Energy Sources and Use

As described by the first law of thermodynamics, energy can be changed from one form to another but can be neither created nor destroyed. The source of all energy for all living things is sunlight. This energy is captured by plants, which use it via photosynthesis to change carbon dioxide, present in the air, and water to oxygen and the carbon compounds that make up the plant. These carbon, or organic, compounds in plants are carbohydrates, fats, and, along with nitrogen, protein. Nitrogen, like the small amount of minerals needed by plants, is taken up from the soil but is originally from the air. Carbohydrates, fats, and proteins are stored sources of energy.

Animals eat plants, or tissues of others that have eaten plants. The stored sources of energy, carbohydrates, fats, and proteins in the plants are digested, absorbed, and transported to the animal’s body cells. Some are used to make up the structural components of the cell and thus the animal; but, if needed in the future, along with oxygen from the air, they can be converted by chemical reaction to carbon dioxide and water, in the process producing energy. Animals use the energy to produce heat and adenosine triphosphate, or ATP, which cells then use to function. Thus, plants and animals have a mutually sustaining relationship in which plants produce carbon compounds (C-cpds)—organic matter and oxygen (O2)—that support animals, and animals produce the carbon dioxide (CO2) and water (H2O) that support plants. Thus, the horse, like us, plants, and all living things on earth, are products of the air and the soil, and function solely as the result of solar energy.

Energy has no measurable dimension or mass, but it can be converted to heat, which can be measured. Oxidation, or burning, converts stored energy (carbohydrates, fats, and proteins) to heat, carbon dioxide, water, and a nitrogen compound. These are returned to the air and soil, where they originated, available to begin the cycle again. When a substance is completely oxidized, the heat produced, called the heat of combustion, is the total or gross amount of energy stored in, and thus available from, that substance. However, the animal cannot use all of the gross energy present in a feed. Some of the stored energy in a feed is not digested and is lost in the feces. Of the remainder, called digestible energy, some is lost in the urine as urea and in the gastrointestinal tract as gases (primarily methane), leaving metabolizable energy, of which some is used in metabolizing feed. What remains is the net energy available for maintenance, growth or fattening, milk production, and physical activity. Most of the total dietary energy needs are for maintenance. Even during heavy lactation or physical activity over 50% of dietary energy is needed for maintenance, and for young horses, 60 to 95% of dietary energy needs are for maintenance, leaving 5 to 40% for their growth.

The amount of heat produced by oxidation (burning) that raises the temperature of one gram of water 1C is defined as one calorie. This is also known as the small, gram, or standard calorie. However, it is not used in nutrition for animals or people. The calorie used in nutrition is the amount of heat required to raise one kilogram of water 1C. It is called the large calorie, Calorie, or kilocalorie (kcal) since it is equal to 1000 small calories. In nutrition, the word calorie always refers to kilocalorie, even if it is not capitalized and neither the kilo- or k- prefix is used. For large animals, such as horses, megacalorie (Mcal), therm, or total digestible nutrients (TDN) are usually used. One megacalorie equals one therm, and both equal 1000 kilocalories. Occasionally, primarily in England, instead of calorie, the joule, or, in the physical sciences, the British Thermal Unit (BTU) is used. One megacalorie equals 4.1855 megajoules and 3968 BTU. TDN is a measure of digestible energy expressed in units of weight or percent, with 1 lb TDN equal to about 2.0 Mcal DE (1 kg TDN 4.4 Mcal). TDN is the sum of digestible carbohydrates, plus digestible protein, plus digestible fats times 2.25, because fats provide about 2.25 times more energy than an equal weight of carbohydrates or proteins. Starch equivalent, or SE, is occasionally used as an energy term by comparing the energy provided by a feed to that provided by starch, which is assigned a value of 100%. Although occasionally used for ruminants, SE is unsuitable for horses because of their different digestive process.

When calculating energy intake, or the amount of feed needed to provide a certain amount of energy, such as that necessary to meet the animal’s energy requirements, any of the various energy terms may be used. Of course, the same units must be used for both the energy content of the feed and the animal’s energy needs. Net energy is the most accurate, followed by metabolizable energy. However, they are the most difficult to determine and, as a result, are not routinely available for most horse feeds. Digestible energy values (or TDN) are generally available for most horse feeds and therefore are the most commonly used energy terms. Energy available from forages is usually 5 to 15% higher for cattle than for horses because of ruminants’ more efficient utilization of fiber. Therefore, if the energy content of forages for cattle, or other ruminants, is used to determine the amount of these feeds needed by horses, the amount determined generally will be erroneously low.

This article is from "Feeding and Care of the Horse", second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.