Diet Selection of Grazing Animals

Written by Rachel Frost and Jeff Mosley, MSU

Think about the wide variety in foodstuffs consumed by people around the world. What people in one region consider a delicacy, people in another region would consider inedible. Even though all people 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.


Factors that Affect Diet Selection

The physical characteristics of animals can have a profound influence on their diet selection. These main characteristics should be considered when designing a grazing plan and calculating stocking rate.

Type of livestock: Graze the type of livestock best suited for the kind of forage available and the forage's nutritional quality. The amount and kinds of forages that livestock consume depend upon a variety of factors, including species, breed, physiological status, and experiences early in life. Understanding why livestock eat certain plants or parts of plants allows managers to use diet selection as a management tool to direct the vegetation change in plant communities toward management objectives.

Grazers. Grazers, including cattle and horses, primarily eat grasses. 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. Browsers focus their foraging on 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 protein and more digestible than the diet of grazers. Many browse plants, however, contain secondary compounds or toxins that limit their intake. 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. 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.

Breed: Breeds of livestock differ in size and production characteristics, which dictate their nutrient requirements, dry matter intake, and digestive ability. These factors influence which plants, and in what proportion, an animal chooses to include in its diet. Livestock selection and breeding may also affect the kind of terrain where animals can effectively forage. Breeds of cattle developed in mountainous terrain may graze rugged rangeland more uniformly than breeds developed in gentler terrain.

Age: Animal age can also profoundly affect diet selection and tolerance to secondary compounds. Metabolic requirements decline with age, so older animals need less food and spend less time foraging. Compared with adults, young, growing animals need diets higher in protein and energy and lower in fiber. Their search for a more nutritious diet takes more energy. This, combined with limited foraging knowledge, may lead younger animals to try novel foods and retry foods that once made them sick. Animals just weaned are expanding their diet selection, so they are also more willing to try novel foods.

As herbivores age, their incisor teeth wear, so they are less able to graze and achieve maintenance requirements, particularly on short forage. Wear on incisors also influences forage selection.

Body Condition: How fat or thin an animal is influences its foraging behavior. Animals in low body condition or on a diet that fails to meet their maintenance requirements may have reduced tolerance for plant toxins. That’s because there is a nutritional “cost” to metabolize a toxic or aversive plant compound. Detoxification most often occurs in the liver, so an animal that consumes chemically defended plants needs a large, healthy liver. Prolonged nutritional stress can reduce liver mass. Protein and mineral supplements can enhance rumen microbial function, liver enzymes, and other compounds for negating toxins, all of which enhance an animal’s detoxification abilities.

Malnourished and thin herbivores generally eat more than animals in good condition. When forage is limited, animals in low body condition may turn to poisonous or less desirable plants to maintain that higher intake.

Gender: Males and females select different diets, in part because of differences in size and overall nutrient requirements during reproduction. Morphological and physiological traits, such as growth rate and feed conversion efficiency, also contribute to differences in diets. Males generally have larger stature and muzzle size than females and may have greater energy needs. Males and females of most species of wild herbivores segregate when it is not breeding season. Therefore, they use different portions of the land area and consume different forages.

Stage in Production Cycle: Animals choose their diets based on nutritional needs, which change dramatically during life stages. The greatest nutrient demands are for lactating females who need more energy and protein to support milk production. However, care should be taken to prevent consumption of certain plants that are particularly harmful to females during gestation, such as ponderosa pine, lupine, and veratrum. Consumption of these plants can cause serious birth defects or even death of the fetus.

Individual Variation and Heritability: “Individuality” is a powerful force that influences dietary preference. Even animals of the same age, sex, breed, and experience will vary in their plant preferences. Some prefer plants high in energy, while others prefer those with medium or low energy concentrations. Just as with humans, animals have unique dental structure, physical abilities, organ size and function, and sensory abilities. Individual differences affect foraging abilities and how an animal metabolizes nutrients. Individuals also vary in responses to plant toxins. Almost every feeding trial with toxic plants has revealed individuals capable of consuming what would be a lethal dose to other animals without showing signs of toxicity.

Grazing selectivity: Grazing animals consume the plant species and plant parts that provide the least disruption to their digestive system and best meet their seasonal nutritional needs. Most plants contain natural chemicals that inhibit digestion when they are consumed above some level. Grazing animals are always trying to optimize nutrient and energy intake and minimize the consumption of chemicals that adversely affect digestion and extraction of nutrients from the forage; hence, the need to selectively consume forage species and plant parts. The performance of individual animals will be best when they are allowed to be picky eaters. However, repeated selective grazing of the same plants typically harms the preferred forage plants. Grazing systems are management tools that attempt to minimize an animal's ability to repeatedly and selectively graze desired plants, while trying to optimize animal performance and plant selection across all forage species.


Role of Learning in Diet Selection

Herbivores forage in a complex environment. How do they learn which foods are nutritious and which foods are toxic or low in quality? Herbivores begin learning about what foods are safe before they are even born and continue the process throughout their life enabling them to survive in a world where toxin and nutrient levels of forages are constantly changing. These same processes allow them to make foraging decisions when they are moved to new pastures with unfamiliar foods.

Mom as a Role Model. A young animal first learns about which foods to eat and which to avoid by foraging with its mother. By the time the animal has to forage on its own, it is already familiar with a number of plants that are nutritious and safe to eat. Thus, an animal divides its foraging world into two food groups, familiar and novel. Animals learn through trial and error about novel foods based on the postingestive consequences of the novel foods they eat.

Novelty. Like most people, herbivores sample novel foods cautiously. If the consequences of eating the food are positive--feedback from needed nutrients---the animal will increase intake of the new food. If the consequences are negative --illness from toxins or lack of feedback because the food is low in nutrients--the animal will decrease intake of the food. When eating a meal of several foods, novelty is the key to figuring out which foods are harmful and which are nutritious. When animals eat a meal of several familiar foods and a novel food and then experience illness, they subsequently avoid the novel food. Conversely, when animals suffering from a nutritional deficiency recover after eating a meal of several familiar foods and a novel food, they learn to prefer the nutritious novel food. Herbivores also reduce intake of familiar foods when the flavor of the food changes. Changes in flavor may occur when forages grow on different sites or as plants mature. If the change in flavor results in illness, the animal avoids the food in the future. If, however, the change in flavor results in positive consequences then the animal will continue to eat the food.

Prior illness. Herbivores continuously sample foods, even foods that made them ill. If an animal gets sick after eating a meal of several familiar safe foods and food that caused illness in the past, subsequently it will avoid the food that caused illness. Animals are able to remember which foods previously made them sick for a long time.

Generalization. Animals use past experiences with familiar foods to make foraging decisions about new foods. If new foods have flavors similar to foods that made the animal ill in the past, it is less likely to eat those foods. Conversely, if new foods have flavors similar to familiar nutritious foods, animals ingest those foods more readily.

Amount and timing. If the foods an animal eats during a meal are equally unfamiliar and the animal experiences illness, how does the body determine which food to avoid? Animals pair feedback--positive or negative--with the food they ate in the greatest amount, provided both foods are equally new. Animals also form aversions to or preferences for foods when food ingestion is quickly followed by either illness or positive postingestive signals, provided the foods are equally familiar to the animal.

Salience. At one time researchers thought animals formed aversions to certain strong flavors more readily than others. They referred to these flavors as salient. Bitter, for example, was thought to be a salient flavor because many toxic compounds are bitter. Further study indicated that the response the scientists observed was simply due to novelty. When animals are reared on bland foods and get sick after eating a meal of several foods, one of which has a strong novel flavor, they form an aversion to the food with the strongest flavor. If, however, they are reared on foods with strong flavors and get sick after eating a meal of foods with strong familiar flavors and a novel bland food, they form an aversion to the bland food. Thus animals associate illness with novelty not necessarily with strong flavors.

Conclusion. Animals depend on the availability of familiar foods to make correct foraging decisions. When animals are moved to new foraging locations that contain only novel foods, it is more difficult for them to select safe nutritious foods and to avoid toxic foods. Understanding how animals discern safe from harmful foods is important information managers can use to help animals make transitions to new locations or train animals to eat new foods.

Adapted from: BEHAVE. Learning What to Eat and What to Avoid. Behavioral Principles and Practices - No. 1.1.3


Nutritional Requirements of Livestock

The essential nutrients required by grazing animals are water, energy, protein, minerals, and vitamins. These nutrients are needed to maintain body weight, growth, reproduction, lactation, and health.

Water. Water is essential for all livestock, and producers should plan for an adequate supply of clean water when designing any type of livestock enterprise. Dirty, stagnant water can lead to inadequate water consumption, which will reduce feed and forage intake and compromise livestock performance. The amount of water required depends on the physiological stage of the animal and the climate. Lactating animals require more water, and the amount of water required increases as atmospheric temperature increases. For example, at temperatures above 35°C (95°F), cattle require about 8 to 15 liters (2.1 to 4 gallons) of water per kg (2.2 lb) of dry matter intake. Generally, cattle require ~2.6% of their body weight in dry matter (DM) intake per day. Therefore, a 1000 lb cow could require as much as 175 liters or 45 gallons of water a day! Daily water consumption of ewes will vary from 0.75 to 1.5 gallons depending on climate and stage of gestation. Water availability should be closely monitored because a deficiency in water will result in death much faster than a deficiency of any other nutrient. Add can cows eat snow

Protein. In most situations, the amount of protein supplied in the diet is more critical than the quality of the protein. Ruminants have the ability to convert low-quality protein sources to high-quality proteins through bacterial action. Microbial protein synthesis is sufficient to supply the protein needs as long as adequate precursors are supplied, except during lactation for high milk producing animals. Protein is required by all grazing animals for tissue growth and repair. Protein required for a 1000 lb nonlactating cow is around 1.6 lb/day or 7% crude protein in the diet. When the cow is lactating, 2.0 lb or 9.6% dietary crude protein is required. If protein is deficient in the diet, grazing animals must break down body tissue to obtain sufficient protein. A protein-deficient animal must break down 6.7 lb of lean body tissue to supply 1 lb of protein, resulting in severe weight loss.

Energy. Insufficient energy probably limits performance of livestock more than any other nutritional deficiency. Energy requirements vary greatly with stage of production, and adequate amounts of energy are extremely important during late gestation and early lactation. Energy deficiencies can cause reduced growth rate, loss of weight, reduced fertility, lowered milk production, and reduced wool quantity and quality. Energy is obtained from carbohydrates in the plant material and can be stored in the form of body lipids. However, heavy demands against fat stores as an energy source to meet daily needs may delay estrus and reduce conception in breeding females. Live weight gain can only occur after the animal’s energy requirements for maintenance and lactation are met.

Vitamins and Minerals. Ruminants require all the fat-soluble vitamins (A, D, E, and K), but they can synthesize the B vitamins in their rumen. Normally, the forage and feed supply contain all essential vitamins in adequate amounts, except vitamin A which is obtained as carotene from green plants and is often deficient in dormant forage. However, vitamin A can be stored in the liver in amounts sufficient to last considerable periods of time, such as winter dormancy or prolonged drought. Salt is essential for many body functions and important to maintain intake of feeds and water. Calcium and phosphorus are needed to maintain growth, feed consumption, normal bone development, and reproductive efficiency. Other nutrients and minerals such as vitamin E and selenium are important for maintenance of healthy bodies and reproduction.

Factors that Affect Nutrient Requirements. The nutrient requirements of domestic livestock have been provided in detail by various National Research Council (NRC) publications. However, these NRC requirements have been developed on pen-fed livestock where maintenance requirements are easily calculated and tend to vary only slightly within a given weight, sex, age, and physiological state.

  • Physiological stage. The greatest influence on nutritional requirements of livestock is their life stage in production. The key physiological stages in the life of grazing animals are growth (i.e., young animals), late pregnancy, lactation, and maintenance, generally during non-lactating periods. In general, the highest nutritional requirements are for lactation, followed by late gestation, growth, and finally maintenance. Managing livestock production schedules to match nutritional demands with forage quality and availability can greatly improve the efficiency of a production system.
  • Topography. The nutrient requirements of grazing animals are also dependent on environmental and climatic variables. Grazing and voluntary travel also require substantial increases in energy expenditure. Range animals walk long distances, climb gradients, and ingest herbage often of low dry matter content, thus spending more time eating and foraging for food. Research estimates that cows grazing rangeland use 30% more energy than confined cows because of longer grazing time and longer travel distance.
  • Climate. Climate, particularly temperature, also affects the amount of feed an animal needs to maintain its body functions. As ambient temperature drops, an animal’s metabolic rate increases, and more energy is needed to maintain internal heat. This effect can be exacerbated by wind or wet hide/hair on the animals.