Blood Percentage: We’ve Been Doing It Wrong

Ballaghmor Class, multi-5* winner for Great Britain’s Oliver Townend. Photo by Tilly Berendt.

One of the most hotly debated topics in modern day eventing centers around the subject of blood percentage: namely, how much blood is the “right” amount for an upper-level horse?
It’s one of those questions where if you ask 10 different people, you’re likely to get 15 different answers. But before you can even begin researching the answer to that question, first you have to ask: is our current standard method of measuring a horse’s blood percentage even accurate?

Historically, a horse’s blood percentage (translated, the amount of Thoroughbred or Arabian lineage found in their pedigree) has been calculated on paper, via its pedigree. To give the most simple example of how this works, let’s say you crossed a Clydesdale with a Thoroughbred. On paper, the blood percentage of the resulting foal would be 50%, because the Clydesdale parent had a blood percentage of zero and the Thoroughbred parent had a blood percentage of 100%.

With warmbloods and sporthorses, however, there are so many generations of part bred or full bred ancestors that these numbers quickly become complex math. Pedigree databases such as Horsetelex, Hippomundo, and Sporthorse Data do the math for you, based on the logged generations of ancestors and their blood percentages, and arrive at an overall blood percentage for the resulting offspring.

With most modern warmbloods and sporthorses, the average calculation tends to be between 40% and 60% blood, with upper level event horses generally trending a bit higher, closer to 50% to 70%.

The Issues with Calculating Blood Percentage

Ballaghamor Class is one horse that actually has unknown lineage on the dam side, making blood percentage calculations impossible. This is not an uncommon issue. Photo by Shelby Allen.

There are two big problems with this method of calculating blood percentage.

First and foremost, many horses – particularly those of Irish descent – have big gaps in their pedigrees where their ancestors were never recorded. Take CCI5* winner Ballaghmor Class for example: we know that his sire was the Holsteiner stallion Courage II, but the only thing that has ever been officially recorded about his dam is her name, Kilderry Place. The rest of her pedigree is not documented, therefore it’s impossible to even begin to come up with an accurate blood percentage for Ballaghmor Class. Cases like this are unfortunately not uncommon.

And then you get to the next problem, which is perhaps the bigger one: the entire basis for the way we’ve always figured blood percentage via pedigree is simply not reliably accurate.

According to Dr. Samantha Brooks, Associate Professor of Equine Physiology at the University of Florida and leading equine genetics researcher, the method for finding a horse’s true and accurate blood percentage just isn’t that simple. To understand why that is, we need to have a better understanding of how genetics work in general.

“All of you out there with a full sibling, I’d like you to picture that sibling,” she explains. “Their hair color, eye color, height, all the things that you recognize as your sibling. Now think of yourself. How many of those traits do you share perfectly with your sibling? Not as many as you might think, right? Yet you have identical pedigrees. If the pedigree told the whole story then all of our full siblings would look like our identical twins.”

With horses as well as with people, every time a parent passes on genetic material to their offspring, they provide half of their genome. However, the specific half, or which gene from each pair gets passed on, is up to chance. The pedigree calculation relies on the average that each parent shares half of its DNA with an offspring. But the real question, and the part that really matters most is: which half?

“That can be a really important detail!” Samantha says. “It comes down to a game of chance, just like a flip of a coin in each generation. Over time, with each round of coin flips, the average can drift quite a long way from that 50/50 estimate in generation 1.”

Is There a Better Way?

Blood percentage has become a critical stat when it comes to event horse suitability, but is there a more accurate way to calculate it? Photo by Shannon Brinkman Photography.

So if simply doing the math of averages based on the horse’s pedigree isn’t necessarily reliable, is there a better and more accurate way? The answer is via genetic testing.
Blood percentage is one of several tests developed by Etalon Equine Genetics as part of their Ancestry series, which provides an in-depth comparison of a horse’s genetic make-up via ancestry and composition analysis.

This testing compares the test subject horse to a large dataset of other horses within various “breeds”, disciplines, and populations around the world, using a “reference” population, one in which the genetics and characteristics have been statistically analyzed and compared to one another for genetic similarity and difference. In addition to blood percentage, the Ancestry series also analyzes traits like genetic diversity, inbreeding, and genetic composition.

This type of genetic testing has been used in the research world for almost 20 years, and the first broad examination of global horse breeds using genetic ancestry analysis and a large set of genetic markers was published in 2013.

“Using thousands of genetic markers is the key to doing this right.”, Samantha says. “The genome is a big set of data: around 2.7 billion base pairs of code! Dozens or even hundreds of markers just can’t cover all of that genome. You need thousands of markers to really get a good picture of where each segment of the genome likely came from.”

My curiosity was especially piqued on this subject earlier this year when I had an Ancestry test done on one of my own horses, my broodmare by a Thoroughbred stallion out of a Mighty Magic mare.

On paper, she’s around 77% blood, but genetic testing showed her to be more like 67%. For this particular instance, that does seem to track – my mare’s warmblood heritage shines through pretty strongly, and she’s a bit heavier than you would expect for a horse that was 77% blood. Having an actual genetic percentage that’s lower than what’s on paper certainly makes sense in her case, and helps give me a better idea of qualities I would be looking for when shopping for a stallion to breed her to.

Several months ago while on a call with Christa Lafayette, CEO and founder of Etalon Equine Genetics, to discuss my own horse’s results, she seemed very unfazed and unsurprised by the variance. In fact, Christa said that through their genetic testing Etalon has seen variances as large as 30% between the blood percentage that’s on paper versus what the horse actually has genetically.

Other examples come from the CCI5* horses Vandiver and Tsetserleg. On paper, Vandiver has a blood percentage of 67%, but genetic testing shows an actual blood percentage of 71%, which is relatively close.

Tsetserleg, however, is a bit more interesting: his pedigree says that he’s 47% blood, but testing shows that he’s actually 62%. That’s a marked difference.

Examining an extremely popular eventing sire, the Irish Sport Horse Cruising, gives you a hint as to just how these types of variances can end up happening generationally. On paper, Cruising is around 53% blood, but genetically he’s really 66%. If you consider that every ancestor could be even just a little bit inaccurate on paper, you can begin to see how wildly inaccurate the final pedigree calculation may actually be.

What Comes Next?

Boyd Martin’s Olympic and World Championship mount, Tsetserleg, was found to have a higher blood percentage than originally thought through genetic testing. Photo by Tilly Berendt.

It also made me wonder: has anyone started keeping statistics regarding the actual blood percentages of upper-level event horses? The age-old argument is “how much is the right amount”, but since we can’t really trust what’s on paper, has any sort of documentation been kept regarding current upper-level event horses?

“Ah, this is what we are all chasing! How to find a good event horse!” Samantha says. “Sadly no, I don’t yet have a comprehensive dataset to test the idea that there should be a perfect mix that will be overrepresented in the elite eventers.”

Key word: “yet”. Samantha and Christa both would love to see some sort of metric or dataset come to fruition and be able to answer that question with certainty. The problem? As with most things, it often comes down to funding.

”I’d love to investigate that,” Samantha says. “But unfortunately there just isn’t enough funding for horse research out there right now. I can say that the availability of this test commercially has provided a fair number of notable examples now shared broadly, and the picture illustrated by those examples suggests that the magic mix might be about 2/3 blood.”

It’s also important to note that there are pros and cons to the blood percentage metric. On one hand, it gives us an idea of how much of a horse’s genome is made up of the breeds known to contribute to athleticism, gallop, and forward-thinking. On the other hand, it doesn’t tell us exactly which genes were transmitted by these blooded ancestors.

Samantha warns that it’s important to keep in mind, particularly for breeders, “These same populations can have a few undesirable traits too” such as poor hoof quality, back issues, smaller frames, or excessively flighty temperaments.

Her statement illustrates the fact that while blood percentage is inarguably a very important metric when it comes to event horses, it’s not the only one that matters. “In the end, blood percentage is also just a more accurate type of estimate. What we really need to know is not just where the genes for athleticism came from, but which genes those are, and which type of each of these genes we need in order to produce stronger, healthier, more successful eventers.”

Samantha went on to explain that the good news is, we have all the genomic technologies needed to identify these genes. The bad news is, scientific research, like most of the things we love about horses, isn’t cheap.

Graphic courtesy of Etalon Dx.

That’s not to say that significant strides haven’t been made, particularly in the last 5-10 years. Etalon Equine Genetics offers other tests that, in combination with their blood percentage testing, could potentially help give a much bigger picture of what other genes the “blood” part of your horse might be contributing. Just to name a few, there are health tests for things like kissing spine, metabolic diseases, and anhidrosis, as well as performance ability tests for traits such as temperament and endurance. These types of tests could be particularly valuable for people looking to source their next top horse, as well as breeders that are striving to produce it.

Still though, additional research is needed to get a more holistic understanding of exactly what a horse’s blood percentage is telling us and how it relates to their potential as an event horse. On that subject, Samantha seems fairly optimistic.

“I think, and admittedly I’m biased, that eventers are the likely candidates to ‘lead the pack’ when it comes to discovering and adopting new scientific technologies like genomics in horse health. We intuitively seek out adventure, know how to manage risk, and can put in the hard work it takes to do something great, be it our sport or scientific research. To all the eventers out there here’s a challenge that needs you: we’ve got to take on the responsibility of supporting and funding scientific research for the sport horse. ”
Part of supporting research and funding is utilizing the tests we already have available to us, which will only serve to increase our understanding of our horses and what traits we should be looking for. Finding out the actual genetic blood percentage of our sporthorses and warmbloods rather than relying on a potentially inaccurate estimate is a solid start in the right direction.

“Investment in scientific research will pay off, in the long term, with new ways to keep our horses healthier and performing at their best, for longer. At the end of the day, isn’t that what we all want most?”

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