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University of Leeds, Leeds LS2 9JT, U.K.
2 Correspondence: Centre for Animal Sciences (phone +44-0-113-3433066; fax: +44-0-113-3433144; E-mail: j.m.forbes{at}leeds.ac.uk).
Three approaches to predicting and understanding food intake by beef cattle are discussed and compared. Although many physiological factors are known to be involved in intake control, these are not sufficiently quantifiable to form an adequate basis for intake prediction. Prediction equations derived from observed effects of animal and feed factors on intake are useful within the range of conditions under which the data were collected, but they do not predict adequately outside this range. The concept of an intermediate approach to intake prediction and understanding is presented, suggesting that proportional deviations of resource supplies from the feed (e.g., energy, protein, fiber) from the animal’s optimal supply ("requirement") generate discomforts. These, when squared and added, yield a total discomfort signal, which the animal minimizes. The fact that feed intakes by individual animals fluctuate considerably from day to day provides a means whereby animals can assess whether an intake somewhat higher or lower than their current average intake will improve their well-being. Examination of intake data from beef cattle fed grass silage suggests that intake is controlled over a period of several days. It is concluded that different levels of understanding of the control of voluntary intake are needed for different reasons, and that no single approach will serve all purposes.
Key Words: Beef Cattle • Physiology • Prediction • Voluntary Intake
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