Embryo transfer first became an accepted clinical procedure in the equine breeding industry in the early 1980s.
In those early stages, embryo transfer was limited by the need to hold recipient mares at the site of embryo collection or to ship the donor mare to a centralized embryo transfer facility.
When an equine embryo cooling technique was identified in the late 1980s, short-term storage (24 hours) made embryo transportation more feasible.
“Embryo transfer is an attractive option and has gained popularity in recent years,” Nebraska veterinarian Erin Schroeder says.
“Use of a recipient mare is a good option for older mares with a tendency to be difficult breeders or to abort.
It’s also an opportunity for horse owners who want to try and maximize the number of foals their older mare is able to have.”
Reproducing the traits of performance horses can also be accomplished through embryo transfer without taking the animal out of the show season.
Erin and her husband, Ben, brought equine embryo transplant services to northeast Nebraska several years ago when they expanded Cedar County Veterinary Services in Hartington, taking over the practice Ben’s father originally established in Coleridge, Nebraska.
Both Ben and Erin specialized in equine reproduction in conjunction with their veterinary training at Kansas State University, where they both obtained their DVMs.
Embryo donor mares may be bred either by natural cover or artificial insemination. Fresh semen results in 60 to 70 percent success rate with embryo transfers.
Use of cooled, shipped or frozen semen has a lower pregnancy rate, thus the success rate with frozen embryos is also lower.
“There are a lot of details requiring attention in order to perform a successful transfer,” Ben says. “The time of the donor mare’s ovulation must be determined to within 12 hours.
Embryo recovery is usually attempted on the seventh day after the mare has ovulated.”
Unlike cows, horses produce only one or at most two embryos per cycle. Superovulation, obtained by administering hormones to increase the number of developing follicles, has been used to produce additional equine embryos. However, drugs used in the superovulation process are not currently available in the U.S.
Another process used to obtain several equine embryos within a season is repetition of embryo collection attempts during a mare’s consecutive heat cycles.
Use of prostaglandins can “short-cycle” mares and produce additional cycles and therefore more ovulations.
“Careful monitoring of follicle development and ovulation is important to the success of the procedure,” Erin says.
“We have several resident donor mares that we flush throughout the summer. With one mare we successfully transferred six embryos in one season and obtained five pregnant recipients.
It was great to see all five foals born the same spring.”
The Schroeders primarily use a non-surgical flushing method for embryo collection. The process begins with utilizing several liters of fluid, which flows by gravity into the mare’s uterus.
Gentle uterine massage usually suspends the embryo in the flush fluid. Once the embryo is suspended, the fluid is drained from the uterus and passed through a fine filter dish where the embryo is retained.
A powerful microscope is used to identify the embryo. Within two to three hours, the embryo is either transferred to a recipient mare or frozen for future use.
Ideal donor mares are young, healthy horses that are free of abnormalities. The mare should be in good physical condition and cycling normally before embryo transfer is attempted.
Erin recommends a thorough reproductive examination of the mare, with treatment of any abnormalities prior to embryo collection.
“We always advise clients who want to attempt embryo recovery from an older mare or one with reproductive disorders that the probability of successfully performing the transfer and producing a foal are decreased,” Ben says.
“The decision to attempt a transfer in this situation would largely be determined by the potential value of a foal resulting from the process.”
“The uterine environment of the recipient mare must be very similar to the donor mare,” Erin adds.
“Levels of hormones in the pregnancy must also be similar.For all these reasons, synchrony of donor and recipient cycles is very important.”
Once embryos are recovered, storing them involves careful planning and monitoring. A special holding solution is maintained at carefully regulated temperatures.
For storage involving anything over 24 hours, the embryo is frozen through cryopreservation methods and stored in liquid nitrogen.
Pregnancy rates involving frozen embryos are rarely as high as those obtained with fresh embryos.
The health of recipient mares is equally important in the embryo transfer process. In the Schroeders’ program, recipient mares receive the same thorough examination as donor mares. Recipient mares are treated with a full vaccination and parasite prevention program as well as routine hoof and dental care.
“Estrous cycles in the recipient mare must be at a similar stage as the donor when the embryo is transferred,” Erin says.
“To help ensure availability of at least one suitable recipient, we usually synchronize two to five potential recipient mares for each donor.
We’ve developed a herd of 60 recipient mares selected for their calm disposition and gentle nature. Most are either quarter horses or paints and are easy to handle.”
Embryo transfer isn’t appropriate for every horse breeder. Because of the technical and tedious nature of the process, it’s a costly procedure.
Horse owners should carefully consult their breed’s registry regulations to ensure the breed will register foals produced through embryo transfer.
The American Quarter Horse Association regulated registration of numerous embryo transfer foals from one mare during a given year until 2002.
At that time a lawsuit regarding the restrictions was settled and all restrictions related to AQHA embryo transfer-related registrations were lifted. Some breeds continue to restrict ET registrations.
Advantages of embryo transfer include producing multiple foals from one mare during a breeding season. Donor mares can remain in training, competition, etc.
Breeding injury risks are greatly reduced and many “problem” mares can produce foals. Since 2007, technological advances made it possible for equine embryos to be vitrified, making them retainable for months with a 60 to 70 percent pregnancy success rate.
Foals valued at $5,000 or more at the time of birth make embryo transfer a cost-effective procedure. Sentimental reasons for use of the procedure may also make it a viable breeding option.
Horse owners can consult their veterinarians for more information about equine embryo transfer and for assistance in locating a transfer facility. 
Additional information about the program offered by Erin and Ben Schroeder is available at www.cedarcountyvet.com
PHOTO
Dr. Erin Schroeder and her husband, Dr. Ben Schroeder, specialized in equine reproduction as they completed their veterinarian training at Kansas State University. They now offer embryo transfer services to clients from Nebraska and several other Midwestern states. PHOTO courtesy of Drs. Erin and Ben Schroeder
