By Ray J. Geor, BVSc, PhD, Diplomate ACVIM
The courtesy of the official website of the AAEP (American Association of the Equine Practitionners) www.myHorseMatters.com

January 23, 2004

Few will dispute that nutrition is important for the optimization of athletic performance in horses. However, there tends to be less agreement among horsemen, nutritionists and veterinarians regarding the most important nutritional considerations for athletic horses. Some will focus on the virtues of the latest fad supplement, while others will emphasize the importance of getting the basics right, i.e. plenty of high-quality forage supplemented by grains or other energy-laden concentrates.

This article discusses some aspects of the feeding management of athletic horses, first focusing on the important fundamentals, and then examining the interplay between feeding management and the development of gastric ulcers and chronic tying-up, disorders that commonly afflict performance horses.

Energy Requirements

As with any feeding program, the main consideration is whether the diet meets the horse’s nutritional needs – meaning adequate water, energy (calories), fiber, minerals (e.g. calcium, phosphorus, selenium) and vitamins. For athletic horses, energy is the most important nutritional consideration.

Daily training and competition burns a lot of body fuel, particularly muscle glycogen, and these fuel reserves must be replenished. Energy is not a nutrient per se but rather a measure of a feed’s potential to fuel body functions.

For horses, energy needs are expressed as megacalories (Mcal) of digestible energy (DE) where 1 Mcal equals 1,000 kcal. The 1989 National Research Council recommendations on the nutritional requirements of horses state that daily DE intake be increased by 25, 50, and 100% above maintenance requirements for horses engaged in light, medium and intense exercise, respectively. These recommendations are based on data from experimental studies, feeding surveys and practical experience. Light work might include equitation and other forms of pleasure riding, while horses engaged in racing, hunting, 3-day events and endurance activities would fall into the intense category (Table 1).

Clearly, if the diet does not provide sufficient energy to meet the increased demands associated with these athletic activities, loss of body condition and performance will ensue. This situation most commonly arises in racehorses, where the adequacy of feed intake can be a problem.

On the other hand, pleasure horse owners have a tendency to overestimate the amount of work (physical activity) performed by their horses – the end result being weight gain due to energy intake in excess of requirements.

For Standardbred and Thoroughbred racehorses, field studies and observations have indicated that the NRC (1989) recommendations are appropriate, at least for horses engaged in heavy training and competition. However, it is important to realize that the NRC recommendations are just a starting point.

There can be substantial variation between horses in terms of calorie needs and how much feed they will eat in a day – some horses become overweight when fed according to these guidelines while others lose weight. Maintenance of body weight and condition are the best indicators of energy sufficiency. So, on a regular basis (every month), it is important to assess body condition and to adjust feed intake accordingly. The body condition scoring (BCS) system assesses flesh coverage – mostly fat – over different areas of the body, where a score of 1 indicates an extremely thin horses and a score of 9 an obese one. For most athletic horses, body condition should be between a score of 4.5 and 6.0.

Recent studies have investigated the relationship between body condition score and completion rate during the Tevis Cup (100 mile) endurance ride – the standard body condition scale of 1 to 9 was used. The mean body condition score of horses that successfully completed the rides was 4.5, whereas horses that were eliminated for metabolic failure (colic, heat exhaustion, synchronous diaphragmatic flutter or “thumps”or tying up) had a mean condition score of 2.9. Horses that were eliminated for non-metabolic reasons such as lameness and over the time limit had a mean condition score of 4.3. The take-home message from these studies is that there is an optimal level of “fatness” for horses competing in endurance events, and that training and feeding programs need to be adjusted accordingly. Thin horses (condition score <3) may be at a disadvantage because of low-energy reserves and muscle mass, while over-conditioned horses could experience detrimental effects due to the insulating effect of a thicker fat cover and the carriage of “dead weight”.

Another fundamental consideration is how much feed we expect a performance horse to eat each day. On average, we might expect a horse to consume somewhere between 2 and 2.5% of body weight as feed per day (22 to 30 lb for an 1100 lb horse). For arguments sake, assume a racehorse that needs 35 Mcal DE/day will eat 28 lbs of feed per day. This means that the overall energy density of the diet must be around 1.25 Mcal DE/lb. Knowing that even good quality hay is usually no more than 0.9-1.1 Mcal/lb, it is obvious that hay alone will not get the job done.

Calorie sources

• Fermentable carbohydrates: components of dietary fiber or roughage that cannot be digested by the horse’s enzymes but can be fermented by microorganisms in the hindgut

• Hydrolyzable carbohydrates: simple sugars and starch that are digested by the horse’s

• Oils and fats

• Protein: not primarily fed as an energy source because metabolism of protein to useable energy is inefficient

Knowledge of the horse’s digestive physiology and the impact of diet composition and meal size on the efficiency of digestive processes are important in the selection of an appropriate ration and feeding strategy.

The horse evolved as a grazing animal and its gastrointestinal tract, with a well-developed cecum and large colon, is highly adapted to the utilization of fiber-rich feeds that are consumed on an almost continuous basis. The hindgut (cecum and colon) comprises approximately 64% of the total (empty) volume of the gastrointestinal tract, whereas the stomach (7%) and small intestine (25%) have a relatively small capacity. Similar to other mammalian species, the small intestine is the major site of digestion of protein, fat and hydrolysable carbohydrates. However, the horse has a limited ability to digest and absorb hydrolysable carbohydrates (particularly starch) in the small intestine. Large concentrate meals may overwhelm the digestive capacity of the small intestine and promote the flow of undigested hydrolysble carbohydrate to the large intestine. This not only reduces the efficiency of feed utilization, but also increases the risk for digestive disturbances associated with excessive and uncontrolled fermentation of the undigested hydrolysable carbohydrate in the large intestine.

Fiber/roughage needs

From the preceding discussion, it is clear that forage (roughage) should always be the foundation of an equine ration. Although a requirement for dietary fiber has not been established in horses, some long-stem roughage is important for maintenance of normal hindgut function and thus for normal digestion. There also is evidence that diets low in long-stem fiber favor development of certain stereotypes (behavioral vices). Therefore, adequate dietary roughage may be important for prevention of some undesirable behavioral traits, particularly in horses kept in confinement. Some trainers prefer to feed low-roughage diets because such rations may reduce the weight of ingesta in the intestinal tract (“dead weight”), thereby providing an energetic advantage during some forms of exercise. The possible benefits of this practice should be weighed against the increased risks of gastrointestinal dysfunction (e.g. colic, gastric ulcers) and behavioral abnormalities when horses are fed low-roughage diets.

This author recommends at least 2 lb (1kg) forage per 200 lb (100kg) bodyweight (i.e. 12 lb (6kg) for a 1200 lb (600kg)  horse). Pastures and different forms of conserved forages (i.e. hay, chaff, hay cubes, haylage) are the primary source of roughage in horse rations. Although several factors can affect the nutrient value of conserved forages, the most important is the stage of maturity at the time of harvest – the energy content, digestibility and palatability of forage all decrease with increasing maturity. Therefore, forages harvested at an early stage of plant maturity should be fed to working horses to maximize nutritional value and intake of the offered quantities. The relative use of grass, legume or cereal (usually oaten) hays, and the different forms of preserved forages made from these species often depends on availability and personal preference.

Energy concentrates

Cereal grains: Traditionally, cereal grains such as oats, corn and barley (alone or in combination) have been a source of energy in rations for athletic horses. Starch, a hydrolysable carbohydrate, is the primary component of cereal grains. Oats are approximately 47-50% starch while the starch content of corn and barley is between 65 and 70%. Digestion of starch in the small intestine yields glucose, the substrate for liver and muscle glycogen synthesis. As muscle glycogen is a primary fuel during exercise, the provision of some hydrolysable carbohydrate (starch and/or sugar) in the diet of an athletic horse is important for replenishment of glycogen reserves. However, there is evidence that the horse has a limited capacity to digest and absorb starch (and perhaps other simple carbohydrates) from the small intestine. With the ingestion of large grain meals, a substantial proportion of the ingested starch may escape hydrolysis in the small intestine, with a resultant delivery of this substrate to the hindgut. Rapid fermentation of starch in the hindgut by lactate-producing bacteria can result in lactate accumulation, excess gas production, cecal and colonic acidosis and increased risk of intestinal disturbances (colic).

Several strategies can be employed to mitigate the risk of digestive disturbances attributable to heavy grain (starch) feeding. First, it is advisable to limit the size of individual grain-based meals to avoid “starch bypass” to the large intestine. Second, only cereal grains with high pre-cecal starch digestibility should be included in energy concentrates for horses. Third, energy concentrates for athletic horses should make more use of non-starch carbohydrates (e.g. sugar beet pulp) and vegetable fats. Inclusion of these alternative energy sources facilitates a reduction in the level of starch feeding without compromising the caloric density of the ration.

A suggested upper limit of starch intake in a single meal is between 2 and 4 g starch per kg bodyweight (0.2%-0.4% of bwt per feeding). Thus, if a concentrate feed contains 50% starch (e.g. plain oats) the maximum recommended amount of concentrate per feeding is approximately 5 lb for an 1100 lb horse. Pre-cecal starch digestibility varies with the type of grain and the nature of any mechanical or thermal processing. For example, whereas oat starch (at up to 3 g/kg per meal) has a pre-cecal digestibility of greater than 90%, approximately 35% of an equivalent dose of cornstarch reaches the cecum undigested. Similarly, the pre-cecal digestibility of unprocessed barley is substantially lower when compared to oats. However, heat treatments such as micronisation, extrusion and steam flaking significantly improve the pre-cecal starch digestibility of barley and corn. Overall, oats appear to be the safest source of starch for horses, although barley and corn are acceptable if they are subjected to some form of heat treatment.

Fats and oils: The addition of fat to horse rations is now commonplace. Most commercial fat-supplemented concentrates for horses contain a vegetable oil such as soy, corn or canola. Other oils that may be used in equine rations include peanut, safflower, coconut, linseed or flaxseed. Stabilized rice bran (18-22% fat), flaxseed meal (40% fat) and copra meal (8-9% fat) are also used in horse rations. Vegetable oils are both highly digestible (90-100%) and palatable. Rice bran is rich in phosphorus and has an inverted Ca/P ratio, but many commercial rice bran products contain added calcium (e.g. calcium carbonate) to correct this imbalance. Alternatively, a mineral supplement can be added to the ration to ensure an appropriate Ca/P ratio (at least 1:1) in rations containing rice bran.

Fat is often added to the diet to increase the energy density of the ration, which can offer an advantage when dry matter intake limits provision of adequate energy to maintain condition (“hard keeper” horses). Alternatively, substitution of fat for a portion of the grain in an energy concentrate allows for a decrease in hydrolysable carbohydrate intake. This strategy is advocated for horses with some forms of chronic exertional rhabdomyolysis (see below).

The ideal amount of dietary fat for horses has not been determined. Commercially, the level of fat or oil added to a concentrate is often limited by manufacturing constraints (e.g. poor pellet quality when large amounts of oil are included in pelleted feeds; greasy appearance of concentrate mixes). Therefore, fat-supplemented concentrates designed for performance horses usually contain between 5 and 14% fat (as fed), providing between 8 and 30% of the DE. However, as these concentrates are fed with forage, the amount of fat on a total diet basis is much lower (3%-8% fat, or 4-15% of the DE from fat assuming a 50% concentrate, 50% forage diet). Many horse owners and trainers add vegetable oil to existing rations. A suggested upper limit of oil supplementation is 1 standard measuring cup (200 g oil) per 250 lb bodyweight per day. There should be a gradual introduction to oil feeding to avoid digestive disturbances (loose and oily feces). Initially, ¼ to ½ cup of oil/day can be added to the ration. Over a two to three week period, the amount of added oil can be increased to 2 to 2½ cups/day, divided into at least two to three feedings.

Non-starch carbohydrates: There are two main types of non-starch polysaccharides used in equine rations: 1) simple sugars and 2) highly digestible sources of fiber (so-called “fermentable fibers”), particularly sugar beet pulp (SBP) and soya hulls and, to a lesser extent, citrus pulp. Simple sugars in the form of molasses (a mixture of glucose, sucrose and fructose) are often added to grain mixes at 6-8% by weight.

SBP or soya hulls can be included as a substitute for cereals (starch) in energy concentrates. Studies in horses have demonstrated that up to 3.0 g SBP per kg bodyweight per day (i.e. up to 3-lb of dry SBP shreds per day for an 1100 lb horse) may be fed to adult horses without any adverse effects on overall nutrient utilization or performance. The inclusion of SBP or soy hulls in the ration can help to ensure adequate fiber intake.

Although the feeding of straight grains or sweet feed mixes to athletic horses remains a popular practice, there is increasing emphasis on use of energy concentrates in which some starch and sugar has been substituted by fat and/or a fermentable fiber such as SBP and soya hulls (so-called “fat and fiber” feeds). Such diets may reduce the risk of gastrointestinal disturbances and, as discussed below, are useful in the nutritional management of horses with chronic exertional rhadomyolysis. Figure 1 compares the sources of digestible energy in a traditional racehorse diet (forage plus grain) and a diet in which a fat and fiber energy concentrate is fed.

Special considerations

Gastric ulcers: Gastric ulcers are of particular concern for athletic horses, and there is some support for the idea that the feeding management typically practiced in training situations is a contributing factor. When a horse grazes at pasture or nibbles on hay for extended periods, a great deal of saliva is produced. Saliva contains sodium bicarbonate (“baking soda”), and this substance helps to buffer acid produced in the stomach and therefore minimize damage to the stomach lining. In contrast, when a horse eats a grain meal, saliva production is less than one-half compared to an equal amount of hay, basically because less chewing is required. Bear in mind that the horses produce gastric acid on a continual basis. Therefore, if a horse is fed morning and afternoon/evening (i.e. two meals/day), the chances are that there will be lengthy periods when the stomach is practically devoid of food but fully exposed to “burning” effects of gastric acid. This may be one factor that contributes to ulcer development in athletic horses on low-fiber/high-grain diets. It has also been proposed that large grain-concentrate meals favor feed fermentation in the stomach and the volatile fatty acids yielded by this process have also been implicated in the development of gastric ulcers.

All of this suggests that horses should be given free access to forage – and in an ideal world, a few hours at pasture each day. Splitting the grain allotment into three to four meals per day, rather than two, also may be beneficial. Finally, there may be some merit in allowing the horse to consume a little hay (1-2 lbs) before exercise. Recent research has shown that the stomach contracts (i.e. shrinks in size) during exercise – this circumstance also may favor a “splashing” of gastric acid over the areas of the stomach prone to ulcer formation. On the other hand, if a little food and saliva are present in the stomach during exercise, the burning effect of gastric acid could be lessened.

Chronic tying-up: Strict control of diet is particularly important for athletic horses that suffer from chronic exertional rhabdomyolysis (tying up) – including recurrent exertional rhabdomyolysis (RER) in Thoroughbreds and polysaccharide storage myopathy (PSSM) in Quarter Horses and related breeds. These horses must be fed concentrates low in starch and sugar. Specialized commercial feeds are now available for horses with chronic tying up – these feeds have a restricted amount of starch and sugar with much of the energy supplied by fat and fiber sources such as beet pulp and soy hulls.

Take-home message

Traditionally, cereal grains have been a dietary mainstay for athletic horses. However, research over the past 20 years has indicated that diets high in hydrolysable carbohydrate increase the risk of colic and increase the incidence of certain medical conditions, e.g. chronic tying up. Therefore, modern rations for athletic horses emphasize alternative energy sources such as fat and fermentable carbohydrates, with lower starch and sugar.

Table 1: Digestible energy (DE) requirements (megacalories per day) for an 1100 lb horse at four different activity levels.

Figure 1: Graphical representation of the sources of energy in a traditional grain and hay diet (Starch) and a contemporary fat and fiber energy concentrate and hay diet (Fat/fiber) fed to an 1100 lb racehorse requiring approximately 32 Mcal of digestible energy (DE) per day. Both diets include 14 lb of mid-bloom timothy hay and, on a total diet basis, provide 12% crude protein. For Starch, the horse is fed ~16 lb of an energy concentrate comprised of oats (57%), cracked corn (37%) and sugar cane molasses (6%). The same quantity of energy concentrate is fed in Fat/fiber (~13% fat), the primary ingredients of which are beet pulp, rice bran, soy hulls and vegetable oil. Graph A depicts estimates of the absolute DE provided by protein, fat, hydrolysable carbohydrate (hCHO; starch and sugar) and fiber carbohydrates (fCHO) in the respective diets. Graph B shows the percentage contribution of the four energy sources to the total DE intake. Note the dramatic reduction in hCHO intake in the Fat/fiber when compared to the Starch diet.