Herbivores select diets from an array of plant species that vary in nutrients and plant secondary metabolites (PSM). The outcome is a diet higher in nutrients and lower in PSM than the average available in the environment. Food preferences in herbivores are controlled by dietary cues (i.e., flavor) associatively conditioned by the food's postingestive actions. The senses of smell, taste, and sight enable animals to discriminate among foods. Postingestive feedback calibrates sensory experiences—like or dislike—in accord with past and present experiences with a food's utility to the body. Thus, food selection can be viewed as the quest for substances in the external environment that provide a homeostatic benefit to the internal environment. Livestock form preferences for foods that supply needed nutrients and medicines and avoid foods that provide excesses of PSM and nutrients. In order to manifest this plasticity, animals need a variety of foods instead of being constrained to a single food or monoculture. Under natural conditions where diversity of plants is the rule, not the exception, eating a variety of foods is how animals meet their nutrient requirements and cope with—and likely benefit from—PSM in their diets. At certain doses, PSM may provide beneficial effects to herbivores and favor plant persistence and adaptability. If herbivores learn to utilize multiple plants, the costs of consuming PSM on animal production and well-being could be minimized and the benefits of PSM enhanced. Once individuals learn about the contextual benefits of consuming diverse foods, social models (e.g., mothers) could train new generations of herbivores by observational learning. We propose that by combining the concepts of animal learning and food diversity, it will be possible to create sustainable grazing systems with less dependence on fossil fuels and with enhanced benefits for soils, plants, herbivores, and people.