Ovulation and Its Relation to Estrus in Cows1

A total of 72 estrual periods on 22, grade dairy cows were studied. The following findings are reported: Ovulation occurred approximately 14 hours after the end of estrus. The mean time of day for estrus to end was between 8 and 9 o'clock in the evening. The mean time of ovulation was approximately 11 o'clock in the morning. Differences were detectable in the times of termination of estrus and of ovulation for cows first noted in heat in the afternoon and those first noted in heat in the morning. There is a slight indication that the interval from end of estrus to ovulation may be slightly shorter. Thirty seven per cent of the total variation in time of ovulation was attributable to differences between cows. The later the hour that heat ended the shorter the interval from the end of heat to ovulation (r = −0.46). The later the hour of ovulation the longer the interval from the end of heat to ovulation (r = 0.46). The later the hour that heat ended the later was the hour of ovulation (r = 0.57). A marked similarity exists in the means, the variation and the interrelations of the different phenomena associated with ovulation for the dairy cattle studied at the Wisconsin Station and the beef cattle studied at the Michigan Station.