By Dr. Juan C. Samper, DVM, PHD, DIPL. ACT

Q: I own a top quality mare that I would love to have a foal out of. However, she is also my primary riding and competition mount, and I’d prefer not to stop riding her so she can have a foal. I do have another mare and am considering using her as a surrogate for embryo transfer. What exactly is involved in embryo transfer? How can I tell if my second mare is a good surrogate candidate?

A: Embryo transfer (ET) is perhaps the best option for this scenario. ET consists of breeding a top quality or special mare, called the donor mare, who will conceive and will carry her own pregnancy for about seven to eight days. At that time the small baby, called the embryo, will be removed (flushed) from the donor mare and will be transferred to a surrogate mare, called a recipient.

The donor mare must be available for breeding. Most people who have a top mare will prefer to breed her with frozen semen, which means that the mare will have to go to a breeding barn or clinic for three to five days.

The surrogate or recipient mare will be examined at the same time that the donor mare is examined for breeding and the cycles will be synchronized. That means that both mares will be ovulating at the same time so that when the baby is moved, the uterus of the recipient is under exactly the same hormones that the donor is.

The embryo at the time of transfer is only three-tenths of a millimeter in size, meaning we cannot see it before we flush the mare. The only way we will know it is there is by looking for it under a microscope. Most of the time, we only get one embryo per try. We have a new protocol to increase the likelihood of getting two embryos in one cycle, which would increase the chances of establishing a successful pregnancy in a recipient. 

The chances of success will be determined by the fertility of the donor mare, the quality of the semen, and the quality of the recipient. If the donor mare is young and has no fertility problems and the semen is of excellent quality, the chances of getting an embryo are 70 to 75 per cent. If the recipient is a young, healthy mare, the chances that the embryo is accepted by the surrogate are about 80 per cent. 

If you have a recipient mare, it is important she gets examined and her cycle followed to ensure that it is normal. Otherwise it is better to use a mare from a facility that has a few to choose from. That way the chances of success are better.

Besides competition mares, other uses of ET are for mares that have repeatedly lost their pregnancies, mares with histories of uterine infections, or mares with other abnormalities of the reproductive tract. When the donor mare has problems, the chances of getting an embryo are about 50 per cent per try. Once the recipient is pregnant the chances of her having a normal pregnancy are exactly the same as if she was carrying her own foal.

Dr. Juan Samper, MSc, PhD, Diplomat ACT, operated JCS Veterinary Reproductive Services in Langley, BC, from 1993-2017. He has consulted with breeders and veterinarians in over 25 countries. He served as the Associate Dean Clinical Affairs at Ross University School of Veterinary Medicine from 2014 to 2017, and is presently the Associate Dean of Students and the University of Florida College of Veterinary Medicine. 

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