Submitted to: Encyclopedia of Animal Science
Publication Type: Book / Chapter
Publication Acceptance Date: 7/7/2003
Publication Date: 11/1/2004
Citation: Ferrell, C.L. 2005. Bioavailability: Energy. In: Pond, W.G., Bell, A.W., editors. Encyclopedia of Animal Science. Marcel Dekker,Inc., New York, NY. p. 111-114.
Technical Abstract: Energy is defined as the potential to perform work and is required by the animal to perform the 'work of living'. Some of the more obvious examples of the 'expenses' of living include thermoregulation, voluntary and involuntary muscular activity, ingestion of food, digestion, absorption, excretion, metabolite transport, cell turnover, tissue or product formation (e.g., wool, milk, eggs), etc. There are inefficiencies or waste associated with all of these processes. Energy requirements depend on the additive needs of individual cells, which vary according to physiological needs imposed upon them. Gross dietary requirement is the sum of all cellular needs plus losses. Bioavailability of energy is an expression of the value of an energy source toward meeting the cumulative energy needs of all cells to perform the 'work of living' of the animal. 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 is 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 are valuable to incorporate knowledge of the entire system for describing animal needs and the value of energy sources to meet those needs.