Keys to successful equine embryo transfer
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 animals.
"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 explains.
"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," McCue says.