Control of estrus with prostaglandin F2alpha in mares: minimal effective dose and stage of estrous cycle.

To determine the minimal effective dose of prostagiandin (PGF2alpha; tromethamine salt) given subcutaneously (SC), mares of mixed breeding (400 kg av body weight) were given 2-, 3-, 5-, and 10-mg doses from 7 to 9 days after ovulation. In some but not all mares given doses of 2 and 3 mg of PGF2alpha, luteolysis occurred, but doses of 5 or 10 mg of PGF2alpha were luteolytic in all mares. The 10-mg dose of PGF2alpha did not cause luteolysis in mares 1 day after ovulation, and caused luteolysis in only 2 of 5 mares on day 3 after ovulation. The same dose of PGF2alpha, however, caused luteolysis in all mares on days 5 or 7 after ovulation. The results indicate that the minimal effective luteolytic dose of PGF2alpha (free-acid equivalent) is about 9 mug/kg, and that PGF2alpha is effective fromday 5 after ovulation.