Intrauterine embryo reduction in the mare

The site of a hypothesized embryo reduction mechanism was studied. The number of normal-sized embryos and undersized embryos (>2 standard deviations below the mean embryo diameter in control mares) was determined at day 7 and 11 post-ovulation in single-ovulating control mares and in multiple-ovulating pituitary extract-treated mares in which all ovulations occurred on a single day. In two additional groups (control and treated), the embryonal enlargement was monitored by rectal palpation until day 49. An average of 0.7 normal-sized embryos per control mare at day 7 and at day 11 and 0.6 vesicles per control mare at day 49 was found. More normal-sized embryos (P<0.05) were recovered per extract-treated mare at day 7 (2.9 embryos/mare) than at day 11 (0.7), and fewer undersized embryos per extract-treated mare (P<0.05) were recovered at day 7 (0.1) than at day 11 (1.1). The total number of embryos (normal-sized plus undersized) per treated mare was greater (P<0.05) at day 7 and at day 11 than the number of palpated vesicles per treated mare at day 49. The number of mares with more than one normal-sized embryo in the day 7 treated group (10/14) was greater (P<0.05) than in the day 11 treated group (1/14) and was greater than the number of mares with more than one palpated vesicle in the day 49 treated group (2/14). Intrauterine reduction was therefore manifested between days 7 and 11 in multiple-ovulating mares, as demonstrated by the number of multiple normal-sized blastocysts recovered at day 7 and by the reduced number of normal-sized and the increased number of undersized blastocysts recovered at day 11.