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Ability of Corpora Lutea Induced Experimentally in Progesterone-Treated Ewes to Prevent Subsequent Ovulation¹

University of Wisconsin, Madison

Abstract

The interval to next spontaneous estrus or ovulation was studied in three experiments when ovulation was induced experimentally during progesterone treatment at the end of the estrual cycle.

In the first experiment interovulatory intervals in six ewes treated with progesterone (10 mg. per day subcutaneously) on days 11 through 22 and given an ovulating hormone (2500 I.U. HCG or 8 gram-equivalents of unfractionated ovine pituitary extract) intravenously on day 19 averaged 8.8 days, while estrual cycle lengths in six contemporary control ewes averaged 17.5 days. This difference was significant.

In the second experiment additional treatments with either estradiol-17 β (500 mcg.) or electrical stimulation (in the rectogenital region) or both on day 19 (six ewes per group) failed to increase interovulatory intervals, as compared with a control group of six ewes treated exactly as in experiment I. Estradiol-17 β treatment was followed by estrus in five of six ewes, but in the group receiving electrical stimulation in addition this response did not occur.

In a third experiment six ewes received progesterone on days 11 through 21, and five ewes received progesterone on days 11 through 18. Both groups were given HCG intravenously on day 19. Short interovulatory intervals were observed in three of the six ewes receiving progesterone until day 21 and in none of the five ewes in which progesterone was stopped on day 18.

Progesterone, acting at the time induced corpora lutea are forming, may shorten this life span.

¹ Published with the approval of the Director of the Agricultural Experiment Station. Paper No. 973 from the Division of Genetics and No. 390 from the Department of Meat and Animal Science. This work was done under a cooperative agreement between the Wisconsin Agricultural Experiment Station and the Dairy Cattle Research Branch, U.S.DA., and was supported in part by the Branch and also by funds from a Ford Foundation Grant.

² Agent of the Dairy Cattle Research Branch, U.S.D.A.