Effect of Exogenous Progesterone on Estrous Cycle Length

Abstract

The effect of exogenous progesterone on estrous cycle length was studied in four species. Injections were begun on day of estrus in all animals. In ewes the injection of 25 mg. of progesterone per day for the first 6 days reduced average length of the estrous cycle from 16.5 days in control ewes to 12.7 days in treated ewes. In cattle 10 daily injections of 100 mg. of progesterone reduced estrous cycle length from 20.7 days in controls to 16.7 days in treated cows. Similarly, 2.5 mg. of progesterone per day for 6 days reduced the average estrous cycle length of guinea pigs from 16.3 days in controls to 15.0 days in treated guinea pigs. However, in swine there was no difference between average estrous cycle length of control gilts (20.7 days) and that of gilts treated with 200 mg. of progesterone per day for 10 days (22.8 days).