Disruption of estrous behavior in ewes by dexamethasone or management-related stress

The effect of dexamethasone administration, isolation stress, or transportation stress on the ability of exogenous estradiol to induce estrus in progesterone-primed, ovariectomized ewes was evaluated in this study. Dexamethasone administered twice daily over a 6-d period, or a single injection at either 2 h before estradiol administration or 8 h after estradiol treatment, delayed or blocked the expression of estrous behavior. In those animals in which dexamethasone did not block the onset of estrus, the average length of estrus was reduced. Isolation stress, which induced significant increases in the plasma concentration of corticosteroids, failed to significantly alter the number of ewes expressing estrous behavior, although the expression of estrus was blocked in three of the nine treated animals. These nine animals all expressed estrus during the nonstress phase. In contrast to isolation stress, 8 h of transportation not only increased the average plasma concentration of corticosteroids, but also significantly (P less than .05) blocked the expression of estrus in five of eight ewes and delayed the expression of estrus in one other. During the nonstressed control phase, all eight expressed estrus. These findings indicate that management-related stress can block estrogen from inducing estrous behavior, however, the role of the adrenal axis response to stress in blocking estrus remains to be clarified.