1
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

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 Terminology

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



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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 1 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 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 Terminology 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

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 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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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 used as the standard for glucose. The basal diet used by Anderson et al. (1958), containing about 50 percent glucose and designated as E9, has been used extensively in determinations of nitrogen-corrected ME (MEn). In the method of Sibbald and Slinger (1963) the test ingredient is substituted essentially for part of the complete basal diet. However, to avoid mineral and vitamin deficiencies, components of the diet containing these nutrients are left intact, The use of two basal diets of differing protein contents was proposed to maintain the protein contents of substituted diets within an acceptable range. An advantage of the substitution method of Sibbald and Slinger (1963) is that the MEn value of the reference basal diet is necessarily determined in each MEn assay. Although samples of glucose are likely to be less variable than samples of regular feed ingredients, the MEn of glucose may vary under different dietary conditions, and its MEn value should be determined under the experimental conditions used (Mateos and Sell, 1980). The test ingredient may be substituted at one or more levels. Regardless of the basal diet used, the accuracy of the MEn value obtained depends to some extent on the proportions of the test ingredient substituted into test diets. In extrapolating to calculate the MEn value of the test ingredient, the error of determination of the test ingredient is therefore multiplied by a factor of 100 divided by percentage of substitution. Therefore the highest proportion of the test ingredient possible in the test diet should be used. Usually, this amount is determined by nutrient balance and palatability. Potter et al. (1960) proposed a linear regression procedure for the calculation of MEn values for ingredients substituted at several levels. The ingredient MEn value is derived by extrapolation to 100 percent inclusion from a regression equation relating test diet MEn values and proportion of test ingredient in such diets. As for most other methods of MEn determination, a criticism of the regression methods is that the extrapolation is beyond the range of experimental data. Sibbald and Slinger (1962) pointed out that this general criticism is of little significance as long as the range of inclusion levels used is within that normally encountered under practical conditions because it is the application of ingredient MEn values in commercial dietary formulation that is of interest. TME was described as an estimate of ME in which correction is made for metabolic fecal and endogenous urinary energy (National Research Council, 1981b). These energy components of excreta are not directly of dietary origin, and, as suggested by Sibbald (1980), correction for their excretion in bioassays leads to TME. It should be noted that ME as determined using the procedure of Anderson et al. (1958) inherently corrects for metabolic fecal and endogenous urinary energy excretion, whereas the method of Sibbald (1976) for determining ME does not. The TME method is quite rapid in that it takes only a 48-hour collection period and, because ingredients are force-fed, there is no need to use a series of basal and test diets. The TME procedure, however, has been subjected to criticism. TME determinations assume that fecal metabolic and urinary endogenous energy excretions are constant, irrespective of feed intake. Data have been presented showing that, to the contrary, metabolic and endogenous energy excretions are influenced by amount and nature of materials passing through the gastrointestinal tract (Farrell, 1981; Farrell et al., 1991; Tenesaca and Sell, 1981; Hartel, 1986). Another criticism is that ingredients are often force-fed alone, thereby preventing synergistic or antagonistic effects between or among ingredients on energy utilization. Synergism is known to occur between fatty acids (Young, 1961; Artman, 1964; Leeson and Summers, 1976a) and there is evidence for synergism between protein concentrates (Woodham and Deans, 1977). A third criticism of the TME method relates to the imposition of 48 periods of feed deprivation, which would result in an abnormal physiological status of the bird. Both ME and TME should be corrected for nitrogen retention that occurs during the assay period. If, during an ME determination, nitrogen is retained by the animal, the excreta will contain less urinary nitrogen and hence less energy would be excreted as compared with an animal that is not retaining N. Because the extent of nitrogen retention differs with age and species, a correction factor is essential if comparisons of ME values for the same ingredient with different animals are to be made. Hill and Anderson (1958), assuming that if nitrogen is not retained it will appear as uric acid, proposed a correction value of 8.22 kcal/g nitrogen retained because this is the energy obtained when uric acid is completely oxidized. This assumption has been criticized because only 60 to 80 percent of the nitrogen of chicken urine is in the form of uric acid (Coulson and Hughes, 1930). However, the assumption that oxidation of varying amounts of protein would yield a consistent pattern of nitrogenous excretory products seems no more correct than the assumption that all nitrogen would be excreted as uric acid (Hill and Anderson, 1958). Thus, from a practical viewpoint, the uric acid value has been used most frequently and is generally quoted (Scott et al., 1982). Sibbald and Slinger (1963) questioned the validity of correcting for nitrogen retention, suggesting that correction does little to improve the usefulness of classical ME values and that the extra work involved is not justified. Potter (1972), however, suggested that correction to zero nitrogen retention is essential for reproducible results when the MEn of a single diet is to be measured

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 with birds of various ages because of differences in rates of protein accretion or protein catabolism. Correction to a species-specific or age-specific nitrogen retention, although having the advantage of applicability for specific circumstances, cannot be used in comparative work because "typical" nitrogen retention varies with species and age. Leeson et al. (1977a) indicated the need for nitrogen correction in interpretation of bioassay data. An alternative to classical bioassay is based on changes in rate of growth in response to dietary energy. Squibb (1971) suggested a method for the "standardization and simplification" of MEn determination procedures. The method is a modification of that described by Yoshida and Morimoto (1970). It is based on the premise that rapidly growing immature animals restricted in terms of energy intake but given adequate protein will show an increase in growth in direct proportion to energy added to the diet. Considering the restricted feeding of the energy-deficient diet used by Squibb (1971), the adequacy of the protein in terms of quantity and quality can be questioned. However, the concept warrants further study as a means of evaluating the energy value of ingredients, such as fats, that are difficult to assay using conventional procedures. Most MEn values reported for feedstuffs have been determined with young chicks. Although adult male chickens have been used to determine TMEn content of many feedstuffs, few studies have been done to determine either MEn or TMEn for poultry of different ages. More MEn and TMEn data are needed for many feed ingredients for chickens, turkeys, and other poultry of different ages. Estimation from Proximate Composition Several researchers have developed prediction equations to estimate the energy content of feed ingredients from their proximate components. Prediction of the "usable" energy value of a feed from its chemical composition has been attempted for many years. The Weende, or proximate analysis, system was developed as an attempt to predict the nutritional value (including the energy value) of an ingredient or of mixed feed from its component parts. Fraps et al. (1940) predicted the ME content of feeds from the values for digestible crude protein, ether extract, and nitrogen-free extract (NFE). Titus (1955) used this concept to derive a series of "percentage multipliers" for the calculation of ME values for different types of feed ingredients. Later, these ''percentage multipliers" were updated and extended to a wider range of ingredients (Titus and Fritz, 1971). Janssen et al. (1979) conducted a series of studies to correlate the chemical composition of different types of feed ingredients to the ME value. By using multiple regression analysis, equations were derived to estimate MEn (kcal/kg dry matter) from chemical composition. More recently, a subcommittee of the European Federation of the World's Poultry Science Association (1989) developed a set of equations to estimate the energy value of ingredients. Data sets from a number of European laboratories were combined to develop the equations. A list of prediction equations that have been published recently is provided in Appendix Table B-1. Dale et al. (1990) developed an equation to estimate the TMEn value of dried bakery products, a blend of various by-products produced by the baking industry. The ME value of grain sorghums is known to be influenced by their tannin content. Sibbald (1977) reported TME values of 3,300 and 3,970 kcal/kg for high- and low-tannin grain sorghums, respectively, and Queiroz et al. (1978) found MEn values of 2,886 and 3,091 kcal/kg for high- and low-tannin grain sorghums. Gous et al. (1982) found a highly significant negative correlation between the MEn of grain sorghums and their tannic acid content, the relationship due to a decreased digestibility with increasing tannic acid concentration. These researchers developed a regression equation to estimate ME from tannic acid concentration. A similar equation was developed by the European Federation of the World's Poultry Science Association in 1989. Although these equations may result in slightly different estimates, they both point out the adverse effects of the tannin content on digestibility of grain sorghums. Moir and Connor (1977) developed equations to predict MEn of grain sorghums using three different types of crude fiber assays. The MEn content of sorghum was predicted from the three fiber assay methods with precision of, respectively, ±117, ±148, and ±126 kcal/kg dry matter. These values correspond to coefficients of variation of 3.0, 3.8, and 3.3 percent, respectively. Thus, any of the three fiber methods could be used to predict the MEn of grain sorghums for poultry. Considerable variation exists in the nutrient composition of poultry by-product meal from various production lots and among producers, depending on raw material used (e.g., proportions of feet, legs, blood, and offal may vary considerably). Pesti et al. (1986) determined the TMEn of a number of samples of poultry by-product and derived several equations to estimate TMEn from various measurements. The equations vary in complexity, some using only one parameter to estimate TMEn and others using two measurements. The coefficients of determination (R2) for the two-measurement equations were similar; thus, persons using these equations may select measurements that are in concert with the capability of their own laboratory. Perhaps the most difficult feed ingredients to analyze for MEn are supplemental fats. Many factors influence the digestibility and subsequent MEn of fats; these have

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 been extensively reviewed by Renner and Hill (1961), Young and Garrett (1963), Lewis and Payne (1966), Hakansson (1974), Leeson and Summers (1976a), Fuller and Dale (1982), Ketels et al. (1987), Ketels and DeGroote (1988), and many others. Prominent among these factors are age of poultry, level of fat inclusion in the diet, and overall fatty acid composition of the diet. Several studies have been conducted to estimate the energy value of a fat from its composition. Janssen et al. (1979) estimated the energy value of fats produced by Dutch renderers (Appendix Table B-1). Huyghebaert et al. (1988) evaluated a wide variety of fats and developed prediction equations for MEn using multiple linear regression analysis involving different characteristics of fats. Several equations were developed for (1) all fats and oils examined and (2) different categories of fats (e.g., animal or vegetable fats). The accuracy of the equations was improved by separating the fats into different categories. It is well known that utilization of saturated fatty acids is improved by the presence of unsaturated fatty acids in the fat blend (Young and Garrett, 1963; Young, 1965; Lewis and Payne, 1966; Garrett and Young, 1975; Leeson and Summers, 1976a). The nature of the fat in the basal diet has a significant effect on the utilization of supplemental fats (Sell et al., 1976; Sibbald and Kramer, 1978; Fuller and Dale, 1982). These interactions between the supplemental fat and the basal dietary fat are especially noticeable at low inclusion levels of supplemental fat (Wiseman et al., 1986; Ketels et al., 1987). Ketels and DeGroote (1989) evaluated the relationship between the ratio of unsaturated to saturated fatty acids (U:S) in the diet and MEn of a number of fats and developed equations relating fat MEn, fat utilization, and the utilization of specific fatty acids to the U:S for young broiler chickens. Best fit regression equations for supplemental fat utilization and fat MEn were exponential. Fat utilization increased rapidly in the U:S range of 0 to 2.5, reaching a near-asymptotical maximum at a U:S of 4. Synergism between added fats, due either to blending vegetable oils with animal fats or to using basal diets with unsaturated lipid fractions, led to increased utilization of animal fats. Utilization of vegetable oils was not influenced by changing U:S ratios. The effect of factors influencing fat utilization, such as level of supplemental fat and basal diet composition, seemed to be primarily through variation in degree of saturation of the total dietary lipid fraction. For young broilers, about 75 percent of the variation in fat utilization and MEn was due to differences in the chemical composition of the fat fraction. Excellent summaries of the use of indirect methods for estimating the ME in feed ingredients have been presented by Harris et al. (1972), Sibbald (1975, 1982), Eackhout and Moermans (1981), Fisher (1982b), Fonnesbeck et al. (1984), and Just et al. (1984). These reports discuss many of the problems associated with the use of indirect procedures to replace conventional bioassays for ME. At this time, the committee cannot recommend the best equation(s) to use to estimate ME from chemical composition. To date, no studies have compared the various equations with a determined value. In addition, some of the chemical determinations are subject to much variability or are relatively complex and may not be easy to adapt to some laboratory situations. Users may wish to calculate ME by using as many of the equations as seem feasible and then evaluating the results before selecting the procedure that is most appropriate for their situation. Setting Dietary Levels In formulating poultry diets, energy level is usually selected as the starting point. An appropriate energy level is one that most likely results in the lowest feed cost per unit of product (weight gain or eggs). The feed cost per unit of product, in turn, is determined by the cost per unit weight of diet and the amount of diet required to produce a unit of product. In areas of the world where high-energy grains and feed-grade fats are relatively inexpensive, high-energy diets are often most economical (i.e., the lowest feed cost per unit of product); however, if a leaner carcass is desired, it may be necessary to consider other levels of dietary energy. In areas where lower-energy grains and by-products are less expensive, low-energy diets are often most economical. The dietary energy level selected is often used as a basis for setting most nutrient concentrations in a diet. This approach to formulation of poultry diets is based on the concept that poultry tend to eat to meet their energy needs, assuming that the diet is adequate in essential nutrients (Hill and Dansky, 1950; 1954; Hill et al., 1956; Scott et al., 1982). Such an assumption, however, must be used with caution and with an understanding of its potential limitations. For example, if a diet is deficient in any nutrient, daily feed consumption may decrease in relation to the severity of the deficiency. One exception may occur with an amino acid deficiency, whereby a marginal deficiency may result in a small increase in feed consumption. If a diet has a gross excess of any nutrient, daily feed consumption usually decreases in relation to the severity of the potential toxicity. The physiological mechanisms by which poultry respond to different dietary energy concentrations are not known, although several possible mechanisms have been proposed (National Research Council, 1987a). Equations that can be used to predict feed and energy

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 intakes of laying hens and coefficients to predict the energy requirements of broiler chickens have been given by the National Research Council (1987a). Although poultry generally adjust feed consumption to achieve a minimum energy intake from diets containing different energy levels, these adjustments are not always precise. Morris (1968) summarized data from 34 experiments and found that laying hens overconsumed energy when fed high-energy diets, and the degree of overconsumption was greatest for strains with characteristically high-energy intakes. Data from a large number of broiler chicken experiments also showed that changes in feed intake were not inversely proportional to changes in dietary energy level, especially when broilers were fed moderate- to high-energy diets (Fisher and Wilson, 1974). More recent studies also illustrated that growing broilers and turkeys consume more energy when fed high-energy diets than those fed low- to moderate-energy diets (Sell et al., 1981; Owings and Sell, 1982; Sell and Owings, 1984; Brue and Latshaw, 1985; Potter and McCarthy, 1985). For laying hens, some combinations of carbohydrates, fat, and protein resulted in more energy intake than others (Rising et al., 1989). Diets with 3 percent fat increased daily feed intake in comparison with diets containing no added fat, and hens fed diets that provided more protein also consumed greater amounts of energy. Generally, regulation of energy intake by laying hens and broilers is more precise when relatively low-energy diets are fed (Morris, 1968; Fisher and Wilson, 1974; Latshaw et al., 1990). In some instances, however, laying hens are fairly accurate in regulating energy consumption when fed high-energy diets (Horani and Sell, 1977). Because the preponderance of data shows that changes in feed intake usually are not proportional to changes in dietary energy concentration, the use of specific protein/amino acid-to-dietary energy ratios (originally termed energy-to-protein ratios) in formulating poultry diets (Baldini and Rosenberg, 1955; Combs, 1961; Scott et al., 1982; Thomas et al., 1986) must be carefully evaluated. Relating nutrient concentrations to dietary energy level seems to have greatest practical application for Leghorn chickens that generally are fed diets of low to moderate energy content. In the instance of growing broiler chickens and turkeys, however, maintaining specific nutrient-to-energy ratios seems questionable. This is particularly true for protein-to-energy ratios intended to support economical growth and feed efficiency (Pesti and Fletcher, 1983; Sell et al., 1985; 1989). If the production of lean broiler or turkey carcasses is of economic importance, appropriate dietary protein-to-energy ratios may be of greater significance. It would be desirable to have mathematical models available that would facilitate the selection of most economical combinations of dietary concentrations of protein/amino acids (and other nutrients) and energy to achieve poultry production goals. Development of such models will be contingent on research designed to obtain more relevant information than is currently available. Factors other than dietary energy and nutrient balance that affect feed intake include bulk density of the diet (Cherry et al., 1983) and ambient temperature (National Research Council, 1981a). The latter can have considerable impact on feed consumption of poultry, especially adult birds, because feed intake decreases as ambient temperature increases. Leghorn-type hens consume approximately 1.5 g less feed per hen daily for each 1°C increase in ambient temperature over the range of 10° to 35°C (Davis et al., 1973; Sykes, 1979). At temperatures above 30°C, the decrease in feed consumption may be 2.5 to 4 g for each 1°C increase (Sykes, 1979; Sell et al., 1983). Similar responses of decreasing feed intake with increasing temperatures have been reported for turkeys (Parker et al., 1972; Hurwitz et al., 1980). Energy Values in the Nutrient Requirement Tables The MEn values heading the lists of nutrient requirements given in Chapters 3 through 6 should not be regarded as energy requirements. The committee chose these as bases of reference. They represent the dietary energy concentrations frequently used under practical conditions of feed formulation and poultry management. For those persons preferring to use TMEn values, the TMEn values of numerous feed ingredients are included in Table 9-1. Generally, MEn values as determined by the method of Anderson et al. (1958) and TMEn values as determined by Sibbald (1983) are similar for many ingredients. However, MEn and TMEn values differ substantially for some ingredients, such as feather meal, rice bran, wheat middlings, and corn distillers' grains with solubles, and so in these instances MEn values should not be indiscriminately interchanged with TMEn values for purposes of diet formulation. CARBOHYDRATES Dietary carbohydrates are important sources of energy for poultry. Cereal grains such as corn, grain sorghum, wheat, and barley contribute most of the carbohydrates to poultry diets. The majority of the carbohydrates of cereal grains occurs as starch, which is readily digested by poultry (Moran, 1985a). Other carbohydrates occur in varying concentrations in cereal grains and protein supplements. These carbohydrates include polysaccharides, such as cellulose, hemicellulose, pentosans, and oligosaccharides, such as stachyose and raffinose, all of which are poorly digested by poultry. Thus, these dietary carbohydrates often

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 contribute little to meeting the energy requirement of poultry, and some adversely affect the digestive processes of poultry when present in sufficient dietary concentrations. For example, the pentosans of rye and beta glucans of barley increase the viscosity of digesta and thereby interfere with nutrient utilization by poultry (Wagner and Thomas, 1978; Antoniou and Marquardt, 1981; Classen et al., 1985; Bedford et al., 1991). Supplementation of rye or barley-containing diets with appropriate supplemental enzyme preparations improves nutrient utilization and growth of young poultry (Leong et al., 1962; Edney et al., 1989; Friesen et al., 1992). PROTEINS AND AMINO ACIDS Dietary requirements for protein are actually requirements for the amino acids contained in the dietary protein. Amino acids obtained from dietary protein are used by poultry to fulfill a diversity of functions. For example, amino acids, as proteins, are primary constituents of structural and protective tissues, such as skin, feathers, bone matrix, and ligaments, as well as of the soft tissues, including organs and muscles. Also, amino acids and small peptides resulting from digestion-absorption may serve a variety of metabolic functions and as precursors of many important nonprotein body constituents. Because body proteins are in a dynamic state, with synthesis and degradation occurring continuously, an adequate intake of dietary amino acids is required. If dietary protein (amino acids) is inadequate, there is a reduction or cessation of growth or productivity and a withdrawal of protein from less vital body tissues to maintain the functions of more vital tissues. There are 22 amino acids in body proteins, and all are physiologically essential. Nutritionally, these amino acids can be divided into two categories: those that poultry cannot synthesize at all or rapidly enough to meet metabolic requirements (essential) and those than can be synthesized from other amino acids (nonessential). The essential amino acids must be supplied by the diet. If the nonessential amino acids are not supplied by the diet, they must be synthesized by poultry. The presence of adequate amounts of nonessential amino acids in the diet reduces the necessity of synthesizing them from essential amino acids. Thus, stating dietary requirements for both protein and essential amino acids is an appropriate way to ensure that all amino acids needed physiologically are provided. Variations in Requirements Protein and amino acid requirements vary considerably according to the productive state of the bird, that is, the rate of growth or egg production. For example, turkey poults and broiler chickens have high amino acid requirements to meet the needs for rapid growth. The mature rooster has lower amino acid requirements than does the laying hen, even though its body size is greater and its feed consumption is similar. Body size, growth rate, and egg production of poultry are determined by their genetics. Amino acid requirements, therefore, also differ among types, breeds, and strains of poultry, as can be seen by comparing the values shown in the requirement tables provided in this report for the different types of poultry. Genetic differences in amino acid requirements may occur because of differences in efficiency of digestion, nutrient absorption, and metabolism of absorbed nutrients (National Research Council, 1975). Although dietary requirements for amino acids and protein usually are stated as percentages of the diet, the quantitative needs of poultry must be met by a balanced source to obtain maximum productivity. Thus factors that affect feed consumption also will affect quantitative intakes of amino acids and protein, and, consequently, will influence the dietary concentration of these nutrients needed to provide adequate nutrition. Factors affecting feed consumption are discussed in the section on "Setting Dietary Levels" and have been reviewed in the National Research Council (1987a) publication, Predicting Feed Intake of Food-Producing Animals. As discussed in the section "Setting Dietary Levels," adjustments in the protein and amino acids concentration of diets may be necessary to compensate for difference in energy concentration of diets. This is especially true for White Leghorn chickens (Morris, 1968; Byerly et al., 1980) and turkey hens (Kratzer et al., 1976). Ambient temperature also affects feed intake of poultry (Hurwitz et al., 1980). Protein and amino acid requirements listed herein generally pertain to poultry kept in moderate temperatures (18° to 24°C). Ambient temperatures outside of this range cause an inverse response in feed consumption; that is, the lower the temperature, the greater the feed intake and vice versa (National Research Council, 1981c). Consequently, percentage requirements of protein and amino acids should be increased in warmer environments and decreased in cooler environments, in accordance with expected differences in feed intake. These adjustments may aid in ensuring required daily intakes of amino acids. Some precautions, however, should be used in increasing the dietary protein concentration for poultry subjected to high ambient temperature. Waldroup et al. (1976d) reported that performance of broiler chicks was improved by minimizing excess dietary amino acids. Information available from research documenting the influence of dietary energy concentration and ambient

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 temperature on feed intake has been integrated with data describing amino acid needs for maintenance, body growth (such as for muscle and feathers), or egg production to derive mathematical models to predict the dietary amino acid requirements of poultry (Fisher et al., 1973; Hurwitz and Bornstein, 1973; Hurwitz et al., 1978; Emmans, 1981; Slagter and Waldroup, 1984). Prediction models may be useful in feed formulation, and they also provide valuable insight into areas of amino acid and protein nutrition where more definitive information is needed on requirements. Dietary protein concentrations can affect the requirements for individual essential amino acids. Generally, as dietary protein level increases, essential amino acid requirements (expressed as a percentage of the diet) increase, although when expressed as a percentage of the protein, essential amino acid requirements are little affected (Almquist, 1952; Boomgaardt and Baker, 1971, 1973a; Morris et al., 1987; Robbins, 1987; Mendonca and Jensen, 1989a). These observations demonstrate the importance of maintaining a balance among the concentrations of essential and nonessential amino acids in poultry diets. Optimal balance is important for efficient utilization of dietary protein. The protein and amino acid concentrations presented as requirements herein are intended to support maximum growth and production. Achieving maximum growth and production, however, may not always ensure maximum economic returns, particularly when prices of protein sources are high. If decreased performance can be tolerated, dietary concentrations of amino acids may, accordingly, be reduced somewhat to maximize economic returns. Specific Amino Acid Relationships Although each amino acid can be metabolized independently of others, relationships between certain amino acids exist. In some instances, the relationship may be beneficial. For example, one amino acid may be converted to another to fulfill a metabolic need. In other instances, a metabolic antagonism may exist with undesirable consequences. A brief description of amino acid relationships that may be of importance in poultry nutrition is given in the following section. METHIONINE PLUS CYSTINE Methionine can donate its methyl group to biological processes, and the resulting sulfur-containing compound, homocysteine, together with serine, can be used to synthesize cysteine via cystathionine. The sulfhydryl groups of two molecules of cysteine are oxidized to form cystine. This conversion cannot be reversed, and two methionine molecules are needed to ultimately supply the two sulfur atoms of cystine (du Vigneaud, 1952; Creek, 1968; Baker, 1976). The requirement for methionine can be satisfied only by methionine, whereas that for cystine can also be met with methionine. The catabolism of methionine and cystine largely leads to conversion of the associated sulfur into sulfate. This sulfate may be used in metabolism, particularly as a part of certain connective tissues. Similarly, methyl groups of methionine may be used in transmethylation and the de novo synthesis of sarcosine, betaine, and choline. Choline is a constituent of phospholipids, and its incorporation into membranes is extensive. During rapid growth, when accrual of connective tissue and expansion of membrane surfaces are great, an increased sensitivity to methionine at levels marginal to the requirement may occur if dietary choline and sulfate are not sufficient (Baker et al., 1983; Miles et al., 1983; Blair et al., 1986). PHENYLALANINE PLUS TYROSINE Tyrosine is the initial product formed during the biological degradation of phenylalanine. In turn, phenylalanine can be used to meet the bird's need for tyrosine on a mole-for-mole basis (Creek, 1968; Sasse and Baker, 1972). Although this conversion may be reversed to a small extent and tyrosine used to form phenylalanine, its contribution is too small to be of practical significance (Ishibashi, 1972). GLYCINE PLUS SERINE Although glycine can be synthesized by fowl, the rate is not adequate to support maximal growth (Featherston, 1976). Serine can be converted to glycine on an equimolar basis. This reaction is reversible, and glycine can be used to form serine (Sugahara and Kandatsu, 1976). IMBALANCE, ANTAGONISM, AND TOXICITYCITY The essential amino acids are related to one another by virtue of need to support production plus maintenance. The combined need for production and maintenance represents the bird's requirement. Requirement for any one essential amino acid represents the combined need for maintenance plus production. Each essential amino acid is unique in its catabolism, and an inadequacy of any one of them (the first limiting) usually necessitates some catabolism of the others. The bird's response can vary with the essential amino acid, the extent of its inadequacy, and existing relationships among the remainder. As an example, Sugahara et al. (1969) fed chicks a purified amino acid diet corresponding to 100 percent of the requirement for all essential amino acids as the positive control and compared the performance response to when all amino acids were reduced to 60 percent of the requirement as opposed to 60 percent reduction

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 with each one alone. Weight gain was better with individual decreases of methionine-cystine, leucine, lysine, and arginine than when a total reduction was imposed, whereas additional weight loss occurred with individual decreases of phenylalanine, tyrosine, tryptophan, isoleucine, valine and threonine. A reduction in dietary histidine gave a similar response to that observed when all amino acids were reduced. Deficiencies of any one of the essential amino acids can be exaggerated by adding purified amino acids and/or combining complete proteins such that the extent of difference between the first and second limiting amino acid increases. The response is generally an additional impairment of body weight gain. Accentuation of the deficiency in this manner usually involves diets of low protein content, and a decrease in feed intake is the fundamental reason for poor weight gain rather than alteration in effectiveness of the first limiting amino acid (Fisher et al., 1960; Fisher and Shapiro, 1961; Netke et al., 1969). Amino acid antagonisms may also accentuate a deficiency of the first limiting amino acid, but these differ from imbalances because utilization of the limiting amino acid is reduced. Antagonisms can occur between amino acids having side chains exhibiting similar structural and/or chemical characteristics, and increasing the dietary concentration of one that is in excess of productive use adversely affects metabolism of the other. In a situation in which one essential amino acid is first limiting, increasing the other's concentration to enlarge the difference antagonizes the use of the first limiting amino acid and induces or exacerbates a deficiency. Antagonisms have been shown to exist for leucine-isoleucine-valine, arginine-lysine, and threonine-tryptophan (D'Mello and Lewis, 1970). The most important of these antagonisms occurs with leucine and isoleucine. Certain feedstuff combinations (for example, corn plus corn gluten meal) can lead to practical diets in which leucine is at particularly high levels while isoleucine is marginal in adequacy. Amino acid levels that would be likely to provoke the other antagonisms probably would not occur in practice unless high levels of supplemental amino acids were used in low-protein diets. An amino acid toxicity requires a particularly high level of one amino acid relative to all others. Such an occurrence is unlikely under practical circumstances because differences of sufficient magnitude do not exist in most protein feedstuffs. Supplemental methionine and lysine are routinely used by the feed industry but usually in quantities low enough to pose no threat of toxicity. Errors in amino acid use may lead to toxicities, however. Methionine is toxic when excessive. Ueda et al. (1981) observed severe depression in feed consumption and growth of chicks given ad libitum access to a diet containing 10 percent protein and 1.5 percent L-methionine. Force-feeding this high-methionine, low-protein diet in amounts equal to the feed intake of controls resulted in death of the chicks. Edmonds and Baker (1987) added excesses of several amino acids to a 23 percent protein corn-soybean meal diet for chicks. Methionine at 4 percent of the diet led to a 92 percent reduction in weight gain, whereas similar excesses of tryptophan, lysine, and threonine were far less toxic. Amino Acid Conversion to Vitamins Niacin is the only vitamin that can be synthesized from an amino acid. Tryptophan can be used to alleviate a dietary niacin deficiency, but the rate of conversion is poor (Baker et al., 1973). When methionine is provided at levels exceeding use for protein synthesis, the additional methyl groups may decrease the dietary choline requirement (Pesti et al., 1980). Using amino acids to spare other nutrients is not currently economical under practical conditions. Amino Acid Availability It is well known that the availability of amino acids varies greatly among feedstuffs. The importance of considering amino acid availability in formulation of poultry diets is discussed in Chapter 9. FATS Fat is usually added to the feed for meat-type poultry to increase overall energy concentration and, in turn, improve productivity and feed efficiency. Oxidation of fat is an efficient means to obtain energy for the cell in large quantity, whereas anabolic use involves direct incorporation into the body as a part of growth. Lipid accrual is most obvious in adipose tissue; however, cell multiplication also requires an array of lipids to form associated membranes. These two uses can occur simultaneously; however, the extent of each may vary considerably. Sources Feed-grade fat may come from many different sources. Grease from restaurants, the rendering of animal carcasses, and the refuse from vegetable oil refining are major sources. These sources represent several types and categories, and each is defined by the Association of American Feed Control Officials (1984). These definitions indicate fat components and limits of nonfat material (Sell, 1988). Moisture (M) and those

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 compounds that are either insoluble in ether (I) or unsaponifiable (U) are usually of no value, and their composite (MIU) essentially acts as a diluent. Total fatty acids contributed by all lipid categories, the proportion that are in free form, and the types of fatty acids present provide information related to expected digestibility as well as how the fat may be used subsequently. Fatty acid chain length, extent of unsaturation, and nature of esterification all influence intestinal absorption (Moran, 1989a). The percentage MIU and percentage digestibility combine to influence the MEn value. All feed fats should be stabilized by an antioxidant to preserve unsaturated fatty acids and routinely monitored for the possible presence of undesirable residues such as insolubles, chlorinated hydrocarbons, and unsaponifiables and for peroxides (Rouse, 1986). Metabolizable Energy Value Factors influencing the MEn value of fat that are not directly associated with fat quality are age of poultry and method of measurement. Improved utilization of dietary fats has been shown to occur after 2 to 6 weeks of life for chickens (Renner and Hill, 1960, 1961; Sibbald, 1978a; Lessire et al., 1982) and turkeys (Whitehead and Fisher, 1975; Sell et al., 1986b). This improvement is particularly evident with long-chain saturated fatty acids and fats containing substantial proportions of these fatty acids (Young and Garrett, 1963; Sell et al., 1986b). The methodology used in obtaining feedstuff energy values has an effect on the values obtained. (See the sections above on procedures for determination of MEn and on estimating the MEn content of ingredients from proximate composition.) Actual digestibility of fat may also be used to estimate energy content, and Sell et al. (1986b) found that values determined by this method agree with concurrent MEn measurements. When the effects of method of determination and age of the bird are superimposed on factors associated with the fat, it becomes evident that assigning a specific MEn value to a fat may be inappropriate. The information in Table 9-9 provides a description of fats that may be used in feeds and their MEn values observed under a variety of circumstances. Data indicate that considerable variation exists and several factors must be considered in determining feeding value. Some of these factors are included in the equations listed in Appendix Table B-1, which can be used to predict the MEn value of fats. Blending Fats When animal tallow is added to feed at a low level, it may be beneficial to blend it with a small amount of vegetable oil. The resulting MEn value of blends is greater than can be explained from the arithmetic combination. A synergism in the absorption of the saturated fatty acids related to the added amounts of unsaturated fatty acids is suspected (Ketels et al., 1986; Ketels and DeGroote, 1987). The properties of animal tallows also may be enhanced by the presence of feed ingredients that contain unsaturated fatty acids. Corn is particularly advantageous in this respect because its fatty acids are mostly unsaturated and it usually constitutes a large portion of a feed. Sibbald and Kramer (1980) noted that the TME for beef tallow was greater when a corn-based carrier was used during measurement than when wheat was used. Extra Caloric Effect Employing high levels of added fat often leads to more MEn than can be accounted for from the summation of ingredients. High level fat feeding evidently increases the intestinal retention time of feed and so allows for more complete digestion and absorption of the nonlipid constituents (Mateos and Sell, 1981; Mateos et al., 1982; Sell et al., 1983). Improved Net Energy of Production All body tissues have an energy value that corresponds to their heat of combustion. The net energy of production corresponds to this energy gained from either body growth or egg formation. Adding fat to feed as an isoenergetic substitution for carbohydrate usually results in an improved productive energy when the same level of MEn has been derived. Such improvement is particularly obvious through that period preceding adolescent development. Sell and Owings (1984) noted that added fat increased the body weight gain of large turkeys, with the greatest advantage occurring between 12 and 20 weeks of age. After 20 weeks, the favorable effect of fat on body weight progressively dissipates, but the effect on feed efficiency remains (Moran, 1982). Fatty acid synthesis within fowl occurs primarily in the liver. Immediately preceding sexual maturity the rate of synthesis increases dramatically, and the rate at which the body's depots accrue fat is great (Moran, 1985b). The provision of fat in feed obviates the cost of synthesis and is more energy-efficient than is synthesis of fat from carbohydrate. Laying hens also may respond to added dietary fat. Most lipid in egg yolk is formed in the liver by using fatty acids obtained from the diet or from de novo synthesis. Providing dietary fat decreases the need for hepatic fatty acid synthesis and generally increases yolk formation and the weight of the egg (Whitehead, 1981;

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 March and MacMillan, 1990). Such advantages are particularly valuable during high environmental temperatures. As feed intake is reduced, the added fat permits the hen to maintain egg formation while minimizing heat generated (Valencia et al., 1980). Fatty Acid Composition Directly employing dietary fat in the assembly of either body or egg lipids results in a fatty acid composition similar to that of the diet. Fat absorbed from the fowl's intestine is transported to the liver, where some modifications may occur. For the most part, the unsaturated fatty acids are unchanged, but the saturated ones may undergo desaturation, especially stearic acid which can be converted to oleic acid. Also, elongation and further desaturation of 18:2(n-6) and 18:3(n-3) may occur in the liver. Depot fat is the tissue most affected by the source of dietary fat. Depot fat of both broiler chickens (Schuler and Essary, 1971; Edwards et al., 1973) and turkeys (Moran et al., 1973; Salmon and O'Neil, 1973) are more influenced by the vegetable oils having high proportions of polyunsaturated fatty acids than by more saturated animal fats. Fatty acid composition in depots can be altered by changing from one dietary fat to another (Watkins, 1988). The extent of influence that each fat has on body composition increases with the level of intake, duration of feeding, and stage of maturity (Bartov et al., 1974; Salmon, 1976). The hen's adipose depots respond to dietary fat in the same way as do those of growing birds, and the yolk lipid exhibits a fatty acid pattern resembling that of the dietary fat (Guenter et al., 1971; Sim et al., 1973). Essential Fatty Acids Linoleic acid (18:2, n-6) and a-linolenic acid (18:3, n-3) are recognized as metabolically essential fatty acids. The position of the double bonds in these n-6 and n-3 polyunsaturated fatty acids (PUFA) is unique because they are not formed in the fowl. The essential fatty acids are converted to long-chain PUFA in poultry through a series of desaturation (addition of a double bond) and elongation steps (chain-lengthening with 2 carbons) to form 20 and 22 carbon PUFA (Watkins, 1991). Membrane phospholipids contain a greater proportion of PUFA than do triacyglyerols although depot fat can contain a reserve of linoleic acid for the fowl. In poultry, specific PUFA are biosynthesized into compounds called eicosanoids which act as potent biological regulators. Linoleic acid is the only essential fatty acid for which a dietary requirement has been demonstrated. Inadequacies of linoleic acid are not readily encountered, but symptoms that result are due to a loss of membrane integrity. An increased need for water and decreased resistance to disease are characteristic deficiency symptoms observed in poultry (Balnave, 1970). A deficiency of linoleic acid in the male can impair spermatogenesis and affect fertility. Insufficient deposition of linoleic acid in the egg will adversely affect embryonic development. The essential fatty acid requirements of growing and adult birds can usually be satisfied by feeding a diet with 1 percent of linoleic acid. Higher levels of linoleic acid may be needed by the laying hen to achieve and maintain satisfactory egg weight. A dietary need for a-linolenic acid (18:3, n-3) has yet to be demonstrated for the fowl. a-Linolenic acid appears to be important, however, in the development of specialized membranes found in the retina and nervous system. These membranes contain relatively high concentrations of n-3 PUFA that can originate from 18:3(n-3) (Neuringer and Connor, 1986). Certain PUFA derived from linolenic and a-linolenic acids are biosynthesized into a multitude of eicosanoids. The primary substrates for eicosanoid production are 20:4(n-6), 20:3(n-6) which are formed from linoleic acid, and 20:5(n-3) a product of a-linolenic acid. Preceding eicosanoid biosynthesis in poultry, the PUFA is released from membrane phospholipids by action of phospholipases. Liberation of PUFA is induced by a number of stimuli. Following a series of different enzymatic steps, several eicosanoids can be formed depending on the tissue and cell type (Watkins, 1991). The eicosanoids are categorized into prostaglandins, prostacyclins, thromboxanes, and leukotrienes. Formation of eicosanoids is widespread in the body and nearly every physiological system is affected by these hormone-like compounds. The eicosanoids are important in embryonic development, reproduction, immunological responses, and bone development in poultry (Watkins, 1991). Eicosanoid production can be modulated depending upon the concentration of substrate PUFA found in tissues. Changing the dietary concentrations of n-3 and n-6 PUFA found in tissues will influence the types and amounts of eicosanoids formed (Watkins, 1991). Elevating the n-3 PUFA content of the diet relative to that for n-6 PUFA alters eicosanoid production in immunocompetent cells (Kinsella et al., 1990). These types of responses also seem to affect inflammatory reactions and blood clotting in animals and humans. To maintain the full spectrum of eicosanoid effects in the body a balanced intake of n-3 and n-6 PUFA is recommended. MINERAL Minerals are the inorganic part of feeds or tissues. They are often divided into two categories, based on the

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 amount that is required in the diet. Requirements for major, or macro, minerals usually are stated as a percentage of the diet, whereas requirements for minor, or trace, minerals are stated as milligrams per kilogram of diet or as parts per million. Minerals are required for the formation of the skeleton, as components of various compounds with particular functions within the body, as cofactors of enzymes, and for the maintenance of osmotic balance within the body of the bird. Calcium and phosphorus are essential for the formation and maintenance of the skeleton. Sodium, potassium, magnesium, and chloride function with phosphates and bicarbonate to maintain homeostasis of osmotic relationships and pH throughout the body. Most of the calcium in the diet of the growing bird is used for bone formation, whereas in the mature laying fowl most dietary calcium is used for eggshell formation. Other functions of calcium include roles in blood clotting and as a second messenger in intracellular communications. An excess of dietary calcium interferes with the availability of other minerals, such as phosphorus, magnesium, manganese, and zinc. A ratio of approximately 2 calcium to 1 nonphytate phosphorus (weight/weight) is appropriate for most poultry diets, with the exception of diets for birds that are laying eggs. When poultry are laying eggs, a much higher level of calcium is needed for eggshell formation, and a ratio as high as 12 calcium to 1 nonphytate phosphorus (weight/weight) may be correct. But high levels of calcium carbonate (limestone) and calcium phosphates may tend to make the diet unpalatable and dilute the other dietary components. If a calcium source contains a high level of magnesium (as does dolomitic limestone), it probably should not be used in poultry diets (Stillmak and Sunde, 1971). Phosphorus, in addition to its function in bone formation, is also required in the utilization of energy and in structured components of cells. Examples of phosphorus-containing compounds are adenosine 5'-triphosphate (ATP) and phospholipids. These forms of phosphorus, if present in plants, can be digested by poultry; however, such digestible forms usually account for only 30 to 40 percent of the total phosphorus. The remaining phosphorus is present as phytate phosphorus and is poorly digested. Only about 10 percent of the phytate phosphorus in corn and wheat is digested by poultry (Nelson, 1976). The phosphorus from animal products and phosphorus supplements is generally considered to be well utilized. Phosphorus supplements for poultry diets are listed in Table 9-10. Sodium and chloride are essential for all animals. Dietary concentrations of salt generally used are those that will just support maximum growth rate or egg production. Higher concentrations lead to excessive consumption of water and attendant problems with ventilation control and wet droppings. Dietary proportions of sodium, potassium, and chloride are important determinants of acid-base balance (Mongin, 1968; Hurwitz et al., 1973; Cohen and Hurwitz, 1974; Sauveur and Mongin, 1978). Other cations and anions such as calcium, sulfate, and phosphate also may be involved. The appropriate dietary balance of these electrolytes is often assessed by the levels of sodium and potassium versus chloride, where each element is expressed in milliequivalents per kilogram of diet. Experiments show that sodium and potassium are alkalogenic (have an alkaline-producing effect), whereas chloride is acidogenic (has an acid-producing effect). Chloride tends to decrease blood pH and bicarbonate concentration, whereas sodium and potassium tend to increase blood pH and bicarbonate concentration. The proper dietary balance of sodium, potassium, and chloride is necessary for growth, bone development, eggshell quality, and amino acid utilization (Mongin, 1981). However, an ideal balance among these electrolytes appropriate for a wide range of environmental situations has not been defined. Trace elements, including copper, iodine, iron, manganese, selenium, and zinc are required in small amounts in the diet. Cobalt is also required, but it does not need to be supplied as a trace mineral because it is a part of vitamin B12. In practical diets, copper and iron are often present at sufficient levels without supplementation. Trace elements function as part of larger organic molecules. Iron is a part of hemoglobin and cytochromes, and iodine is a part of thyroxine. Copper, manganese, selenium, and zinc function as essential accessory factors to enzymes and, in the case of zinc, DNA structural motifs (zinc fingers). If one of these minerals is deficient, the functional activity of the organic moiety requiring the presence of the mineral will be decreased, as has been described in detail for each mineral by Mertz (1986). The requirements for trace minerals are often fulfilled by concentrations present in conventional feed ingredients. Soils vary, however, in their content of trace minerals, and plants vary in their uptake of minerals. Consequently, feedstuffs grown in certain geographic areas may be marginal or deficient in specific elements. Thus, poultry diets may require supplementation to ensure adequate intake of trace minerals. Because of the interactions that occur between various minerals such as copper and molybdenum, selenium and mercury, calcium and zinc, calcium and manganese (Mertz, 1986), excessive concentrations of one element may result in a deficiency in the amount available to the bird of some other element. Formulators of poultry diets should be aware of these possible mineral interactions and of the

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 potential effects that the chemical form (cation-anion combination) of mineral sources may have on their utilization by poultry (Allaway, 1986). Mineral salts used as feed supplements are not usually pure compounds but contain variable amounts of other minerals. The concentrations of minerals that may be present in feed-grade mineral supplements are shown in Table 9-10. Experimental diets may sometimes be formulated from purified or chemically defined ingredients. Under these conditions, silicon and boron may be inadequate and biological responses may occur with the addition of these elements to the diet (Carlisle, 1970, 1980; Nielsen, 1986). VITAMINS Vitamins are generally classified under two headings: fat soluble vitamins, A, D, E, and K, and water-soluble vitamins, that include the so-called B-complex and vitamin C (ascorbic acid). Vitamin C is synthesized by poultry and is, accordingly, not considered a required dietary nutrient. There is some evidence, nevertheless, of a favorable response to vitamin C by birds under stress (Pardue et al., 1985). The requirements for most vitamins are given in terms of milligrams per kilogram of diet. Exceptions are vitamins A, D, and E, for which requirements are commonly stated in units. Units are used to express the requirements for these vitamins because different forms of the vitamins have different biological activities (Anonymous, 1990). Requirements for vitamin A are expressed in either International Units (IU) or U.S. Pharmacopeia units (USP) per kilogram of diet. The international standards for vitamin A activity are as follows: 1 IU of vitamin A = 1 USP unit = vitamin A activity of 0.3 µg crystalline vitamin A alcohol (retinol), 0.344 µg vitamin A acetate, or 0.55 µg vitamin A palmitate. One IU of vitamin A activity is equivalent to the activity of 0.6 µg of ß-carotene; alternatively, 1 mg ß-carotene = 1,667 IU vitamin A (for poultry). Vitamin D for poultry must be in the form of vitamin D3, which is found naturally in fish liver oil or may be synthesized by the irradiation of animal sterol. Vitamin D2, which is from plant sources, is active for rats and most mammals but has very low activity for poultry. One unit of vitamin D3 (USP or IU) is defined as the activity of 0.025 µg of vitamin D3 (cholecalciferol). The requirements listed herein for vitamin D are based on diets containing the stated requirements for calcium and available phosphorus. One IU of vitamin E is the activity of 1 mg of synthetic DL-a-tocopheryl acetate, 0.735 mg D-a-tocopheryl acetate, 0.671 mg D-a-tocopherol, or 0.909 mg DL-a-tocopherol. The dietary requirement for vitamin E is highly variable and depends on the concentration and type of fat in the diet, the concentration of selenium, and the presence of prooxidants and antioxidants. Vitamin K activity is exhibited by a number of naturally occurring and synthetic compounds with varying solubilities in fat and water. Menadione (2-methyl-1,4-naphthoquinone) is a fat soluble synthetic compound that can be considered the reference standard for vitamin K activity. Two naturally occurring forms are K1 or phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone) and K2 or menaquinone (K1 substituted with 2 to 7 isoprene units). Water-soluble forms include menadione sodium bisulfite (MSB), menadione sodium bisulfite complex (MSBC), and menadione dimethylpyrimidol (MPB). The theoretical activity of these compounds is 33, 50, and 45 percent, respectively, as calculated on the basis of the proportion of menadione present in the molecule. Dietary supplements frequently contain, as a factor of safety, levels of vitamins in considerable excess of the minimum requirements. Vitamin tolerances have been reviewed by the National Research Council (1987b). Maximum tolerances for vitamins are of the order of 10 to 30 times the minimum requirement for vitamin A, 4 to 10 times for vitamin D3, and 2 to 4 times for choline chloride (possibly because of the chloride). Niacin, riboflavin, and pantothenic acid are generally tolerated at levels as great as 10- to 20-fold their nutritional requirement. Vitamin E is generally tolerated at intakes as great as 100-fold the required level. Vitamins K and C, thiamin, and folic acid are generally tolerated at oral intake levels of at least 1,000-fold the requirement. Pyridoxine may be tolerated at 50 times or more of the requirement (Aboaysha and Kratzer, 1979). High levels of biotin and vitamin B12 have not been tested. WATER Water must be regarded as an essential nutrient, although it is not possible to state precise requirements. The amount needed depends on environmental temperature and relative humidity, the composition of the diet, rate of growth or egg production, and efficiency of kidney resorption of water in individual birds (Medway and Kare, 1959). It has been generally assumed that birds drink approximately twice as much water as the amount of feed consumed on a weight basis, but water intake actually varies greatly. Several dietary factors influence water intake and water:feed ratios. Increasing crude protein increases water intake and water:feed ratios (Marks and Pesti, 1984). Crumbling or pelleting of diets increases both water and

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 feed intake relative to mash diets, but water:feed ratios stay relatively the same (Marks and Pesti, 1984). Increasing dietary salt increases the water intake (Marks, 1987). The data given for water consumption in Table 1-1 are for environmental temperatures of about 21°C except for brooding chicks and poults. With broilers, water consumption increases about 7 percent for each 1°C above 21° C. Laying hens may consume from 150 to 300 liters (40 to 80 gal) per 1,000 birds daily, depending on temperature and other factors. Survival under extremely hot conditions is influenced by the ability to consume large quantities of water or, more precisely, the ability to use water to remove heat from the respiratory surfaces of the body. This ability varies from strain to strain. Water intake data for broilers listed herein are based on studies using modern commercial broilers (Marks, 1981; Ross and Hurnik, 1983; Gardiner and Hunt, 1984; Pesti et al., 1985; Miller et al., 1988). Most of the studies were carried out under moderate temperature conditions, with corrections for evaporative losses. In most of the studies, data also were collected on feed intake, allowing for calculation of water:feed ratios. Documented water intake data for laying hens are limited, especially data related to cage systems. Dun and Emmans (1971) compared the water consumption of caged hens on trough and nipple watering systems in a 3-year study. Feed and water consumption were 126 g and 254 ml with the trough system and 124.9 g and 166 ml with the nipple system (four hens per nipple). Hearn and Hill (1978) compared feed and water consumption of hens on trough and nipple watering systems, with varying numbers of birds per nipple. During the study, that was conducted from 20 to 72 weeks of age, hens on trough waterers consumed an average of 115 g of feed and 213 ml of water. Hens with 2.5, 5, and 10 birds per nipple consumed 109, 109, and 108 g of feed and 182, 169, and 165 ml of water, respectively. Gardiner (1982) examined the water intake of individually caged hens for a 336-day period beginning when they were 32 weeks of age. Over this period of time, mean feed consumption of laying hens was 109 g and daily water intake was 183 ml, for a feed:water ratio of 1.68. There was no indication of type of drinker used. It is evident that the type of watering system used will influence water consumption (or, more correctly, water disappearance) of laying hens. Although many tables of estimated water consumption can be found in the literature, the sources of the data used to compile these tables cannot be documented. Water consumption data for turkeys obtained from experimental studies are meager (Enos et al., 1967). Thus, the data on water consumption of turkeys shown in Table 1-1 are based mainly on information obtained recently from commercial turkey production companies. TABLE 1-1 Water Consumption by Chickens and Turkeys of Different Ages Age (weeks) Broiler Chickens (ml per bird per week)a White Leghorn Hens (ml per bird per week)a Brown-Egg-Laying Hens (ml per bird per week)a Large White Turkeys (ml per bird per week)a,b       Males Females 1 225 200 200 385 385 2 480 300 400 750 690 3 725 – – 1,135 930 4 1,000 500 700 1,650 1,274 5 1,250 – – 2,240 1,750 6 1,500 700 800 2,870 2,150 7 1,750 – – 3,460 2,640 8 2,000 800 900 4,020 3,180 9 – – – 4,670 3,900 10 – 900 1,000 5,345 4,400 11 – – – 5,850 4,620 12 – 1,000 1,100 6,220 4,660 13 – – – 6,480 4,680 14 – 1,100 1,100 6,680 4,700 15 – – – 6,800 4,720 16 – 1,200 1,200 6,920 4,740 17 – – – 6,960 4,760 18 – 1,300 1,300 7,000 – 19 – – – 7,020 – 20 - 1,600 1,500 7,040 – NOTE: Dash indicates that information is not available. a Varies considerably depending on ambient temperature, diet composition, rates of growth or egg production, and type of equipment used. The data presented apply under moderate (20° to 25°C) ambient temperatures. b Based on data obtained from commercial turkey production units. Water deprivation for 12 hours or more has adverse effects on the growth of young poultry and egg production of laying hens, and water deprivation of 36 hours or more results in a marked increase in mortality of young and old poultry (Bierer et al., 1965a,b; Haller and Sunde, 1966; Adams, 1973). Water restoration, after extended periods of water deprivation (36 to 40 hours), may cause a ''drunken syndrome" or "water intoxication," leading to death (Marsden et al., 1965). Young turkeys are especially susceptible to this condition. The salt content and pH of water may influence the use of the drinking water to administer vitamins and drugs. Turkeys are known to detect minor differences in the flavor of medicated water and may accept drugs in one water supply but not in another. Intermittent provision of water is sometimes used to reduce the water content of the droppings and to control feed intake in laying hens without reducing egg production (Maxwell and Lyle, 1957). Because birds differ in their ability to conserve body water by increasing kidney resorption, there is a danger of causing dehydration of some birds by practicing water restriction of a flock. Some water supplies contain considerable concentrations of sulfur or sulfates, nitrates, and various trace minerals. These are usually readily absorbed from the intestine and may be either useful or harmful to the bird, depending

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 TABLE 1-2 Guidelines for Poultry for the Suitability of Water with Different Concentrations of Total Dissolved Solids (TDS) TDS (ppm) Comments Less than 1,000 These waters should present no serious burden to any class of poultry. 1,000–2,999 These waters should be satisfactory for all classes of poultry. They may cause watery droppings (especially at the higher levels) but should not affect health or performance. 3,000–4,999 These are poor waters for poultry, often causing watery droppings, increased mortality, and decreased growth (especially in turkeys). 5,000–6,999 These are not acceptable waters for poultry and almost always cause some type of problem, especially at the upper limits, where decreased growth and production or increased mortality probably will occur. 7,000–10,000 These waters are unfit for poultry but may be suitable for other livestock. More than 10,000 These waters should not be used for any livestock or poultry.   SOURCE: National Research Council. 1974. Nutrients and Toxic Substances in Water for Livestock and Poultry. Washington, D.C.: National Academy of Sciences. on concentration. Table 1-2 gives the guidelines suggested by the National Research Council (1974) for the suitability for poultry of water with different concentrations of total dissolved solids (TDS); that is, the total concentration of all dissolved elements in water. XANTHOPHYLLS A number of carotenoid pigments are responsible for the yellow-orange coloration of egg yolks and poultry fat and also may contribute to coloration of the skin, shanks, feet, and beak. The xanthophylls, which are characterized by the presence of hydroxyl groups, are the carotenoids of most interest in poultry nutrition. The most commonly considered xanthophylls are lutein in forages such as alfalfa and zeaxanthin in corn. Relative xanthophyll contribution by various xanthophyll-rich ingredients is shown in Table 1-3. Individual xanthophylls differ in their ability to impart color. Although ß-carotene has little pigmenting value, other xanthophylls and synthetic products are effective in influencing yolk and skin color. Less than 1 percent of dietary ß-carotene is deposited in the yolk, but for zeaxanthin, as found in corn, the value is closer to 7 percent, and for some synthetic products, such as ß-apo-8-carotenoic acid ethyl ester, the incorporation rate may be as high as 34 percent (Roche Vitamins and Fine Chemicals, 1988). Fletcher et al. (1985) and Saylor (1986) reported that natural sources of xanthophyll differed in their ability to pigment egg yolk and the skin of broilers. Alfalfa meal contains several types of xanthophylls, but TABLE 1-3 Xanthophyll and Lutein Content of Selected Ingredients Ingredient Xanthophyll (mg/kg) Lutein (mg/kg) Alfalfa meal, 17% crude protein 220 143 Alfalfa meal, 22% crude protein 330 — Alfalfa protein concentrate, 40% crude protein 800 — Algae meal 2,000 — Corn 17 0.12 Corn gluten meal, 60% crude protein 290 120 Marigold petal meal 7,000 — NOTE: Dash indicates that information is not available. the one of greatest abundance and importance is lutein, which tends to impart a yellow color, whereas corn and corn gluten meal contain primarily zeaxanthin, which tends to impart an orange-red color. Avian tissue normally accumulates xanthophylls, although the retina may accumulate other carotenoids (Goodwin, 1986). In the laying hen, 50 percent of total body zeaxanthin (as derived from corn) is found in the ovary (Scheidt et al., 1985). Goodwin (1986) indicated that body stores of xanthophylls in the muscle and skin are transferred to the ovary at onset of sexual maturity. Presumably, this transfer occurs throughout the egg production cycle and contributes to the gradual loss of pigment from the shank and beak as egg production continues. Synthetic carotenoids that have been approved for use by regulatory agencies are used in poultry diets, because levels of desired pigments in natural feedstuffs are not always constant and many of the carotenoid-containing natural feedstuffs are relatively low in energy content. Approval of use of these synthetics varies among countries. Synthetic pigments, such as canthaxanthin and ß-apo-8-carotenoic acid (usually as an ethyl ester), can be used to control pigmentation more precisely to yield varying degrees of yellow-orange-red coloration. In natural products, xanthophylls are unstable, and effective levels may decline as a result of oxidation during prolonged storage. This decline can be reduced by the inclusion of antioxidants in the feed. A number of factors can adversely affect absorption of xanthophylls and thus lead to reduced pigmentation. Broilers infected with Eimeria sp. exhibit reduced pigmentation and blood xanthophylls (Bletner et al., 1966), and the viral infection that may be responsible for malabsorption syndrome also results in altered xanthophyll status of the bird (Winstead et al., 1985). Exposing feed to light may have variable effects on subsequent pigmentation (Fletcher, 1981). The presence of certain mycotoxins in feeds seems to be detrimental to pigmentation (Tyczkowski and Hamilton, 1987).

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Nutrient Requirements of Poultry: Ninth Revised Edition, 1994 UNIDENTIFIED GROWTH FACTORS So-called unidentified growth factors have been reported throughout the history of poultry nutrition studies. Natural ingredients claimed to contain such factors are most often animal proteins or fermentation by-products (Summers et al., 1959; Al-Ubaidi and Bird, 1964; Dixon and Couch, 1970; Waldroup et al., 1970). Ingredients containing unidentified growth factors are claimed to improve chick growth and reproductive performance (Morrison et al., 1956; Touchburn et al., 1972). Bhargava and Sunde (1969) described a chick assay for quantitation of such unidentified factors. The mode of action of these unidentified factors is far from clear, however. With the identification of vitamins and consideration of the significance of trace minerals, many nutritionists now disregard the importance of growth factors. That responses may still occur could relate to truly unidentified nutrients or, more likely, to changes in feed palatability and/or quality (Alenier and Combs, 1981; Cantor and Johnson, 1983), mineral chelation, or simple improvement in the balance of available nutrients. ANTIMICROBIALS Antimicrobial feed additives, although not nutrients in the sense that they are required by poultry, are included in diets to improve growth, efficiency of feed utilization and livability (Stokstad et al., 1949; Coates et al., 1951; Libby and Schaible, 1955; Milligan et al., 1955; Bird, 1968; Begin, 1971; Morrison et al., 1974). Antimicrobial agents are included in diets at relatively low concentrations (1 to 50 mg/kg), depending on the agent and stage of development of poultry. They are, accordingly, classified as additives and as growth promoters. Egg production is also frequently improved by dietary supplementation with antimicrobial agents (Carlson et al., 1953; Balloun, 1954; Andrews et al., 1966). The mechanisms by which antimicrobials improve performance are not clearly understood. Because antimicrobials do not stimulate growth of chicks kept in a germfree environment (Coates and Harrison, 1969), it is likely that stimulation of growth results from either suppression of microorganisms that may cause adverse effects or encouragement of other microorganisms that may have favorable effects on poultry performance. There is some concern that feeding of low concentrations of antibiotics may favor the proliferation of antibiotic-resistant microorganisms, which could have serious consequences for disease control in humans or domestic animals. A study by the National Research Council (1980a) examined this concern and concluded that "the postulations concerning the hazards to human health that might result from the addition of subtherapeutic antimicrobials to feeds have been neither proven nor disproven." Continued monitoring of bacterial resistance in humans and animals has not provided clear-cut answers to this concern. Constraints and regulations on use of particular antimicrobials in poultry feeds vary among countries and are subject to change. Detailed information on specific antimicrobial agents, levels of usage, and legal requirements for use in the United States and Canada may be found in the Feed Additive Compendium (published each year by the Miller Publishing Company, 2501 Wayzata Boulevard, Minneapolis, MN 55440) and in the compendium of "Medicating Ingredient Brochures" (Plant Products Division, Canada Department of Agriculture, Ottawa, Ontario, Canada). For official information concerning Food and Drug Administration approval of antibiotics and other animal drugs, the Code of Federal Regulations (CFR), Title 21, should be consulted. Title 21 is revised at least once each year as of April 1. The CFR is kept up to date by the individual issues of the Federal Register. These two publications must be used together to determine the latest version of any given rule. Title 21 is published in six parts: Part 500-599 covers animal drugs, feeds, and related products and is available from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. The Federal Register is available from the Superintendent of Documents and includes monthly issues of the "List of CFR Sections Affected" and "The Federal Register Index."