Predicting when the mare will ovulate is one of the key skills we need to hone as veterinarians working with mares. Regardless of the breeding system used (natural cover, fresh semen, chilled semen or frozen semen) it is vital we breed the mare at the correct time. We have two major tools to enable us to do this. Palpation and ultrasound of the follicle. Follicular size is important, but only during the first days of the follicular phase. When we are within 36 to 48 hours of ovulation, size is too variable to be the crucial parameter and we must use changes in the ultrasonographic appearance of the follicle.