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Embryo Transfer in Mares

By: Dr. Sylvia J. Bedford-Guaus

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Embryo transfer is an assisted reproduction technique by which an embryo is transferred from the uterus of a donor mare to the uterus of a recipient mare that will carry the pregnancy to term. The embryo is usually recovered from the donor mare by uterine lavage (wash) at around 6 to 8 days after ovulation. The embryo is then placed in the uterus of a synchronized recipient mare.


  • A valuable mare can produce more than one foal per season.

  • Mares can be in competition while producing foals.

  • Young, genetically valuable mares can start producing foals before reaching physical maturity or while in training.

  • Aged mares incapable of carrying a foal themselves due to pathology based on uterine biopsy can continue to produce foals.

  • Mares with physical disabilities preventing pregnancy or foaling such as tearing of abdominal muscles or old pelvic fractures can still produce offspring.


  • The procedure is costly.

  • Restrictions or prohibition are enforced by some breed associations.

  • Performing embryo transfer in mares requires specialized training, and therefore not all equine facilities or practitioners are prepared to offer this service

    The success of this procedure depends on the ability to control any of the following factors:

  • The inherent fertility of the donor mare, which cannot be controlled. Furthermore, in many cases, subfertility is the reason why some mares become embryo transfer candidates.

  • The inherent fertility of the recipient mares. These mares should be chosen based on young age and a normal reproductive tract.

  • Breeding management or timing should be appropriate when performing embryo transfer.

  • The embryo recovery and transfer technique. Experience of the veterinarian plays an important role in the success of the procedure.

  • Degree of synchronization of donor and recipient. The donor and recipient mares are synchronized with the use of hormones; tight synchronization is important to ensure that the embryo is transferred into an appropriate uterine environment.

    Equipment for Embryo Transfer

    Starting an embryo transfer program requires setting-up a specialized laboratory. Embryo transfer equipment is commercially available from most companies selling equine breeding supplies. Apart from the usual equipment used for routine assisted breeding, such as an ultrasound machine, the following is required:

  • Foley catheters to flush the donor's uterus. This is a special type of catheter used to flush the uterus of the donor mare for embryo recovery. A balloon at its end allows lodging of the catheter in the cervix of the mare, preventing loss of fluid during the flushing procedure.

  • Extra tubing and connectors that are attached to the end of the Foley catheter to introduce and flush out fluid from the mare's uterus.

  • Special filters to recover the embryo from the flushing fluid.

  • A dissecting microscope to search for the embryo.

  • Sterile culture dishes, capillary glass pipettes, and miscellaneous laboratory utensils.

    The flushing medium most commonly used for embryo transfer is Dulbecco's phosphate buffered saline (DPBS) supplemented with 10 percent fetal calf serum and antibiotics. This medium is also used to wash and transfer the embryo into the recipient's uterus. The medium may be purchased in liquid or in powder form and reconstituted in the laboratory. As an alternative to DPBS, which is expensive, some practitioners use Lactated Ringers Solution instead.

    Prior to attempting embryo recovery, all utensils and media should be warmed to body temperature in an incubator to avoid cold shock to the embryo. All equipment should be either disposable or washed properly and sterilized prior to use. No detergents or disinfectants should be used for washing embryo transfer equipment, since they may leave residues that might compromise viability of the embryo. Equipment is washed with warm tap water only and is then thoroughly rinsed with distilled water. Reusable catheters, tubing and filters should be gas sterilized prior to use.

    Separate Foley catheters should be reserved for embryo recovery procedures and not mixed with those used to lavage the uterus of mares with infections.

    Choosing Recipients for Embryo Transfer

    It is well worthwhile to invest time and money in choosing optimal recipients, as it would be wasteful to invest resources and expertise in breeding a donor mare and recovering an embryo and then place it in the uterus of a mare that could not carry a foal to term. Therefore all mares purchased with the purpose of becoming recipients for an embryo transfer program should be submitted to a complete breeding soundness exam. All mares that make it into the recipient program should pass all aspects of the BSE and have a healthy endometrium (lining of the uterus) when evaluated in a biopsy sample.

    The age of the recipients is also important. It is more likely that a mature but relatively young mare (5 to 8 yr old) will be able to carry a foal to term. Often, mares purchased as recipients are acquired through public auctions and the age is unknown. One should be confident about assessing age by teeth evaluation.

    Ideally, mares in a recipient program should be chosen in accordance to a body size similar or slightly larger than the breed size of the donor mare. It is known that size of the mare will influence size of the foal, regardless of the genetic input by the donor.

    Finally, it is well worthwhile to choose mares with a good predisposition, not only because they will be much easier to handle throughout the process, but also because they may make better mothers. In this regard, knowing the background of a mare and choosing a mare that has foaled uneventfully in the past should be ideal. Not only would you know for sure that the mare is capable of carrying a foal to term but also the motherhood of the mare. Unfortunately, unless provided by the client, the past of recipient mares is often unknown.

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