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Ovulatory Response of the Ewe to Repeated Injections of PMS¹

U. S. Department of Agriculture

Abstract

Mature Rambouillet, Targhee and Columbia range ewes were given four injections of 1,000 I.U. of PMS. The first and third injections were given following estrous synchronizations with progesterone. The second and fourth injections were given 16 days after the first and third injections, respectively. Varying intervals of time were allowed to elapse between the first two injections which were at 16-day intervals and the last two injections (33, 74, 166 and 340 days). The ovulatory response in these ewes following the last two injections of PMS was compared with the response in ewes having no previous history of PMS treatment.

The experiment was repeated a second year with certain modifications. The treated ewes were given a total of six or seven injections of PMS, the number of refractory recovery periods was reduced to three (16, 136 and 319 days) and an additional control group was added which received no PMS. A total of 279 ewes were included in both studies.

A comparison of the percent of ewes ovulating and the mean ovulation rate following initial PMS injections, and after short and long PMS-refractory recovery periods, and a further comparison with the ovulation rate in injected and uninjected control ewes indicates that the ewe rapidly develops a refractory reaction. This reaction appears to gradually dissipate with time. After approximately 1 yr. the response was intermediate between the uninjected control and the PMS injected controls.

A highly significant increase in ovulation rate following the second PMS injections (first vs. second) was believed to be due to the influence of exogenous vs. endogenous progestogen influence immediately prior to ovulation. PMS given at 16-day intervals following estrous synchronization appears to have a leuteotrophic effect on those CL which have not started to degenerate at the time of the second PMS treatment.

¹ U. S. Sheep Experiment Station, Dubois, Idaho in cooperation with the University of Idaho, Moscow and from Utah State University, Logan. Published as Utah Agricultural Experiment Station Research Paper No. 802.

The authors express appreciation to Eli Lilly Company for the PMS, and to the Upjohn Company for MAP used in this study.