The National Research Council (NRC) has published an atlas of feed composition (34) and a more recent table of feed composition (35) tabulating the composition of feeds. There are also many other tables of feed composition (28,36,37). Feed composition tables, along with actual feed analyses, are the basis for formulating nutritionally complete diets for livestock.

Feeds can be analyzed for many constituents. It is convenient to divide these into chemical and biological constituents (37). Chemical analyses for nutrients, antinutri-tional factors, toxicants, contaminants, and residues can be performed, but these results do not necessarily indicate the bioavailability of these constituents when consumed by animals. Thus, biological constituents are measured using a biological response such as digestibility, absorption, metabolic efficiency, or some biochemical response. Gross energy (GE or heat of combustion) indicates the total energy value of a feed but only a portion is digestible (digestible energy, DE), less is available for metabolism (ME), and the net energy (NE) value of a feed is the energy available for maintenance of body tissues and growth (meat), wool, milk, and egg production. Similar bioavailability considerations apply to protein, amino acids, minerals, vitamins, and other constituents. Feeds are commonly analyzed for moisture, crude protein, ash, fiber (crude, acid detergent, neutral detergent fiber), and crude fat. Additional analyses for individual amino acids, minerals, and vitamins can be performed.

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