Follicular Development during Four Stages of the Estrous Cycle of Beef Cattle

Estrogen and progestin concentrations in plasma and follicular fluid (FF) of follicles within the small (5 to 100 µl), medium (101 to 400 µl) and large (>400 µl) range, numbers of follicles and volume of FF per follicle within these ranges were determined in beef heifers. There were differences (P<,01) in mean concentrations (±SE) of steroids among and within follicles from the small, medium and large ranges during the estrous cycle. Concentrations of progestins were highest in follicles within the small range (178 ± 12 ng/ml); whereas, concentrations of estrogens were highest in follicles in the large range (55 ± 8 ng/ml). Mean ratio of progestins: estrogens was 12:1 in small follicles, 4:1 in medium follicles and 2:1 in large follicles. Volumes of FF from follicles within the small, medium and large ranges varied (P<.01) throughout a cycle. Mean volumes of FF from follicles in the medium and large range increased 130% and 180%, respectively, as time of ovulation neared. In contrast, mean volumes of FF from follicles within the small range decreased 170%. Total numbers of follicles per pair of ovaries and heifers with follicles in the large range increased (P<.01) toward the end of the estrous cycle.