Effects of Exogenous Progestogens on Reproductive Phenomena in Mares

Progesterone in oil solution injected into mares intramuscularly in midcycle blocked estrus and ovulation at levels of 100 mg. per day or higher. Fifty milligrams per day prevented estrus but not ovulation. The interval from end of treatment to estrus appeared to depend upon dosage, and the effects on subsequent cyclic patterns were confounded by season of the year. Neither 50 nor 100 mg. per day stopped estrus or blocked ovulation when treatment was started on day 1 of estrus. Injection of 100 mg. per day of progesterone in oil delayed postpartum estrus and ovulation, but 50 mg. per day did not. A single injection of 500 mg. of crystalline progesterone did not delay postpartum estrus and ovulation consistently. MAP administered orally to mares in midcycle or to mares showing erratic estrus in the early breeding season failed to inhibit estrus or ovulation at levels of from 400 to 1782 mg. per day. Melengestrol acetate was ineffective when fed at 10 or 20 mg. per day to cycling mares. Neither a single injection of 500 mg. of crystalline MAP nor daily injection of 100 mg. per day in oil solution was effective in blocking postpartum reproductive activity.