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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