Measuring Digestible Energy in Horse Feeds By Dr. Joe Pagan · August 16, 2012
One of the most important measures of a horse feed’s value is its energy content. Energy density determines how much feed must be fed to meet an animal’s energy requirement. Level of intake in turn dictates the concentration of all other nutrients in the feed. Therefore, horse feeds cannot be properly formulated without knowledge of their energy contents.
Unfortunately, it is not possible to directly measure the amount of useful energy contained in feed as you can with other nutrients such as protein and minerals. Instead, energy content must be estimated from other measurements. In the United States, the energy content of horse feed is expressed as digestible energy (DE). There are several different ways that DE content of horse feed can be estimated.
One method looks at the heat of combustion (gross energy). When a substance is completely burned to its ultimate oxidation products (carbon dioxide, water and other gases), the heat given off is considered its gross energy or heat of combustion. This measure is the starting point in determining the energy value of feed. The determination is usually carried out in a bomb calorimeter. The bomb calorimeter consists of a chamber, or “bomb,” in which the feed is burned. It is enclosed in an insulated jacket containing water that surrounds the bomb and provides the means of measuring heat production.
The basic unit of heat energy is the calorie (cal), defined as the amount of heat required to raise 1 g of water one degree Celsius. This unit is too small for use in horses, so the energy content of horse feed is usually expressed as kilocalories (kcal) or megacalories (Mcal). There are 1,000 calories in a kcal and 1,000,000 calories in a Mcal. Measured in terms of the joule, the international unit of work and energy, 1 Mcal = 4.185 megajoules (MJ).
Fats contain over twice as much gross energy as carbohydrates. These differences are due to the relative amount of oxygen contained in each molecule, since heat is produced only from the oxidation that results from the union with oxygen from outside the molecule. In the case of carbohydrates, there is enough oxygen present in the molecule to take care of all the hydrogen present, and thus heat arises only from the oxidation of the carbon. In the case of fat, however, there is much less oxygen present and combustion involves the oxidation of hydrogen as well as carbon. There is a difference in gross energy between glucose and starch because there is relatively more carbon in 1 g of starch than in 1 g of glucose, and thus starch has a higher energy value.
The energy-producing component of horse feed can be divided into three classes of nutrients: protein, fat, and carbohydrates. Gross energy for carbohydrates is 4.15 kcal/g; for fats, 9.40 kcal/g; and for proteins, 5.65 kcal/g. These gross energy values for the three classes of nutrients explain the gross energy differences in various feeds. For instance, oats have more gross energy than corn because they contain more protein and fat. Whole soybeans are high in protein and fat, so their gross energy is even higher.
Gross energy is not a good indicator of overall feed value because it does not differentiate between various carbohydrate sources. Digestible energy (DE) is a far better indicator of the energy a horse will derive from a particular feed.