Embryo Transfer Technique

Advanced reproductive technologies that can be used in horses are expanding every year. Geography is no longer a limiting factor in choosing a mate for your mare, as fresh cooled semen can be shipped across North America with ease. Frozen semen also means stallions from other continents can be used. Frozen semen allows for the continued breeding of a stallion even after he dies (for those breeds that allow it). Embryo transfer--once uncommon and expensive--is now commonplace and relatively affordable. These techniques allow horse breeders more options for their breeding programs, and are helping preserve endangered species. In this article, we will discuss the ins and outs of embryo transfer, the success rates, and the approximate costs involved.

A Brief History

Embryo transfer is the removal of a six- to eight-day-old embryo from one mare and the transfer of it into a recipient mare's uterus to continue development. This procedure has been performed in the United States since the mid-1970s, and the recovery of the embryos has changed very little in the last 15 years. However, new procedures are being used for the implantation of the embryo, which will be discussed later.

With the approval of several breed associations to register foals from embryo transfer, the demand and success of embryo transfer has grown.

Why Embryo Transfer?

Embryo transfer is a very important technology in equine reproduction because it allows breeding of mares which can no longer carry foals to term. That could be due to a previous foaling injury, infection of the uterus, or just old age. Furthermore, mares with a heavy performance schedule which can't take a year or more off for pregnancy are great candidates for embryo transfer. Some people also use this technology to produce multiple foals from a single mare in one year.

The desired mare need only be able to carry a pregnancy for seven days post-ovulation to be suitable for an embryo transfer. Mares which have other problems that prevent them from conceiving, maintaining a healthy embryo, or which are older and have fertility problems are not suitable candidates for embryo transfer.

What is Involved?

Embryo transfer is made possible by the synchronization of the estrous or heat cycles of the donor and recipient mares. Once a decision is made to attempt an embryo transfer, usually a minimum of two potential recipient mares are begun on an estrus synchronization program. The recipient mares need to be within 24-48 hours of the donor mare's ovulation schedule. This must occur for the recipient mare's uterus to be ready to accept an embryo.

The donor mare (and recipient) are monitored daily for closeness to ovulation, and when the donor is ready, she is inseminated and she and the recipients are monitored for ovulation. Seven days following ovulation, the donor mare's uterus will be flushed with a special saline-type solution to collect the embryo(s). To do this, a special catheter is used to prevent the loss of any of the fluid (or embryos). The fluid coming out of the uterus is put through a special filter cup to help collect the embryo(s), then the filtered solution is examined under a microscope.

Once the embryo is located, its quality is scored. The grades are from 1-5; one being excellent and 5 being a non-viable embryo. The grades are based on the morphologic appearance and apparent health of the embryo. Grade 1-2 embryos provide the best chance for a viable pregnancy. Once the embryo is recovered and scored, it can be transferred into a recipient mare immediately, or within 24 hours if it's being shipped to another facility. This whole process is called the recovery stage of embryo transfer.


At this step there are a few options to consider before beginning the process. Depending upon your veterinarian's facilities, the embryo can be implanted immediately into a recipient mare on site, or the embryo can be shipped to a commercial facility that houses potential recipient mares. The embryo will be shipped overnight to the facility and within 24 hours (or hopefully less) be implanted into the recipient mare.

The recipients are housed and monitored for pregnancy, then are usually leased or purchased for the year. After a recipient has been confirmed in foal, she is housed at that facility until she is 90 days in foal, then she can be released to your farm. Discussion with your veterinarian will determine which is most appropriate for you and your breeding program.

The Transfer

In the past, the transfer was performed most commonly by surgically implanting the embryo into the recipient mare's uterus via flank incision. It was thought that there was less chance of contaminating the uterus with surgery. In some studies, there was a higher success rate of pregnancies in surgically implanted mares.

Today, however, the transfer is usually carried out vaginally. Research has shown the success rates of surgically implanted vs. vaginally implanted embyos to be very similar, and the less-invasive approach is not only simpler, but less time-consuming, less expensive, and obviously less invasive.

The transfer is carried out by placing the embryo in a "transfer gun." The mare's perineum is cleaned thoroughly and her tail pulled out of the way. The transfer gun is then gently advanced through the vagina and the cervix and into the uterus, where the embryo is deposited.

Following this, the recipient mare is checked for pregnancy within five to six days and she is monitored frequently.

Success Rates

In young, healthy donor mares, the embryo recovery rate per cycle is about 60%, and the successful establishment of a pregnancy in a recipient mare is also about 60% when a grade 1-2 embryo is transferred, according to Chris Schweizer, DVM, Dipl. ACT. "Success rate for both embryo recovery and successful pregnancy decrease as the donor mare's innate fertility decreases and embryo quality declines," she says.


The cost of embryo transfer often depends upon the facility and veterinarians involved. However, universally it is not cheap. Furthermore, the cost can be related to how lucky you are with the transfer. Schweizer recommends being ready to attempt three cycles with your mare.

"Sometimes everything works great and you get a viable embryo with one cycle, other times, after three cycles, you have not achieved a viable pregnancy," Schweizer says. "These are often the older, infertile mares that someone wants just one more foal from, and this is often the worst candidate for an embryo transfer."

Most reproductive veterinarians recommend that you should not consider embryo transfer unless the resulting foal would be worth $10,000-$12,000. For more accurate information on cost in your situation, you should contact the facility where the recovery and implantation will take place. Also remember that many facilities require that you purchase the recipient mare, so make sure you ask about that cost as well.

Saving Endangered Species

The most exciting part of embryo transfer in horses and other species is the potential for the preservation of endangered species through embryo transfer. Using one female to produce numerous siblings could in a matter of a few years increase a species population significantly. Furthermore, researchers are using donated eggs and sperm from endangered animals to create "test-tube" embryos. Once the embryos are ready, they can be implanted into to a closely related domesticated or more common species to be used as a surrogate mother. There have already been several cases of successful interspecies embryo transfers, such as a domestic cat giving birth to an African Wildcat and a common Holstein cow giving birth to a Gaur. Embryo transfer technology, coupled with the techniques of freezing embryos, could go a long way in helping preserve endangered mammalian species.

For example, researchers at Monash University in Victoria, Australia, in 2002 took a step forward in protecting the world's most endangered donkey breed when a rare Poitou donkey foal was born to a surrogate Standardbred mare. This was accomplished through embryo transfer from the donkey's biological mother, which was bred by artificial insemination.

The foal brings the number of Poitou donkeys in Australia to three. The foal was born after a 364 days under the guidance of Angus McKinnon, BVSc, MSc, Dipl. ACT, Dipl. ABVP, an honorary research fellow at Monash's Institute of Reproduction and Development (MIRD), and co-author of the world-renowned veterinary textbook Equine Reproduction (see article #3437 at www.TheHorse.com).

The Future

New techniques are being perfected and are commercially available for acquiring unfertilized eggs from mares, which can then be fertilized outside the mare and implanted into a recipient mare (gamete inter fallopian tube transfer or GIFT). However, this technique is still quite expensive and only has a pregnancy rate of around 30% at this time.

Veterinarians are experimenting with hormones to create multiple ovulations in mares, so that many embryos can be obtained during one cycle, instead of the usual one embryo per cycle.

So, if you have a mare whose offspring is worth at least $10,000-$12,000--which isn't uncommon in many breeds and disciplines today--or a mare which needs to remain in training but at the same time could be producing a foal or foals, then you should discuss embryo transfer with your veterinarian or have him/her refer you to a local equine reproductive specialist. 


1 McKinnon, A.; Voss, J. Embryo Transfer. In Equine Reproduction. Lea & Febiger, Philadelphia. 1993, 357-367.

2 Equine Research, Inc. Embryo Transfer. In Breeding Management & Foal Development, eds. Evans, J.W.; Torbeck, R.L. Equine Research Inc. Tyler, TX. 1982. 351-358.

3 Foss, R.; Wirth, N.; Schiltz, P.; et al. Nonsurgical Embryo Transfer in a Private Practice. 45th Annual AAEP Convention Proceedings, 1999; 210-212.

About the Author

Christina S. Cable, DVM, Dipl. ACVS

Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.

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