Embryo Transfer

Equine Embryo Transfer at South Coast Equine Veterinary Practice,  Meroo Meadow NSW

What is Embryo Transfer? Embryo Transfer is the process of fertilising an egg (from a mare who is a successful competition or show horse, or perhaps from one who is not able to carry a foal to term), harvesting the resulting embryo and transferring it into a recipient mare, who can then carry the foal to term, give birth and care for the foal until it is weaned.


What are the advantages of ET?
The main advantage with Embryo Transfer is the fact that mares who are not able to carry a foal to term may still reproduce. If your mare is a particularly valuable horse, putting her in foal does come with some associated risks. Through using a recipient mare, you are still able to have a foal from your mare without the risk of complications to your mare during pregnancy or foaling.

If you have a mare with a particularly intense competition schedule, ET might be preferable to taking her out of work until the foal has been weaned. If you’re breeding on a large scale, it is possible to have more than one foal from your mare in a season, and you can use different stallions. ET also allows foals from older mares to be carried by recipient mares – or mares who have previously had complications foaling or carrying a foal to term.

What does the recipient mare contribute to the process? The recipient mare is the mare who will carry the embryo (foal) to term, foal, and be a surrogate mother to the newborn until weaning. The recipient mare does not contribute genetic information to the embryo. As the recipient mare raises the foal she may ‘pass on’ some behavioural characteristics and mannerisms to the foal.

What qualities should a recipient mare possess? Ideally, recipient mares will be proven breeders in good health, and will not have had any prior foaling problems. The history of a recipient mare should be carefully considered before she is used for the purpose of embryo transfer. We use young standardbred mares at SCEVP because they are strong, great mothers, and cope well with living in a herd situation, and generally are quiet and tractable.

It is important to note that not all good recipient mares (who are healthy and physically able to carry the foal to term) make ideal surrogate mothers. The recipient mare is also required to raise the foal until it is of weaning age.

How does the quality of semen affect the end result of embryo transfer? As with ‘conventional’ breeding methods, and indeed, artificial insemination, the quality of the semen used does have an effect on the success rate of Embryo Transfer. Poor quality semen (with less viable or small numbers of sperm) can greatly reduce the success rate of breeding a foal.

Fresh semen is usually the best,with chilled and frozen semen having slightly lower conception rates in general. Frozen semen has the advantage of being easily transported – interstate or overseas, provided it is kept in optimum conditions.

How many embryos are you likely to get from the donor mare? Donor mares can have embryos collected at each ‘cycle’. Most mares only ovulate once, so you will only get one embryo if the mare conceives. Some mares will have ‘double ovulations’ and may produce two embryos in a cycle. Embryos are not guaranteed at every flush. Once embryos are harvested, they are washed, cleaned and examined for viability.

When is a mare a good candidate to be an embryo transfer donor? Donor mares must be healthy and in good condition – and they should be thoroughly examined before the ET process is undertaken. Ideally, younger donor mares (under twelve years of age) who cycle regularly have the highest success rate and produce better quality embryos. The quality of the embryo from the donor mare has a lot to do with your chance of success. If preserving the mare’s bloodlines is very important to the breeder, the time and expenses involved should be weighed up against the possibility of no resulting foal. The reasons the mare may not have been able to successfully conceive may or may not be overcome using the embryo transfer process. In this case, an examination and veterinary advice should be sought for the best course of action.

What is the process for ET? The donor mare should be examined by a vet to determine her cycle and time of ovulation. Artificial lighting may be used to regulate the cycle. The aim is to determine the day of ovulation, optimum breeding times, and hopefully, to be able to co-ordinate a recipient mare. The donor and recipient should be synchronised for the best results.

The donor mare is bred (either by fresh, chilled or frozen semen). Embryos are then flushed from the donor mare at around day 7 or 8 after ovulation in a non-surgical procedure. They are graded according to viability. Embryos that appear viable are scored a ‘1’ or a ‘2’, and are more likely to “stick” in the recipient than a poor quality embryo.

Embryos can be transferred to the recipient mare immediately (either surgically or non-surgically), or they may be chilled if transporting an embryo from one stud or property to another! Embryos can also be frozen – for which they are usually collected on day 6 after ovulation.

Embryos are then implanted into a recipient mare who has ovulated as close as possible to the donor mare. A pregnancy test can be performed as close to five days after the embryo transfer has occurred.

Is it an option for my mare? While it might seem like the ideal solution if you compete with your mare every weekend, Embryo Transfer isn’t something to be taken lightly. As with breeding a foal of your own the conventional way, there are associated costs, financial outlay, and there is no 100% guarantee that you’ll end up with a foal. Success rates are improving, and if conditions are right (a viable egg from a donor mare, good quality semen, and a healthy recipient mare) there is obviously a greater chance of the process resulting in a live foal.