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/A), medium (101 to 400 gd) and large (>400 gtl) 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. (