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Some Effects of Progesterone on Estrous Behavior and Fertility in the Ewe¹

Utah State University, Logan

Abstract

Seventy mature, white-faced ewes were divided at random into four groups as follows: group 1 served as controls and received no treatment, and groups 2, 3, and 4 received 17 daily intramuscular injections of 10 mg. progesterone dissolved in corn oil. Injections were started on the same day for all ewes regardless of stage of cycle. One treatment group each was bred at the first, second, and third post-treatment estrous periods. Breeding in the control group was begun at the same time as the first treatment group. Each ewe was bred to two or more rams of high fertility.

Progesterone treatment was effective in synchronizing estrus. Ovulation occurred in all ewes during the estrus just preceding autopsy. Progesterone had no significant effect on post-treatment estrous periods. The lengths of time required for all of the animals in the various groups to come into estrus during the post-treatment estrous periods were 16.5 days for the control group, and 4.0, 3.5 and 6.0 respectively, for animals in the first, second and third post-treatment periods. Progesterone did not affect size or number of follicles or ovulation rate. The incidence of abnormal ova was significantly greater in the group bred at the first post-treatment estrus than in the other groups (43.5, 16.1, 4.4 and 8.3% for the first, second and third post-treatment estrous groups and the control groups, respectively). Fertility of the ewes bred at the first post-treatment estrus was significantly lower (28.0%) than that of the ewes in the control group (66.7%) or those of the treated groups at the second (67.9%) or third (86.4%) post-treatment estrous periods. The high incidence of abnormal ova was interpreted to pose a barrier to fertilization. Nonsignificant correlations were found between number of days under the influence of progesterone (endogenous plus exogenous) and interval from the end of injection to onset of estrus (0.19), ovulation rate (–.05), and percent fertility (0.20).

¹ Published as paper No. 349 from the Department of Animal Husbandry with the approval of the Director of the Agricultural Experiment Station. Supported in part by NIH Grant No. GM-06949.

² Present address: Department of Animal Husbandry, University of Missouri, Columbia, Missouri.