Embryo transfer in horses

This is achieved by the removal of a six to eight-day-old embryo from the uterus of a donor mare and its implantation into the uterus of a recipient mare. This efficient, non-surgical process can also allow the mare to produce more than one foal per breeding season.

It has been used extensively in South America, the USA and Australia commercially since the 1980s. In South America, it is commonly used in polo ponies, where most of the top horses are females.

At the end of the playing season, some mares will be transferred to a specialist centres where, over several breeding cycles, embryos can be flushed and transferred into recipient mares during a 'rest' period.

This allows the mare to continue competing the following season, while several progeny can be produced in one season.

The procedure has also become popular in the sports horse population (eventing, dressage and showjumping horses) where it allows the donor mare to continue competing and training following embryo transfer.

By performing embryo transfer in a young, promising mare, the interval to obtaining the next generation is shortened. High class proven horses are extremely valuable and difficult to find.

Due to pressures of competition, top quality mares are not usually sent to stud until late in their lives. Embryo transfer enables these mares to reproduce and continue their careers without the risks associated with pregnancy.

An owner can also have the chance to take embryos out of a valuable competing mare and then sell her at her prime without losing her bloodlines.

Furthermore, fertility in older mares may be reduced, thus reducing the chances of achieving a pregnancy and carrying a foal to term. Therefore, by obtaining embryos from these mares at a younger age and using recipients to carry the foal, we increase the chance of successful breeding.

The major disadvantage of ET is the cost associated with it. It is usually only carried out at specialist centres where there are a large number of recipient mares available for implantation.

What is required in the process of embryo transfer?

Donor mare

As highlighted, due to the costs involved the donor mare selected for an embryo transfer programme will usually be a performance mare of high genetic quality.

Stallion

The stallion must be proven to be fertile. Fresh or chilled semen will help maximise conception rates and increase the chances of recovering an embryo.

Although frozen semen can be considered, the likelihood of success may be lower than that of fresh or chilled semen (as is the case for regular AI of mares) and expectations should be adjusted accordingly.

Recipient dams

The recipient mare (the mare the embryo is transferred into) is an important factor in the programme. Ideally, she should have a clean reproductive history, be fully vaccinated and have a sound temperament. Those selected are usually aged between five and 12 years.

Maiden mares can be used and, although the birth weights of their foals are often lower, due to their milk producing abilities the foals generally catch up by weaning.

Several recipient mares are ideally lined up to allow for perfect synchronisation with the donor. A ratio of three recipients to one donor is common.

Recipients come in all shapes and sizes. Native mares are generally thought to be hardier, more fertile, produce better milk and make excellent mothers.

The procedure

The donor mare and the recipient are hormonally co-ordinated so that their oestrus cycles are synchronised.

This is to allow the collected embryo to be transferred into a uterus at a similar stage of cycle to the uterus from which it has been removed.

Synchronisation is usually achieved by giving prostaglandin injections to the mares when they are not in season (they will need to be scanned to assess the ovaries and uterus and the stage of their cycle).

Theese injections will 'short-cycle' the mares and bring them into season approximately three to seven days later.

As highlighted above, it is common to synchronise two to three recipient mares for each donor mare to allow for discrepancies in individual mares. The most closely synchronised mare is chosen as the eventual recipient.

The donor mare is inseminated at the optimum time to produce a pregnancy (dependent on type of semen used). The donor and recipient mares will be closely monitored through ultrasound examinations of their reproductive organs.

The recipient mare is given a hormone injection to cause her to ovulate 24-48 hours after the donor mare. The embryo transfer is performed seven to eightdays after the donor mare has ovulated.

The donor mare is sedated and a uterine flushing catheter is passed into her uterus. One to four litres of specialist warmed embryo flushing solution is transferred into the uterus which then fills it. This fluid is then allowed to run out and pass through a filter which catches the embryo(s).

The embryo is located by searching under a microscope. When located it is 'washed' and kept in a holding solution.

The recipient is also mildly sedated and her perineum and vulva thoroughly cleaned (as in regular AI). The embryo is loaded into a special transfer pipette, which is then passed through her cervix and the embryo deposited in her uterus. The recipient mare is then scanned seven days later to confirm if she is pregnant.

ET has become increasingly popular in the UK over the past 10 years. Pregnancy rates of 60-80% of recovered embryos resulting in a pregnancy have been reported.

It should, however, be remembered that not every insemination of the donor mare will result in conception and the subsequent flushing of an embryo. It is important that mare owners have realistic expectations of success rates.

As embryo freezing techniques improve, a new industry in the sale of frozen embryos has also arisen. These embryos are of specific high genetic merit and may be bought and implanted into a suitable recipient mare in the same way as described in this article.