Embryo Transfer: Who's Your Mama?

Embryo transfer (ET)--recovering the embryo from a bred donor mare for placement into a recipient mare that will carry and raise the foal--has become a useful reproductive tool for a growing number of breeders. Mares that are competing, or mares that can't carry a foal to term because of physical or age-related problems, now can have registered offspring.

Breed registries that allow registration of foals through ET include Quarter Horse, Paint, Arabian, Appaloosa, Peruvian Paso, and several draft and Warmblood breeds.

The first ET foal in the United States was produced at Texas A&M University (TAMU) in 1978 by graduate student Stephen Vogelsang, BS, MS (now the breeding manager at Wichita Ranch in Texas), working with Duane Kraemer, DVM, PhD, professor in TAMU's Department of Physiology and Pharmacology, who had just completed the first primate ET.

Reasons for Using ET

Originally felt to be a way to get foals from old or problem mares that could not carry a pregnancy, ET is now used mainly in young mares, allowing them to produce foals while continuing their careers.

Vogelsang says ET is also a way to "back up" a mare that foals late in the season without losing a year of production (letting that mare skip a year while another mare raises that year's foal). Some breeders use ET to get more than one foal a year from an outstanding mare.

"Probably the main reasons people use ET are so they can keep showing the mare, or to get several foals from her by multiple stallions," says Vogelsang.

"People book mares to more than one stallion, and breeding contracts now address this issue--regarding which stallion the mare will be bred to first, at what stage she'll be bred to the second, etc.," says Vogelsang. "For instance, in our contract for Mecom Blue (standing at Wichita Ranch), it states that he's the first stallion the embryo is produced by, and that the mare won't be changed to another stallion until we develop a viable pregnancy. Otherwise you get into a situation where you have multiple stallions involved and if there's a hang-up along the way, then you have multiple returns."

ET is also a tool to find out what stallion crosses best with a mare. You can get some foals from an exceptional performance mare on the ground and get them trained, rather than having to wait until she retires. You might know her best cross by the time she's seven or eight, and you can raise more of those foals if the stallion is still available.

ET is also an option for a mare with a physical problem--such as a torn cervix--that makes it difficult for her to carry a foal.


Collecting multiple embryos at one time has been commonplace in cattle breeding for a number of years, but only recently has it become readily available for horse breeding. Products to stimulate the mare to produce multiple follicles (superovulate) were still experimental until a few years ago. James Bailey, DVM, of Royal Vista Southwest, an assisted equine reproductive service center in Purcell, Okla., says that eFSH (equine follicle stimulating hormone, produced by Bioniche) came into commercial use about three years ago and made superovulation feasible in horses. Prior to this, crude pituitary extract (using pituitary glands from cadavers) was used.

"It had rather crude results, and some of the research mares had ill effects from it," says Bailey. "This is our fourth year using the eFSH, and we have not experienced any ill effects from it.

"Colorado State University (CSU) has done a lot of work with it and published some good protocols for using eFSH," he says. "Dr. Jay Hudson (DVM, MS), an associate of mine at Royal Vista, worked with eFSH at CSU during his thesis work in his Masters program. It's a twice-a-day intramuscular injection that you start at a specific point during the mare's diestrus (between breeding cycles), continuing until she produces follicles of a certain size. Then you stop the injections and use hCG (human chorionic gonadotropin) and breed the mare. The more experience a person has with using this, the better the success rate."

Vogelsang said there's tremendous individual variation in success rates.

"Some mares, depending on how they build follicles, respond very positively," Vogelsang says. "When you stimulate a mare to superovulate, you might get three to six ovulations. In most cases you get about three. And there's a significant percentage of mares that won't respond to the drug. Cost will range from $300 to $400 for the treatment."

Bailey says at Royal Vista they have the best success in recovering embryos gleaned from superovulation in mares that produce three to four eggs, as opposed to five or six.

"The mares that produce five or six eggs don't have as high a final success rate," says Bailey. "It's always a bit of a gamble, but with young, normal mares (bred to fertile stallions), the chance of success is increased."

Sometimes overstimulation of the ovaries will set a mare back and she won't cycle again as promptly. If the mare owner wants foals from multiple stallions, this technique doesn't help, explains Vogelsang.

"If a mare will go to several different stallions, you don't want to mess that up," he says. "You can follow and map mares over multiple cycles and tell whether a mare will respond well to the drug, deciding which ones would be a good risk for this technique and which ones won't."

"Frozen" Embryos

Some mare owners are "freezing" embryos so they can be stored or shipped and put into recipient mares later. "We don't actually freeze embryos; we vitrify them," explains Bailey. Freezing forms crystals that would destroy the embryo.

"The success rate was very low at first," says Vogelsang. "When I was working with this technique at Granada, Spain, we had international clients interested in buying embryos, but only a 30-40% success rate. Now it's more like 60-70%. Today there's a growing interest for international trade and for collecting embryos for transfer at a more opportune time. We can vitrify an embryo in November and transfer it in March. If the donor mare has a heavy spring show schedule, we can collect an embryo in the fall and save it for a better time."

For vitrification to work, it's necessary to obtain younger embryos than those used for normal ET. "We need embryos that are only 6-6½ days old, under 250 microns in size," says Bailey. "Once they are larger than this and form the blastocele (fluid-filled cavity produced in the center of the dividing mass of cells), they don't work well."

Hudson did a trial project in late 2005 at Royal Vista looking at a new product for vitrifying embryos. Current vitrifying products use fetal calf serum, but vitrified embryos are not eligible for international export because of that ingredient. The new product uses polyvinyl alcohol.

"We compared the old solution to the new one, harvesting and vitrifying 13 embryos, then thawing them and putting them into recipient mares," says Bailey. "We had an 80% conception rate, so it's working well."

Many of the embryos his facility transfers are shipped in from other places. "People have the option of cooling and shipping the smaller embryos to us to be vitrified when they get here," he says.

Some mares naturally double ovulate prior to breeding. The mare owner might choose to transfer one embryo and vitrify the other so the mare won't need to be flushed and bred the following year. Vitrification gives a mare owner more options.

"This was approved in March 2006 at the AQHA (American Quarter Horse Association) convention for registering foals from vitrified embryos," says Bailey.

Success or Failure

Success of embryo transfer can hinge on a number of factors. First is being able to create a pregnancy and recover an embryo, and this can depend on whether you are dealing with cooled or frozen semen, or semen from an average or a subfertile stallion, says Vogelsang.

"With the average, healthy mare, you can expect an 85% or higher recovery rate," he explains. "ET success (after the embryo is collected and prepared) will range from 50-70% on old mares, up to more than 85% on young, healthy mares. Overall success rate for ET has improved about 20% over the past 10 years."

The main factors that affect success are the donor mare, the quality of her embryo, the reproductive health of the recipient mare, and season of the year, says Vogelsang. Stage of the embryo is also important--the older it is, the less likely it will result in a term pregnancy. Ideally the embryo should be six to seven days old. When you get past Day 8, it is larger and more easily damaged.

"If the donor is an old mare with a problem like chronic founder (and she's in pain), that's not a good situation for trying to get her pregnant," says Vogelsang. "For ET to work, she must get pregnant first."

Other difficult donors are those with chronic uterine infections and old mares that accumulate fluid in the uterus.

"At first we thought ET would be a panacea for old mares, but found they have a lower rate of success," says Vogelsang. "Even though you put their embryos into young mares with a healthy uterus, the embryo from the old mare may not be as viable as that from a younger mare; there's more risk. In a mare that's 20 years old, for instance, her eggs are 20 years old.

"The female does not reproduce eggs like a male is reproducing sperm; all her eggs have been there since birth," he says. "Even with good management, pregnancy rates with embryos from old mares are 10-15% lower than with healthy young mares."

Recipient Mares

Vogelsang says the main criteria for a recipient mare is that she be reproductively healthy with adequate pelvic area for easy foaling and good vulva conformation. Her breed doesn't matter. The best recipients are five to seven years old.

Vogelsang says a good recipient mare will stay in his herd until she's 12 to 14, and he offers to buy back the recipients once they have foaled and raised the babies.

Bailey says he likes to get good mares back because, "They are accustomed to the program, and we have a history on them. We like mares that are good mothers, easy foaling, and that clients get along with them. Right now we have 630 recipient mares and strive hard each year to eliminate any that are difficult to handle. When mares are returned, we look at their past history and have a form for clients to fill out about about how they liked the mare or if they have any complaints."


The cost of embryo transfer varies from $2,000-$5,000, which usually includes the recipient mare as part of the package. This does not include the stud fee. "Many people charge a $1,000 enrollment, which includes a number of embryo collections." says Vogelsang. "I charge a $300 collection fee for each embryo collection. Then if someone gets one or two collections and we haven't had any success, they have the option to quit."

At Royal Vista the first transfer on a mare costs about $4,500, including purchase of the recipient mare (which the client can return later for full credit).

"The second transfer is about $3,500 since we drop the booking fee," says Bailey. "Again, the client takes ownership of the recipient mare. The fee breakdown is $1,000 booking fee, $2,500 when we successfully accomplish a pregnancy of 26 days, and the client buys the recipient mare for $1,000.

"If they choose, they can leave the recipient mare with us and we provide year-round care and foaling, but most recipients go home with the client to foal out," he notes. "When they return the mare, they are given $1,000 back as a credit on their account for future work."

Take-Home Message

If you have a mare that cannot carry a foal to term, a young mare that is competing successfully and you want to keep her in training, but still have a foal from her, or you want more than one foal a year from a successful mare, embryo transfer might be the solution to your problem.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.

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