Food preferences in farm animals: why don't they always choose wisely?

While people can express their preferences in abstract form, animals can only demonstrate which food they prefer by which food they eat, or the ratio of intakes of each food, when given a choice. Thus, the food preferences of animals are monitored by their diet selection and there is now a very substantial amount of evidence to support the concept that farm animals, like laboratory animals, are capable of choosing a balanced diet from two (or more) foods, one of which has more, the other less, of an essential nutrient than is required for optimum metabolism. This is to be expected, as an excessive intake of a nutrient incurs penalties arising from its excretion-metabolism which may be as disadvantageous as a mild deficiency of a nutrient. The general background to diet selection is provided by Emmans (1991) and a catalogue of successes is reported by Illius &