terminology used in the U.S. poultry industry and there is no difference in accuracy between the two terms.

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 atmospheres 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 × dry weight of food] - [E of feces per unit dry weight × 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. A correction for nitrogen retained in the body is usually applied to yield a nitrogen-corrected ME (MEn) value. MEn, as determined using the method described by Anderson et al. (1958), or slight modifications thereof, is the most common measure of available energy used in formulation of poultry feeds.

True metabolizable energy (TME) for poultry is the gross energy of the feed consumed minus the gross energy of the excreta of feed origin. A correction for nitrogen retention may be applied to give a TMEn value. Most MEn values in the literature have been determined by assays in which the test material is substituted for part of the test diet or for some ingredient of known ME value. When birds in these assays are allowed to consume feed on an ad libitum basis, the MEn values obtained approximate TMEn values for most feedstuffs.

Net energy (NE) is 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 1-1 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 1-1 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) (Fraps, 1946; Hill and Anderson, 1958; Titus, 1961). 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 (Scott et al., 1982).

Procedures for Determining Metabolizable Energy

Metabolizable energy is determined by various bioassay procedures whereby feed intake and excreta output are related over a 2- to 5-day test period. Apparent metabolizable energy is most commonly determined through actual measurement of feed intake and excreta output, or by determining the ratio of dry matter intake to output through use of an inert dietary marker, such as chromic oxide (Cr2O3). A number of potential problems arise with use of markers (Kane et al., 1950; Vohra and Kratzer, 1967; Duke et al., 1968; Vohra, 1972a), and thus the latter method often leads to more variation in final determined ME values (Potter, 1972).

When the ME value of an ingredient is to be determined, two or more diets must be used, since feeding an ingredient by itself can cause palatability problems and fails to accommodate potential synergism between nutrients. The two methods most frequently used in substituting the test ingredient into a control basal diet are those described by Anderson et al. (1958) and Sibbald and Slinger (1963). In the former method the test ingredient is substituted for glucose, but in the latter method the test ingredient is substituted for all the energy-yielding ingredients of the basal diet. Anderson et al. (1958) proposed that the value of 3.65 kcal/g be

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