Energy Value Of A Feed Ingredient In Poultry Diets
Energy is not a nutrient but a property of energy-yielding nutrients when they are oxidized during metabolism. The energy value of a feed ingredient or of a diet can be expressed in several ways. Thus, a description is presented below of terminology associated with dietary energy values, including units of measure (digestible energy, metabolizable energy etc...).
Here are a few brief description of the terms most frequently used when describing the energy value for animal feed.
A calorie (cal) is the heat required to raise the temperature of 1 g of water from 16.5°C to 17.5°C. Because the specific heat of water changes with temperature, however, 1 cal is defined more precisely as 4.184 joules.
A kilocalorie (kcal) equals 1,000 cal and is a common unit of energy used by the poultry feed industry.
A megacalorie (Mcal) equals 1,000,000 cal and is commonly used as a basis for expressing requirements of other nutrients in relation to dietary energy.
A joule (J) equals 10^7 ergs (1 erg is the amount of energy expended to accelerate a mass of 1 g by 1 cm/s). Although joule is defined in mechanical terms (the force needed to accelerate a mass), it can be converted to calories. The joule has replaced calorie as the unit for energy in nutritional work in many countries and in most scientific journals.
A kilojoule (kJ) equals 1,000 J.
A megajoule (MJ) equals 1,000,000 J.
Gross energy (E) is the energy released as heat when a substance is completely oxidized to carbon dioxide and water. Gross energy is also referred to as the heat of combustion. It is generally measured using 25 to 30 atmostpheres of oxygen in a bomb calorimeter.
Apparent digestible energy (DE) is the gross energy of the feed consumed minus the gross energy of the feces. (DE = [E of food per unit dry weight x dry weight of food] - [E of feces per unit dry weight x dry weight of feces]). Birds excrete feces and urine together via a cloaca, and it is difficult to separate the feces and measure digestibility. As a consequence, DE values are not generally employed in poultry feed formulation.
Apparent metabolizable energy (ME) is the gross energy of the feed consumed minus the gross energy contained in the feces, urine, and gaseous products of digestion. For poultry the gaseous products are usually negligible, so ME represents the gross energy of the feed minus the gross energy of the excreta.
True metabolizable energy (TME) for poultry is the gross energy of the feed consumed minus the gross energy of the excreta of feed origin.
Net energy (NE) is the metabolizable energy minus the energy lost as the heat increment. NE may include the energy used for maintenance only (NEm) or for maintenance and production (NEm+p). Because NE is used at different levels of efficiency for maintenance or the various productive functions, there is no absolute NE value for each feedstuff. For this reason, productive energy, once a popular measure of the energy available to poultry from feedstuffs and an estimate of NE, is seldom used.
Disposition of Dietary Energy
Figure 1 illustrates the proportional relationships in the disposition of dietary energy ingested by a laying hen. Energy is voided or used at various stages following consumption of 1 kg feed by the hen.
Figure 1. Disposition of dietary energy ingested by a laying hen
Of 4,000 kcal provided in 1 kg of this particular diet, 2,900 kcal are capable of being metabolized by the hen and about 2,300 kcal are available for maintenance and transfer into body tissue and egg (net energy). The relative amounts of both metabolizable and net energy will, of course, vary with the composition of the feedstuffs in the diet. Other factors, such as the species, genetic makeup, and age of poultry, as well as the environmental conditions, also influence the precise distribution of dietary energy into the various compartments.