Modern horse breeding: Embryo transfer in horses

Isabell Werth's top mare Weihegold can already look back on more than a dozen foals as her offspring - without having borne any of them herself. Embryo transfer makes it possible. Many german horse breeders are still skeptical about modern reproduction technology, even though it is not such a new breeding method.

Modern horse breeding: What is embryo transfer actually?

Embryo transfer is an established technique in reproductive medicine in which an embryo is transferred from the mother mare to the surrogate mare. The embryo, which is just a few days old, is flushed out of the uterus of the donor mare (mother mare) with a flushing solution and, after being cleaned, inserted into the recipient mare (surrogate mare). The recipient mare then carries the unrelated foal and also takes over the rearing of the foal.

The donor mare can be inseminated by the stallion by natural or artificial insemination with fresh or frozen semen. Embryo transfer is most frequently used for high-performance mares in top-class sports. Several offspring can be bred from them at the same time without the mares having to be taken out of sport.

‍Embryo transfer sounds like a very modern procedure in horse breeding. However, the British zoologist Walter Heape carried out the first embryo transfer back in 1890. Back then, Heape transferred embryos from hares and the first ET foal was born in 1974. However, embryo transfer was not used commercially in horse breeding until the 1990s by polo horse breeders in South America. ET was then introduced in Germany in 2003. However, the German horse world is still cautious when it comes to embryo transfer. ET has its stronghold mainly in the north of Germany.

The reasons for opting for this breeding method vary greatly. So do the opinions that are critical or favorable towards embryo transfer. In the video, we clarify the question of whether embryo transfer could be the future of horse breeding:

Reasons for and benefits of embryo transfer in horses

As embryo transfer is a relatively complex and cost-intensive breeding method, it is mainly used for mares with a high breeding value. The breeding result must be in proportion to the cost of the embryo transfer. In most cases, these are top sport mares from which several foals are to be bred.

Embryo transfer in mares is also permitted:

• Taking the mare out of training or competition for only a few days
• Breeding a foal from mares that have not bred or aborted in the past
• Breeding a foal from mares that are no longer able to carry a foal themselves due to their age
• to use mares as broodmares even before the start of their sporting career
• Preserving endangered horse breeds by freezing the flushed embryos or conducting research

Donor mare and recipient mare: What are the requirements for an embryo transfer?

Embryo transfer requires some preparation, which can be very time-consuming compared to natural breeding and artificial insemination. After all, at least two, ideally even several mares are involved.

Both donor and recipient mare must first undergo a breeding suitability test with a swab sample. This involves an examination for infectious anemia and a uterine biopsy to determine the quality of the uterine mucosa and the probability of foaling.

Both mares come to the same clinic for the period of heat, mating, and embryo transfer. The sexual cycle of both mares must be synchronized so that the embryo transfer can be carried out successfully. To this end, the mares receive gynecological care, and ovulation is determined to the day. Hormone stimulants are administered to help synchronize the mare's heat and ovulation. Ideally, the recipient mare ovulates one to two days after the donor mare so that the embryo has more time to implant in the uterine lining.

As cycle synchronization is very complex, ideally there should be several receiver mares to choose from.

With regard to the semen used, vets prefer fresh semen. Fertilization with frozen semen is also possible, as the quality of the semen is crucial.

Which recipient mare is suitable for the embryo transfer?

• Especially young (3-12 years)
• Ideally had already had a successful pregnancy
• Healthy and suitable for breeding
• Good milk yield and above-average maternal traits
• Approximately the same size as the donor mare, as the size and development of the foal depends on the size of the uterus of the pregnant mare.

If the breeder does not have his own recipient mare available, he can fall back on a partner station of the clinic carrying out the procedure or look specifically at a recipient mare herd. The leasing of mares has now become established in horse breeding with embryo transfer.

How does an embryo transfer work in a mare?

1. cycle synchronization

The hormone prostaglandin is administered to the donor and recipient mare for synchronization. Only small amounts of the hormone are required to trigger estrus. This results in only minor side effects for the mares. Finally, ovulation is triggered with the hormone HCG.

‍2. donor mare is inseminated by the stallion

As in normal breeding, the mare is first inseminated with a suitable stallion by the vet.

3. uterine lavage

7 to 8 days after insemination or ovulation, the embryo is in the vesicle stage as a blastocyst and is ready to be flushed out. A balloon catheter is attached to the cervix and then 700 to 1,000 ml of lukewarm flushing solution is introduced and drained again. To ensure that the embryo has been flushed out, the process is repeated three to four times. The solution runs through a fine filter that collects the embryo.

‍4. the embryo is purified

The pre-filtered irrigation fluid is examined for the embryo under a light microscope. If it is found, it can be cleaned and transferred.

‍5. the embryo is transferred

The transfer to the recipient mare can take place surgically or transcervically. The non-surgical route is usually chosen due to the low cost. 8 days after the transfer, the mare can be checked for pregnancy by ultrasound.

The chances of obtaining a fertilized egg are between 50 and 80 %, provided that the semen used is fresh. The chances of fertilization decrease significantly with frozen semen.
Afterward, the chances of the recipient mare becoming pregnant are 25-64%.
If the transfer is successful, the mares can go home from the clinic immediately.

For and against embryo transfer in horses

Although embryo transfer has proven to be an efficient technique in horse breeding, the focus is abroad. The USA, Australia, Argentina and Brazil in particular are increasingly relying on ET foals, while German horse breeders are holding back. In Germany, two opinions on embryo transfer clash: breeding progress vs. moneymaking.

‍ProEmbryo transfer

The biggest argument in favor of embryo transfer is that the donor mare only has to take a maximum of one week out of sport or training. As she only provides the fertilized egg, she does not have to bear the health and injury-related risks of pregnancy. She can therefore be a sport mare and a broodmare at the same time. It is also possible to increase her breeding value by having several foals per year.

Even young mares at the beginning of their sporting career can become mothers thanks to embryo transfer. If they were to start breeding at the end of their sporting career, they are usually too old to withstand the physical strain of a pregnancy. In the case of promising sport mares, embryo transfer is an obvious choice.

Due to the increased reproduction of maternal genetics, the dam side of horse breeding is moving further into the foreground. Horse breeding, which is otherwise very stallion-heavy due to artificial insemination, regains some of its balance through embryo transfer.

‍Contraembryo transfer

While one side sees the opportunity to focus on the dam's side, the negative voices see the danger of arbitrary reproducibility of maternal blood in embryo transfer. However, the higher foal rate per mare can hardly counteract the flooding of paternal genetic material.

Where embryo transfer is inferior to conventional breeding methods is the financial and organizational effort involved. Both should be worthwhile in terms of the breeding result and should therefore be assessed well in advance.

And finally, embryo transfer remains an intervention in the nature of the horse. The donor mare is not able to go through a pregnancy herself. Despite the elimination of the entire pregnancy, it is a hormonal and therefore also a physical burden for the mare. This is because every aborted pregnancy also means an abrupt hormonal change. For this reason, vets recommend having a donor mare carry a foal herself every four to five years so that the uterus can refresh itself.

The rollercoaster ride of hormones is therefore an immense double burden, especially for the donor mare who remains in competition. Some experts therefore advise giving the donor mare a break from competition despite ET. This is exactly what was done with Isabell Werth's mare Weihegold: she was given a break from sport during the breeding season.

How expensive is an embryo transfer for horses?

In addition to the insemination by the stallion (stud fee depends on the stallion), the costs for the swab sample, the stabling in the clinic and the actual embryo transfer (approx. € 500), there are also the costs for the rental mare, which must either be purchased or leased. Roughly speaking, the costs for an ET foal are up to €5,000 higher than for a normal foal. These costs must be recouped with the sale of the foal, whereby the foal market in Germany is difficult. On the German foal market, if an above-average foal and a financially strong buyer come together, you won't get more than €10,000.

A look into the future: effects of embryo transfer on horse breeding

Anyone who does not rely on embryo transfer in their horse breeding must first send their mare into sporting retirement. For this reason, there is little breeding with top mares, as they have to produce yields in sport first. Instead, mares with a low breeding value are used for breeding. Here, embryo transfer can provide new opportunities to breed with the very best bloodlines. The opportunities lie not only in breeding but also in sport: by combining the advantages of both breeding and sporting use of young top mares, a higher quality in equestrian sport and competition is achieved.

Nevertheless, there is never a guarantee of a live foal. Breeding is and remains a constant hope, even with embryo transfer. Even an ET foal out of a top mare is no guarantee that it will sell for high prices or later bring home titles and gold medals.

Embryo transfer can potentially utilize the valuable genetics of mares and thus bring more balance of maternal and paternal genes to horse breeding. However, the number of ET foals from a mare is not in proportion to the number of foals produced by a stallion through artificial insemination. Experts therefore do not see embryo transfer in horse breeding as a revolution, but rather as additional potential for sport mares and old mares.

‍Onestep further: What is an ISCI horse?

However, developments in embryo transfer go even further and bypass artificial insemination in the mare. Intracytoplasmic sperm injection, or ICSI for short, involves in vitro fertilization. For this, up to 10 follicles are removed directly from the ovaries of the donor mare outside of heat. This process is called ovum pick-up, or OPU for short. The follicles are then flushed to obtain the egg cells. These mature in the incubator for 24 to 32 hours before being injected with a sperm cell. Back in the incubator, the fertilized egg matures into a blastocyst, which is then implanted into the recipient mare.

ICSI is mainly practiced in North America, in Europe in countries such as Italy and the Netherlands. In Germany, less than 1% of pregnancies are induced via ICSI, as it is a very expensive undertaking. At least in horse breeding - the situation is different in cattle breeding.

ICSI is particularly suitable for mares with low fertility or where embryo transfer was unsuccessful. In vitro fertilization is particularly interesting for stallions with only a few sperm cells left. This is the case, for example, with sires who have already died and whose frozen semen must be used sparingly.

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Author: Mirjam-Sophie Freigang