Emergency Equine Vet:
Breadstone: 01453 811867
Failand: 01275 392956
Stretcholt: 01278 229129
Willesley: 01666 880501
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 10 years. It is now a routine gynaecological procedure for competition 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 at day 7 or 8 after she has ovulated and is 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 supplied for syncronisation by the owner of the donor mare. If suitable recipient mares are not available, then the embryo can be flushed out of the donor mare at day 6 post-ovulation and stored frozen for transfer at a later date, or alternatively the embryo can be transported for transfer into a mare at an alternative ET centre.
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.
Semen selection
Fresh semen routinely gives the highest embryo retrieval rates so choosing a local stallion is the preferred choice. If a stallion further afield takes your fancy then ultimately the most important thing is stallion fertility and semen quality - the better the semen the higher the success rate of the embryo flush.
Donor mare fertility
Although your mare doesn’t have to carry the foal until term multiple elements should be considered when determining the likely success of embryo retrieval.
The oocyte quality decreases as mares age, resulting in lower embryo survival. A mare’s 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 can negatively influence both the embryo retrieval and transfer success rates. 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. Excessive or prolonged fluid present within the uterus can negatively impact embryo survival when it descends into the uterus around day 6 post-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. Donor mares with cervical issues may require careful management around the time of breeding.
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.
Temperatures of above 30°C can result in as much as 50% lower embryo retrieval rate.
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 affect this process, including exercise and transportation. Management from the time that the mare is in season until the embryo flush is crucial; stress and heavy exercise should be kept to a minimum.
Recipient mare selection
Recipient mares are a very important part of the transfer success 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 recipient and donor mares need to have their oestrus cycles synchronised and a minimum of two are required to ensure that if an embryo is retrieved from the donor mare that a suitable recipient mare is available (also useful in the rare case that twin embryos are detected).
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.
