Identification of the Ovulating Follicle by Its Size on Different Days of the Estrous Cycle in Heifers

Abstract

The largest and the second to the largest follicles were marked in 24 heifers at various days of the estrous cycle to determine the proportion of heifers that ovulated a marked follicle at the next estrus. The ratio of number of heifers which ovulated the largest follicle to those which did not ovulate the largest follicle varied significantly among the 9 different days of the cycle studied. The probability of the largest follicle being the one which ovulated increased markedly beginning at day 18. Furthermore, the largest follicle as determined three days before estrus was the one that ovulated whereas prior to that time another follicle was the one that ovulated. The second to the largest follicle became the ovulating follicle in only three animals (days 15, 18 and 18 of the cycle). No pattern of ovulation occurrence for the second to the largest follicle was found. Interval since previous estrus was a better predictor of the time of the next estrus than the size of the largest follicle.