Dr. Stephen Duren, PhD.
Kentucky Equine Research, Inc.
Versailles, Kentucky
USA
Dr. Peter Huntington BVSc
Kentucky Equine Research
112B Martin Street, Brighton 3186
Australia

Introduction
To understand how to feed an endurance horse, it is necessary to understand the sport of endurance riding and the nutritional needs of the endurance horse. The following paper will provide a brief description of endurance riding while outlining the critical nutrients and feeding management strategies and tips for endurance horses.

An endurance horse could potentially complete a 25 mile endurance course in about 3 hours, a 50 mile endurance in just over 5 hours and a 100 mile course in approximately 11 hours. Given these estimates for "competition time", a tremendous opportunity exists for nutrition to influence performance. Contrast this with the racing Thoroughbred or the Standardbred racehorse which can complete races in just under two minutes. The real opportunity for a diet and dietary manipulation to influence performance lies in events which last longer than a few minutes.

The speed at which the exercise bout is performed also will influence the ability of diet to modify performance. In the case of short, intense (anaerobic) bouts of exercise, the animal is forced to generate energy for muscle contraction as fast as possible. This limits the type of fuel a muscle can utilise and the method by which the fuel is burned. The endurance horse, on the other hand, performs an extended exercise b ' out at a much slower (aerobic) speed. This provides an opportunity for the muscles to select a fuel and obtain the maximum energy production from that fuel. In endurance situations, fuel (feed) can actually be ingested, absorbed and circulated to the muscle for conversion to energy while the exercise is still being performed. Fit Endurance horses can become fatigued due to a variety of reasons including:
1. Low glycogen
2. High body temperature
3. Electrolyte losses
4. Water losses

Critical Nutrients
There are several key nutrients which will directly influence the performance capability of the endurance horse. These nutrients include: energy, fibre, electrolytes and water.

Energy
The main productive function in endurance horses in work. This work may vary from relatively slow speed exercise over long distances, to exercise conducted at faster speeds over the shorter courses. Energy is the dietary nutrient which will directly influence whether an endurance horse can go the distance. Energy is not a nutrient per se, but rather a measure of a feed's potential to fuel body functions and muscle contraction during exercise. Muscle contraction, in turn, will move the legs and ultimately the horse across the ground during the ride. The endurance horse takes in a variety of feed types (fibre, starch, fat, protein) which can be used to fuel muscle contraction (Figure 1). Since horses are not able to eat continuously during a ride, feed must be digested and stored within the body to be used later as fuel during exercise. Stored energy in the form of muscle and liver glycogen (sugar), intramuscular and adipose triglycerides (fat) along with feed taken in during the ride will provide for muscle contraction. For muscle contraction to occur, the chemically bound energy from feed must be converted into mechanical energy. This conversion process occurs in the muscle cell, and utilises adenosine triphosphate (ATP) as the "currency" for muscle contraction (Figure 1).

Figure 1.
The Energy Supply for muscle contraction.

There are several factors which will determine both the choice of fuel and the pathway used to generate ATP.. These factors include: muscle fibre type, the speed and duration of exercise, type of feed/animal fitness. Two fundamental reactions resynthesise ATP:

1) Aerobic Oxidative Phosphorylation, breaking down carbohydrates, fats and protein, in the presence of oxygen, producing energy (ATP).

2) Anaerobic Glycolysis, breaking down glucose or glycogen into lactic acid.

The horse has three basic types of muscle fibre (Table 1) - Type I, IIA and IIB. Type I fibres are slow contracting fibres while Types IIA and IIB are fast-contracting. Type I and IIA fibres can utilise fuels aerobically while Type IIB fibres have a low aerobic capacity and depend on anaerobic glycolysis for energy generation.. It is not surprising that different breeds of horses will have different percentages of muscle fibre types. For example, Quarter Horses typically have more Type IIA and IIIB fibres and fewer Type I fibres than an Arabian horses. This would help explain why, the Arabian, is known for endurance.

Table 1.
Metabolic characteristics of different muscle types.

While walking, the muscles contract very slowly and expend relatively small amounts of ATP. During this type of exercise, Type 1 fibres are primarily recruited and energy generation is entirely aerobic. At this speed, the muscle bums predominantly fat. As speed increases from a walk to a trot to a canter, Type 1 fibres alone are unable to cope. At this point, Type IIA fibres are also recruited. These fibres are alsol aerobic, but they use a combination of glycogen and fat for energy generation. Glycogen (glucose) can be metabolised twice as fast as fat, and as speed increases, fat becomes simply too slow a fuel. At the canter , Type IIB fibres are recruited. At these speeds, the requirements for ATP has exceeded the ability of the horse to deliver enough oxygen to the muscle to produce the energy by aerobic means. Anaerobic glycolysis takes over as a rapid metabolic pathway and results, however, in lactic acid accumulation. Fatigue soon develops as the pH in the muscle begins to fall.

The speed at which endurance horses typically travel is within the range which can be maintained almost entirely through aerobic energy production. Only during the "controlled runaway" some riders use at the beginning of a ride, the "end of race" sprints and during hill climbing would the energy production shift toward anaerobic means and then, only for a short time. Therefore, fatigue in an endurance horse is much more likely to result from depletion of glycogen and/or triglyceride stores than lactic acid accumulation.

Digestible energy (DE) refers to the amount of total energy in the diet t hat is actually absorbed by the horse. The DE requirements for different types of horses are calculated based on the horse's maintenance DE requirements plus the additional energy expended during exercise. Table 2 lists the amount of DE required above maintenance at various speeds. For example, a 450 kg endurance horse would have a maintenance DE requirement of 62 MJ. Using the values in table 2, this same endurance horse ridden at a medium trot (250 metres/min) by a 75 kg rider for 3 hours, would have an additional energy requirement of 62 MJ. The total energy requirement would be nearly 124 MJ of DE/day, a value the NRC, 1989, would classify as intense work.

Table 2:
Digestible Energy (DE) Requirements of 450 kg horse with 75kg rider (& gear) ridden for 3 hours at Various Speeds

* Weight of horse plus rider and tack.

The total DE requirement (maintenance + exercise) can be provided by four different dietary energy sources: starch, fat, fibre and protein.

Starch
Starch, a carbohydrate composed of a large number of glucose (sugar) molecules, is the primary component of cereal grains. Of the grains commonly fed to endurance horses, corn has the highest starch content, followed by barley and then oats. Starch is the dietary energy source of choice for glycogen synthesis. Starch digestion results in a direct rise in blood glucose and insulin - two of the most important factors involved in glycogen synthesis. Muscle glycogen is a versatile fuel for energy generation during endurance exercise, since glycogen can be metabolised either aerobically (with oxygen) or anaerobically (no oxygen). In addition, glycogen stored in the liver is available for the production and release of glucose levels during exercise is of prime importance since glucose is the only fuel that is available to the central nervous system. In endurance horses, hypoglycaemia (low blood sugar), as a result of prolonged exercise, can be a cause of fatigue.

Fat
Various oils, sunflower seeds and commercial supplements eg EQUIVIT EQUI-JOULE are the most common sources of fat in the horses diet. These fat products contain 3 times as much DE as an equal weight grain. Numerous digestion studies have confirmed that fat is both very palatable and extremely well digested. Fat is, however, an extremely useful dietary energy source. Research studies have concluded that feeding fat to horses resulted in a greater mobilisation and utilisation of fat during long distance exercise In essence, it appears horses trained their enzyme systems to utilise fat, thereby, sparing the use of muscle and, liver glycogen. Further, endurance horses in heavy training have a very high daily DE requirement. Often these endurance horses can not or will not eat enough feed to meet their energy requirements. The result is a steady decrease in body condition. In these instances, adding fat will increase the energy density of the diet so that less feed is required to maintain body weight.

Fibre
Fibre (hay/pasture chaff) is an energy source that is often overlooked in horse nutrition. Horses have a highly developed hindgut which houses billions of bacteria capable of fermenting large quantities of plant fibre. Volatile Fatty Acids (VFA's), the end product of fibre fermentation, are absorbed from the hindgut and transported to the liver. Once in the liver, VFA's can be converted to glucose and be stored as liver glycogen or be converted to fat, and be used to fortify the bodies fat stores, Fibre, therefore, can be used as an energy source throughout the endurance ride since fermentation of fibre and absorption of VFA's continues long after a meal has been eaten.

An endurance horses' intestinal health is critical to success. Research conducted in Germany and the USA has underscored the importance of fibre in maintaining gut health. Their experiments have shown that a diet high in fibre resulted in an increased water intake. Further, animals supplemented with a simple hay and salt diet had 73% more water in their digestive tracts after exercise and approximately 33% more available electrolytes than horses on a low fibre diet. More importantly, the water and electrolyte pool created by a high fibre diet can be used to combat dehydration and electrolyte imbalances which derail so many endurance horses. Another important attribute of a digestive system full of fibre is maintenance of blood flow to the digestive system during exercise. The physical presence of fibre in the digestive system will help ensure that blood is not totally diverted away from the digestive system with the onset of exercise and could prevent colic.

Fibre in the form of hay/pasture & chaff has been discussed as an excellent energy source for endurance horses. In addition to these fibre sources, there are so-called "super-fibres". These super fibres have the same beneficial aspects of forage fibres for maintaining gut health and fluid/electrolyte balance, but contain more energy. The additional energy is the result of both a high fibre content and low lignin (non-digestible fibre) component. Therefore, these ingredients have more fibre which is available for microbial digestion. These super fibres (eg soybean hulls and lupins hulls) contain energy equivalent to oats, but they would be safer to feed because they do not produce the problems of grain overload. Lupins are an example of high energy feed which contains a lot of digestive fibre.

Protein
If the protein intake of an endurance horse exceeds its requirement, then the extra protein can be used as a source of energy. Excessive protein intake should be avoided in endurance horses for a number of reasons. First, water requirements increase with increased protein intake. This can be devastating for endurance horses which typically struggle to maintain proper hydration. Second, accumulation of nitrogen end-products (ammonia and urea) in the blood can lead to nerve irritability and disturbances in intestinal function and carbohydrate metabolism. Further, increased ammonia excretion in the urine may lead to respiratory problems associated with ammonia in the stable. Excess protein commonly occurs in horses with high lucerne intakes.

Electrolytes and Water
Energy metabolism within the body is not 100% efficient and 80% of the energy of muscle contraction is released as heat. In order for the horse to remain healthy and continue to exercise, excess heat must be dissipated from the body. If the horse is unable to rid itself of this heat, body temperature can rise to the point where it becomes life threatening. For horses, the main route of heat dissipation is through a form of evaporative cooling known to everyone as sweating. Once the hot fluid (sweat) is on the skin, it spreads out and evaporates taking heat away from the body. Unfortunately, sweating also takes water and electrolytes away from the body. With intense exercise, water loss can become so extreme that blood volume is decreased and further sweating is not possible. If the horse is not re-hydrated, death from the heat stroke will occur.

The National Research Council (NRC, 1989) has cited research that indicates non-working horses require 2-3 litres of water/kg of dry matter intake. This would equate to 20-30 litres (6 to 8 gallons) of water for a 450 kg horse. It is thought that exercise conducted in a hot, humid environment may increase the water requirement by 3000%, giving a total water requirement of 90 litres (24 gallons). Electrolyte losses means that a dehydrated horse may not drink enough water to replace water losses. Remember that horses drink more readily from a bucket than an automatic waterer.

Electrolytes are substances that dissociate in solution into electrically charged particles called ions. In horses, electrolytes play an important role in maintaining osmotic pressure, fluid balance and nerve and muscle activity. During exercise, chloride (Cl-), sodium (Na'), potassium (K'), calcium (Ca'+), and magnesium (Mg') are lost in the faeces, urine and sweat (Table 3: Electrolyte losses related to sweat loss). Loss of these electrolytes causes fatigue and muscle weakness, and decreases the thirst response to dehydration. As you can see the greatest losses by far are of chloride, sodium and potassium. Therefore, it is vital to replenish electrolyte losses in competitive endurance horses by the use of a salt block and a well formulated electrolyte eg EQUIVIT RESTORE. A suitable electrolyte should have a high chloride content, approximately a 2:1 sodium:potassium ratio and no bicarbonate or other alkaline agents. Remember to give adequate quantities, so adjust electrolyte intake to match sweat losses.

Return to Top of Page

Feeding, Management Strategies
Forage
Of the feeds offered to endurance horses, forage is by far the most important. Horses have evolved as grazing animals and have a unique ability to take in large amounts of forage (up to 3.5% of body weight). The horse, in concert with the bacteria in the hindgut, utilise this forage primarily for energy production. The ability of the horse to effectively utilise forages is evident if one considers that many horses are maintained on all-forage (hay/pasture) diets. Occasionally, a competitive endurance horse can be maintained solely on good pasture, but this is certainly the exception and not the rule. In addition to being a steady source of energy for the endurance horse, forage is essential to maintain intestinal health. A diet containing large amounts of good quality fibre can increase the water consumption, and provide a reservoir for both water and electrolytes. This water and electrolyte reservoir can be utilised throughout the ride to minimise dehydration and electrolyte imbalances. Finally, the presence of fibre in the digestive system can help ensure the blood is being distributed to the digestive system during the ride. This maintenance of blood flow to the digestive system will aid the ability of gut tissue to remain active and could help prevent colic.

A logical question often asked is :What type of hay and chaff do I feed my endurance horse?" To actually determine which hay to buy, one should consider the fibre, digestible energy (DE), protein and calcium content of the hay. First, to prevent digestive upset, it is absolutely essential to provide the horses' hindgut with an adequate source of digestible fibre. This would mean maximising the use of good quality hay/pasture with a high neutral detergent fibre (NDF) value and a low (ADF) value. The terms NDF and ADF are a form in which the fibre -content of roughages is expressed. The measurement of the entire cell wall fibre content, including the cellulose, hemicellulose and lignin, is known as the NDF. Whereas the ADF is a measurement of the cellulose and lignin. Second, the digestibility and thus the DE content of any given plant decreases with maturity. Therefore, horse owners should avoid feeding extremely mature forages. Since endurance horses do not have high protein requirements, and have to expend energy and use extra water to get rid of excess protein, select a hay with a lower protein content (8% to 14% As-Fed). Finally, since chronic over-supplementation with calcium can cause problems with endurance horses, avoid high calcium hays. With these selection criteria, the endurance horse would be well suited with free-choice access to a good quality grass hay. A mixed clover/grass hay is also acceptable provided it is predominantly grass, but avoid feeding a lot of lucerne, hay or chaff. If you feed lucerne chaff feed oaten or grass hay. If you feed some lucerne hay feed oaten or wheaten chaff.

Grain
Most competitive endurance horses are unable to maintain body weight on all forage diets. These horses need additional sources of energy that come in the form of starch, fat, fibre and protein, These energy sources are found in most commercial grain concentrates. With the information presented in this paper outlining the benefits of starch for energy production, it may sound like :the more starch the better". This is NOT the case. There is a limit to the amount of starch which an endurance horse's diet should contain. If a large amount of starch is fed in a single meal (ie. greater than 2.5kg of grain/meal), the small intestine's ability to digest and absorb the starch may be overwhelmed, and a substantial amount of the starch may pass into the large intestine. Once in the large intestine (caecum, colon), a cascade of reactions occur which can result in laminitis (founder) or colic.

There also is a limit to the amount of fat which can be added to the diet. First, from a palatability standpoint, horses will indicate when they have reached their peak level of fat intake by refusing to eat the feed. The threshold level of fat necessary to reach this stage varies with the horse and the type of fat. however, grain concentrates with over 10% added fat (top dressed) are prone to feed refusal.

The other limitation on the amount of fat that can be added to the diet occurs in situations where calories from fat are replacing calories from starch These high fat, low starch diets can limit the amount of starch available for glycogen synthesis and actually decrease liver and muscle glycogen stores (Pagan et al 1987). Grain concentrates which have between 6 and 10% total fat appear optimum for endurance horses. To obtain the best results with the addition of fat to the diet, begin adding the fat during the conditioning phase of training and continue throughout the season. This will expose the muscles to high levels of fat and condition the body to use fat as an energy source. The combination of dietary fat and fitness will allow endurance horses a greater mobilisation and utilisation of fat during long distance exercise. The addition of fat only on "race days" will be of limited value. If you are adding fat from oil or EQUIVIT EQUI-JOULE it may be good to taper intakes in the few days before a ride. This will allow to top up muscle glycogen stores from the extra starch.

Just as there are limitations in the amount of starch and fat appropriate for endurance horses, so are there limits on the amount of protein. The actual protein requirement for the endurance horse is only about 8 to 10% of the total diet. This is much lower than is actually fed to endurance horses because there are few ingredients that are this low in protein. For example, maize is around 8% protein, oats 8-10%, grass hay 6-8% and lucerne hay 1520% protein. Therefore it is not practical to restrict protein intake to the horse's actual requirement. Instead, protein content of the ration should be monitored and not allowed to become extremely high.

Time of Feeding
Specific information relating time of feeding prior to exercise with endurance horse performance is not available. Research efforts have mainly concentrated in time of feeding prior to a relatively short bout of exercise. In these studies, a diet is typically fed at a given time prior to exercise and blood indicators at metabolism and stress are monitored throughout the exercise protocol. Pagan et al (1995) has demonstrated changes in plasma glucose and plasma insulin prior to, and during exercise as a result of feeding time. These differences in plasma metabolises did not result in any marked differences in exercise performance. Further, Stull and Rodiek (1995) reported the composition of the diet and the timing of the meal prior to exercise. However, stress measurements (lactate and cortisol) did not respond to feeding time. Both of these studies are interesting since they were able to change plasma glucose levels as a result of pre-exercise feeding. However, it is possible the. duration of the exercise protocol was not long enough to see a difference. With endurance horses, the exercise protocol would definitely be long enough to determine the affect of feeding. In addition, endurance horses are fed at rest points during ride so time of feeding and the composition of the meal may have a dramatic influence on exercise performance. Kentucky Equine Research is currently working on the design of an experiment which could evaluate feeding time prior to exercise in endurance horses but our current recommendations would be no grain within 8 hours of a ride and small frequent meals of hay or chaff which avoids rapid intake of large amounts of hay.

Choosing an Endurance Feed
To choose a grain concentrate for endurance horses, several points need to be addressed. First, the energy content of the feed is probably the most important factor. Energy should be available from a combination of sources. Fortification with processed starch in the form of cracked corn, steamed rolled barley or rolled oats will be the basis of the feed. Expanded, Micronized or Extruded corn and barley would be especially useful. Addition of energy in the form of vegetable oil or a quality animal fat product is the next step. Finished grain concentrates for endurance horses should contain between 6 and 10% fat eg HYGAIN TRACKTORQUE OR HYGAIN POWATORQUE. With the many benefits that fibre has both for energy production and for maintenance of proper digestive function, inclusion of additional fibre is justified. Adding one or a combination of the "Super Fibres" (lupins, soybean hulls or lupins hulls ) would be beneficial. The next criteria the finished grain concentrate will be judged on its protein content. For endurance horses, high protein and high performance do not go together. Finished grain mixes should contain 12% protein or less. Electrolytes would seem like an obvious group of minerals to add to an endurance horse feed. Look for a feed with a high salt content and little alkaline electrolyte content - keep it simple. Other nutrients which warrant consideration include: vitamin E, selenium, and B-vitamins. Vitamin E and selenium are both involved in antioxidant reactions within the body and vitamin E should be added at a rate of 165-250 IU/kg of finished feed to supply at least 1000 IU Vitamin E/day with selenium adjusted to provide 2-3 mg of total selenium. B-vitamin fortification should be included in the grain concentrate to account for any potential deficiencies which may occur due to stress as these vitamins are involved in energy utilisation. These products are water soluble and need to be added daily. Finally, the total grain concentrate must be balanced for other essential nutrients including vitamins and minerals.

Supplements for Endurance Horse
If you are mixing your own feed it would be best to use oats, lupins, steam flaked or micronized barley , cracked or extruded corn as the major grains. Avoid soybean meal. Add fat from oil, EQUI-JOULE (35% fat), rice pollard (20% fat) or sunflower seeds (20% fat). Use 1-3 cups oil per day or 50Og-1000g EQUI-JOULE. Use a well formulated electrolyte such as EQUIVIT RESTORE. To supply high levels of Vitamin E and selenium add EQUIVIT PRESERVE OR E CONCENTRATE. A live yeast culture can help aid feed digestibility and Yea-Sacc 1126 from EQUMT PERFORM or EQUIVIT GOLD PELLET will be useful. These products also supply trace minerals and B-vitamins necessary to balance the diet and utilise energy effectively. As you aren't using much lucerne you may need some extra calcium and magnesium from EQUIVIT BONE FOOD.

FOR MORE SPECIFIC DETAILS
ON FEEDING YOUR HORSE
CONTACT DR PETER HUNTINGTON
PH 03 9530 6334
0418 108 946
FAX 03 9530 6339

Return to Top of Page