Pastures that provide alternatives enable individuals to better meet needs for nutrients and to better cope with toxins. All plants contain toxins, but at the appropriate dose, many toxins have medicinal benefits. Furthermore, how much of a food an animal can eat depends on the other foods it selects because at the biochemical level, nutrients and toxins interact - 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 in needs for nutrients and abilities to cope with toxins. Thus, individuals can better meet their needs for nutrients and regulate their intake of toxins when offered a variety of foods that differ in nutrients and toxins, and feeding and grazing practices that allow producers to capitalize on the individuality of animals are likely to improve performance of the herd by enabling the uniqueness of individuals to be manifest. Finally, past experiences play a crucial role in an animal's propensity to learn to eat different foods. When herbivores are allowed to eat only the most preferred plants, they are not likely to learn to mix foods high in nutrients with foods that contain toxins. Conversely, herbivores encouraged to eat all plants in an area are more likely to learn to eat mixes of plants that mitigate toxicity. Experienced animals who have learned to eat a variety of foods that differ in nutrients and toxins do so even when nutritious alternatives are available, whereas naive animals familiar only with the nutritious alternatives eat only that subset of familiar foods. Continuous grazing at low stock densities can encourage selectivity and reduce diet breadth, whereas grazing at high stock densities can increase diet breadth.