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Tipping Point for the Friesian Horse?

Friesian Mare and Foal. Photo: Katya Druz /

The Friesian horse is beautiful. The shiny black coat, proud neck, flowing mane and impressive movements delight enthusiasts worldwide. An icon for Friesland and for the rest of the Netherlands. The Friesian horse is also an important export product, with enthusiastic buyers in the United States, Mexico, South Africa and China, among others. Still, many breeders and owners of Friesian horses are concerned about the future of the breed. This is mainly due to health problems resulting from inbreeding. In this long-read an investigation into those concerns, the backgrounds and possible solutions.

Author: Mirjam Hommes

Supported by The Netherlands Support Fund for Freelance Journalists Original article in Dutch:

This English translation was largely carried out by translation software and checked for errors by Angie DePuydt and the author.

The Friesian horse has been on the verge of extinction a number of times in the course of history. The breed survived only thanks to a number of persistent breeders and studbook administrators. At the lowest point, there were about 500 broodmares and only three studbook stallions. These animals form the basis of the current Frisian population, which is estimated at about 75,000 horses, 50,000 of which are in the Netherlands. Because all these horses descend from a narrow base and because the population has not received any blood from outside for 44 years, the mutual kinship is high. This poses a risk of hereditary disorders and of a so-called ‘inbreeding depression’, in which the horses are less healthy, fertile and sustainable.


There have long been concerns about the genetic health of the Friesian horse. At the end of the twentieth century, those concerns became increasingly acute. Two highly visible hereditary conditions plagued breeding: dwarfism and hydrocephalus. Dwarf foals are born with too short limbs. Often these animals survive, but their lives are usually not long and healthy. A foal with hydrocephalus is not viable and also poses a serious threat to the mare.


When the science was ready, genetic testing of the DNA of dwarf and hydrocephalus foals pinpointed the genetic markers for these conditions within a few years. In both cases it turned out to be simple recessive abnormalities. That is to say: only if a foal receives the defective gene from both parents will it be sick.

The Inheritance of Dwarfism & Hydrocephalus. Source: KFPS

Tests have been available since 2014 to check hydrocephalus and dwarfism carriers so that breeders can avoid mating two carriers. This way, the risk of a hydrocephalus or dwarf foal can be excluded. All young star mares and studbook stallions are now tested and for other breeding animals the owner can easily arrange this through the studbook. The studbook explicitly chooses not to exclude carriers from breeding, because you also lose other favorable characteristics. The fewer breeding stock you use, the smaller the genetic diversity becomes. You don’t want to make that genetic diversity narrower than necessary. All breeders are asked to test their mares and to avoid ‘risky’ pairings. The responsibility is thus placed with the mare owner. Several breeders suspect that the genetic information for hydrocephalus lies in a gene that also brings a number of positive properties for the sport. Remarkably often, animals that carry the hydrocephalus gene carry favorable characteristics that are rewarded in sports and inspections. For that reason, they are often used as breeding animals. Some breeders even feel that for this reason there are more and more carriers of the hydrocephalus gene in the population. However, figures on the numbers of carriers are not public.

ARE DWARFS AND HYDROCEPHALUS FOALS STILL BEING BORN? Several foal buyers I spoke to over the past few months said they still occasionally encounter dwarf foals. The vets who contributed to this article say they see little or no abnormal foals. Marco de Bruijn, internist at the Wolvega veterinary clinic: “I have practically not seen dwarfs and hydrocephalus foals for a long time, since the genetic tests came into being, but actually before that.” Former veterinarian Siebren Boerma was very concerned in the past about the genetic abnormalities in Frisians: “Together with Professor Bak from Utrecht, I was first to collect genetic material from foals that were born with hydrocephalus or dwarfism. That was in the late 90s. The aim was to develop a reliable gene test so that these two disorders could be bred out of the population. The studbook and many keepers of Friesian horses wanted to know absolutely nothing about the large amount of likely hereditary defects within the Friesian breed. The problem did not exist, they thought. Or at least I was exaggerating. But it was at that time abundantly clear that hydrocephalus and dwarfism were much more common in Friesians than in other breeds and that this indicated a hereditary problem. Vets who were paying attention also knew, that there was much more going on. In 2007 a bill was passed to ban breeding with animals that have a genetic defect. After a lecture by me about the various genetic problems in the Friesian horse, the press translated this to me saying ‘we should stop breeding Friesian horses’. That’s not what I said, but it caused a lot of fuss and negativity at the time.” Boerma retired from his veterinary practice in April 2021. Lately he has observed a turnaround: “I think the number of deviations has fallen sharply in the last five or six years. The number of foals born with dwarfism and hydrocephalus has become very low since the tests started. If I may be optimistic, I think things are going in the right direction. But to be sure you would like to have numbers about how often something occurs. Dwarfs and hydrocephalus were covered up in the past, so we didn’t have good numbers on that.” Stories from the Friesian community on the internet seem to indicate that sufferers from one of these conditions still occasionally occur, but that this is in fact always the result of not testing the mare or of a ‘calculated risk’. The latter sometimes occurs when a mare does not become pregnant with the chosen stallion. Before the end of the breeding season another stallion from the same stud is then used. A gamble on things not getting out of hand too quickly, but with negative consequences in some cases.

THE RISKS OF INBREEDING The two best-known hereditary disorders in the Friesian horse, dwarfism and hydrocephalus, now appear to be reasonably under control. Unfortunately, that does not mean that all problems for the breed are over. The narrow genetic basis of the Friesian breed continues to have an impact. There is therefore a lot of scientific research into genetic disorders, with the complex of connective tissue disorders currently appearing to be the most urgent. Before we talk further about the risk of inbreeding in a breed, let’s talk a little more about the basic principles of inbreeding and how inbreeding and kinship are registered in the Friesian studbook.

MAKING INBREEDING AND KINSHIP VISIBLE Inbreeding is the result of mating between animals that are related to each other. Animals share genetic information when they have a common ancestor. How strong the inbreeding is, depends on the degree of kinship between the parents. This relationship is relatively high in the Friesian horse, because the breed has been on the verge of extinction a few times. Inbreeding and kinship are confusing terms that are not always used in the same way. This creates uncertainty, even among breeders. When KFPS members log in to the studbook website, they can see for their own horses what the inbreeding percentage of the animal itself is over five generations, the relatedness percentage compared to the entire population and they can calculate what those numbers will look like for their foals, when they breed their mares to specific studbook stallions. I asked Bart Ducro, university lecturer and geneticist at WUR, to provide interested readers with some more information about how these key figures are generated. If you find this information a bit ‘too much’, the main text continues below.

WHAT DOES THE INBREEDING PERCENTAGE SAY? Bart Ducro: “Whether an animal is inbred or not, has to do with the relationship between the parents. The inbreeding percentage indicates the chance that an offspring will receive the same information for a certain gene from the father as from the mother (is homozygous). For the Friesians, we actually use the level of inbreeding over all known generations as the kinship percentage. That number says something about what you can expect from inbreeding in the next generation. The Friesians are around 18.0% in the youngest generation of stallions. There is often confusion about these numbers because there are different definitions for the relationships between animals and how they are measured. Internationally, another measure is often used, the ‘average relatedness’. It is defined in a different way and that is why confusion sometimes arises. This international definition resembles a doubling, but should not be used to make a prediction for the next generation. For the Friesians, we opted for a practical explanation of the kinship percentage. You can use that number to make a statement about inbreeding in the next generations. It is your steering mechanism.”

HOW IS THE KINSHIP PERCENTAGE AMONG THE FRISIANS CALCULATED? Bart Ducro: “The relatedness percentage says to what extent there is overlap in the DNA of the horse in question with the future breeding population. We always calculate that number against the breeding population for the future. You can also choose to compare the relationship with the mares that are currently being used, for example the list of mares that are now producing foals. You then look at how much DNA they share. A common percentage is then derived from that analysis; the relatedness percentage. But not all mares in the population are used for breeding. Star and crown mares often produce more foals. You want to take that into account. That is why we have chosen a figure that points to the future to calculate the kinship percentage. We therefore use the last three years of foals as the reference population. This is because the next population of mares comes from those horses and we want to know what the relationship of that population is. By using the foals for the calculation, you also take into account popular mares more. It’s about that overlap in the DNA and it’s an estimate based on the family tree. When calculating the kinship percentage, we look at the last eight generations, as far as we know. In horses, a generation is about 10 years. If you were to read the DNA, you would get a more accurate picture, but the use of the family tree in itself is a suitable method. The registration is also quite good for the Friesian population. And if we notice that information is missing or incomplete, then the horse in question gets a little penalty, so that he doesn’t get a lower kinship based on incomplete information. Over time, you see that families that have more offspring become more and more closely related.”

WHAT DOES THE NUMBER FOR INBREEDING RATE SAY? Bart Ducro: “The inbreeding rate indicates the increase in the average level of inbreeding in a population, from one generation to the next. The increase in inbreeding level is not linear and is expressed relatively, how much is left to complete inbreeding. Complete inbreeding means a percentage of 100%. That is a fictitious ceiling, in practice you will never achieve that. The figure we calculate for the inbreeding increase is the difference in increase for this generation and for the next generation, compared to the inbreeding level at that moment. So it is a relative number, a quotient.” Because the inbreeding rate is a relative number, this number also sometimes leads to some confusion. In the Genetics manual for BSc level it is summarized as follows: “The magnitude of the increase in inbreeding gives an indication of the risk of inbreeding depression and the decrease in genetic diversity. The more the animal is inbred, the more characteristics of the father and the mother will be equal. The two alleles in the DNA will then be the same, the animal is homozygous for that trait. The rate at which the remaining genes also become homozygous in the following generations decreases because there are fewer and fewer places where the animal is not yet homozygous. This means that the higher the inbreeding, the more the inbreeding increase will level off.” Bart Ducro: “That is why the increase in inbreeding is related to the remaining part of the population that has not yet been inbred. In the formula you divide by 1-F, where the F is the coefficient of inbreeding and indicates which part of the genome is homozygous if derived from descent. For example: Suppose the inbreeding in the parents is 0 and in the next generation is 0.008 (so in percentage is 0.8% inbreeding) then the inbreeding increase is equal to 0.008. That is below the limit of 1% inbreeding increase and therefore more or less safe. In another population in which the parents are 20% inbred (ie F=0.2) and the offspring 20.8% (=0.208), the difference is also 0.008. But since the parents are already inbred you have to divide this difference by 1-0.20 to calculate the inbreeding increase and then it is 1% and that is, the risk of inbreeding problems is equal to a population increasing from 0% to 1% inbreeding.”

STATE OF AFFAIRS: INCREASE IN INBREEDING A commonly used number to weigh whether the inbreeding of a population is getting worse, is the increase in inbreeding per generation. The United Nations Food & Agriculture Organization (FAO) uses this number to determine whether a variety is in danger of extinction. The FAO urges studbooks to keep the inbreeding rate well below 1% per generation and asks to aim for a growth rate of less than 0.25%. The organization finds a temporary increase to 0.5% acceptable. The textbooks state that an increase in inbreeding of more than 0.5% per generation poses a lot of risk: hereditary disorders manifest themselves and an accumulation of adverse effects on fertility, health and longevity can be expected. An inbreeding increase above 1% increases the chance that the population will not survive in the long term. Around the year 2000, the increase in inbreeding among Friesian horses was around 2% per generation, in 2012 that number fell below the critical 1% increase. At the moment, the increase in inbreeding within the Frisian studbook has been around 0.5% for several years and therefore within the FAO standards.

WHAT THE NUMBERS SAY Figures on inbreeding and kinship are one thing, but even if they were completely undisputed and crystal clear to everyone, the question remains: what do those numbers mean? The kinship percentage will probably soon rise above 20% among Friesian horses, but is that really bad? Hydrocephalus and dwarfism appear to have become a fringe problem within the breed, thanks to the testing. But what are the other risks of a related, closed population where only a limited part of the animals are used for breeding? Genetic diversity allows for flexibility in a population. If a bottleneck occurs, it results in a drastic decrease in population size, followed by a recovery. A bottleneck often has an enormous negative influence on genetic diversity and that effect is irreversible. Such bottlenecks have happened several times in the Friesian breed. Steven Janssens, geneticist at KU Leuven: “At one point there were very few approved stallions. Then you have a greater chance of certain health effects and hereditary diseases.” However, genes are not static, although change is slow. Bart Ducro: “Because the Friesians form a closed population, there will always be an increase in inbreeding. That’s not a problem, as long as it doesn’t go too fast. If the increase in inbreeding is not too great, the loss of genetic diversity through inbreeding is compensated by all kinds of other genetic processes, of which mutations are the most important. Those mutations bring new information to the population and therefore more diversity.”

GENES OPERATE IN A COMPLEX WAY Fighting genetic disorders and undesirable hereditary characteristics is difficult. Most hereditary diseases do not work as ‘simple’ as dwarfism and hydrocephalus. In fact, it is almost never a matter of one gene that only produces a sick animal if it is passed on to the offspring by both parents. Often several genes are involved and they also influence each other.

“Genes can be on or off, or a little on or off, or only on or off if another gene is at least a little off or a lot on… A handful of genes make an entire organism, just like eight notes are enough for all the symphonies of the world.” ~ Midas Dekkers

INBREEDING DEPRESSION OR NOT? In genetics, one speaks of the risk of an ‘inbreeding depression’ if animals are too closely related. With inbreeding depression, genetic abnormalities have arisen and the animals have become less durable and less fertile. A decreasing height can also be a signal. The problem with this is that you only really notice when it’s actually too late. Inbreeding depression is also described by geneticists as a kind of snowball effect, you do not know at the beginning that you are already in it. It is therefore very difficult to know whether there is currently an inbreeding depression in the Friesian horse. It is even very difficult to research that. Because the Frisian population has been very small on a number of occasions, the kinship among all Frisians is quite high. This limits the research possibilities, because truly unrelated animals simply do not exist. You cannot therefore make a scientific comparison between animals with very little and a lot of inbreeding. That makes it difficult to draw conclusions. The answer to the question of the real status of the Friesian breed can therefore only come from practice. Are the horses healthy, fertile and fit for the purpose people keep them for? However, there is no central registry of conditions and opinions about the general health of the breed are varied. This makes the evidence anecdotal and the impression you get about the status of the Friesian breed varies enormously with each person you ask. Known current problems in Friesian horses are: connective tissue disorders such as aortic rupture, dilatation of the esophagus and esophagus blockage (food plugs get stuck), gastric impaction (the stomach is not emptied into the intestines), fertility problems and retained placenta (the placenta does not detach on its own after birth).

CONNECTIVE TISSUE AND COLLAGEN: HOLDS EVERYTHING TOGETHER Connective tissue (fascia) has a supporting and caring function. It protects the organs and determines their shape and mobility. Connective tissue also leads the blood vessels and nerves to the organs. Connective tissue disorders are still under investigation, both in humans and in animals such as horses and dogs. Aortic rupture, esophageal dilatation (megaoesaphagus) and gastric impaction are examples of connective tissue problems in Friesian horses. Hypermobility is also related to the connective tissue. At the moment it seems that the connective tissue problems in Friesians are related to the incorrect production and breakdown of collagen. Collagen is a glue-forming protein and is a very important part of connective tissue. It seems that collagen formation in Friesian horses is often chaotic, which results in the connective tissue not obtaining the right structure. The Fenway Foundation reported in early February that there may be a common collagen problem in many of the genetic disorders that affect Friesian horses. These include aortic rupture, esophageal dilatation or paralysis, and gastric impaction and gastric rupture. Also with hydrocephalus and dwarfism there seems to be an error in the production of the collagen fibers. In addition, (often at a later age) the ‘bear-footedness’ of Friesian horses has a link with weak collagen production in both the superficial and the deep flexor tendon. The structure of the tendon tissue is different in these horses. A previous scientific study found that Frisians in general have more elastic tendons than English Thoroughbreds and ponies. Friezes with dwarfism were found to have extremely elastic tendons.

‘CONNECTIVE TISSUE PROBLEM IS UNDERESTIMATED’ Marco de Bruijn sees about 30% Friesian horses in his clinic. He is most concerned with the connective tissue problem in the Friesian horse. De Bruijn: “I have been collecting the cases of aortic rupture that I come across for years. I send blood samples from these horses to Ghent. Initially, the horses also went there for section. Cases of aortic rupture are also sent in from Utrecht. The same goes for esophageal dilatation. We work together with Ghent University, Utrecht University, WUR and the University of Kentucky on genetic research into Friesian horses. Since a few years I regularly see gastric emptying problems. The stomach stretches to the point where it can no longer empty itself. This leads to a blockage of the stomach, and sometimes even to a stomach rupture.” Breeder and former stallion selection jury member Bauke de Boer is also concerned about the disorders related to connective tissue and collagen. “The health problems are underestimated. I regularly meet people who have had to say goodbye to several young horses in quick succession. All with underlying conditions, which are mainly collagen-related. People are very sad about that, but there is also a lot of money involved. People often don’t choose a Friesian horse again, after such experiences.”

MEGAESOPHAGUS MOST URGENT Research into the connective tissue problem is currently taking place under the wings of the American Fenway Foundation. Former KFPS studbook director and geneticist Ids Hellinga is now scientific advisor there. Hellinga: “We are currently fully focusing on research into dilatation of the esophagus (megaoesophagus). We see that this is an increasing problem. It seems that aortic ruptures are somewhat less common. It is unclear why this is, but it could be because individual stallions have a lot of influence. The faculty in Utrecht has tried to find out the incidence, but in the Netherlands it has not been officially registered how many horses suffer from this. The Fenway Foundation has kept a reasonable record of that in the US. There you see that the esophagus problem happens more often than aortic rupture.”

FENWAY STUDY ON MEGAESOPHAGUS Ids Hellinga explains what the American research on esophageal dilatation entails: “In the US, they are very good at DNA technology and detecting the genetic mutations that cause conditions such as esophageal dilatation. We have already conducted research into this in the Netherlands and Belgium in the past, but it came to nothing. The phenotyping for this disease is more difficult, by which I mean that esophageal dilatation is much less recognizable than, for example, dwarfism or hydrocephalus. You do not always see that a horse suffers from this condition. Sometimes it only becomes apparent at a later age, for example we recently had an 18-year-old mare and she had already had twelve foals. That is also the dangerous thing about this condition: sufferers of megaesophagus could in an extreme case, be breeding stallions. Therefore, this problem may be even greater than previous genetic disorders in the Friesian horse.” This concerning suspicion of Hellinga seems to be confirmed by the many stories about esophageal dilation and paralysis that have appeared in recent years in, for example, Facebook groups of Friesian horse lovers. Many horses don’t show the first symptoms until later in life, although the problem certainly also affects foals and younger horses. Hellinga: “We now only have foals in the study and use the newer and more accurate technique of whole genome sequencing. Our starting point is that it comes down to one gene, but we are also expressly looking more broadly for more complex forms of inheritance. Fortunately, there is a good chance that it is a single recessive gene, which would make research easier. Thanks to the new techniques, we can limit the research to a smaller number of families. In America in particular, we have collected quite a few families, although we could add a few more. The research does not focus on individual horses, but on father, mother and some brothers and sisters, preferably young horses and foals.” Hellinga calls on owners of horses who have esophageal dilatation to participate: “If owners can provide us with data and hair samples from multiple family members, they are very much invited to come forward.” Hellinga continues: “I used to have the feeling that this was an unsolvable sudoku, but every time we add a family to the study, we observe that we are getting closer to a solution. When that will be, I can’t say. We will also have to validate the results and that takes time. But we’re doing everything we can, and the investigation is going well. I think we can develop a gene test for esophageal dilatation.”

“Megaesophagus may be an even bigger problem than previous genetic disorders in the Friesian horse.” ~ Ids Hellinga

TAIL AND MANE ECZEMA Bart Ducro: “Both aortic rupture and esophagus blockage are not pleasant at all for a horse owner, but we don’t really have an idea of what the biggest problems are with the breed. For example, tail and mane eczema will also not make an owner happy, but that disease is not life-threatening. It is an allergy and it is already clear that not one single gene plays a role, but several genes. This means that a gene test such as for dwarfism or hydrocephalus is simply not possible for tail and mane eczema. Research has contributed somewhat to the extent to which genetic predisposition plays a role, but without knowledge of the DNA behind it, breeding eczema-free is still very difficult.”

FERTILITY PROBLEMS AND SPERM QUALITY Fertility is an important characteristic by which you can tell if inbreeding is causing problems in a population. Steven Janssens of KU Leuven: “One way to investigate inbreeding depression is to plot a hereditary characteristic, such as height or fertility, against the degree to which the animals are inbred. The more inbred an animal is, the lower the height at the withers or fertility could be. This is quite difficult to do for fertility.” Bart Ducro of WUR: “Currently, about 75% of the matings with Friesian mares actually lead to a registered foal. That may not seem very good, but compared to other pedigree breeding farms it is not very bad. I don’t see a very big problem with mare fertility yet, the concerns are greater on the stallion side. A relatively large number of young Friesian stallions are removed from the stallion selection process, because their semen quality is insufficient in the tests. That percentage is much higher than in other breeds. That is a real shame and it diminishes the selection of stallions available. Years ago I myself investigated whether there is a relationship between sperm quality and inbreeding, but I could not prove any. Such research fails because all Friesian horses are inbred to some extent. For such a study you compare stallions with a higher and lower inbreeding, but that is not possible if there is not a big difference between much lower and much higher inbreeding percentages. You simply can’t see the difference. What struck me ten years ago in that study, is that the parameters of sperm quality, such as the number of cells and motility, were much worse in Frisians than in warmbloods and even Shetlanders. In my opinion, the fertility problem of the Frisians is more on the male side.”

IMMUNE SYSTEM AND DURABILITY In 2021, the breeding goal of the Friesian horse breed has been adjusted at the request of the KFPS Members’ Council. From now on, a healthy horse is the first goal, before racial type and sports aptitude. Health and sustainability go hand in hand. Obviously, many breeders and owners are concerned about the durability and longevity of their horses. Horses are sometimes unusable at a young age or have to be put down early. There is also no good overview of all this, because the registration of mortality is not (yet) in order. Many people do not hand in their horse passport after their horse has died and do not report the cause of death to the studbook. Other horses disappear abroad. This leads to a lack of really reliable figures about the average life expectancy of the Friesian horse. It means that we continue to rely on anecdotal ‘evidence’. Veterinarian Marco de Bruijn: “In our clinic I observe that the Friesian horse is less able to solve problems by itself these days, whether that is a respiratory infection, roundworm or a recovery after colic surgery; it is very difficult. To be honest, I am increasingly dreading operating on a Friesian with colic. Another breed often returns home within a week, but with Friesians you often first see diarrhea or an infection, if the intestines start up at all. The horse is just not that strong. I understand that people want a beautiful Friesian horse, but in my experience that comes at the expense of health and immunity. There are also problems that may be related to the immune system, such as poor uterine emptying and fluid in the uterus. I think it all comes down to immunity and little genetic diversity.” The afterbirth can also be related to inbreeding, explains De Bruijn. “A placenta is basically foreign tissue and is therefore rejected by the mare. But relatedness is high, the mare’s immune system may no longer recognize the placenta as foreign.” In 2004, M. Sevinga and colleagues from Utrecht University already investigated the influence of inbreeding on the mare’s retention on the afterbirth, after the number of cases increased spectacularly between 1980 and 2000. The researchers found a connection with the relatedness percentage of the foal. “A high incidence of afterbirth is at least partly due to inbreeding,” they concluded. Veterinarian Siebren Boerma says that the number of problems with retained placenta seems to be decreasing in recent years. “But that’s not necessarily because the hereditary factor has decreased,” says Boerma. “Today hygiene is better and people are also more aware of the problem than before, which means that interventions are carried out faster and fewer big problems arise.”

HOW DID WE GET HERE? If you want to come up with solutions and make the future Friesian horse healthier and more sustainable, it is wise to be aware of how the breed ended up in its current situation. The history of the Friesian studbook and the development of the association culture provide insight into this.

‘FRIESIAN TYPE’ HORSES Stallion owner Erwin Spliethof likes to delve into the studbook history: “In the early days, the horses were registered in two registers: register A for the ‘Native breed’, which was the Friesian breed, and register B for the ‘Foreign and crossed breed’. Those were more typical of the Groningen type of horse as it occurred at the time. In the period between 1907 and 1915, the Friesian breed seemed doomed, after the then board decided to include all horses in one register. Stallions were limited to Friesian and Oldenburger or East Friesian breed and all possible crossing products could be registered in case of mares, as long as they met the breeding goal. That was: ‘a harness and agricultural horse with smooth and powerful gaits.’ In 1915, after persistent scrambling by the followers of the Friesian breed, the books were again split into ‘Friesian breed’ on the one hand and ‘Bovenlands or crossed breed’ on the other. Moreover, from 1918 onwards, horses could only be entered in register A or B if the pedigree was known. That is why the Support Book was opened for mares that were ‘Friesian type’ and met the breeding goal, but whose pedigree was unknown. In the first decades after the studbook was established, the association repeatedly went through ups and downs. It was difficult for the studbook to protect the breed. You can only have respect and admiration for the people who at the time worked hard to keep things going. But if you understand what that initial period looked like, you also understand that the term ‘racial purity’ is quite relative. ‘Friesian typical’ is a much better term. I think it’s important to realize that, especially with a view to future measures to curb inbreeding in Friesians.”

FROM PEDDLING FOR MEMBERS TO A CLOSED STUDBOOK Because of this history, for a long time the studbook did mention thoroughbred Friesians, but used the description: a ‘Friesian-typical’ horse. In the 1960s, racial purity wasn’t much of a concern. When a board member of the studbook saw a Friesian-looking horse standing in a meadow, he walked into the yard to convince the farmer to register the horse in the studbook and use it as a breeding animal for the endangered Friesian breed. Slowly the numbers of horses grew and by 1978 the situation had completely changed. The association believed that there were now enough Friesian horses and the studbook was closed. This means that since then no more animals from outside can be registered in the Friesian Horse Studbook (KFPS). From 1978 it is therefore no longer allowed to breed with animals with ‘foreign blood’. Foals with a sire or dam that are not registered with the KFPS no longer have the opportunity to enter the studbook. Not even if these ‘half’ Friesians are continued to be bred using registered breeding animals, and not even if this takes place over several generations. There is no more room in the statutes of the studbook for this kind of ‘lateral entrants’. This means that the gene pool – the total of hereditary characteristics within the breed – has not been supplemented since 1978. New combinations are always made, but from a limited stock of genetic material. This makes the Friesian studbook different from, for example, the largest sport horse studbook in the Netherlands, the Royal Warmblood Horse Studbook of the Netherlands (KWPN), but also from a studbook such as the Groninger Paard, which is an example of a registered rare breed.

‘FULL PAPER’ AS THE HOLY GRAIL When you read the advertisements for Friesian horses on the Dutch online ‘Market Place’ or the KFPS website these days, you immediately notice that people often talk about ‘full paper’. This is an important recommendation for a Friesian horse. ‘Full paper’ means that the horse comes from a maternal line of at least three generations of mares with a star predicate or higher (crown or model). Mares receive a star predicate when they receive a minimum average of 7 for conformation, walk and trot from the jury on a studbook inspection or breeding day. Horses born from a studbook mare (less than a 7 on average at the inspection) or a foalbook mare (never attended the inspection as an adult or underperformed there) do not have a ‘full paper’. These are still full Friesians, who are in the closed studbook or in the foal book, but they are often worth a lot less than the ‘full paper’ animals. Even for horses that are only used for sport and not for breeding, such as geldings, a ‘full paper’ has become a distinguishing feature that often makes thousands of euros difference in the resale value. As a result of this common obsession with a full paper, breeding is mainly done with star mares. That limits the breeding population. In recent years, only about 3,200 foals were registered per year. The studbook aims for 4,500. It is expected that several hundred more foals will be born this year.

REGISTRATION AND OPENNESS REQUIRED Vets, owners and geneticists observe a number of problems with the Friesian horse breed, but how often these occur and where they come from, remains unclear. Bauke de Boer is a big proponent of registration and openness: “You have to map out which lines are sustainable and which ones tumble. Of course there is luck involved in breeding, but I hear too much doom and gloom. There are breeders and stallion owners who want to keep everything under wraps, but I think that’s stupid. It has to be open and exposed, in order to get out of the misery. Breeding is foresight. We must all work together to ensure that you breed a healthy horse, which does retain the breed characteristics, but which you can do something with. It is a duty to breed a healthy horse.”

‘TAKING STRIDES’ Until now, there has been no systematic registration of genetic problems, disorders and longevity of the Friesian horse. This despite previous recommendations from scientists. For example, in 2011, Siem Korver argued for a completely closed (and mandatory) registration of matings, pregnancy and birth defects. Bart Ducro: “At the moment we lack a good overview of what exactly is going on and what the priorities should be. If it had been properly registered, I could now tell you: ‘These are the big problems’. It would be preferable to have some sort of database of the identified problems, with added DNA samples from blood or hair. This would allow us to take strides and move forward, because the techniques are getting better, cheaper and more reliable. You could even make such a database anonymous. Once you have the DNA profile, it doesn’t matter which horse it is. As long as you know what condition was seen in that horse. The DNA pattern is a kind of barcode, which we can then use to investigate which gene is behind which condition. But now we have little material or only material that has been selected very strongly.”

COVER-UPS AND CONSPIRACY THEORIES The old culture in the Frisian studbook, which veterinarian Boerma also encountered in the 1990s, meant that there was no registration and that data on the occurrence of genetic abnormalities was not shared. It remains unclear how often deviant foals are born. Some breeders and stallion owners still choose to keep problems under wraps as much as possible, because they fear openness would be bad for reputation and trade. A hotline on the internet that claims to register defects in the Friesian horse remains completely unreachable for comment, despite repeated attempts, and does not even want to anonymously indicate how many deviations they have registered in recent years. Over the years, the supposed cover-ups of hydrocephalus or dwarfism foals has led to suspicion among members of the association and sometimes even the spread of conspiracy theories among some of the breeders. For example, the theory regularly crops up that there are several types of dwarfism and that the existing test only detects one of those types of dwarfism. Geneticist Bart Ducro of Wageningen University earlier commented on this in De Paardenkrant: “Disproportionate dwarfism (chondrodysplasia) occurs in Friesian horses, that is the form we have always seen. This shape is specific to the Friesian breed.” This form of dwarfism is therefore tested. However, the lack of transparency and (public) registration of problems gives critics room to argue that the test is unreliable.

DEVELOP KNOWLEDGE AND IMPROVE REGISTRATION Veterinarians such as Marco Bruijn have been keeping their own lists for years and sometimes data has been collected for specific investigations, but the studbook does not have a numerical and structural overview of what is going on in terms of hereditary disorders in the population. Even the death rates are largely incomplete. In 2021, the studbook therefore started a campaign to encourage people to deregister their horses from the studbook if they die. In such a report, the cause of death can also be entered. Studbook director Marijke Akkerman: “The deregistration of deceased horses is still not done enough by our members. We hope to gain more insight thanks to the European law on the Identification and Registration of Horses that was implemented in 2021. The studbook can link to that data and thus gain a better insight into the mortality rates.” The studbook also wants to link the data on the causes of death to other data registered about the horses. Akkerman: “A working group from the Breeding Council is working on this. We want to compare certain linear scores for, for example, leg position and height with lifespan. This research into sustainability is still in its early stages. But of course everyone wants to enjoy their horse for a long time. In addition to sustainability and longevity, there is also a study into fertility, together with Utrecht University. That study focuses specifically on sperm quality.” Akkerman indicates that the studbook now also wants to keep better track of which studies are being conducted in different places in the world and how far along these studies are. “We also want to be more on top of this ourselves and in 2022 we will communicate more about this,” the director promises.

“WE KNOW TOO LITTLE” A growing group of breeders and enthusiasts has lately been pressing for transparency. The current board and the breeding council also seem to want to move in that direction. Tjitze Bouma, the new chairman of the KFPS breeding council, said in the online ‘College Tour’ of March 31, 2022: “You will have to collect more data about health and sustainability in breeding. In fact, we still know far too little about our horses. Few horses are deregistered when they are deceased. And if they have died, you would also like to know the reason. Not to denounce people in public, but because you want to be able to discern from the data which bloodlines have greater longevity, for example.” Bouma also said that they are also considering measuring older horses. At the moment, the young horses are assessed mostly, but that does not provide any information about durability. “One possibility is to ride and test the horses again sometime between the ages of 12 and 15, to see how they are doing.”

‘TACKLE PROBLEMS HEAD-ON’ Systematic registration of deviations would also be an important step. Marco de Bruijn: “I don’t think we have made any progress with the Friesian race in the past 20 years. In the context of preventive medicine, you have to see if you can do something up front, because management alone will not save you. So you have to find the solution in breeding. The question then is whether there is still enough genetic diversity in the breed to solve the problems. In my opinion, to be able to answer that question, you must first map out the connective tissue problem, after which you can examine whether the current diversity is still sufficient to get to a solution. And if the answer is no, then you have to think outside the box. That’s not something everyone wants to hear, I know. And will hurt some people. But the problem is: Now that pain is also there, but it is distributed among the individuals, among my customers. I stand against that.”

Systematic registration of abnormalities is an important step towards understanding the inbreeding problem and the prevention of genetic disorders

TRACKING HEALTH CHARACTERISTICS Stallion owner Erwin Spliethof would also like to see more attention being given to the health of the existing population: “I think it would be good to formulate more health characteristics, in addition to X-rays, cornage examination and the tests for dwarfism and hydrocephalus. Just as we once came up with the characteristics for the linear score, you can now start with a list of health characteristics. There are two major advantages to doing it this way. In the first place, health is universal and equally important to every owner or user of the horse. Whether you want to go for a ride in the forest, harnass your horse, or ride dressage at Grand Prix level, everyone benefits from a vital horse that can age healthily with little veterinary costs. A second important consideration with health, is that it provides hard data. Unlike jury judgments, which always contain a certain amount of subjectivity. I would therefore argue in favor of placing much more emphasis on health, especially in the stallion selection, and less on sports aptitude and exercise. Because however capable the jury corps may be, no one, absolutely no one, can tell from a three- or four-year-old horse what he has to offer by the time his talents reach their full maturity. Let alone estimate at such an early stage what his qualities could be as a breeding stallion. So it is also quite pointless to judge and test to the max, and prematurely knock a large number of stallions overboard in the process. You will probably miss a number of good breeding stallions that way. My advice: raise the lower limit for health traits considerably, leave the rest of the selection largely to the market and rely on the knowledge and skills of your breeders. Over the years, they have shown that they are capable of building successful mare lines, each following their own insight.”

HOW TO CURB INBREEDING In addition to collecting more information about the current population of Friesians, there is a lot of thought and discussion within and outside of the studbook about possible solutions to limit inbreeding in the next generations of horses. Some breeders have dropped out of the KFPS and will now breed their mares to stallions from other studbooks, but many involved think it is still too early for that, or warn of unwanted side effects. Over the years, scientists have provided several reports about inbreeding in the Friesian breed and how to control it. Although a number of recommendations from their reports have since been adopted by the studbook, such as publishing the kinship figures and assigning a breeding value to kinship, a lot still remains to be done.

STALLION SELECTION PROCESS Breeding decisions revolve around a mother’s or father’s genetic predisposition, which is what is passed on to the offspring. Breeding values and total indexes are frequently used in Friesian breeding when choosing a stallion. A breeding value is a substantiated estimate of the genetic predisposition. The number indicates to what extent a parent animal will pass on a particular trait. Breeding values are always calculated relative to the population average and are reassessed every year. The average breeding value is always 100, in practice this is a variation between 96 and 104. The more offspring an animal has, the more reliable the breeding value. For the Friesian breed, breeding values are calculated for breed appearance (race), build, legs, walk, trot and canter. In addition, a breeding value is calculated for each characteristic in the horse’s linear score. The information that is used to calculate breeding values comes from studbook inspections and talent tests, of the animal itself and its offspring. Unique to the Friesians is that since 2020 breeding values for character have also been published for the studbook stallions. This arose from the desire to maintain the gentle nature of the breed. In addition, a breeding value for relatedness is included in the rankings of active studbook stallions and of the 1,500 best mares. The more closely related the horse is, the lower the breeding value for that trait. In the so-called ’total index’, all breeding values are taken together in order to make a ranking of the most interesting broodmares. Unfortunately, the mare list also includes several animals that are no longer alive or have disappeared abroad. A problem with these rankings is that you are comparing very reliable breeding values of older animals with the much less reliable breeding values of young horses. As a result, breeders may sometimes place too much value on this index.