasonic technique using a 5 MHz scanner 480 VET (Pie Medical, Colorado, USA). Mares were checked twice a day,starting from the first observation of behavioral estrus until rupture of an ovarian follicle was registered. Ovulation time was assigned as the time when half the interval had elapsed between an ultrasonic assessment where the ovulatory follicle was still present and the time the rupture of a follicle was first registered.