Embryo Transfer in Equine Breeding Programs

Once an experimental technology, embryo transfer has become an integral part of breeding programs throughout the world. Experience and technological improvements have made it a productive and relatively reliable procedure. Unlike the in-vitro fertilization procedures commonly used in human infertility treatments, both conception and early embryo growth occur in the donor mare. Approximately seven days after conception, the embryo is transferred to a recipient mare.

Embryo transfer has three common purposes:

  • To produce more than one foal in a season from a particularly valuable mare.
  • To produce foals from mares involved in another activity such as showing or racing.
  • To produce foals from mares otherwise unable to carry a foal to term successfully.

There are embryo transfer centers that can perform on-site embryo recovery and transfer, as well as supply recipient mares for embryos that are recovered elsewhere and shipped for transfer.

Donor Mare Selection

The main requirement for a successful donor mare is that she be able to produce an embryo and carry it for seven days. The more fertile a donor mare, the easier it should be to recover an embryo.

One recent study divided donor mares into three groups: young show mares which were to produce foals while maintaining their careers; broodmares to produce multiple foals that year; and mares with a history of fertility problems. Embryos were recovered from the show mares on more than 80% of attempts, the multiple foal group on approximately 60% of attempts, and mares with a history of fertility problems on 30% of attempts.

This does not mean that a mare with a history of fertility problems would be a bad donor; each mare's individual problems need to be assessed. While some make excellent donors, some take additional work. There is a small portion of mares from which it is impossible to recover an embryo.

Embryo Recovery

Standard breeding practices can be used with the exception that the mare should have an ultrasound exam at least daily during breeding to provide optimum monitoring and determination of the day of ovulation. Embryo recovery and transfer success rates are considerably higher when a competent professional is actively involved in breeding and monitoring the mare. Mares with reproductive problems might need additional treatment before, during, or after breeding.

Several days following ovulation, the uterus is flushed. A filter is used to recover the embryo. The donor mare usually receives prostaglandin following the flushing procedure to return the mare to heat and prepare her for additional breeding. The typical mare can be flushed for embryo recovery every 17-18 days.

Embryo recovery requires attention to detail to recover and maintain an embryo successfully. Therefore, one should choose an equine practitioner with training and experience in the recovery of equine embryos.

Embryo Transfer

Once an embryo is recovered, it can be cooled and shipped to an embryo transfer center or transferred on-site. The transfer center has the advantage of having a large group of recipient mares from which to select, as well as experienced personnel.

Careful selection of the recipient mare can "make or break" the procedure. Not only must her reproductive tract be evaluated thoroughly and be in good health, her cycle must be synchronized with the donor mare. There is only a certain segment of a mare's cycle in which she is receptive to the transfer of an embryo, so the recipient mare should ovulate at about the same time as the donor.

The transfer of the embryo to the recipient mare can be done surgically or non-surgically. Transfer to a recipient mare through a flank incision has been the most common method because of relatively consistent results.

Recent studies have shown that non-surgical transfer, when performed under controlled conditions by someone experienced, can give excellent results. One should select an embryo transfer program with a 70% or higher success rate in producing pregnancies from transferred embryos.

How Does it Work for Me?

The first step in starting embryo transfer is to determine if you will send your donor mare to a transfer center or have the work done on your farm. Consultation with your veterinarian probably is the best way to start, as your veterinarian will understand your horse, your needs, and the resources available in your area.

If the mare is shipped to a transfer center, she might need to be there for some time, depending on how effective she is at producing embryos, the number of foals desired, and the success of each transfer. An average donor produces an embryo 50% of the time, and when an embryo is transferred, a pregnancy is established 70-80% of the time. She can be flushed approximately every 2 1/2 weeks.

Embryo transfer is not a cure-all for reproductive problems, but when used judiciously, it can benefit most breeding programs. Proper evaluation of donor mares and an understanding of the chances of success should help owners determine if embryo transfer is right for their programs.

About the Author

Rob Foss, DVM

Rob Foss, DVM, is president, practitioner, and practice manager of Equine Medical Services in Columbia, Mo. This general equine practice includes one of the larger embryo transfer programs in the country.

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