Embryo transfer is accepted today as a valuable tool in equine practice to increase the number of progeny from genetically
valuable mares and for obtaining foals from mares incapable of carrying a pregnancy to term.
Embryo transfer: Donor-recipient synchrony is considered the most important factor in achieving success with this procedure.
(PHOTOS: COURTESY OF DR. ED KANE)
Most major breed associations (Quarter Horses, Arabians, Paint Horses, Morgans, American Saddlebreds) allow both embryo transfer
and registration of multiple offspring produced by a given mare in a given year, including foals produced by assisted reproduction.
A notable exception is the Jockey Club, which does not allow the registration of Thoroughbred foals produced by embryo transfer
or other forms of assisted reproduction.
Timing is everything: Two of the most important considerations in embryo recovery are the timing of insemination and the fertility
of the donor mare. Roughly 50 percent to 70 percent of embryo flushes should yield a healthy embryo.
In the 1990s, embryo transfer gained commercial value in South America, where it was developed as a means of producing offspring
of polo ponies. Using nonsurgical embryo transfer methods reduced the need for more complicated surgical procedures. Also
critical to the advancement of equine embryo transfer was the ability to transport equine embryos at 5oC for up to 24 hours
without appreciable decrease in viability.
Unfortunately, commercial expansion of embryo transfer was hindered because mares were unusually resistant to the induction
of superovulation and equine embryos were poorly tolerant of cryopreservation.
Even the most basic requirement for successful equine embryo transfer (i.e, adequate synchronization of the estrous cycles
of donor and recipient mares) can be labor-intensive and difficult because of the significant variation in estrus length between
"The majority of embryo transfer clients that I have want more than one foal from a particular mare, usually two or three,"
explains Pat McCue, DVM, PhD, the Iron Rose Ranch Professor of Equine Reproduction and director of the Equine Reproduction
Laboratory at Colorado State University. "They'd like one foal from one stallion, the second pregnancy from a different stallion
and a third foal from a third stallion," "An advantage of superovulation and embryo transfer is that by getting three or four
ovulations, it increases our odds of getting at least one embryo out of that donor mare in a given cycle."
The second use of embryo transfer is for subfertile mares, ones that have had difficulty in maintaining pregnancies. "By transferring
the embryo to a reproductively healthy recipient, hopefully that recipient will carry the foal successfully to term," McCue
"The potential benefit of embryo transfer with a subfertile mare helps us get foals out of mares that we might not otherwise
get foals from, but the aged subfertile mare may not be an ideal candidate for embryo transfer," explains Dirk Vanderwall,
DVM, PhD, chief, Section of Reproduction, University of Pennsylvania's New Bolton Center. There is good evidence that inherent
declining oocyte quality in the older mare — hence declining embryo quality in the aged mare — is likely a primary contributing
factor to declining fertility in the mare that is in her late teens or 20 years and older.
"Twenty years ago, I think the feeling was that we could rescue embryos from a poor reproductive tract in an aged, subfertile
mare, put the embryo into a young, fertile mare, and that was going to solve all of our problems," Vanderwall says.
Done experimentally in the 1980s at Cornell University, collecting embryos from aged, subfertile mares and putting them into
young, fertile mares had a higher embryonic-loss rate than embryos collected from young, fertile mares transferred into other
young, fertile mares.
A third purpose is for horse owners who want a foal from a mare to show, yet keep that mare open (i.e., not pregnant). "We
do embryo transfer to give the pregnancy to a surrogate and allow the donor mare to remain open and continue to be shown,"