Effect of reproductive status on twinning and on side of ovulation and embryo attachment in mares

Brood-farm records were used to test several hypotheses concerning twinning and inequalities in the side of ovulation and embryo attachment. Ovulation occurred more frequently (P<0.05) from the left ovary (61% versus 39%) in maiden mares, but with equal frequency from either ovary in lactating and barren mares. The embryo attached more often (P<0.05) to the left horn in lactating mares (60%) and to the right horn in barren (59%) and maiden (67%) mares. Double ovulations and twin embryos were diagnosed more frequently (P<0.05) in barren mares (11% and 6%, respectively) than in lactating mares (5% and 1%). Pregnancy rates for double ovulations were affected (P<0.01) by the length of the interval between the two ovulations (0 days, 83%; 2 days, 87%; 4 days, 67%; 6 days, 54%; 8 days or more, 38%). Pregnancy rates were higher (P<0.01) for double ovulations 0 or 2 days apart (84%) than for single ovulations (54%). Twins were diagnosed more frequently (P<0.05) when double ovulations were two or more days apart (9 32 ) than when the ovulations were synchronous (0 19 ). These results support the conclusion that an embryo-reduction mechanism exists in mares for the elimination of excess embryos and indicate that the mechanism loses its effectiveness when the embryos originate from asynchronous ovulations.