Alteration of Ovarian Activity in Cycling Beef Heifers with Small Daily Injections of Estradiol

Daily injections of 20, 40, 80, 160, 320 or 640 mcg. of estradiol-17β were administered to cycling beef heifers beginning on day 6 of the estrous cycle. Corpus luteum weight, progestin content, progestin concentration and follicular fluid weight at day 12 tended to be lower in the heifers injected with the higher levels of estrogen than in the control heifers. Similar estrogen treatment given daily from days 6 to 18 reduced luteal and follicular activity. Heifers receiving the higher levels of estradiol showed the greatest reduction. There was no evidence obtained in this study to support the hypothesis that low levels of estrogen have a differential effect on the hypophysis of the heifer. In general, all levels of estrogen administered reduced all measures of ovarian activity taken and in no case was a stimulation of ovarian activity apparent.