Embryo transfer is an increasingly popular technique with many advantages for the breeder. Newmarket Equine Hospital is pleased to offer this service and several of our stud vets are experienced in this procedure. If you wish to discuss your requirements please call us on 01638 782000.

What is embryo transfer and what are the benefits?

Embryo transfer (ET) is essentially a surrogate pregnancy. A donor mare is inseminated, but instead of carrying the pregnancy herself, the embryo is flushed from her uterus and implanted into a recipient mare, who then carries the pregnancy to term and delivers the foal. The main advantage of ET is that it allows the production of increased numbers of high quality foals with excellent bloodlines, with minimal disturbance to the sports careers of the biological parents.

A stallion has the potential to produce hundreds of foals each year, especially if his semen is used for artificial insemination (each collection can be divided and inseminated into several mares). In contrast, the mare’s pregnancy lasts around eleven months so she can carry a maximum of one foal per year and in reality this can be hard to achieve as it requires her to become pregnant very soon after delivering her last foal. Embryo transfer allows us to flush multiple embryos from one mare and transfer them into multiple recipients. These embryos will all be genetic descendants of the donor mare and the procedure allows a mare to produce multiple foals per year. This allows a high quality mare to breed more foals throughout her lifetime than would be otherwise possible.

Another huge advantage of ET is that the donor mare can continue to be ridden and compete whist the recipient mare carries her pregnancy. In the past, a top class competition mare would only be retired to stud due to advancing age or injury. She cannot continue to compete whilst heavily pregnant or nursing a foal, so breeding would mean long periods out of work. Of course this disrupts training and competition schedules, so the mare may not achieve her full competitive potential. The disadvantage of allowing a mare to finish her athletic career before breeding from her is that fertility declines with age –sport horse mares may compete well into their teens and, at this point, may not conceive easily or there may be increased risks associated with pregnancy. ET allows excellent competition mares to produce foals from a young age.

Breeders are increasingly recognising the value of establishing superior dam lines – it doesn’t not matter how good a stallion is if the mare isn’t exceptional herself. ET has been a key tool in improving the quality of British bred sport horses as breeders have focused on the mare. It is also a very valuable technique to improve numbers and genetic diversity in rare breeds.

ET also allows mares to produce foals if they are physically unable to carry a foal themselves. For example, if they have a gynaecological problem that makes them able to conceive but unable to carry a foal to term, or perhaps an orthopaedic problem such as a pelvic fracture, embryo transfer may be an option.

What is the process?

1. Pre-breeding disease screening – it is important that your mare is screened for contagious diseases which can affect fertility. We require that all mares be swabbed for CEM and blood tested for EVA before further examinations are started.
2. Syncronisation– the recipient mare’s reproductive cycle needs to be closely synchronised to the donor mare. We can manipulate both mares’ cycles by administering certain hormones, to ensure the recipient mare’s reproductive tract is ‘ready’ to receive the embryo from the donor
3. Insemination– The donor mare is set up for breeding in exactly the same way as if she were to carry the foal herself. She can be bred by natural cover or artificial insemination. Her cycle needs to be monitored closely by one of our vets as it is important that we know her ovulation date
4. Flushing– this takes place 6-8 days after the donor mare ovulated. The embryo is recovered from her uterus in a non-surgical, well tolerated procedure. The uterus is flushed with a sterile solution that is then filtered and examined for the presence of the embryo. If the donor mare conceived successfully, the embryo recovery rate can be as high as 70%; of course this depends on both the mare and stallion fertility.
5. Transfer– the embryo is carefully processed and is then transferred into the recipient mare. The embryo transfer success rate is around 50-70%.
6. Pregnancy scans – the recipient mare is then scanned approximately 7 to 9 days later, to see if the transfer was successful. Further scans are usually performed at 28 days post donor ovulation (the heartbeat scan) and again at 45 days

Recipient mares

The recipient plays a vital role in the ET process. The recipient should be as closely matched to your donor as possible in terms of height and build. It is important that she has a good temperament, is in good condition and is settled in her environment. A relaxed, happy recipient with no vices is essential – remember this mare will nurse and raise your foal and although they will not be genetically related, the recipient will have a huge influence on the foal’s early learned behaviour! The recipient needs to have excellent reproductive health, ideally she should be a proven broodmare although maidens can also be used. She should be young – 4-9 years is preferable. It is advisable to have at least three potential recipient mares available – we will select the one that is best synchronised to your donor mare.

Transported embryos

We appreciate that managing and keeping recipient mares can be difficult, so we work closely with several embryo transfer providers who manage large herds of recipient mares and loan them out to clients. We will inseminate and flush your mare as described above. The embryo is then packaged and transported by courier to be transferred into a recipient at a geographically distant location. The benefit of this service is that the donor mare does not have to travel significant distances and you only take over the care of the recipient once she is confirmed in foal.

What are the downsides?

There are increased costs associated with ET, associated with management of the recipient mare and additional veterinary procedures that are required. As with all veterinary procedures there are some associated risks, although all the gynaecological techniques associated with ET are very similar to those used in routine stud medicine and are generally very well tolerated by the mares. Due to the additional manipulation of the embryo during flushing and transfer, overall pregnancy rates are lower compared to a mare that carries her own pregnancy. Nevertheless, excellent results can be achieved with the correct management, and you can have a beautiful foal on the ground whilst your mare continues her competition career.

Fiona Hogg, MRCVS