The State of Energy in Horse Feeds: Three Decades

Horses require dietary energy to fuel growth, performance, gestation, lactation, and other physiologic demands. Joe Pagan, Ph.D., founder and president of Kentucky Equine Research, has investigated equine energetics for many years, beginning when he was a student in the early 1980s. Since then, much has changed in the field of energetics. In a one-on-one with Pagan, he explains pertinent milestones in nutrition, specifically the field of energetics.

Before we talk about some of the innovations in feeding performance horses, in particular energetics, what did horse-feeding look like prior to the advent of commercial horse feeds?

As herbivores, horses are content to wander and consume plant fiber. They have digestive tracts designed to process huge volumes of forage nearly continuously. Once domesticated, horses were used in different ways, usually made to work hard, and they could not meet their energy requirements on forage alone. Today, we think of energy requirements as the horse’s ability to perform the work asked of it and maintain at least moderate body condition. Back then, energy requirements probably means the ability to put in a full day’s work without shutting down. To support this kind of work, cereal grains became a staple in diets.

Cereal grains are actually a good source of energy. The primary energy source in cereal grains is starch. Cereal grains have different levels of starch: corn has the highest, barley has intermediate, and oats has the lowest. While starch is useful for many horses, it comes with its fair share of hurdles.

When did you start looking at other ways to provide horses with energy aside from carbohydrates?

Fat became a popular alternate energy source in the 1980s. Adding fat to feeds was really the first time feeding performance horses changed significantly; before that, it was just carbohydrates. Fat has a lot going for it, including energy density. It has two and a half times as much energy as corn and three times as much energy as oats, so we can pack a sizeable punch with fat. Horses can digest fat pretty well, and fat proved palatable. One of the most significant research efforts during the 1980s concerning fat came from Texas A&M University, where the team there was feeding beef tallow to horses.

We noticed some performance benefits from feeding fat. A few years later, after starting Kentucky Equine Research, we did a lot of work with feeding fat. We found we could teach muscles to burn fat, so that other fuels, like glycogen, could be spared. This was an especially important finding for endurance athletes.

What other energy sources popped up after the introduction of fat?

 We began looking for other sources of fuel for the performance horses not long after fat came into the spotlight. We came upon beet pulp in the 1990s, which turned out to be a great alternative energy source for horses. We use the term “super fiber” to describe beet pulp because it contains fiber that is extremely digestible, nearly twice as digestible as fiber found in normal hay.

We then realized that we could use multiple energy sources in a single feed. During the 1990s, the popular performance horse feed became one that had a mix of energy sources. We used cereal grains as a base and then added fat and fermentable fiber. To this day, this is the most popular type of feed for racehorses.

What else happened in the 1990s that changed how energy was delivered to horses?

A huge change came in the mid-1990s, and it came through some collaboration with Dr. Stephanie Valberg, a veterinary professor who was at the University of Minnesota at the time. She is a specialist in muscle physiology and muscle disease. Stephanie was working on her doctorate in Sweden at the same time I was there working on my post-doc.

Stephanie and I have collaborated on research throughout our careers, and that continues to this day. At that time, she was interested in some very specific muscle disorders, and she asked for our help to see if there was a nutritional connection. The types of muscle disorders that she was particularly interested in were forms of exertional rhabdomyolysis, better known as tying-up.

Now we know there are several forms of the disease. The one that we were most interested in at the time was called recurrent exertional rhabdomyolysis or RER. This form was found in racehorses, especially Thoroughbreds and Standardbreds.

Stephanie found that the disorder is in the ability of the horse to regulate the release of calcium in its muscle to make it contract and then relax. These horses have normal insulin sensitivity, and they don’t have abnormal polysaccharides. What we know about this type of tying-up in Thoroughbreds is that it is more common in females than males. About 5% of racing Thoroughbreds have it, and there is a common behavioral characteristic: they have a nervous temperament. Typically, these horses are exercised pretty hard, and you have to feed them quite a bit of feed to fuel performance and maintain body condition. They usually have these episodes when they’re getting really fit, when there’s an uptick in their training. As they become fit, they become more excitable. Resting creatine kinase, a measure of a muscle enzyme, are normal. After they tie-up, though, the amount of creatine kinase in the blood increases dramatically. We also know that lack of routine daily exercise and high-starch diets make it worse.

Stephanie started to do some pioneering work looking at the genetics of tying-up. What she found is that there are several prominent Thoroughbred family lines that have offspring with RER. They did breeding trials that confirmed that it is genetic. But, it doesn’t happen in every horse. That was the mystery: what was the trigger? She came to us and said, “Well, if high starch causes it, could the trigger factor actually be the diet that the horses are eating?” We got together with Stephanie, and did one of many studies on that topic. The first one we published was in 1998.

In the end, what we did was make a completely different type of performance horse feed. We took out all of the grain and used nothing but fat and fermentable fiber. We got the nonstructural carbohydrates (NSCs) down to 10%; we took away all of that and replaced it with these other types of energy. As a point of reference, typical performance feeds of that time derived 43% of their calories from NSCs, so this change was radical.

We tested these feeds on some horses that we knew had a genetic predisposition to tying-up and had actual problems with tying-up previously. When we kept these horses on low intakes of traditional performance feeds, they didn’t tie-up, even after a standardized exercise test in which we tried to bring about a tying-up episode. If we raised their intake of traditional grain to the caloric intake they needed to maintain their body weight, the horses tied-up and had concomitant increases in the muscle enzyme creatine kinase. When we fed the same number of calories with Re-Leve, the new low-starch feed we had created, we couldn’t get them to tie-up. We were able to prevent the clinical expression of tying-up in these horses by altering the source of energy. That was a pretty big deal!

Why did this work?

In Stephanie’s subsequent trials, we found that the feed works not because of the change in muscle glycogen, and not because of the old-wives’ tale that these horses have high levels of lactic acid. She measured lactic acid in horses that tie-up, and the lactic acid is no higher. That is not what happens. Tying-up is not affected by dietary cation-anion balance, nor does it have anything to do with calcium intake.

The feed works because the horses are calmer. During the research trials, the horses had a calmer demeanor during standardized exercise tests. Before we started exercise, they had lower heart rates. So it seems then, at the end, what we did was change behavior by the type of energy we fed.

Stephanie is way down the pike in terms of trying to figure out a genetic basis for this. There are multiple genes involved. Why was it more prevalent in fillies instead of colts? Is there another gene that occurs more often in fillies than colts that’s a trigger gene? Is there an anxiety gene in these horses?

The creation of the really low-carbohydrate feeds literally changed the way that we look at feeding sport horses today. Modern sport horse nutrition is really a balancing act. When we are trying to feed a horse, we’re trying to get maximal athletic performance but, at the same time, it’s a balance against behavior. We didn’t used to worry about that 20 or 30 years ago. Is it because we know more about different energy sources? Are the horses different or are we riding a different type of horse today than before? Or are the riders different than they were 20 or 30 years ago?

To that point, not all horses are the same and not all horses are asked to do the same kind of work. Can you touch base on that in regard to energy?

Just as cars need different levels of octane, horses need different levels of octane. Some horses run very well on diesel fuel, and some need high-performance fuels. So, we make different types of feeds. That’s the reason why there is more than one performance feed in the marketplace. We have designed different performance products for many feed manufacturers, and across those lines we often have a low-starch versions and then feeds that have progressively more starch. All of those feeds are appropriate for performance horses, but they are not suitable for every single performance horse. The challenge is in figuring out which feed is best for the horse being fed.

Glycogen is the primary fuel used by horse muscle, so we need to make sure that we offer feeds that can replete muscle glycogen. The amount of muscle glycogen that’s used during exercise starts out very low. As the horse engages in higher speeds, it starts to rely upon anaerobic metabolism, and muscle glycogen utilization skyrockets. Glycogen is used very inefficiently, and it’s used up quickly.

So what does that mean for different types of performance horses? What does it mean for an endurance horse versus a driving horse? We don’t know for sure in some of these horses. For instance, we don’t know how much muscle glycogen is used in jumping. Certainly a lot is used. We don’t know how much extra muscle glycogen is used in driving. Certainly there is quite a bit. We know a little bit more about horses that go for longer amounts of time. For instance, the endurance horse is going to run in a speed range between 250 and 500 meters per minute, depending upon where it is in the course of a 100-mile race. The amount of muscle glycogen that is being used is actually pretty small at that point. But, they go a long, long time. Fortunately, as I mentioned previously, endurance horses know how to switch fuel. They can switch over to fat. They can change the fuel tank and preserve glycogen.

If we look specifically at three-day-event horses, glycogen burn becomes greater. As you go into higher and higher levels of eventing, and cross-country gets faster and faster, the burn gets higher. The amount of muscle glycogen that is actually used is going to matter to a certain degree on what level you are actually competing. And again, we have got to throw in a little extra for all of the jumping efforts.

Switch to Thoroughbred racing, though, and you’re in a completely different ballgame. Thoroughbred racing is done at this speed. In training and in racing, they use massive amounts of muscle glycogen. They don’t run very long, but when they’re running, they burn a lot of muscle glycogen. We have to make sure that we replete that muscle glycogen.

We did a trial a few years ago with Stephanie. One of the tools that we use extensively is the muscle biopsy, which allows us to figure out how much muscle glycogen is actually burned by a horse. In this study, we measured muscle glycogen. We exercised the horses hard enough to deplete 30% of their muscle glycogen, which is quite a bit. We then fed them for three days afterwards with either a high-NSC feed or a no-NSC feed. We found the horses repleted muscle glycogen when they had some carbohydrates in their diet, but they could not replete muscle glycogen when they did not have any carbohydrates in their diet. If you’ve got a horse that’s exercising hard enough to utilize muscle glycogen, you need to make sure you feed something that can replete that muscle glycogen. This research trial solidified the idea that you have to match the right horses with the right feed.

For readers that want to know more about your work, what would you suggest them to read?

The work I did at Cornell was published in the Journal of Animal Science, back in 1986. Like I mentioned previously, it’s the foundation for many energy recommendations that stand today. What’s more, the second paper even has some sketches of the calorimeter I used to measure energy expenditure during exercise.

Pagan, J.D., and H.F. Hintz. 1986. Equine energetics. I. Relationship between body weight and energy requirements in horses. Journal of Animal Science 63:815-821.

Pagan, J.D., and H.F. Hintz. 1986. Equine energetics. II. Energy expenditure in horses during submaximal exercise. Journal of Animal Science 63:822-830.

The other work, including the collaborative work with Stephanie Valberg, has been reported in the three volumes of Advances in Equine Nutrition, all of which are available online at ker.com.