Components of Poultry Diets
Poultry diets are composed primarily of a mixture of several feedstuffs such as cereal grains, soybean meal, animal by-product meals, fats, and vitamin and mineral premixes. These feedstuffs, together with water, provide the energy and nutrients that are essential for the bird's growth, reproduction, and health, namely proteins and amino acids, carbohydrates, fats, minerals, and vitamins. The energy necessary for maintaining the bird's general metabolism and for producing meat and eggs is provided by the energy-yielding dietary components, primarily carbohydrates and fats, but also protein.
Poultry diets also can include certain constituents not classified as nutrients, such as xanthophylls (that pigment and impart desired color to poultry products), the "unidentified growth factors" claimed to be in some natural ingredients, and antimicrobial agents (benefits of which may include improvement of growth and efficiency of feed utilization). Each of these components of poultry diets is considered in the following sections.
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.). Because metabolizable energy values are most commonly used to define the dietary energy available to poultry, several procedures for determining metabolizable energy values, by using bioassays or estimates based on proximate analysis, are described. An example of the disposition of dietary energy ingested by a laying hen and some general considerations regarding setting dietary energy concentrations of diets follow. Finally, some caveats are given concerning the energy values listed in the nutrient requirement tables in this report.
Energy terms for feedstuffs are defined and discussed in detail in Nutritional Energetics of Domestic Animals and Glossary of Energy Terms (National Research Council, 1981b). For a more in-depth discussion of energy terms related specifically to poultry, the reader is referred to Pesti and Edwards (1983). A brief description of the terms most frequently used in connection with poultry feeds appears below.
A calorie (cal) is the heat required to raise the temperature of 1 g of water from 16.5° 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 107 ergs (1 erg is the amount of energy expended to accelerate a mass of 1 g by 1 cm/s). The joule has been selected by Le Systéme International d'Unites (SI; International System of Units) and the U.S. National Bureau of Standards (1986) as the preferred unit for expressing all forms of energy. Although the joule is defined in mechanical terms (that is, as the force needed to accelerate a mass), it can be converted to calories. The joule has replaced the calorie as the unit for energy in nutritional work in many countries and in most scientific journals. In this publication, however, calorie is used because it is the standard energy
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
Nutrient Requirements of Poultry:
Ninth Revised Edition, 1994
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