The use of early pregnant mares as embryo recipients

Fourteen normal, cyclic mares, treated to synchronise oestrus and ovulation and inseminated artificially with fresh semen, were assigned to a donor or a recipient group after ovulation, with the aim of obtaining a degree of synchrony of > or =2 days. Ten embryos, collected on Day 6 or 7 after ovulation (Day 0), were transferred nonsurgically to inseminated recipient mares (IRM) that had ovulated up to 5 days after the respective donors, or to pregnant recipient mares (PRM) that had ovulated 2-7 days before the donors. Embryonic size and development, as determined by ultrasound examination, were used to distinguish embryos derived from the recipient (recipient embryo = RE) or from the donor (transferred embryo = TE) mare. In cases of twin pregnancy, the RE was manually squeezed on Days 14-16. Abortion was induced in all mares on Day 30. Three of 6 TE developed in IRM. Two of 6 IRM developed a twin pregnancy (RE+TE), while 4 of 6 IRM developed a singleton pregnancy (1 TE and 3 RE). None of 4 TE developed in Day 9-14 PRM and one of these PRM lost her own embryo following ET. The experiment demonstrated that a mare can carry her own embryo and a transferred embryo simultaneously. However, the status of pregnancy does not improve conception rate in recipient mares that ovulate prior to the donor.