By Roger Dixon BVM&S CertAVP(ESM) MRCVS, Ashbrook Equine Hospital.
Embryo transfer is an advanced reproductive technique that is now well established in the UK. It involves the removal of a 6-8 day old embryo from the uterus of a donor mare and its non-surgical transfer into the uterus of a recipient mare.
Embryo transfer was first described in horses in 1972 and has been performed commercially since the 1980s. It has been popular in countries such as Argentina, Brazil, USA and Australia for many years, largely because both the costs of recipient mares and land are relatively cheap.
Why Use Embryo Transfer?
There are several applications for embryo transfer. In South America it is frequently used in polo ponies. The ponies are admitted to an embryo transfer unit at the end of the polo season and several embryos may be flushed and transferred over a 2 month period. As well as allowing the mare to continue competing the following season, it allows the birth of several progeny in one breeding season.
This is also a major advantage in Sports Horses where it allows the donor mare to continue competing following a successful transfer. A second advantage is that by performing embryo transfer in the young, competing mare, the interval to the next generation is shortened, whereas traditionally many competition mares are only put in foal once they finish competing. Fertility of these older mares may be impaired, reducing the chances of getting in foal.
Embryo transfer is also of some use in older mares who are unable to carry a foal to term e.g. those mares who are scanned in foal at the 16 day scan but repeatedly resorb the pregnancy at a later date.
There are several factors to consider when contemplating embryo transfer. These include the donor mare, the recipient mare and the choice of stallion.
The Donor Mare
The donor mare will be a high performance mare of superior genetic quality, with the aim to obtain more high quality foals. The quality of the donor mare must justify the expense of the embryo transfer.
The Recipient Mare
The recipient mare (the mare the embryo is transferred into) is arguably the most important factor in an embryo transfer programme. The ideal recipient mare is 3-10 years old, the same size as the donor mare, fully vaccinated and of good temperament. A detailed breeding history is very important; ideally the recipient will previously have carried a foal to term and at no point resorbed a pregnancy.
The stallion must be a proven fertile stallion. Either fresh or chilled semen is preferred in order to maximise conception rates and increase the chances of recovering an embryo when flushing the donor mare. Although frozen semen may be used (and this will depend on how the stallion is available), it should be recognised that the likelihood of success may be lower than that of fresh or chilled semen and expectations should be adjusted accordingly.
Procedure for Embryo Transfer
It is very important that the Donor mare and the Recipient mare are at a similar stage of their oestrus cycle so that the embryo is transferred into a uterus very similar to the uterus from which it has been removed. The two mare’s oestrus cycles must therefore be synchronized. This may be achieved by administering prostaglandin injections to the mares when they are not in season. So long as the mares have been out of season for at least 6 days, the prostaglandin will ‘short-cycle’ the mares and bring them into season approximately 3-7 days later. Because there is variation in the time it takes each mare to come back into season, it is common practice to synchronize 2-3 Recipient mares for each Donor mare. The most suitable mare is then used as the Recipient.
Once in season, the Donor and Recipient mares are closely monitored by ultrasound scanning of their uterus and ovaries. The Donor mare is then inseminated at the optimum time to produce a pregnancy; this timing depends on whether fresh, chilled or frozen semen is being used. The Recipient mare is given a hormone injection to cause her to ovulate 24-48 hours after the Donor mare.
The Embryo Transfer itself is performed 7-8 days after the Donor mare has ovulated. After being mildly sedated, a uterine flushing catheter is placed inside her uterus and 1-4 litres of warmed special embryo ‘flushing solution’ are infused into her uterus. When the uterus is full, the fluid is let out and run through a filter cup which catches the embryo.
The embryo is searched for under a microscope, and when found it is ‘washed’ in a ‘holding solution’ which enables it to survive for much longer than it would otherwise. The Recipient mare is then sedated and her perineum and vulva thoroughly cleaned. The embryo is loaded into a transfer pipette, which is passed through her cervix and the embryo deposited in her uterus. The Recipient mare is then scanned 7 days later to confirm if she is pregnant.
Embryo Transfer is an exciting technique which is becoming increasing popular in the UK. Pregnancy rates reported by different centres vary from 60-80% of recovered embryos resulting in a pregnancy. However, not every insemination of the Donor mare will result in conception and the subsequent finding of an embryo. It is therefore important that mare owners have realistic expectations of success when considering embryo transfer. Fresh or chilled semen A.I. will improve the rates of embryo recovery.
The advantages of breeding multiple foals from top-quality mares that are able to continue competing will be attractive to many. Also, as embryo freezing techniques improve over the next 5-10 years, it is likely that a new industry in the sales of frozen embryos will emerge. This is certainly an exciting time to be involved in the equine breeding industry and we all look forward to the challenge of using these reproductive techniques to increase the quality of UK-produced Sports Horses over the coming months and years.