ABSTRACT
Amino acid requirements for growing and adult animals have been studied and debated for many years. In most studies, point estimates of requirements have been determined based on the relationship of animal performance or N retention to the dietary supply of the test amino acid. The point at which performance ceases to increase as dietary supply increases is the breakpoint in the relationship considered to be the requirement. Amino acid requirements typically have been expressed in proportion to dietary dry matter or, more recently, dietary energy content. The latter expression is an attempt to adjust for differences in feed intake which can result if energy concentration in the diet is altered. Desirable methods to express requirements on a scientific and on a practical basis will be discussed. Usefulness of the breakpoint from a specific experiment as an index of the requirement under various genetic and environmental conditions has been questioned by Whittemore 1 who suggested that nutrient requirements should not be stated in fixed terms but instead should describe levels of energy, protein, or other nutrients that are required to satisfy a target response in the animal consistent with the animal’s genetic potential and environment. If requirements can be expressed on the basis of a targeted response, fixed point supply estimates need to be replaced by other estimates of amino acid utilization.
