The mare’s reproductive or estrous cycle has a conventional length of approximately 21 days through the breeding season. Throughout her cyclic activity, fluid-filled follicles develop on the mare’s ovaries. Towards the end of the ‘heat’ period or estrus during which time the mare is receptive to a stallion, a follicle or occasionally two follicles, grow, mature and release an egg or ovum. The ovum is enthused into the uterine tube where it can be fertilized by a sperm made available by natural or artificial breeding. The fertilized egg or zygote undergoes cell division to become an embryo that is then propelled along the uterine tube towards the uterus. In six days after ovulation the embryo is comprised of a mass of cells and is less than 0.2 mm in diameter when it enters the uterus. The mare is unique in that usually only viable embryos are transported into the uterus. About 25 days after ovulation an embryo that continues to develop moves freely in the uterus before attaching to the uterine lining.

The ideal embryo donor is a young hale and hearty mare free of reproductive irregularities. A potential donor mare should be in good physical condition and should be cycling typically before embryo recovery is attempted. A systematic reproductive examination must be performed and any abnormalities treated prior to embryo recovery being commenced. Whereas various clients desire to attempt embryo recovery from old, barren mares or mares with reproductive disorders, it should be understood that the possibility of success in an infertile mare will be decreased. In those cases the decision to continue is largely ascertained by the potential value of any foal that may result. Basic to the success of an embryo transfer program is the selection of healthy recipient mares. Recipients should be young mares of verified fertility and normal reproductive health. Essentially, recipients should undergo the same level of scrutiny as donor mares. Recipient mares must be at a comparable stage of their estrous cycle as the donor at the time of embryo transfer. Guaranteeing the availability of at least one suitable recipient is reasonable to synchronize two potential recipient mares for each donor.

Embryo transfer involves the use of a donor and recipient mares that must be coordinated in their estrous cycles. Both mares should be healthy, reproductively sound and cycling regularly. Seven days after the donor mare is bred her uterus can be flushed to recover the embryo. Several liters of Dulbecco’s phosphate buffered saline are thawed out and infused into the uterus through a catheter. The uterus is cleansed numerous times and drained into a filtered collection bottle. The collected embryos are assessed for possibility based on their size, grade, morphology and developmental stage. Only the most important embryos are drawn on for transfer in order to maximize the use of the recipient mare and not waste her time on poor quality embryos. Non-surgical embryo transfer is the method most normally used in private practice, but has a slightly lower success rate than surgical transfer. When transferring the embryo to the recipient mare, it is important she ovulated no more than one-day before and three to five days after the donor mare. The embryo is placed in a straw connected to an insemination gun. The gun is positioned in transcervically into one uterine horn sustained by rectal palpation. The embryo is deposited and the gun is detached and checked to make sure no embryo remains in the straw. After the transfer is complete, the recipient mares should be examined for pregnancy by ultrasound on days 12 (five days after transfer), 14, 30 and immediately before being shipped to the client. A fifty percent (or greater) success rate is expected if the procedure is carried out properly and suitable donor and recipient mares were selected. A level of synchrony between embryo and uterine environment is essential for the establishment of pregnancy when performing embryo transfer. The ability to extend the acceptable degree of asynchrony would allow more efficient use of recipient mares. Selection of the mares, synchronization, flushing the embryo and then transferring it all take time, but the rewards of success will be well worth the wait when a mare unable to carry a foal for various reasons is able to produce a viable offspring.