Ovarian response, pregnancy rate, and incidence of multiple fetuses in mares treated with an equine pituitary extract.

The ability of a pituitary extract to induce ovulation and the effect of multiple ovulations on pregnancy rate and incidence of multiple fetuses were studied in seasonally anovulatory mares. The extract induced ovulation in 95% of mares having a greater than or equal to 25 mm follicle at the onset of treatment and in 64% the ovulations were multiple. The extract was less effective when the largest initial follicle was less than 25 mm. Approximately 50% of the treated mares that ovulated did not become pregnant or enter prolonged dioestrus before returning to an anovulatory condition. In treated mares with one ovulation the pregnancy rate (33%) was lower than in controls (60%) and was further reduced in mares with greater than 2 ovulations. There was a pronounced discrepancy between the number of palpated conceptuses and number of palpated ovulations. Only one conceptus and one fetus was found on Days 28 and 50, respectively, in every pregnant mare (11 treated, 1 control) in which multiple ovulations were palpated.