EMBRYO-TRANSFER IN THE MARE - A STATUS-REPORT

Embryo transfer in the horse has progressed enormously since the first successful transfers in the early 1970s. Currently, the embryo recovery rate in young, fertile mares is 55-80% and the pregnancy rate after transfer is about 70%. Recovery rates, pregnancy rates and pregnancy maintenance in recipients are lower for embryos from subfertile mares. There is little repeatable difference in pregnancy rate between surgical and transcervical transfer. Recipient mares have the best pregnancy rates if they ovulate from 1-2 days before to 2-3 days after the donor. Ovariectomized mares treated with progesterone in synchrony with donor ovulation may also be used successfully as embryo recipients. Embryo recovery rates are essentially doubled when mares spontaneously double ovulate; however, mares do not respond significantly to superovulation regimes such as administration of pituitary extract, pulsatile gonatrophin releasing hormone (GnRH) (in anestrous mares) or immunization to inhibin, all of which result in an average of only three follicles ovulated per cycle. Twins can be created by bisection of equine embryos but the success rate is low, probably because of interference by the capsule during manipulation. Similarly, while small embryos freeze well, freezing of large equine embryos (those with capsules) results in a poor pregnancy rate. Equine embryos may be stored for 24 h at 4 degrees C with no loss in viability. Intensive gamete manipulations such as in vitro oocyte maturation and fertilization are in their infancy; however, there have been foals produced by transfer of oocytes to the oviducts of recipient mares and by in vitro fertilization. Blastocysts have been produced by transfer of in vitro matured equine oocytes to the oviducts of recipients and by transfer of immature oocyte to the preovulatory follicles of mares.