True, the diets of horses afflicted by chronic muscle disorders such as recurrent exertional rhabdomyolysis (RER) and polysaccharide storage myopathy (PSSM) should be lower in starch (one type of carbohydrate). As well, it is always prudent to restrict the size of grain and grain concentrate meals fed to horses to avoid digestive upsets. However, it is also true that the horse is very much dependent on glucose, the body’s store of carbohydrate, for energy during exercise. For horses undertaking prolonged or high-speed exercise, a critically low supply of glucose will impair performance. In this sense, the exercising horse can be regarded as carbohydrate-dependent–the practical implication is that the diet must supply enough glucose to replenish the body’s reserves.
During exercise, the horse is reliant on body fuel stores to provide the energy needed for muscle contraction and locomotion. The horse can use three types of fuel:
Glucose–provided by stores of glycogen in the liver and in muscle (glycogen is the storage form of glucose);
Fatty acids–from fat stores in adipose (fatty) tissue throughout the body as well as fat stored in muscle cells; and
Amino acids–to a very limited extent, body proteins can be broken down into amino acids (the building blocks of protein), which in turn can be burned for energy.
The use of amino acids for energy is a very inefficient and metabolically expensive process, and generally the body only resorts to proteins for energy when glucose and fat are in short supply. From a nutritional perspective, the analogy I like to use is that protein (amino acids) in the diet is used to “build the engine,” while glucose and fat in the diet will ultimately be used to “run the engine.”
Up to 2% of skeletal muscle weight is in the form of glycogen, which provides a readily available source of energy. As well, blood glucose derived from liver glycogen stores can be used by muscle during exercise. However, the horse’s “glycogen tank” is a fairly limited resource when compared to the “fat tank” since glycogen sources provide less calories per gram than fat does. On average, 6-8% of the horse’s body weight is fat and, in the context of exercise, the horse’s store of fat is virtually inexhaustible.
Although it would make sense for the horse to utilize the fuel present in greatest supply (i.e., fat), in reality he is somewhat glucose-dependent during exercise. At rest and very slow speeds (walk and trot), fat will provide a large proportion of the energy used, particularly when the horse exercises at these intensities for a long period of time (one to two hours). Still, even during light exercise, as much as 30-40% of his energy will come from carbohydrate (glucose) stores. And during prolonged, endurance-type exercise, it is the depletion of glucose, not fat, that contributes to fatigue.
As you “step on the gas,” there is an even greater reliance on glycogen (glucose) for energy. For example, during galloping exercise lasting only a few minutes, virtually all of the horse’s energy is provided by glucose–mostly muscle glycogen. As shown in the graph on page 70, the faster the horse runs, the greater the rate of muscle glycogen utilization.
At speeds greater than 22-26 mph (600-700 meters per minute), there is a dramatic increase in glycogen breakdown. At these very high exercise intensities, much of the energy must come from anaerobic metabolism (energy production that doesn’t use oxygen), and glucose (from glycogen) is the only fuel that can be used for anaerobic metabolism. This helps to explain why there is limited use of fat during intense exercise.
There are several reasons why adding fat to the diet of athletic horses is beneficial. For example, in hard-keeper horses, the calorie boost from fat helps maintain condition. Using fat as an energy source also allows for a reduction in the starch content of the ration (more on this in a moment). But one of the fundamental rules of biochemistry is that “you cannot make glucose from fat.” This means the diet must have an adequate supply of glucose for refueling the glycogen tank. For horses in regular training and competition, a shortage of dietary glucose could result in sub-optimal glycogen stores and, as a result, poor exercise performance.
Remember that all starches and sugars are carbohydrates, but not all carbohydrates are starches and sugars!
Not all dietary carbohydrates are the same, and certainly not all carbohydrates will supply the horse with the glucose needed for glycogen replenishment. Approximately 75% of all plant material (e.g., hay, grains) is comprised of carbohydrates. This should dispel the notion that a “low-carb” diet is just the thing for an equine athlete!
As forage (grass, hay, etc.) is always the most important part of any horse’s diet, it is evident that a “low-carb” diet for horses is simply unrealistic. In fact, horses consume high-carbohydrate diets regardless of whether they receive any grain in their diet.
Dietary carbohydrates can be divided into two broad categories:
Hydrolyzable carbohydrates–These include the simple sugars (such as glucose) and starch. Enzymes in the small intestine break down these carbohydrates into simple sugars that can be absorbed into the bloodstream. Hydrolyzable carbohydrates are also termed non-structural carbohydrates.
Fermentable carbohydrates–These carbohydrates, also termed structural carbohydrates, are primarily found in plant cell walls (i.e., dietary fiber). Examples include cellulose, hemicellulose, and pectin. These carbohydrates are resistant to the horse’s own digestive enzymes; instead, breakdown of these fibrous feed components relies on the action of microorganisms that live within the gastrointestinal tract, mostly in the hindgut. The horse utilizes starches and sugars as glucose, digesting them with his own enzymes (which act in the small intestine). Fiber (structural carbohydrates in the plant cell) is fermented by the microbes in the hindgut. This process produces short-chain or volatile fatty acids that are absorbed and used as energy sources. (Note that all carbohydrates are potentially fermentable. For example, if starch or sugar is not digested in the small intestine, it will enter the hindgut and be fermented, potentially compromising the health of the intestinal tract and the horse.)
Glucose for glycogen replenishment will come from two major dietary sources. The first is simple sugars (like sucrose and glucose) found in molasses and pasture grasses (such as perennial ryegrass). Molasses is probably the best recognized source of dietary sugar for the horse, and some have attributed the so-called “sugar high” in horses to the inclusion of molasses in the diet. However, for the most part, molasses is not a significant source of sugar for the horse.
For example, let’s say a horse is consuming eight pounds (3.6 kg) of a sweet feed per day that is 10% molasses by weight. Only half of the molasses is actual sugars–the remainder is water and other solid constituents. Therefore, of this eight pounds (3.6 kg) of sweet feed, only 0.4 pounds (181 g) is sugar.
Contrast this situation with a horse grazing a lush spring pasture that, on a dry matter basis, has approximately 3.2 ounces per pound (200 grams per kilogram) of sugar. Assuming that the horse has access to pasture all day, he can consume upwards of four to five pounds (1.8-2.3 kg) of sugars per day.
No wonder many horses get fat when given full access to lush, spring pasture!
The other major source of glucose for the horse is grain. Grains such as oats, corn, and barley contain a lot of starch. Corn and barley are 65-70% starch, while oats are slightly more than 50% starch (starch is the plant’s equivalent to glycogen in the body; that is, the storage form of glucose). In the small intestine, the horse’s digestive enzymes break down the starch into glucose, which is then absorbed into the bloodstream and can be delivered to the liver and skeletal muscle where it can be used for glycogen synthesis.
A third potential dietary source of glucose is maltodextrin. Like starch, maltodextrins are made up of many hundreds of glucose molecules. However, compared to starch, they are more easily digested and represent a useful way to give a quick boost in glucose supply. “Carbo loader” supplements designed to help replenish muscle glycogen before, during, and after exercise contain maltodextrins.
For a horse on an all-hay diet, there is little in the way of simple sugars. Some legume forages (e.g., alfalfa), especially when cut early, contain a little starch. However, by and large, fiber is the major dietary carbohydrate when the horse is eating only hay. Fermentation of these fiber components by the microbes in the large intestine does provide energy for the horse. This energy is in the form of the volatile fatty acids (VFAs) acetate, butyrate, and propionate. Without going into all of the details, just remember that the only VFA that can be used to make glucose (and therefore glycogen) is propionate. However, in most cases the amount of glucose provided this way won’t be enough for a horse in regular work–his levels of muscle glycogen will get very low.
Everything in Moderation
By now, it should be evident that the athletic horse needs dietary starch and/or sugar to replenish glucose (glycogen) reserves. The questions are, how much and in what form? To sum up, the options are starch from grain or sugars from molasses, pasture, or a “carbo loader” supplement.
Before addressing this issue, the importance of dietary fiber as the foundation of any horse diet must be emphasized. Without adequate fiber, there is serious risk of digestive upsets and other health disorders. The general rule is that, at the bare minimum, one pound (0.45 kg) of hay or equivalent forage should be fed per 100 pounds (45 kg) of body weight (10 pounds for a 1,000-pound horse or 4.5 kg for a 450-kg horse). More ideally, the horse should receive 1.5 pounds (0.68 kg) per 100 pounds (45 kg) body weight (i.e., 1.5% of the horse’s body weight) as forage. Contrary to popular belief, good-quality hay is not just a “filler.” It will provide a substantial proportion of the horse’s daily energy needs.
Grain (i.e., oats, corn, barley, or a combination), with its high starch content, is the traditional high-energy feed for athletic horses. Today, those feedstuffs remain a dietary mainstay in terms of rebuilding glycogen reserves. However, one concern with grain, especially with heavy grain feeding, is the potential to overwhelm the digestive capacity of the small intestine. The result of this is the delivery of undigested starch to the large intestine, rapid fermentation of the starch, and the accumulation of lactic acid in the bowel. Too much acid accumulation can result in digestive upsets including gas colic and diarrhea.
Therefore, caution is necessary when feeding grain to horses, especially “straight grains” (i.e., oats, corn, barley, or a mix as the only component of the meal). The main concern isn’t so much the total amount of grain per day, but rather the size of individual meals. As a general guide, for a 1,100-pound (500-kg) horse, feed no more than five pounds (2.3 kg) of straight grain per meal. Also, keep in mind that oats are more digestible than barley or corn and thus easier on the horse’s delicate digestive system.
More modern feeds and feeding systems help alleviate concerns with “starch overload” while still providing enough starch and sugar for glycogen replenishment. Today’s sweet or textured feeds for athletic horses have a greater diversity of energy sources–simple sugars from molasses, fat from vegetable oils, starch from grains, and in some feeds, fermentable fiber in the form of beet pulp or soy hulls. The overall effect is a much lower starch content.
Commercial pelleted feeds also achieve the goal of lowering starch content while maintaining adequate glucose supply. In addition, these feeds include processed grain products that are more easily digested compared to whole grains. Although these commercial feeds are lower in starch content, the size of individual meals (for the 1,100-pound, or 550-kg, horse) should be no more than six to seven pounds (2.7-3.2 kg). For the high-octane athlete requiring more than 12 pounds (5.4 kg) of a grain concentrate per day to meet energy needs (such as a racehorse), the ration should be divided into three meals per day rather than the traditional two.
Of course, in order to follow these guidelines, it’s necessary to feed by weight rather than by volume. However, it’s not necessary to weigh the ration at each feeding. If you routinely use a scoop for dishing out your charge’s ration, measure the weight of a single scoop of the feed (and half scoop) and use this number to determine the volume to be fed at each meal. You will need to repeat this process if you change feeds.
Also remember that rapid changes in diet, or even in the amount of the usual grain or grain concentrate fed, can result in digestive upsets. The amount of grain fed should not be increased by more than one pound per day (for a 1,100-pound, or 500-kg, horse). Some trainers and competitors favor boosting the size of grain or grain concentrate meals for a few days before a big event in an attempt to maximize muscle glycogen reserves. To avoid digestive upsets, it is advisable to keep such increases within these limits. A safer way to boost glucose supply is to use one of the maltodextrin-based “carbo loaders” as a supplement to the main ration during the days leading up to competitions.
So remember, carbohydrate is not a bad word in the horse’s nutritional language. His body was built to run on the byproducts of carbohydrates. However, you need to learn how to supply him with those energy needs to avoid digestive upset while fulfilling the nutritional requirements of your horse.