Effects of Ovariectomy during Pregnancy and of Prematurely Induced Parturition on Progesterone, Estrogens, and Calving Traits

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Abstract
Cows (7) and 1 heifer were ovariectomized bilaterally on day 218 of gestation. Intact peers (12) served as controls until day 266 when 6 were treated with 25 mg of estradiol-17.beta. to induce birth prematurely. Profiles of ovarian steroids in blood plasma and urine under experimental conditions known to cause dystocia and retention of fetal membranes more than 12 h were compared. Ovariectomy caused an immediate decrease in progesterone of blood plasma to less than 1 ng/ml in 4 days. Estrogens in plasma and urine were not changed significantly from controls at similar stages of gestation except for 2 ovariectomized cows that calved at 251 and 257 days of gestation. In the latter cows, urinary excretion of total estrogen increased to greater than controls coincident with decreased progesterone and about 5 days prior to any increase in estrogens of plasma. Among the remaining ovariectomized cows, 2 aborted on days 222-232 and 4 calved between days 269-278. Four of 8 ovariectomized cows had dystocia, and all retained the fetal membranes as did the 6 peers induced to calve prematurely on day 268 of gestation or 42 .+-. 2 h after treatment with dexamethasone and estradiol-17.beta.. Progesterone increased gradually from .8 ng/ml on day 222 of gestation to over 1.5 ng/ml in ovariectomized cows that did not calve prior to day 251. Because both groups of treated cows calved earlier in gestation, they averaged lower in estrogens prepartum than control cows. Fetal membranes may be retained if the circulating estrogens, principally estradiol-17.beta. and estrone, remain low until at least 4 days prepartum. If progesterone in plasma is chronically low for several wk prepartum as in the ovariectomized cows, dystocia may occur.