Written by Rachel Frost and Jeff Mosley, Montana State University
Think about the wide variety in foodstuffs consumed by people around the world. What people in one region consider a delicacy is considered garbage by people in another region: and all humans are the same species! Now, think about the many species of herbivores that graze rangelands. Each species (and often individual animals) has very different dietary habits and preferences that are molded from their environment, their genetics, and their social interactions. In general, animals consume foods that they are physiologically adapted to digest and that meet their nutritional requirements. These inherent dietary differences result in herbivores being classified into three major groups: grazers, browsers, and intermediate feeders. In addition, physiology alone does not dictate diet selection in animals. The diets of animals are strongly influenced by 1) social interactions with mother, peers, and people; 2) feedback from nutrients and toxins in plants; and 3) interactions with their physical environment including location of water and predators.
Grazers, including cattle and horses, primarily consume grasses and have digestive systems capable of handling large quantities of forages with relatively low nutritional quality. The sheer size of the mouth of these herbivores limits their ability to select individual parts (leaves, twigs) of plants. The large rumen of cattle and the active cecum of horses are well suited to consuming large quantities of low-quality, fibrous forage like dormant grasses. They obtain the nutrients they need by consuming a large quantity of low-quality forage.
Browsers focus their forage selection on the leaves, flowers, and twigs of woody species. They typically have a smaller, more pointed mouth than grazers. The narrow muzzle and other dental adaptations of browsers help them select individual plant parts of higher nutritional quality. In general, the diet of browsing animals is higher in crude protein and more digestible than the diet of grazing animals. Many browse plants, however, contain secondary compounds or toxins that limit their intake by herbivores. Browsers have developed several physiological characteristics that help them either metabolize or avoid exposure to these compounds. For example, many species of browsing herbivores have a large liver in relation to their body size, which aids in metabolism of harmful plant toxins. Some browsers are equipped with salivary glands that bind tannins, an anti-quality compound found in some browse plants.
Intermediate feeders have adaptations of both grazers and browsers. They typically possess a narrow muzzle and a large rumen relative to body mass, which allows them to graze selectively and still tolerate substantial fiber in their diet. Sheep are intermediate feeders that possess a relatively small mouth allowing them to graze relatively close to the ground and to strip leaves or flowers from stems. The diet of intermediate feeders generally is dominated by forbs, although they will readily consume grasses when grass plants are succulent or when other forage has limited availability.
Attributes That Affect Diet Selection
Remember that animals consume foods that they are physiologically adapted to digest and that meet their nutritional requirements. However, the ability to digest foods and the nutritional requirements of individual animals can vary greatly depending on their age, body condition, gender, and reproductive status.