Absence of Any Specific Diurnal Pattern in the Time of Ovulation in the Mare

Abstract

This study investigates a long-held belief that mares tend to ovulate during the night. If this were so, a strategic examination regimen might replace multiple daily examinations when the time of ovulation needs to be known to within a few hours. Over 500 mares were examined at least three times daily to determine the time, to within ±4.25 hours, of 896 ovulations. The time of ovulation was determined as midway between the pre-ovulatory and post-ovulatory examinations. Three broad peaks of incidence of ovulation occurred at 5:00 am, 2:00 pm, and 10:00 pm. The interval between peaks was 9, 8, and 7 hours. during which 34.2% (5:00 am to 2:00 pm), 44.3% (2:00 pm to 10:00 pm), and 21.3% (10:00 pm to 5:00 am) of ovulations occurred. The percentage of ovulations occurring in the period 2:00 pm to 10:00 pm was significantly higher ( P < .001) than in the period 10:00 pm to 5:00 am and significantly higher ( P < .01) than in the period 5:00 am to 2:00 pm. The data showed that mares ovulated at all times of the day and night, with some increased incidence between 2:00 pm and 10:00 pm. Therefore, no strategic examination regimen was possible.