A brief synopsis of classical approaches for assessing the availability of energy for meeting the cumulative energy needs of all cells to perform the ''work of living'' of the animal has been presented. These concepts and approaches remain the standards on which animal requirements and the value of energy sources to meet those requirements are based. As knowledge of energy transactions in the animal increases, and systems to describe those transactions become more refined and complex, the classical notions of requirements and feed value become less clear. The classical separation of energy from other nutrients, such as amino acids, becomes more difficult to justify. Mathematical modeling approaches, such as those described by Baldwin, are valuable to incorporate knowledge of the entire system for describing animal needs and the value of energy sources to meet those needs.
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