The current status of equine embryo transfer

Abstract

The use of embryo transfer in the horse has increased steadily over the past two decades. However, several unique biological features as well as technical problems have limited its widespread use in the horse as compared with that in the cattle industry. Factors that affect embryo recovery include the day of recovery, number of ovulations, age of the donor and the quality of sire's semen. Generally, embryo recoveries are performed 7 or 8 d after ovulation unless the embryos are to be frozen, in which case recovery is performed 6 d after ovulation. Most embryos are recovered from single-ovulating mares. Because there is no commercially available hormonal preparation for inducing multiple ovulation in the horse, equine pituitary extract has been used to increase the number of ovulations in treated mares, but FSH of ovine or porcine origin is relatively ineffective in inducing multiple ovulation in the mare. Factors shown to affect pregnancy rates after embryo transfer include method of transfer, synchrony of the donor and recipient, embryo quality, and management of the recipient. One of the major improvements in equine embryo transfer over the last several years is the ability to store embryos at 5°C and thus ship them to a centralized station for transfer into recipient mares. Embryos are collected by practitioners on the farm, cooled to 5°C in a passive cooling unit and shipped to an embryo transfer station without a major decrease in fertility. However, progress in developing techniques for freezing equine embryos has been slow. Currently, only small, Day-6 equine embryos can be frozen with reasonable success. Additional studies are needed to refine the techniques for freezing embryos collected from mares 7 or 8 d after ovulation. Demand for the development of assisted reproductive techniques in the horse has increased dramatically. Collection of equine oocytes by transvaginal, ultrasound-guided puncture and the transfer of these oocytes into recipients is now being used to produce pregnancies from donors that had previously been unable to provide embryos. In vitro fertilization, however, has been essentially unsuccessful in the horse. One alternative to in vitro fertilization that has shown promise is intracytoplasmic sperm injection. However, culture conditions for in vitro-produced embryos appear to be inadequate. The continued demand for assisted reproductive technology will likely result in the further development of techniques that are suitable for use in the horse.