Embryo transfer (ET) in horses has become more popular in recent years, particularly in high quality sport horses. The process involves breeding a donor mare to a stallion and then transferring the resulting embryo into a recipient mare which carries the foal to term and nurses it until weaning. Rossdales now offer this service in association with Dr Sandra Wilsher from the Paul Mellon Laboratory of Equine Reproduction. Dr Wilsher previously oversaw the highly successful ET programme, which ran at the Equine Fertility Unit in Newmarket.
Most breed societies will register foals produced by embryo transfer. However, the rules and regulations vary so 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 embryo transfer may be registered.
The main advantage of ET is that it enables the production of foals from superior sports horse mares while allowing them to continue in their competitive careers. Using ET, it is possible to produce more than one foal from a valuable donor mare in the course of a year. Some older mares are incapable of maintaining a pregnancy themselves, or they 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.
Potential recipient mares 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 the procedure 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. Previously it was thought that mares much larger than the donor mare would make good recipients but there is increasing evidence to show that this can be detrimental to postnatal growth and development. It is also important that recipient mares have a good temperament and are good mothers. Recipient mares can either be your own or they can be hired. They will require a full gynaecological examination performed to ensure that they are suitable for the job.
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 that from which it has been removed.
The 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 veterinary surgeon inseminates your mare, it is important that he or she scans her daily after insemination to determine accurately the day of ovulation.
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 Diagnostic Centre and involves flushing your mare's uterus with a special medium which 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.
Embryo transfer is a labour-intensive procedure so is relatively expensive. You should also remember that although pregnancy rates from ET have improved greatly in recent years, they are understandably still slightly below straightforward methods.
If you wish to discuss sending your mare to us, or need any further information regarding embryo transfer at Rossdales, contact Chris Phillips, Ollie Pynn or Andrew McGladdery. 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 also contact us.