Embryo Transfer

Embryo Transfer is the technique of removing (flushing) an embryo from one mare - the biological dam - and placing it in the uterus of a recipient mare.

The recipient mare carries and delivers the foal and nurses it until weaning. This allows the biological dam to continue in work and potentially breed more than one foal per season. Success rates with Embryo Transfer (ET) have improved enormously over the last 15 years. It is now a routine gynaecological procedure for mares that are unable or unsuitable to carry their own pregnancy.

The donor mare is inseminated as normal with fresh, chilled or frozen semen. The embryo is then flushed from her uterus 7 or 8 days after ovulation and transferred non-surgically into a recipient mare whose reproductive cycle has been syncronised to that of the donor. A minimum of two potential recipient mares should be synchronized to the donor mare.

Pregnancy scans of the recipient mare are performed initially at 5 to 7 days after transfer when the embryo is 12 to 14 days old. Follow up scans are performed as necessary until a healthy embryo and heartbeat have been confirmed.

At B&W Equine Vets we offer the comprehensive embryo transfer package, including provision of  a suitable recipient mare if you do not have your own available.

Several factors can influence the success of performing embryo transfer with your mare:

Donor mare fertility

Although your mare doesn’t have to carry the foal until term multiple, donor mare fertility does still have an impact on the likelihood of embryo retrieval.

• The oocyte quality decreases as mares age, resulting in lower embryo survival. A mare is most fertile under the age of 6 years and fertility begins to significantly decrease when she reaches 12 years old and embryos are noticeably smaller in size at the point of flushing in mares 18 years and older. Mares less than 10 years of age have the highest retrieval rates.
• Endometritis (uterine inflammation and infection) can influence embryo retrieval rates and the transfer success rate. If an infection or inflammation of the uterus is detected it may need to be treated before the mare is inseminated.
• A tight cervix, commonly found in maiden mares of all ages and older mares that have not had a foal for many seasons, can cause problems with fluid retention. It is necessary to ensure that all fluid is removed and the cervical seal is intact before the embryo descends at day six after ovulation. A compromised cervix from, for example, a traumatic foaling also has a negative effect as there is not a tight seal to create a safe uterine environment for the embryo. 

Semen selection

Stallion fertility and semen quality are an important factor in embryo recovery rates - the better the semen quality the higher the success rate of the embryo flush. Fresh and chilled semen can give higher embryo recovery rates than frozen semen.

Donor mare management

Many mare owners utilise embryo transfer to breed their competition horses. Heat, stress and exercise can all have a negative effect, however, on the process.

Stress causes an increased level of circulating cortisol which influences the hormones required for the mare to produce a dominant follicle and subsequently ovulate. Many factors can contribute to this cortisol release, including exercise and transportation. Management from the time that the mare is in season until the embryo flush is crucial and exercise and stress should be kept to a minimum.

Recipient mare selection

Recipient mares are a crucial factor in the success of embryo transfer, and the mares in the recipient herd at the B&W Willesley Stud are carefully selected.

The recipient mares should have excellent reproductive and general health, be no older than 14 years old, at least the same size or bigger than the donor, of good temperament and have a normal udder.

The oestrus cycles of the recipient and donor mares must be synchronised and a minimum of two recipients are required to ensure that if an embryo is retrieved from the donor mare, a suitable recipient mare is available (also necessary in the rare case that twin embryos are retrieved).

Embryo Freezing

In addition to regular embryo transfer, B&W Stud & Reproduction Services also offers an embryo freezing service.

Embryo freezing or ‘vitrification’ is a method of rapid cryopreservation, resulting in an embryo that can be preserved indefinitely in liquid nitrogen for transferal to a recipient mare at a later stage. Embryo vitrification provides a means of preserving the genetics of a particular mare/stallion combination, for transfer to a recipient when convenient. This can be particularly useful for embryos that are flushed from the donor mare later in the Autumn. These embryos can then be frozen, stored and transferred to recipients early in the spring, resulting in early foals.

Embryo vitrification also removes the need for synchrony of the donor and recipient since the embryo can be frozen until transfer is convenient. In addition, export worldwide of embryos reduces the need for transport of live animals and allows a worldwide market if selling embryos at auction. Please contact the stud team on 01666 880501 if you require further information on embryo vitrification or routine embryo transfer.