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Equinews: Feeding Management of Athletic Horses in Stressful Conditions
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Feeding Management of Athletic Horses in Stressful ConditionsBy Dr. Joe Pagan · March 1, 2012

Most performance horses train and compete under a variety of stressful conditions that adversely affect health and performance. Feeding management is of critical importance to reduce many of these problems. Additionally, pre-competition feeding can significantly affect performance. Feeding management affects a number of different aspects of equine health and performance including gastrointestinal function, hydration, electrolyte status, and substrate selection during exercise. This article will review these key areas of performance horse nutrition and give practical recommendations about how to feed horses under stressful conditions.

Horses have evolved over millions of years as grazers, with specialized digestive tracts adapted to digest and utilize diets containing high levels of plant fiber. They are capable of processing large quantities of forage to meet their nutrient demands. In an attempt to maximize growth or productivity, horses are often fed diets which also contain high levels of grains and supplements. Unfortunately, this type of grain supplementation often overshadows the significant contribution that forages make in satisfying the horse’s nutrient demands.

Horses are classified anatomically as nonruminant herbivores or hindgut fermenters. The large intestine of the horse holds about 80 to 90 liters (21 to 24 gallons) of liquid and houses billions of bacteria and protozoa that produce enzymes which ferment plant fiber. The by-products of microbial fermentation provide the horse with a source of energy and micronutrients. The equine digestive tract is designed in this way to allow the horse to ingest large quantities of forage in a continuous fashion. The small capacity of the upper part of the tract is not well suited for large single meals, a fact which is often ignored by horsemen. Large single meals of grain overwhelm the digestive capacity of the stomach and small intestine resulting in rapid fermentation of the grain carbohydrates by the microflora in the hindgut. This fermentation may result in a wide range of problems including colic and laminitis.

The fact that horses are hindgut fermenters has several implications for the person feeding the horse. First, since horses are designed to live on forages, any feeding program that neglects fiber will result in undesirable physical and mental consequences. Horses have a psychological need for the full feeling that fiber provides. Horses fed fiber-deficient diets will in extreme cases become chronic wood-chewers. It is also important to maintain a constant food source for the beneficial bacteria in the hindgut. Not only does their fermentation of the fiber provide a great deal of energy for the horse, but their presence prevents the proliferation of other, potentially pathogenic bacteria. Horses, like humans, need a certain amount of bulk to sustain normal digestive function. Horses have an immense digestive system designed to process a large volume of feed at all times. Deprived of that bulk, the many loops of the bowel are more likely to kink or twist, and serious colic can result.

The optimal quantity of forage intake varies by discipline. Endurance horses benefit from high forage intakes during both training and competition. Research conducted in Germany has underscored the importance of fiber in maintaining gut health. These experiments have shown that a diet high in fiber 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 animals on a low-fiber diet. The additional water and electrolytes in the digestive tract of the animals fed high-fiber diets is probably due to the high water-holding capacity of plant fiber. More importantly, the water and electrolyte pool created by a high-fiber diet can be used to combat dehydration and electrolyte imbalances which derail so many performance horses.

In Thoroughbred racehorses, excess gut fill during competition may be detrimental because additional energy must be expended to carry the extra weight of the ingesta. Therefore, a feeding strategy must be followed that tapers forage intake before a race. Kentucky Equine Research (KER) conducted a study to determine the effects of restricted hay intake on the metabolic responses of horses to high-intensity exercise. We hypothesized that, compared to ad libitum hay intake, a regimen of restricted hay feeding starting three days before a standardized exercise test would decrease body weight and reduce energy expenditure during running.

Four conditioned Thoroughbred horses were studied in a 2 x 2 crossover design. Initially, the length of time required for adaptation to ad libitum (AD LIB) intake of grass hay was determined. Thereafter, the metabolic responses to sprint exercise (SPR) were examined in two dietary periods, each five days in duration: (1) AD LIB, where horses had free-choice access to hay; and (2) Restricted (RES), where hay intake was restricted (~1% of body weight) for three days before the exercise test. Feed and water were removed four hours before the exercise test.

After measurement of body weight, horses completed a warm-up followed by 2 min at 115% of maximum oxygen uptake, then a 10-min walking recovery (REC).

During the three days before SPR, hay intake in AD LIB averaged (± SE) 10.1 ± 0.9 kg, whereas intake during RES was 4.3 ± 0.2 kg. Pre-exercise bodyweight was significantly lower in RES (528 ± 5 kg) than in AD LIB (539 ± 4 kg). During SPR, total mass-specific VO2 was higher (P=0.02) in RES (243 ± 8 ml/kg/2 min) than in AD LIB (233 ± 10 ml/kg/2 min). Conversely, accumulated oxygen deficit was higher (P<0.01) in AD LIB (89.4 ± 2.2 ml O2/kg) than in RES (82.4 ± 1.7 ml O2/kg). Peak plasma lactate was also higher in AD LIB (22.2 ± 1.2 mM) than in RES (19.1 ± 2.1 mM), and VO2 during recovery was 10% higher (P=0.12) in AD LIB.

The main findings of this study were: (1) compared to ad libitum hay feeding, three days of restricted (1% of body weight) hay intake was associated with an approximately 2% decrease in body weight, and (2) the reduction in body weight associated with restricted hay feeding resulted in an increase in the mass-specific rate of oxygen consumption during sprint exercise, with a corresponding decrease in anaerobic energy expenditure. The anaerobic contribution to energy expenditure during exercise was lower in RES than in AD LIB as evidenced by lower values for accumulated oxygen deficit and peak plasma lactate concentrations.

Currently, it is recommended that performance horses receive hay at a minimum of 1% of body weight per day to satisfy requirements for long-stem fiber and to minimize digestive upsets. In this context, relative to the restriction protocol used in this study, more severe or long-term restrictions of hay intake are not recommended. Nonetheless, on the basis of our results, further studies that examine the relationship between fiber intake, body weight, and exercise metabolism and performance are warranted.