Professor Dr Tom Stout is an expert on embryo transfer. As such he gave an exciting clinic at the 2012 KFPS Stallion Show, that was food for lots of afterthought. “With sport mares embryo transfer is an ideal way to avoid maternity leave.”
When asked where his fascination for fertility and reproduction in horses comes from, Tom says: “I grew up in England, where I rode for years just like many other Englishmen. Working at a stud farm as a student my attention was drawn to reproduction in horses. After my degree in Veterinary Medicine I wanted to do “something” with reproduction. Although I’m also into other animals a lot, I decided to focus on horses. As far as reproduction is concerned horses offer a lot more possibilities. Cows, pigs and sheep are slaughtered when they fail to reproduce and a dog owner will think it a pity if it doesn’t work, but that’s it. People usually don’t breed dogs and cats but they do horses. That’s why the veterinarian has more freedom when it comes to horses and that what’s makes it interesting.”
Embryo transfer for dummies
In embryo transfer, an embryo is taken from the donor mare, being the biological mother of the foal to come. A flush tube is inserted into the mare’s uterus. Several litres of warm liquid literally flush the embryo from the mare’s uterus. The liquid is collected and filtered, so the embryo can be retrieved. Next, the embryo is transferred into a so-called recipient mare, that carries, brings forth and nurtures the foal. It’s important for an embryo transfer program to have ample potential recipient mares (at least two per donor mare) as the recipient mare’s cycle must coincide with the donor mare’s.
Embryo 7 days
One of the big pros of embryo transfer is that you can breed several foals a year with one good mare. Tom: “This way, a sport mare can produce foals without having tot take “maternity leave”. Also, when a mare has reproductive organs issues, embryo transfer may be a solution. Say a mare has a damaged cervix and is not able to carry the foals to full term. You might consider taking flushing the mare and transferring the embryos into recipient mares. Thanks to embryo transfer you can breed several foals with your best mares. If all goes well, you can try and flush the mare during the breeding season every two to three weeks. For most mares this is not a problem, so you could “create” a foal about seven times a year. Some mares aren’t particularly fond of the process though. Some riders notice their mares respond differently because of the hormone changes and witness behavioural changes. You could opt for a less frequent flushing in such cases.”
Are there any risks associated with flushing donor mares? Tom: “The flushing itself is safe. We do feel it’s good for a mare to have her own foal at least once. Flushing a mare over a period of years tightens the cervix, which might induce fertility problems. This goes for every mare, also mares that are never flushed. Like I said, some mares react strongly to the interference in their cycle, but that will settle if you give it a rest for a period of time. An embryo is very rarely damaged in the flushing process, but it happens. Once the foal is born, it has a different dam, but if the recipient mare is suitable, is does not pose any risks.
A flushed embryo is transferred into the recipient mare. This mare has the responsible task of carrying, bringing forth and nurturing a foal. Naturally, the demands are high. Tom explains: “At our clinic there are fifteen recipient mares. They are healthy and in good shape and are of ample height. We prefer them having had a foal before. Of course the recipient mare’s height should match the donor mare’s height. It’s not a good idea to transfer a Friesian horse embryo in a pony mare. We know that a maiden mare’s first foal is often small. An experienced dam knows how to deal with foals. We are really happy with harness horse mares, in this respect, especially when combined with Friesian horses. Harness horse mares are relatively inexpensive, big and they usually have broad bellies. We avoid mares with behavioural issues. We want them to be friendly and easy to handle. Before they’re introduced into the group, they go into quarantine for a while. If all goes well, the chance of gestation after the transfer is 80%.
Not just the recipient mares, the embryos also are carefully monitored. Tom: “Luckily horses score a 90% average on good-quality embryos. We usually flush on the eighth day. The embryo is then between a quarter of a millimetre and half a millimetre in size. A normal embryo looks like a light-coloured, tight, transparent little ball. It’s sometimes surrounded by darker dead cells, which are often bigger than healthy cells. It goes without saying that many dead cells and delayed development of the embryo are not good signs.”
The average costs for producing an embryo transfer foal are 5,000 euros, excluding service fee.
In cows you can induce “super ovulation” by administering hormones. This means that instead of one to two embryos per flush, you can get to six to as much as thirty embryos per flush per cow. “We’d like that in horses as well. However, they are less sensitive to the hormones than other species are. The hormone involved is extracted from the brains of slaughtered horses. It is not available in the Netherlands. A US lab is working on a synthetic equivalent that might be available in the near future.” But for now, the super ovulation is out of reach for horses.
Frozen versus fresh semen
Research shows there is a much bigger chance of an embryo developing with fresh semen than there is with cooled or frozen semen, a 70% versus a 35 to 50% chance. Tom: “It’s not like I advise against frozen semen, but it’s important as a breeder to be aware of the difference in success rates. It also varies enormously from stallion to stallion. Some stallions show very low frozen semen conception rates, others have a 70% success rate. In the Netherlands frozen Friesian horse semen is hardly used yet, but colleagues abroad have lots of difficulty inseminating mares with frozen semen from Friesian stallions. This might be the stallion, or the accuracy with which the semen is frozen. It’s very important to choose a diluent that fits the stallion. All stallions are different and sometimes the diluent does not match the stallion’s semen. It’s essential to try different diluents beforehand. Not everyone does, so unfortunately a lot of low-quality semen is being sold. It’s bad for a country’s reputation and for a breed’s reputation and that’s a real shame.
Like semen embryos can also be frozen. “But it is much harder to freeze an embryo than it is semen,” Tom explains. For freezing, an embryo is best flushed the sixth day. As soon as it gets any bigger, the chance of success decreases rapidly. You should check on the mare several times a day to make sure she’s flushed at exactly the right moment. Good results call for lots of experience. There are many advantages, especially for Friesian horses. You could flush the embryos of a two-year-old mare that is subsequently started and prepared for competitions. If she turns out to be successful, the embryo is there to use. There are no clear rules for bringing embryos outside Europe yet, so it’s not easy. But who knows, perhaps we can sell embryos to far away foreign countries in the future.”
Tom Stout reflects on an option in the near future: performing a biopsy on a flushed embryo without compromising its viability and having it tested for genetic disorders. Such a procedure could be of use for the Friesian horse. It will also be possible to determine the embryo’s sex. Tom: “We’re working on mapping out genetic anomalies, common also in Friesian horses. We do not know exactly which gene causes what problem nor where it is located. We have however pinpointed several genetic disorders in Quarter Horses as well as Arabians. Once you know which genes are responsible for the disorders, you can have the embryos tested for their presence. This way, you can rule out breed-related anomalies prenatally by embryo selection. Utrecht and Wageningen Universities, among others, are working on this closely together with universities outside the Netherlands. We know now in which chromosome area we’re supposed to look, but I reckon it’ll take another three to four years before we’ve located the genes and their exact location.”
UK-born Tom Stout moved to Utrecht to become Lecturer and, later, Head of the Clinic for Equine Reproduction at the Faculty of Veterinary Sciences and Professor at Utrecht University.