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

Embryo Transfer (ET) is an excellent solution for breeding your top mare while allowing her to continue competing. This process involves breeding your donor mare—the mare you want to breed—and allowing her to carry the pregnancy for approximately seven to eight days. At this point, the embryo (the early-stage foal) is carefully removed (flushed) from the donor mare and transferred to a recipient mare (surrogate) for the remainder of the pregnancy.

Embryo transfer allows breeders to preserve the competition career of a valuable mare while still obtaining a foal from her. Here's how it works:

1. Breeding the Donor Mare: The donor mare must be bred with high-quality semen (often frozen semen), so she needs to be brought to a breeding facility or clinic. This typically involves 3 to 5 days of preparation for optimal breeding conditions.
2. Flushing the Embryo: After the donor mare becomes pregnant, the embryo is carefully flushed from her uterus, typically around 7-8 days after conception.
3. Transferring to the Surrogate: The embryo is then transferred to a recipient mare, who will carry the pregnancy to term.

The surrogate mare must meet specific criteria to successfully carry the embryo to term. Some important considerations for a good surrogate include:

• Good Reproductive Health: She should be in excellent condition and able to carry a pregnancy.
• Appropriate Age: Surrogate mares are usually between 4 and 12 years old for optimal fertility and health.
• Proper Uterine Environment: The surrogate's uterus must be able to support embryo implantation and growth.

Embryo transfer can be an effective solution for breeding without interrupting your top mare’s career. By carefully selecting both the donor and recipient mares, you can maximize your breeding success and ensure a healthy foal.

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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