Producing foals: Is embryo transfer your best option?
December 9, 2010
Equine embryo transfer (ET) has been a useful tool for horse owners since the first successful transfer was reported in 1972. Drs. Ben and Erin Schroeder brought the service to northeast Nebraska several years ago when they expanded Cedar County Veterinary Services in Hartington, NE, taking over the practice Ben’s father originally established in Coleridge.
Both Ben and Erin specialized in equine reproduction in conjunction with their veterinary training at Kansas State University, where they both obtained their DVMs. They offer equine ET for horse owners who want to reproduce superior traits found in their animals without taking them out of competition or training to do so.
“Embryo transfer is an attractive option for horse owners and has gained popularity in recent years,” Dr. Erin says. “Use of a recipient mare is also a good option for older mares with a tendency to be difficult breeders, abort or those that people want to try and maximize the number of foals they are able to have in their ‘golden years.'”
Embryo donor mares may be bred either by natural cover or artificial insemination. Fresh semen results in 60-70 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,” Dr. 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. Super-ovulation, obtained by administering hormones to increase the number of developing follicles, has been used to produce additional equine embryos. However, drugs used in the super-ovulation process are not currently available in the United States.
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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 also “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,” Dr. 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.”
A non-surgical “flushing” method is Erin’s primary embryo collection method. To accomplish the collection, she utilizes several litres of fluid which flows by gravity into the mare’s uterus. Gentle uterine massage usually suspends the embryo in the flush fluid, which is then drained from the uterus and passed through a fine filter dish where the embryo is retained. A powerful microscope allows for embryo identification. Within two to three hours the embryo is then either transferred to a recipient mare or frozen.
Ideally, donor mares are young, healthy horses free of reproductive abnormalities. The mare should be in good physical condition and cycling normally before embryo recovery is attempted. Dr. Erin completes a thorough reproductive examination and treats any abnormalities prior to attempting embryo recovery.
“If a client wants to attempt embryo recovery from an older mare or one with reproductive disorders, we always emphasize that the probability of successfully performing the transfer and producing a foal are decreased,” Dr. 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,” Dr. Erin adds. “Levels of hormones in the pregnancy must also be similar. For these reasons, synchrony of donor and recipient cycles is very important.”
Short-term storage of equine embryos requires careful regulation of temperatures and use of a special holding solution. Longer-term storage, anything in excess of one day, requires freezing the embryo using cryo-preservation methods and storage in liquid nitrogen.
“It’s unusual for pregnancy rates involving frozen embryos to be as high as those obtained with fresh transferred embryos,” Dr. Erin says.
In the Schroeders’ program, recipient mares receive the same thorough exam as the donors. They’re treated with a full vaccination and parasite prevention program and 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,” Dr. 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 ET-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 60-70 percent pregnancy success rate.
Foals valued at $5,000 or more at the time of birth make ET a cost-effective procedure. Sentimental reasons for use of the procedure may also make it a viable breeding option.
ET 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 donor mares 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.
editor’s note: additional information about the program offered by drs. erin and ben schroeder is available at http://www.cedarcountyvet.com.