Starch and Fat Provide Extra Energy to Horses

The horse’s energy requirements determine the amount and type of feed needed. Forage, primarily pasture grass, is the most natural food for horses. Horses at rest or only lightly exercised can meet their energy requirement from pasture alone, but young growing horses, pregnant or lactating mares, and performance horses usually need more energy than can be supplied by roughage. Additional digestible energy (DE) is commonly provided by adding dietary starch, fat, or both to the horse’s ration.

Starch
The most common way of supplying energy in the ration is by feeding grain. Utilising energy from starch is very efficient because of the simple enzymatic process involved in its digestion. It takes fewer kilograms of grain to get the same amount of energy that is in a much larger amount of roughage. Grains are an excellent source of starch, which comprises 50–70% of the grain’s dry matter, but they can be hazardous if fed to excess.

The starch molecules in grains are complex polysaccharides that are digested in the small intestine by the enzyme amylase, which breaks up the polysaccharides into simple sugars (glucose). These sugars are easily absorbed into the bloodstream and once in the blood, the glucose units are used for a number of different purposes. Some are oxidised directly to produce adenosine triphosphate (ATP), the fuel for active muscular contraction. Others are stored as muscle glycogen, liver glycogen, or body fat.

Muscle glycogen is an important source of energy during exercise. The glycogen stored in the liver is available for the production and release of glucose into the blood during exercise. Maintaining blood glucose levels during exercise is very important because glucose is the only energy source available to the central nervous system.

Starch is the dietary energy source of choice for glycogen synthesis. The digestion of starch directly increases blood glucose and insulin, two of the most important factors involved in glycogen synthesis. The limiting factor of starch digestion is the production of amylase in the intestinal tract, which can vary between horses. If amylase production is too low or the amount of starch is too high, undigested starch passes through to the large intestine. The fermentation of large amounts of grain in the hindgut creates lactic acid, which lowers the pH of the hindgut and upsets the microbial balance by killing other bacteria, leading to the release of endotoxin into the blood and possibly laminitis or colic.

Starch molecules come in different sizes, some of which are more easily digested than others. The starch molecules contained in oats, for example, are smaller than those in corn and barley and are more easily digested by amylase. However, if corn and barley are treated with heat, the nature of the starch molecule is altered and it becomes more easily digested. Thus, processing increases the nutritional value of some grains.

Fat
In some cases, particularly the horse in heavy training, even cereal grains will not provide enough energy because the horse cannot eat enough to meet its requirements. If a concentrated source of energy is required, then a fat supplement may be added to the diet. Fat contains three-fold more digestible energy (DE) than oats and 2.5-fold as much DE as corn (maize). Fat is a less versatile energy source than starch because it must be oxidized aerobically to produce energy or be stored as body fat. Fat is an extremely useful dietary energy source for horses for a number of reasons. Performance horses in heavy training have a very high daily requirement for DE and may not be able to eat enough grain and roughage to meet it. Adding fat will increase the energy density of the diet so that less feed is required. Fat supplements are either vegetable oil (e.g., corn oil, canola oil, and soybean oil) or high-fat feedstuffs such as sunflower seeds, full-fat soy, or rice bran. The dry fat sources are easier to feed than vegetable oil.

There is a small percentage of fat in the basic diet of grass (1–4%) and in the diet modified by the addition of oats (4–5%) and hay (1–3%), so fat in the diet of the horse is not unnatural. The horse is better able to digest fat than cattle or sheep and can tolerate up to 20% of the diet as fat, although in practice that quantity is rarely fed.

Fat must be digested completely in the small intestine. If too much fat passes into the hindgut, the microbial balance will be upset, leading to digestive disorders and interference with the absorption of some nutrients. Signs of too much fat in the diet include diarrhoea or loose droppings that have a soapy appearance.

After the fat has been absorbed from the small intestine, it is taken in the bloodstream to the liver where its future use is decided. If the body needs immediate energy for muscle contraction, the fat is sent by the bloodstream to the muscle cells where it is further broken down into two carbon units that are used as an energy source. If the body does not need energy from fat at that particular time, it is stored in the adipose tissue (body fat) distributed throughout the body.

As with all changes to the horse’s diet, the introduction of fat must be done gradually to allow the digestive processes to adapt. Beginning with 50 ml of oil, add another 50 ml every three days, taking two weeks to reach 250 ml/day. If adding high-fat feeds, change the diet slowly over 7–10 days. The digestive system will take 14–30 days to adapt to the new diet.

Horses also need time for their metabolic processes to adapt to a higher-fat diet. A minimum of 30 days is needed before the metabolic processes in the muscles switch to using fat as an energy source in preference to glucose. It takes 3–4 months for the optimum development of energy production in the muscle, so you will not get benefit of a high fat diet overnight.