The monotonous diets fed in confinement (total mixed rations) and on pasture (monocultures) often contain excesses of nutrients, nutrient imbalances, and toxins that adversely influence animal welfare. How much of any food an animal can eat will depend on the other foods it consumes, because at the biochemical level, nutrients and toxins interact one with another—nutrients with nutrients, nutrients with toxins, and toxins with toxins. Food intake and preference also depend on differences in how individual animals are built morphologically and how they function physiologically, and marked variation is common even among closely related animals relative to the needs for nutrients and tolerance to toxins.

An integral part of an animal's ability to meet its particular nutritional requirements and consume substances that improve health depends on having a variety of foods available so each animal can select a diet that best meets its homeostatic needs. Food choice may also offer animals a means to cope with toxins, as certain food combinations have the potential to ameliorate the negative effects of toxins. We suggest that the availability of alternatives may not only contribute to maintain homeostasis but also reduce levels of stress. Thus, food choice is necessary for individual animals to have freedom to express their normal behaviors. We contend this freedom enables the uniqueness of individuals to be manifest, thereby promoting animal welfare and performance and increasing profitability of the people who manage animals.