There are 4 main components that go together to make up a poultry diet. While protein, vitamins and minerals are referred to as nutrients, energy the 4th and most costly part of the diet is not a nutrient but the property of energy yielding nutrients. Dietary nutrients that yield energy are protein, fat and carbohydrates.

Protein is not commonly thought of as a source of dietary energy but it does result in a significant contribution to the energy requirement of the bird, and can, if fat and carbohydrate are in short supply, be used by the animal as it's main source of energy.

Dietary protein is a source of amino acids which are the building blocks for body tissue, hence growth, and the production of a product — eggs. Thus it's use as a source of energy must be kept to a minimum.

Protein, carbohydrate and fats all contain carbon, hydrogen and oxygen and thus can be burned as a source of energy in the body. While proteins and carbohydrates yield around 4 calories of energy per gram, fats yield 2 times as much or 9 calories per gram. Thus, when formulating for high energy diets it is usually necessary to add a source of fat to poultry diets.

Diets with high levels of energy are referred to as having a higher nutrient density. This means that the same amount of nutrients are available in a smaller volume of with less weight. It follows that if the diet is more dense the bird will have to eat less of it to obtain it's nutrient requirements and thus feed:gain or feed:egg mass ratios are reduced. Hence, improved feed efficiency or improved feed utilization results.

Dietary energy level is the main factor influencing feed intake, as birds will, under normal circumstances, eat to satisfy their energy needs. Therefore the dietary nutrients, protein vitamins and minerals should vary in relation to the dietary energy content of the diet, if they are not to become deficient, with low feed intakes, or overconsumed, with low energy diets.

While there are a number of factors, such as level of protein, balance of essential amino acids and perhaps level of some of the other dietary nutrients, that can influence the cost of a diet, the level of dietary energy is usually the main factor influencing diet cost. Hence, by and large, the higher the level of energy the higher the diet cost and usually the lower is the feed consumption in relation to gain.

The energy content of a diet is usually given as so many calories per kilogram of diet. Thus diets are said to contain, for example, 2800 or 3200 kcal (a thousand small calories) per kilogram.

Energy content of a feedstuff is measured by burning it in an oxygen saturated environment and measuring the amount of heat or total, (gross energy) produced. However, all the energy, or heat produced from, for example, burning a gram of corn is not available to the bird. When the bird consumes corn, some of it is not digested and this undigested material is lost via the feces. Since birds excrete feces and urine together it is not possible to get a simple measure of fecal material. Thus for poultry the energy content of the feces and urine are measured together. Since this energy is unavailable to the bird it is subtracted from the gross or total energy value of the corn to yield what is referred to as a metabolizable energy value. This is the energy that is available to the bird for productive purposes, eg for growth, the production of eggs, for maintenance, activity, etc.

While the above is a simple explanation of energy and how it is utilized by the bird, there are many factors which can interact to influence dietary energy utilization. One of the main factors increasing energy requirements of the bird is pen temperature. Birds use dietary energy as a fuel to maintain body temperature. Hence, in cold pen situations a significant amount of dietary energy can be used to maintain body temperature rather than be used for more productive purposes like weight gain or egg production.

Feed manufactures are continually looking for ways to improve the energy utilization of ingredients. Such things as:

  • The addition of fat to diets — which slows down the rate of food passage in the gut and thus allows enhanced digestion by digestive enzymes.
  • Steam pelleting and conditioning — which through chemical and physical action improves the utilization of certain nutrients.
  • The use of dietary enzymes to help break down some of the poorly digested dietary components.
  • Mixing saturated and unsaturated fats together, in proper proportions, to enhance fatty acid absorption.
  • The use of synthetic essential amino acids to give a better balanced lower protein diet and thus reduce nitrogen excretion — a high energy cost function.
  • Precision grinding of cereal grains to increase surface area. This allows more efficient enzyme action resulting in enhanced nutrient availability.

The above all help to improve feed utilization of the bird by making nutrients more available.

Energy is the fuel that keeps the many different body functions operating, every minute of the day. It is a vital feed component, a costly feed component and the most wasted of the feed components. Hence, everything should be done to enhance the utilization of dietary energy for productive body functions, as improvements readily show up in increased monetary returns which are readily apparent by improved feed:gain or egg mass ratios.