Although an assisted reproductive technology, equine embryo transfer is used with such regularity and is so integral to countless breeding programs around the world that it is now very much considered a conventional means of producing offspring. The procedure involves removing a fertilized egg, or embryo, from a biological dam and transferring it to another mare who will carry it to term and raise the foal as her own.

Alberta veterinarian Dr. Chris Berezowski is well-versed in embryo transfer. The CEO and part-owner of Moore Equine in Calgary is board certified in both equine practice and theriogenology (the branch of veterinary medicine that deals with reproduction). He says there are a few reasons a horse owner might opt for embryo transfer versus other reproductive routes, but primarily, it allows maximization of a mare’s genetic material.

“Most of the time it’s done on younger mares that are still competing and performing and the owner wants to have some offspring from that mare, but they don’t want to take her out of training or competition.” And with healthy, fertile females capable of producing upward of six or eight embryos a year, it also opens the door to obtaining multiple foals per season, often from different stallions.

Other mares are candidates because they have a reproductive condition or might not be able to handle the physical stresses of carrying and/or caring for a foal due to lameness or other issues. Then, says Dr. Berezowski, there are older mares “that have their own problems, where they can get pregnant, but they can’t carry a pregnancy to full term for some reason.”

No matter what brings a horse owner to choose embryo transfer, they can be assured that since the first published reports of successful procedures in 1972, it has become an extremely safe and effective reproductive option. In fact, Dr. Berezowski says, the procedure has become quite routine for practitioners, particularly in the past decade
or so.

“The equipment hasn’t changed. The products being used haven’t really changed. It’s a very successful thing and it probably  gets more and more so because of people getting more experienced at it. The success  rate continues to be quite good.”

Equine Embryo Transfer Process

In the first part of the transfer process, the biological dam, known as the donor mare, is bred normally by either live cover or artificial insemination of fresh, cooled or frozen semen from the chosen stallion. Fertilization (hopefully) then occurs and the embryo  enters the uterus, usually about five days  after ovulation. Meanwhile, a recipient  (or surrogate) mare is at the ready, her heat cycle synchronized with the donor. Getting both mares on the same agenda isn’t always smooth sailing.

“It can be difficult, or at least take some time, if you only have one mare that’s getting bred and one recipient. If you have a group of recipients available, then it can be easier to have mares that are on similar cycles,” says Dr. Berezowski. “And we’ll have quite a few clients that have their own recipient mare, so it’s like that one-to-one relationship where we have to match up the two mares and their cycles. Sometimes it works and it’s really quite easy just based on where they are naturally. Other times, it takes a little bit of time and some hormonal manipulation to get them there.”

An embryo flush is performed about seven or eight days after ovulation. This non-surgical procedure is “very safe and straightforward,” says Dr. Berezowski. It involves filling the mare’s uterus with a specialized sterilized fluid before the embryo has a chance to implant. The fluid is then drained out and run through a small filter which catches the embryo. After three or four flushes, the filter is examined under a microscope to check for the presence of one or more embryos, which, at this point, are usually only about a third of a millimetre to a millimetre in size. For young, fertile mares, about 50 to 70 per cent of flushes yield an embryo.

Recovered embryos are collected, washed and assessed for viability. They’re usually transferred immediately or shipped cooled to another location for insertion into a recipient mare within 24 hours (at which point viability begins to diminish). They can also be frozen and stored indefinitely for later use, although the chance of obtaining successful pregnancies does decrease.

Transferring embryos involves placing them in a straw, which in turn is put in a transfer pipette that goes through the vaginal canal and cervix and deposits the embryos into the uterus. Embryos can also be implanted through a simple surgery via a flank incision.

The pregnancy will be confirmed by ultrasound around day 12. If all continues to go well, the recipient mare will carry the foal, deliver, and nurse it until weaning.

A Key Component: The Recipient Mare

One of the imperatives in successful embryo transfer is the selection and management of the recipient mare. Berezowski says the main criteria are age and fertility. “Normally, we like to use younger mares, ideally less than ten years of age. If they’ve had a foal or two in the past, that’s super, but it’s not absolutely necessary. A young, healthy three- or four-year-old is usually quite a good recipient.”

Size is also “somewhat important,” says Dr. Berezowski, to ensure the foal has ample room to develop and grow within the uterus. “We usually try not to have a size discrepancy, but we definitely have used little Quarter Horse recipient mares for big warmblood donor mares and that all works out fine.”

The mare’s temperament is also important. “You want something that’s easy to work with, because they’re the ones that are going to be raising the foal,” he says. “We like to use a lot of horses from the racetrack – Thoroughbreds or Standardbreds. It’s a good opportunity for another job for these horses to do when they don’t cut it as a racehorse or they’re not worth breeding for the race industry. That seems to work well, especially the Standardbreds – they’re really good moms.”