When and what should I feed my horse before exercise? This question is very familiar to equine nutritionists, and generally sparks considerable debate about feeding management of horses prior to competition. While it generally is agreed that feeding practices before competition exercise have an important bearing on performance, there is little consensus as to what is most beneficial. Indeed, several surveys of horse trainers and owners have indicated that the horse's diet usually is altered on the day of competition. However, the nature of this change in diet varies greatly. Some trainers will withhold all feed, some will decrease the amount of hay fed, while others will increase the amount of grain or other high-energy feed.

So, just what is the best feeding practice for horses before competition exercise? As you might expect, there is no easy answer to this question, and probably no one approach is optimal. Of greatest importance is the type of exercise required of the horse--pre-exercise feeding management of the Thoroughbred or Standardbred racehorse will differ from that of an endurance horse.

In this article, we will consider how feeding affects energy metabolism during exercise and provide recommendations for feeding different types of horses.

Is Change Good?

The first question to address when discussing pre-competition feeding management is "why should the horse's diet be altered?" We all are aware of the potential perils associated with rapid alterations in the horse's diet or pattern of feeding--such changes can precipitate digestive upsets and colic. Thus, drastic changes in diet are not recommended. On the other hand, more subtle alterations in management and feeding practices safely can be used to maximize the horse's fuel stores and optimize use of these fuels during exercise. There are two time periods to consider: 1) the one to two days before competition exercise; and 2) the six- to eight-hour period before the horse begins to exercise.

In the days leading up to competition, there should be a tapering of the horse's training program. As discussed in a previous column (see Body Fuel in The Horse of January 2000), hard and prolonged exercise places heavy demands on the horse's carbohydrate reserves--liver and muscle glycogen. This type of exercise places a much higher emphasis on fat utilization--both from volatile fatty acids and body stores. During prolonged exercise (e.g., endurance rides), inadequacy of carbohydrate reserves can be a factor that limits exercise performance especially if fat utilization is limited. For shorter duration exercise (e.g., racing), carbohydrate supply normally is not limiting. However, recent studies have demonstrated that very low muscle glycogen stores impair sprint exercise performance (Lacombe et al. 1999).

Thus, regardless of the discipline, adequacy of body carbohydrate stores is important for optimal exercise performance. A gradual reduction in training efforts, together with maintenance of the horse's regular diet, will help ensure that liver and muscle glycogen stores are near their optimal levels before the horse is asked to undertake hard exercise.

The type of feed consumed and the interval between feeding and the start of exercise will have profound effects on the horse's response to exercise. It must be emphasized that relatively few research studies have addressed the effects of pre-exercise feeding in horses. Even fewer studies have determined whether different pre-exercise feeding practices alter the horse's performance during exercise. Nonetheless, there is sufficient information to make some recommendations. First, though, it is important to understand how feed type and the interval between feeding and exercise influence fuel use in the horse during exercise.

Effects Of A Grain Meal

The fuel used by muscle during exercise largely is dictated by the intensity and duration of the work effort (see Body Fuel, January 2000). During very fast exercise (e.g., racing), fuels stored within muscle, particularly glycogen, provide most of the energy. When a horse undertakes lower intensity exercise, energy is derived from both carbohydrate and fat. Indeed, it is to the horse's advantage to utilize fat during prolonged exercise--maximal use of fat will conserve carbohydrate stores and delay onset of fatigue.

Both the availability and utilization of fuels are influenced by hormones, particularly the blood concentration of insulin. This is where time of feeding becomes important. When horses are fed a grain meal, starch within the grain is broken down into glucose units in the small intestine and absorbed into circulation. Following a 1 to 2 kg meal of oats, corn, or sweet feed, the peak in blood glucose concentration generally occurs about two hours after feeding. This delay reflects the time required for movement of food from the stomach into the small intestine, breakdown of starch to glucose, and absorption of glucose.

Insulin, a hormone synthesized in the pancreas, is released into circulation in response to this rise in blood glucose. Insulin has a profound effect on body metabolism. When blood glucose concentrations are increased, insulin facilitates entry of glucose into various tissues, resulting in a lowering of glucose levels. In muscle, this glucose is stored as glycogen, while in adipose tissue, glucose is converted to fat. Consumption of a grain meal (oats, corn, or a grain mix such as sweet feed) one to four hours before exercise will result in elevated blood concentrations of glucose and insulin at the onset of exercise (Lawrence et al. 1993, Pagan and Harris 1999). On the other hand, if a grain meal is consumed five or more hours before exercise, blood glucose and insulin concentrations will be near baseline when exercise begins.

If blood glucose and insulin are elevated at the start of exercise, there can be a rapid fall in blood glucose, reflecting a marked increase in the rate of glucose uptake by muscle (see Figure 1 on page 82). Some researchers believe that this decrease in blood glucose is detrimental to performance. However, this drop in glucose levels can be transient and does not necessarily have a negative effect on performance.

In a recent study, horses either were fasted for 24 hours or given a glucose solution one hour before exercise, which provided an amount of glucose equivalent to feeding 2 kg of oats to a 450 kg (1,000 pound) horse (Geor et al. 2000). Horses then completed one hour of moderate intensity treadmill exercise. Although blood glucose concentrations decreased in the glucose-fed horses during the first 10 to 20 minutes of exercise, blood glucose was not different from the fasted horses for the remainder of the trial. Overall, utilization of blood glucose in the fed horses was double that of the fasted horses (see Figure 2 below). But if they were performing the same amount of work this implies that the fasted horses did just as well using their body fat stores and volatile fatty acids, which may be desirable since fat utilization generates less metabolic heat than carbohydrate, which is of major concern in three-day and endurance horses.

Early research in human athletes on the effects of pre-exercise carbohydrate feedings demonstrated a sparing of muscle glycogen. However, it now is believed that carbohydrate feedings before exercise have no effect on the rate of muscle glycogen utilization. Instead, the increase in blood glucose use is associated with a decrease in fat utilization (see Figure 2 at left). Again, insulin plays a role in this response--insulin blocks release of fatty acids from stores in adipose tissue and limits the rate of fatty acid metabolism in muscle.

When compared to horses which are fasted or fed forage, horses fed grain or given glucose before exercise have lower blood fatty acid concentrations during exercise. Thus, grain meals that result in a large increase in blood glucose (a high glycemic response) before exercise will shift fuel utilization away from fat toward carbohydrate utilization. As carbohydrate stores are more limited than fat reserves, this shift in metabolism is not desirable when horses are required to undertake long exercise tasks.

So, is grain feeding before exercise good or bad? The answer is, it depends!

Horses scheduled for afternoon competitions should receive their usual morning meal. If there is a five- to six-hour interval between feeding and exercise, blood glucose and insulin concentrations will be near baseline and there should be no residual effects on substrate metabolism during exercise. In fact, the glucose in this meal might "top off" liver and muscle glycogen stores (see Figure 3 on page 88) and be beneficial to performance. However, feeding of high glycemic grain meals one to four hours before an event is not recommended, particularly for disciplines such as endurance rides, where utilization of fat is important.

Much less is known about feeding one hour or less before exercise. Of course, endurance horses routinely are fed at rest stops during rides. In this circumstance, the interval between feeding and the start of exercise usually is less than 30-45 minutes. Even when grain is fed, there will be minimal increase in blood insulin during this short interval between feeding and exercise. Whether or not the starch in this meal is digested and absorbed (as glucose) during exercise is not known. Continued digestion and absorption of glucose would help to maintain the supply of blood glucose during a time when the horse's supply of liver and muscle glycogen is waning.

The potentially negative effects of pre-exercise grain feeding (i.e., suppression of fat utilization and increased reliance on carbohydrates for energy) are manifested through the effects of insulin. Thus, feeds that can provide fuel without causing a large increase in insulin (low glycemic feeds) would be desirable in a pre-exercise meal for horses.

The best example of a low glycemic feed is forage (hay or grass). Consumption of a hay meal before exercise does not affect blood glucose, insulin, or fatty acid concentrations (see Figure 1 on page 82). However, as discussed below, such meals can alter fluid balance and cardiovascular function during exercise. Further research is required to identify feeds that, when consumed one to three hours before exercise, have minimal effect on fat metabolism, but provide additional energy to the horse during exercise.

Hay Feeding Before Exercise

Horses secrete large amounts of saliva during chewing, and this fluid is swallowed along with the food material. In addition, arrival of food in the stomach and intestinal tract stimulates fluid secretions into the gut. These salivary and intestinal secretions are drawn from the horse's extracellular fluid compartment (see April 2000 column), and this movement of fluid results in a decrease in plasma (blood) volume. This decrease in plasma volume is temporary because virtually all of the secreted fluid is reabsorbed by the large intestine. Nonetheless, consumption of a large hay meal (six to seven pounds or more) within two hours of exercise will result in up to a 15% decrease in plasma volume that persists until the start of exercise.

Such reductions in plasma volume might compromise cardiovascular function during both low- and high-intensity exercise. Presence of a large meal within the digestive tract also alters the pattern of blood flow within the body--more blood is directed to the intestinal tract, potentially limiting blood flow to muscle during exercise. Large meals, whether forage, grain, or a combination, are not recommended within five hours of competition exercise. However, small hay (or grass) meals do not result in large fluid shifts and can be fed during this time interval before exercise.

For endurance horses (or horses engaged in other types of prolonged exercise), consumption of forage four to six hours before an event can be beneficial. For every one kilogram (2.2 pounds) of dry hay intake, horses will consume 2.5 to 3.5 kg (up to one gallon) of water. Within the large intestine, the fiber in hay and other forages "holds" this water. This reserve of water (along with electrolytes) then is available for absorption during exercise, helping offset fluid and electrolyte losses in sweat. In general, high-fiber diets are recommended for endurance horses because of the associated increase in size of the gut water and electrolyte reservoir.

In contrast to the endurance horse, racehorses will not benefit from consumption of a high-fiber meal in the hours preceding competition. In fact, such practices might adversely affect performance. The amount of energy expended by the horse at any speed depends on how much total weight it carries--at any given intensity or speed of exercise, additional weight requires more energy.

If a racehorse consumes 2-3 kg of hay, and has access to water, gut fill and body weight will increase by up to 12 kg. In Thoroughbred racing, increasing weight carriage (adding lead weights to the tack) is a tried and true method of handicapping. Therefore, such a large increase in body weight almost certainly will be detrimental to high speed exercise performance. In fact, among racehorse trainers, it is common practice to limit or eliminate hay feeding on the day of the race.

Recent studies at Kentucky Equine Research have demonstrated that, compared to free-choice hay feeding, restriction of hay intake to 1% of body weight (10 pounds of hay for a 1,000 pound horse) for three days results in a decrease in body weight of approximately 10 kg. Presumably, this decrease in body weight is due to a reduction in gut fill. Such feeding practices might be beneficial for racehorses. A word of caution--to minimize digestive disturbances, forage intake should not be lower than 1% of body weight.

Fasting Before Competition?

If feeding grain and hay before exercise (especially in large quantities) can be detrimental to exercise performance, some might think it logical to conclude that complete withdrawal of food (fasting) is the best strategy. However, this is not the case--even short periods of fasting can result in a marked decrease in liver glycogen stores. Figure 3 on page 88 shows liver and muscle glycogen concentrations in horses in response to fasting and re-feeding. Note that muscle glycogen is largely unchanged, whereas liver glycogen is markedly reduced after one day of fasting and is quickly replenished after re-feeding. Also recall that water intake often is linked to food ingestion. Therefore, prolonged fasting before exercise can limit water intake and compromise hydration. In general, horses should not be fasted more than six hours before hard exercise.

Pre-Exercise Feeding Strategies

As stated at the outset, there is no easy answer to the question, "What should I feed my horse before exercise?" Nonetheless, based on our current level of understanding, the following recommendations can be made.

Gut fill is a prime concern when feeding the racehorse. Reducing hay intake to 1% of body weight for a three-day period before a race effectively will reduce body weight without causing digestive disturbances. Timing of grain feeding is not as critical for racehorses as other competitive horses, but pre-exercise grain meals should be small in size (0.5-1 kg) and fed no later than four hours before a race.

Three-day event horses
As the speed and endurance test normally commences early in the day, morning grain feeding is not recommended--high pre-exercise insulin and glucose might limit fat oxidation and increase reliance on carbohydrates for energy. On the other hand, feeding the horse a high glycemic meal (grain) or administering glucose after completion of Phase D might help to restore muscle glycogen more quickly. More rapid replenishment of muscle glycogen stores can be beneficial to performance during stadium jumping the following day.

Endurance horses
Forage intake should be high because of the associated increase in size of the water and electrolyte reservoir in the hindgut. However, as with other disciplines, large hay meals should not be fed within four hours of the start of a ride. Similarly, grain meals should be fed four to six hours before competition. A high glycemic meal the night before a race is warranted to "top up" liver glycogen stores. Unless the horse is used to receiving the "hyglycemic" grain, there may be a danger of early exercise tie up, especially if training has been reduced and the horse has not been warmed up adequately. The best rations are "complete" feeds based on beet pulp or other readily fermentable roughage source. High-quality forage such as alfalfa should be offered at rest stops/check points. Feeding small grain meals (~1 kg) immediately before exercise or at rest stops might be beneficial--this practice will not disrupt fat utilization and could supply carbohydrates during exercise. Many top competitors feed a beet pulp or bran-based slurry at reststops. Many horses will not consume 1 kg of regular grain at such stops but will eat the slurries (carrots and apple included) to increase water intake in addition to boosting energy.



Geor, R.J.; Hinchcliff, K.W.; McCutcheon, L.J.; Sams, R.A; Epinephrine inhibits exogenous glucose utilization in exercising horses. Journal of Applied Physiology 2000 (in press).

Lacombe, V.; Hinchcliff, K.W.; Geor, R.J.; Lauderdale, M.A.; Exercise that induces substantial muscle glycogen depletion impairs subsequent anaerobic capacity. Equine Veterinary Journal Supplement 30: 293-297, 1999.

Lawrence, L.; Soderholm, L.V.; Roberts, A.; Williams, J;, Hintz, H. Feeding status affects glucose metabolism in exercising horses. Journal of Nutrition 123: 2152-2157, 1993.

Pagan, J.; Harris, P. The effects of timing and amount of forage and grain on exercise response in Thoroughbred horses. Equine Veterinary Journal Supplement 30: 451-457, 1999.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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