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Embryo Transfer


By Leigh Ballard

Artificial breeding practices are used extensively in the equine industry, and especially in the performance and show horse world. Artificial insemination (AI) is the most widely used procedure, making it relatively easy to breed to a stallion from out of the geographical area. AI with cooled fresh semen has a 60-70% conception rate. Frozen semen has a conception rate of about 30%, but offers the advantage of possible breedings to stallions in Europe and other places where the timeliness of shipping fresh semen is not a factor.

Equine embryo transfer (ET) is an advanced reproductive technology. Much has been learned about equine embryo transfers in recent years to improve the process, and it is sometimes used for the highest levels of performance horses. Breed registries such as the American Quarter Horse Association, Tennessee Walking Horse Breeders and Exhibitors Association, and others accept this type of breeding. Required DNA testing verifies parentage, and sometimes there is a limit on the number of foals produced this way.

 Embryo transfer breeding is not for the faint of heart. It’s an involved process which is highly technical and very expensive. It is time consuming and has a relatively low success rate. It requires a large commitment to a special mare. The success rate of an embryo flush is about 65-70% - not too bad by artificial breeding standards. But the acceptance rate of the embryo by the recipient mare who, hopefully, will carry it to term, is about 50%, thus reducing the overall success rate of the procedure.  Success of the ET procedure has been increased with better management of recipient mares, but even so, a breeder should count on 3 flushes and transfers before achieving success. There are highly specialized reproductive clinics like Equine Medical Services in Columbia, MO ( and Hartman Equine Reproduction Center in Whitesboro, TX ( where the ET procedure is constantly being performed and perfected.

The process of ET is as follows: Between 6 and 8 days after ovulation and breeding, a mare who is assumed to be in foal is “flushed” with a specially formulated solution in her uterus. The tiny embryo has not attached to the uterine wall at this stage (the embryo attaches at day 18) and is mobile in the uterus. Through a sterile, closed system, the fluid containing the hoped-for embryo is flushed out and collected into a container. The embryo is invisible to the naked eye on day 6 or 7, but at day 8 is about the size of the ball of a ball-point pen and can be seen in the solution. The solution is checked under a microscope, and if an embryo is visible and viable, the solution is carefully packed in a climate controlled container and sent immediately to a “recip” or recipient mare. Herds of recip mares are maintained at specialized breeding clinics, some herds as large as 700 or 800 mares.  About 10 mares are necessary to be available per one flushed embryo. A group of recip mares will have been managed so as to synchronize their estrus cycles with those of the biological or donor mare. These mares need to be at the same stage of their heat cycles as the donor mare so that their bodies will be most likely to accept the embryo. The recip mares are monitored continuously, and when the time comes to implant the embryo, a recip mare is chosen based on exacting details of readiness. Then it’s a wait-and-see period. The recip mare is checked frequently until she is determined to be in foal or not, and then rechecked to be sure she is maintaining the pregnancy.

There are several reasons to use embryo transplants, and none of them are sentimental. ET is a way to begin reproduction in valuable mares; it is primarily a show horse or performance horse business decision. A highly competitive mare can continue showing while starting to produce valuable offspring. For example, a top cutting horse mare could win $100,000 in the cutting pen, so her owner may elect to try ET rather than give her time off for breeding. If the value of a mare’s offspring is very high, she could feasibly produce more than one foal per year with embryo transplants. Another reason to use embryo transplants is that the mare may have a poor uterine environment and not be able to carry a foal. If she is known to produce good foals, especially if there’s a favorite cross with a certain stallion, she can continue producing one or more foals per year using ET.

 Dr. Colin Anderson, attending veterinarian at Masterson Farms in Somerville, TN, performs several embryo flushes per year on the Masterson show mares and breeding clients’ mares. He sends these embryos to Dr. Joe Noble of Noble Equine Veterinary Service in Purcell, OK, another leading equine reproductive facility. The embryos are sent to Dr. Noble’s recip mares in early spring, and in summer, the pregnant recip mares come to Tennessee for gestation and to deliver their foals under supervision of the Masterson breeding manager, Janné Stewart. The foals are raised by their surrogate mothers until weaning, at which time the mares are returned to Oklahoma, and a new set of pregnant recip mares is sent to Tennessee.

More information about equine embryo transfer, as well as other equine reproduction articles, canbe found at     

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