The effect of embryo reduction and transfer on luteostasis in the mare

This study investigated the effects of embryo reduction and transfer of Day 11 embryos, with or without subsequent reduction, on luteostasis in the mare. In Experiment 1, reduction of embryos at Days 10 (n = 15), 11 (n = 47), 12 (n = 36), 13 (n = 27), 14 (n = 5) and 16 (n = 2) of pregnancy resulted in luteostasis in 13%, 47%, 78%, 89%, 80% and 100% mares. Mares undergoing > 1 embryo reduction showed consistency in when luteostasis occurred. In Experiment 2, transfer of Day 11 embryos to recipient mares 10 (n = 9), 11 (n = 8), 12 (n = 9) and 13 (n = 8) days post ovulation resulted in luteostasis in 78%, 87.5%, 78% and 37.5% of mares. Only 22%, 37%, 0% and 12%, respectively, of these mares remained pregnant. In the Day 10, 11 and 12 recipients luteostasis occurred on at least one occasion when an embryo was detected at 24 h but not at 48 h post transfer. In the Day 12 recipients luteostasis occurred on three occasions (3/9;33%) when the transferred embryo was not detected at 24 h. In Experiment 3 reduction of a Day 11 embryo 24 h after transfer to a Day 10 (n = 4), 11 (n = 6), 12 (n = 6) or 13 (n = 6) recipient resulted in luteostasis in 100%, 83%, 100%, and 83% of mares. All five Day 11 recipients that had an embryo reduced 12 h post transfer became luteostatic. These results suggest there is plasticity overall, but individual rigidity, in the timing of maternal recognition of pregnancy. Furthermore, an intact embryo need only be present in the uterus for 12 h to cause luteostasis.