The need to assess the adequacy of nutrition arises frequently. It occurs in relation to individual animals and to groups of animals in both experimental situations and in commercial practice.

The measurement of food intake poses certain difficulties and involves a considerable amount of time and effort, particularly with grazing animals, but also with housed animals which are group–fed. Even where food intakes can be measured, the assessment of the adequacy of that nutrition necessitates a knowledge of the animal's requirements – and this too can present problems in some situations. Changes in live weight and in body condition are obvious and useful indices of the adequacy of nutrition, but in many cases the time taken for such changes to become manifest is too great. By the time an excessive rate of live–weight loss has been detected it may be either too late to effect a change in the level of feeding, or an unacceptable production penalty may have already been incurred.