The Energy Content of Horse Feeds

The Energy Content of Horse Feeds

The use of calories in equine nutrition is twofold: We discuss how many calories are required in a given day and how many calories are in feed.

Photo: Anne M. Eberhardt/The Horse

Understanding the concept of weight management requires an understanding of energy. Very often horse owners confuse the term “energy” with “spirit” or “activity level.” As it relates to nutrition, however, energy refers to calories, which are units of energy (described further below). And with respect to weight management, if any animal takes in more calories than it expends, it will gain weight (mostly as fat); if it expends more calories than it takes in, it will lose weight.

So what is a calorie? A calorie is defined as the energy required to heat one gram of water one degree centigrade. One calorie is a very small amount of energy; for instance, a 500 kg horse trotting for one minute burns 56,000 calories. Because a calorie is such a small unit of energy, most feeds and requirements are reported in kilocalories (kcal, where 1 kcal = 1,000 calories) or even megacalories (Mcal, where 1 Mcal = 1,000 kcal). To put it in perspective, humans refer to Calories (note the capital “C”), which actually represent kilocalories. Most equine diets are built on the range of megacalories. For example, a mature horse might require 16 Mcal of energy per day. In Europe and elsewhere in the world, the unit of measure for energy is the joule (J), where 1 calorie is equal to 4.184 joules.

The use of calories in equine nutrition is twofold: We discuss how many calories are required in a given day and how many calories are in feed.

Energy Content

Horses derive calories by breaking down feed through processes of digestion, absorption, and metabolism. Different feeds will generate different amounts of calories per unit weight based on these processes, so it is important to have an understanding of how feeds are broken down. Some biochemistry is involved to fully understand these processes, so we’ll just cover the basics.

Horses can generate energy from three main classes of feedstuffs: carbohydrates (derivatives of sugar), fats, and proteins. These are types of organic compounds because their structures have three key elements: carbon, oxygen, and hydrogen. (Note “organic” in this case does not mean “not treated with pesticides or herbicides.” In the realm of nutrition, organic refers to compounds containing these elements.) Structurally, carbohydrates, proteins, and fats are very different, but the ultimate energetic end product of their digestion and metabolism is the same: adenosine triphosphate (ATP), with a difference among these energetic compounds being the number of ATPs each generates.

Continuous work (such as walking or trotting or even beating the heart) requires multiple contraction and relaxation cycles and substantial amounts of ATP.

Photo: Anne M. Eberhardt

ATP is the compound required for muscle to function, though in reality it relaxes the muscle after a contraction. Continuous work (such as walking or trotting or even beating the heart) requires multiple contraction and relaxation cycles and substantial amounts of ATP. Thus, without ATP the muscle would contract but not relax. For example, when rigor mortis sets in after death, it is due to the body’s inability to reproduce ATP to relax the muscles. (Eventually, the muscle proteins begin to break down so that rigor mortis only lasts a few hours.)

ATP is the energy currency of all cells, similar to gas in a car. Without gas the car wouldn’t go very far. Likewise, the body needs constant replenishment of ATP, which we can generate through the metabolism of feedstuffs. Carbohydrates (found in forages and grains) and fats (found in small amounts in all feeds, but primarily in oils) are the most important sources of energy for a horse because of the ability of carbohydrates and fats to generate ATP through their metabolism. The use of one unit of ATP generates 7 kcal (or Calories) of energy.

Carbohydrates are extremely important to the horse for two main reasons. For one, they are a major component of forages, a staple of the horse’s diet and required for digestive health. Secondly, forages provide ample energy for the horse after its digestion and metabolism to ATP.

One of the simplest carbohydrates is a monosaccharide, a structure comprising six carbon atoms arranged with hydrogen and oxygen atoms. Types of monosaccharides include glucose, fructose, and galactose, depending on the actual structure. Another type of carbohydrate is a disaccharide, such as sucrose and lactose, which contains two monosaccharides. For example, sucrose is made of the monosaccharides fructose and glucose while lactose is made from galactose and fructose.

A polysaccharide is a long chain of hundreds to thousands of monosaccharides bound together. Polysaccharides such as starch and cellulose (a type of fiber) are digested by the body differently, largely because of the specific bonds between the individual saccharides.

Digestion of carbohydrates such as starch and lactose is through the action of digestive enzymes, which break these compounds into monosaccharides which are then absorbed by the digestive tract. Therefore, monosaccharides, such as glucose, are the primary form of carbohydrates ultimately used for energy production. Because of their relatively simple structure, sugars don’t generate large amounts of energy (ATP) per unit weight. However, as horses can consume significant amounts of carbohydrates, particularly when fed a high starch diet (for example, one that is high in cereal grains), energy derived from them can make up a substantial portion of their intake.

Note that fiber, a type of polysaccharide, is not digested by enzymes produced by mammals. These carbohydrates must be fermented by microbial organisms found in horses' digestive tracts. The end products of microbial fermentation of fiber are the volatile fatty acids (VFAs). These compounds are absorbed and metabolized within the body and can be used to produce ATP. The VFAs do not produce substantial energy per unit weight, but have the ability to provide a large portion of the horse’s energy intake because of the high intake of fiber in the horse’s typical diet. The concept of fiber digestion is explained in more detail within the digestion section below.

As indicated earlier, fats are another important source of energy to the horse because of their ability to produce large amounts of ATP. Fats (lipids) are a category of nutrients that generally consists of triglycerides. Triglycerides are structures consisting of three fatty acid chains and one glycerol (a short carbohydrate) unit. Different types of fat are derived from differences in the fatty acid chain. For example, saturated fats have single bonds between all of their carbon units while unsaturated fats have double bonds between many of their carbons. The chain location of these double bonds gives rise to the “omega” fatty acids; for example, omega-3 fatty acids have their first double bond after the third carbon. Because of the complex structure of fats, their metabolism generates relatively large amounts of energy (ATP) per unit weight.

Proteins can also be used for energy production, though they are not a very efficient source of energy. Protein is found in varying amounts in most equine feeds, with feeds such as soybean meal and legume hays having high amounts (soybean meal is 44% protein and legume hays are typically 15% to 18% protein). Proteins are unique organic compounds in that along with the elements oxygen, hydrogen, and carbon, they also have nitrogen in their structure.

Amino acids are the building blocks of protein, similar to letters in a word. There are approximately 20 different amino acids due to different side chains in their structure. Proteins (amino acids) are primarily used to build hormones and other important functional compounds within the body, but they can be metabolized to generate energy if fed in excess. When protein is fed in the diet, it will first be used to satisfy body protein requirements; any excess protein is metabolized for energy. Therefore, while protein can be used for energy, it is not a very efficient source of ATPs.

Almost all equine feeds are going to have a mixture of some carbohydrate, fat, and protein.

Photo: Anne M. Eberhardt

Almost all equine feeds are going to have a mixture of some carbohydrate, fat, and protein, and the proportions of these will partially determine how much energy can be generated. However, an important part of the picture is how well these compounds are digested and absorbed into the body.

The concept of how well energy is produced is relatively simple in theory. All energetic compounds (carbohydrate, fat, protein) can generate energy; that is, they can generate calories or heat when they are combusted (burned). The amount of heat (energy) generated by combusting a particular feed component is termed the gross energy. The gross energy of 1 gram of carbohydrate (straight table sugar, starch, or cellulose) is approximately 4 Calories; 1 gram of fat generates approximately 9 Calories; and protein generates approximately 5 Calories. Remember, however, that most horse feeds (such as grains or hay) are going to be made of plant material that already has a mixture of some carbohydrate, protein, and fat in it, so the actual gross energy of a feedstuff may vary.

As the animal digests feed, some components may not be digested well and will be lost from the body as feces. The amount of energy digested (and therefore available) by the horse after energy is lost through feces is termed the digestible energy. Thus, the amount of digestible energy in a particular feedstuff is going to depend primarily on the energetic nutrients in the feed (fat, carbohydrate, and protein breakdown) but also on the amount digested. When we refer to how much energy a horse needs in its diet, we use digestible energy values. It should be noted that some countries use the net energy system, which accounts for losses of energy in urine, gas, and heat.

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