When it comes to nutritional requirements, racehorses are in a class of their own. Broodmares need large quantities of high-quality protein and minerals to grow a large healthy foal during gestation and to produce gallons of nutrient-rich milk daily through lactation. Likewise, young horses require plenty of protein and minerals to add hundreds of pounds of muscle and bone during their first year of life. Racehorses must produce one thing–performance–and it requires heaps of energy. While racehorses certainly have a requirement for other nutrients, diet formulation for them usually begins and ends with energy.
In the wild, horses survived by grazing relatively poor-quality grasslands. Their digestive system evolved to efficiently utilize this type of diet, but their nutrient requirements were also fairly low. Today’s racehorse has energy requirements that cannot be met by forage alone. This article will review the best energy sources for racehorses and make recommendations about how to minimize problems related to feeding.
When a horse exercises, its muscles use small molecules called ATP (adenosine triphosphate) to stimulate muscle contraction. Horse muscle has very little stored ATP, but it has numerous metabolic pathways that can produce ATP as quickly as it is used. These pathways use several fuels to produce ATP, and one objective of feeding a racehorse is to optimize the stores of these fuels so the muscle can continue to contract without fatiguing.
The most important fuels for the racehorse are glycogen (a string of glucose molecules), which is stored predominantly in the muscle and to a lesser degree in the liver, and fat, which is stored mostly in adipose tissue and to a lesser degree in the muscle. During a race, muscle glycogen is the major fuel used by the racehorse, but fat is important to supply calories for lower intensity training and to meet the racehorse’s maintenance energy requirement.
Dietary Energy Considerations
Dietary energy is usually expressed in terms of megacalories (Mcal) digestible energy. Digestible energy (DE) refers to the amount of energy in the diet that is absorbed by the horse. Requirements for DE are calculated based on the horse’s maintenance DE requirement plus the additional energy expended during exercise. An idle adult horse requires about 16-17 Mcal of DE per day. A racehorse’s requirement is doubled.
Fibre is an energy source that is often overlooked in horse nutrition. Horses have a highly developed hindgut that houses billions of bacteria and protozoa capable of fermenting large quantities of fiber. The end products of fiber fermentation can be used as energy sources throughout the day because fermentation continues long after a meal has been eaten.
Since proper gut function is essential to the health and well-being of the horse, fibre-rich forage should be considered the foundation of a racehorse’s feeding program. Racehorses should be fed 7-9 kg per day of clean grass hay such as pasture hay or a pasture/rye/clover mix. Smaller quantities of lucerne hay 0.9 to 1.8 kg per day may also be offered. This level of hay intake will meet the racehorse’s maintenance DE requirement and help protect against gastric ulcers and colic.
Feeding hay alone would leave the racehorse with an energy deficit of around 15-20 Mcal DE, and this must be supplied from other energy sources. Traditionally, these extra calories needed to fuel the racehorse were supplied by straight cereal grains such as oats, corn, and barley. The main fuel source in cereal grains is starch. Starch is the dietary energy source of choice for glycogen synthesis because starch digestion results in a direct rise in blood glucose and insulin, two of the most important factors involved in glycogen synthesis.
There is a limit, however, to the amount of starch that a racehorse’s ration should contain. When large grain meals are fed to horses, a portion of the starch may escape digestion in the small intestine and rapidly ferment in the cecum and colon. Changes in the pH of the hindgut due to alterations in the microbial populations and acid profiles may result in hindgut acidosis. Horses with hindgut acidosis may develop anorexia, colic, or display stereotypical behaviors such as wood-chewing and weaving. Kentucky Equine Research (KER) has developed a hindgut buffer called EquiShure® that is designed to buffer excess acids produced in the hindgut. In a controlled study, Thoroughbreds fed timothy hay (equivalent to pasture hay in Australia) and an all-grain sweet feed had significantly lower lactic acid concentrations in their feces when supplemented with EquiShure compared to unsupplemented horses.
In addition to feeding hindgut buffers, hindgut acidosis can be controlled by limiting meal size and by substituting alternative energy sources for a portion of the starch in a racehorse feed. As a rule of thumb, never feed more than 2.5 kg of grain in a single meal to a racehorse.
Racehorses that experience exertionalrhabdomyolysis (RER) or tying-up should be fed a very lowstarch diet. Research conducted at the University of Minnesota in conjunction with KER demonstrated that replacing the grain in the diet of Thoroughbred horses with a history of tying-up with a low-starch, high-fat feed will significantly decrease the amount of muscle damage they experience.
In a feeding trial, Thoroughbred horses with RER were exercised on a treadmill for five days a week while they consumed hay and a variety of energy supplements. When the daily caloric intake of a high-starch ration was kept low, the horses had lower post-exercise serum CK (a measure of muscle damage) than when this feed was increased to a level commonly fed to racehorses. In contrast, if extra calories were provided from a low-starch, high-fat feed (Re-Leve®), no increase in post-exercise serum CK activity occurred. Since these studies were conducted, Re-Leve has been used to successfully manage RER in hundreds of Thoroughbred racehorses around the world (currently not available in Australia).
Fat is an attractive alternative energy source for racehorse rations, supplying a large number of calories in a concentrated form. Even though horses do not consume large quantities of fat in the wild, they are able to digest fats efficiently, particularly vegetable oils. Most vegetable oils contain long-chain (16-18 carbon) unsaturated fats. These fats are liquid at room temperature and are used extensively as human foods for cooking and salad oils. Once adapted, horses will digest over 90% of the vegetable oil in a ration, even when fed at levels as high as 2 to 2.5 cups (500-600 ml) per day. High levels of oil intake should be reached slowly, however, since some horses may develop loose, greasy feces when switched to a high-oil diet too quickly.
The energy density of vegetable oils is quite high, averaging about 2.25 times that of starch. Vegetable oil has about 2.5 times as much digestible energy (DE) as maize and 3.0 times as much DE as oats. Because of its high digestibility, fat is a very safe energy source. Even if some oil escapes digestion in the small intestine, it will not cause major disruptions of fermentation in the hindgut since bacteria cannot ferment long-chain oils.
Highly Fermentable Fibre
An interesting alternative energy source for performance horses is beet pulp, a byproduct of the sugar beet industry, made by drying the residual pulp after the sugar has been extracted. It contains a high percent of fermentable fibre, and its DE content is similar to oats.
If the protein content of a racehorse’s ration exceeds its requirement, then the extra protein can be used as a source of energy. The amino acids from this extra protein are broken down by the liver, and the nitrogen from the protein is excreted as ammonia. The carbon “skeletons” that are left can be oxidized to produce ATP or used to make glucose or fat.
Excessive protein intake should be avoided in the exercised horse for a number of reasons: (1) water requirements increase with increased protein intake; (2) urea levels increase in the blood leading to greater urea excretion into the gut, which may increase the risk of intestinal disturbances such as enterotoxemia; and (3) blood ammonia increases cause a number of problems such as nerve irritability and disturbances in carbohydrate metabolism.
Increased ammonia excretion in the urine may also lead to respiratory problems because of ammonia buildup in the stall.
Over the past decade, there has been a shift away from feeding straight cereal grains towards the use of commercial products formulated specifically for racehorses. These manufactured feeds use alternative energy sources such as fat and fermentable fibre to reduce the proportion of energy supplied by starch. They are also fortified to meet all of the racehorse’s nutrient requirements, including energy, vitamins and minerals when fed with an appropriate amount of forage.
A sweet feed consisting of only grain and molasses would derive 60-65% of its calories from starch and sugar. A well-formulated racing feed would limit the caloric contribution of starch and sugar to less than 50%, deriving 20-30% of its calories from fat. These feeds will usually contain between 6-10% fat along with highly fermentable fibre sources such as beet pulp. They are typically fed at levels of intake equal to 4.5 – 7 kg per day.
By Kentucky Equine Research Staff