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Alteration of the Bovine Estrous Cycle with Oxytocin¹

Department of Animal Husbandry, Cornell University, Ithaca, New York

ABSTRACT

The daily administration of natural or synthetic oxytocin to post-puberal nonlactating dairy heifers, during the first week of the estrous cycle, resulted in marked shortening of the diestrual period, the next heat occurring from eight to 12 days after the previous estrus, instead of the usual 21 days. These precocious heat periods appeared normal in every respect, being followed in all cases by ovulation at the expected time. In all cases, the subsequent diestrual periods were of normal lengths.

The corpora lutea formed during the oxytocin treatments failed to attain the expected size and contained a decreased number of normal luteal cells and an increased proportion of connective tissue elements. Concurrent injections of prolactin failed to overcome the diestrum-shortening effect of oxytocin. Daily progesterone injections prevented the oxytocin-treated heifers from returning to estrus until from four to six days after cessation of the progesterone injections. It was concluded that the observed effects of oxytocin were caused by inhibition of normal corpus luteum function, possibly by interfering with the secretion of a luteotrophic hormone from the anterior pituitary.

Oxytocin was less effective in causing diestrual shortening when either atropine or reserpine was administered concurrently, and was completely ineffective in inducing precocious estrus in hysterectomized, but not in sham-operated, heifers.

¹ The data in this paper are, in part, from a thesis submitted by the senior author to the Graduate School of Cornell University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Supported in part by funds from the regional project N.E.-1, "Causes and Prevention of Reproductive Failures in Dairy Cattle," a cooperative study by agricultural experiment stations in the northeastern region and the Dairy Husbandry Research Branch, U. S. Department of Agriculture.

² Present address, Department of Biology, Brookhaven National Laboratory, Upton, L. I., New York.

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