Embryo Transfer (ET)

Embryo transfer (ET) is a popular method of breeding from high quality sport horse mares while allowing them to continue their competitive careers. Enormous progress has been made in ET over the last decade and it’s routinely performed by members of our dedicated stud team, with great success.

The process involves breeding a (donor) mare to a stallion and then transferring the resulting embryo into a reproductively competent (recipient) mare, which carries the foal to term and nurses it until weaning.

Most breed societies will register foals produced by ET.  However, before embarking on an embryo transfer programme it is important that if you wish to register your foal, you check with the individual breed society concerned that foals produced by ET may be registered.

Why choose ET?
The main advantage of ET is that it enables owners to breed from superior sports horse mares while allowing the mares to continue their competitive careers. Using ET, it is possible to produce more than one foal from a valuable donor mare during the course of a single breeding season. Some older mares are incapable of maintaining a pregnancy themselves, or produce inferior foals due to age-related endometrial degeneration. In these cases, an embryo can be recovered and transferred to a younger recipient mare with a healthy uterus.

Recipient mares
Potential recipient mares will require a full gynaecological examination to ensure that they are suitable for the job and must have their oestrus cycle synchronised with the donor mare. To maximise the chance of synchronising the reproductive cycles of the donor mare and recipient mares, it is best to have at least 3 potential recipient mares available, although it can be done with fewer. Ideally, recipients should be proven young broodmares (4–12 years old), of known high fertility and of a similar size to the donor mare.

Transferring an embryo to a mare smaller than the genetic mother can result in a smaller foal being born. It is also important that recipient mares have a good temperament and are good mothers.

Synchronisation of the donor and recipient
This involves regular ultrasound scanning and the use of hormone injections or implants. The aim is to get the recipient mare to ovulate either at the same time as the donor mare or up to 3 days afterwards. Correct synchronisation of the recipient is very important as it ensures that the uterus receiving the embryo will be at an equivalent stage to the one from which it has been removed.

Insemination of donor mares
Insemination of your donor mare can be undertaken at Rossdales or at a local AI centre. It is best to use fresh or chilled semen from a stallion of known high fertility. Frozen semen can be used, but this may yield a lower embryo recovery rate. If your own vet inseminates your mare, it is important that he or she scans her daily after insemination to determine accurately the day of ovulation.

Flushing and transferring the embryo
Recovering the embryo from your mare is a simple, non-surgical, painless process that is carried out between 6 to 8 days after ovulation. Embryo recovery is carried out at Rossdales Equine Hospital and involves flushing your mare's uterus with a special medium that is then drained through an embryo filter. The embryo is isolated, either visually or using a microscope. It is then "washed" and immediately transferred into the recipient mare using a catheter inserted through the cervix. Embryo recovery rates are equivalent to conception rates and are, therefore, dependent upon the age and inherent fertility of the mare and stallion. The donor mare can return home immediately after the flushing procedure. The recipient mare should be scanned approximately 7 to 9 days later to ascertain if she is pregnant. If she is, further scans at 28 and 45 days following ovulation should then be performed to ensure that the pregnancy is healthy and developing normally. There may be a requirement to place the mare on Regumate during this initial period.

If you wish to discuss sending your mare to us for embryo transfer, contact Camilla Scott, Annalisa Barrelet or Andrew McGladdery (Newmarket clients), or Sarah Moore or Steph Fitzgerald (Hertfordshire clients). Similarly, if you have a mare you think would be suitable as a recipient and would be willing to hire her out for this purpose, then please contact our Newmarket practice on 01638 663150.