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Written by Rachel Frost and Jeff Mosley, Montana State University
The nutrient value of rangeland forages depends upon their ability to meet the grazing animal's nutritional requirements throughout the year. Livestock (or any animal) are a production unit, and each unit has different nutrient requirements based upon its physiological status (yearling steer, cow-calf pair, pregnant cow, dry cow, etc.). Plant nutritional values should be compared with the corresponding animal requirements for the animal's physiological status. The nutrient evaluation of rangeland forage is based upon the plant's content of protein, phosphorus, energy, and carotene (vitamin A). These four principal nutrients are those mostly likely to be deficient in rangeland forage, although localized deficiencies of other nutrients or minerals are possible.
Protein is calculated from the amount of nitrogen contained in plants. Grasses decline in digestible protein rapidly as they mature. Nitrogen is moved by the grass plant from aboveground parts available to the grazing animal to storage organs below the ground as the current year's grass growth matures. Shrubs, on the other hand, are good sources of protein even after they reach full maturity because nutrients remain in branches and leaves as well as below ground. Forbs, in general, are intermediate between shrubs and grasses with respect to protein content during most seasons.
Phosphorus, a macro-mineral, is often limiting in range forage plants. Grasses are low in phosphorus soon after they form seed. Shrubs are generally considered good sources of phosphorus for general animal maintenance and gestation, even when mature. Most forbs have a phosphorus content only slightly lower than that of shrubs. Phosphorus content of plants can fluctuate depending on the soil status. Soils high in phosphorus will allow plants to contain more phosphorus than where soils are limiting in phosphorus content.
Energy values of forage are commonly reported as total digestible nutrients (TDN) or digestible energy (DE). Grasses are generally considered good sources of energy primarily because of their high content of cellulose. In very mature grasses however, digestibility will be so low as to reduce intake and thereby reduce total energy intake. Digestibility is the proportion of a dietary nutrient available for animal metabolism and indirectly tells us something about intake (as digestibility goes down, intake may go down). Shrubs are not considered good sources of energy after they reach fruit development. Again, forbs are intermediate between grasses and shrubs in furnishing energy.
The single biggest problem however, especially when forage plants are mature, is maintaining intake so that the animal gets enough total nutrients each day. Other factors may also affect the nutritive value of range plants. Range condition, for example, may alter total forage intake of grazing cattle. Research shows that protein and phosphorus are about the same in plants growing on good- versus poor-condition range. However, plant species on poor-condition range may be less digestible than plant species on good-condition range, which can reduce total forage intake by grazing animals. The animals either can’t or won’t eat enough. An appropriate mix of grasses, shrubs, and forbs, is necessary to provide nutritious forage to livestock on a yearlong basis.
Classification of Range Forage Value: To facilitate management, range plants are commonly classified according to their forage value.
- High forage value designates plants that are nutritious, palatable, and produce abundant forage.
- Medium forage value denotes a plant that will provide adequate nutrients if eaten; however, it is not preferred by animals or does not produce abundant forage.
- Low or poor forage value describes plants that simply do not provide adequate nutrients to the grazing animal. Additionally, most plants containing anti-quality compounds that reduce intake or poisonous plants containing toxins that cause illness or death in herbivores are classified as having "low" forage value.
Ways to Manage Your Forage Value Management factors such as stocking rate and specialized grazing systems can also influence grazing animal nutrition. Heavy stocking reduces individual animal performance and can result in damage to the forage resource. Although the influence of animal numbers can be altered by controlling the time the plants are exposed to grazing and allowing for adequate recovery periods, proper stocking rates are essential to long-term rangeland health and healthy, productive grazing animals. Grazing systems may reduce or improve forage nutritive value. Although forage reserves are a necessary part of ranch planning, and some amount of plant material should be left for resource protection, animal production may suffer if pastures are allowed to accumulate too much old plant growth. This can be offset by adjustments in stocking rates or changes in range condition. Carefully planned grazing can help increase diet quality. In grazing cells, for example, the longer animals stay in a particular paddock, the further diet quality is reduced.
Adapted from: Ruyle, G. 1993. Nutritional Value of Range Forage for Livestock. Arizona Rancher's Management Guide.
In this section you'll find addtional information regarding:
Seasonal Changes in Forage Quality and Quantity
Written by Rachel Frost and Jeff Mosley, Montana State University
Forage quality and quantity are both important to maintaining livestock and wildlife production. Quality and quantity both change substantially throughout the year, and it is important to understand how to balance these attributes. Supplementation programs should be designed to specifically address a deficiency in quality or a lack of quantity to be effective.
Forage Quantity Forage quantity can be limiting even when there appears to be plenty of available standing crop. This occurs because herbivores have very definite forage preferences and dietary requirements. For forage quantity to be adequate, there has to be sufficient quantity of the preferred plant species for the specific herbivore and sufficient forage of acceptable quality. For example, forage quantity can be a problem in the spring when the quality of the forage is high, but the availability of the green plant material is limited. Drought conditions and overgrazing are the most common causes of insufficient forage quantity.
Forage Quality Forage quality can be affected by a variety of biological and environmental factors. In general, the nutritional value of forages is highest when the plant has an abundance of young, actively growing leaves and declines as the plant nears maturity. Understanding how and why forage quality changes throughout the year can help producers match the nutritional requirements of their livestock — or wildlife managers matching vegetation to the wildlife species — to the nutrient content of the forage resource. This permits producers to target forage supplementation to the specific needs of their livestock, which should reduce supplementation costs.
See the fact sheet "Why Forage Quality Changes" for a more detailed discussion on all factors.
Supplemental Feeding of Livestock on Rangelands
Written by Rachel Frost and Jeff Mosley, Montana State University
Under certain conditions, livestock grazing western rangelands are not able to consume enough nutrients from forage alone to meet their nutritional requirements. At these times, supplementary nutrients are often provided to maintain production levels. Supplementation is one of the biggest expenses of range livestock production, accounting for up to 70% of the operation's total variable expenses. Providing the proper supplements in the correct amount and at the appropriate time can save producers money and conserve forage resources.
Determining the proper supplementation program requires livestock managers to know the following:
- nutritional requirements of the grazing animal
- nutrient content of the forage
- cost of supplementation and expected benefits
The decision to provide a supplement should be based on forage supply, protein content, and animal body condition.
Basic Ruminant Nutrition. Ruminants differ from pigs and humans in their ability to digest fibrous plants because they have a rumen that allows for fermentation before the food enters the abomasum (stomach) of the animal. The rumen houses microorganisms that are capable of breaking down cellulose through fermentation. These microorganisms break down consumed feedstuffs for their own nutritional requirements and in return release volatile fatty acids that are a major energy source for ruminants. The microorganisms eventually die, and their bodies pass into the small intestine, where they are digested and contribute to the protein supply of the animal. This symbiotic relationship, while essential, also adds to the complexity of predicting and effectively meeting the nutrient requirements of ruminant animals.
Relationship of Protein and Energy within the Ruminant. Ruminants need the microorganisms to "unlock" the energy in forage, allowing them to harvest and make use of cellulose that is unavailable to non-ruminants. The existence and growth of microorganisms depends on an adequate supply of nitrogen, primarily found in protein. Supplementing ruminants with protein increases the number and activity of microorganisms in the rumen, which improves forage digestion and increases passage rate and intake of forages. Increasing forage intake improves energy availability; therefore, correcting a protein deficiency is generally the first supplementation priority.
Types of Protein Supplements. Escape protein does just what its name implies and escapes digestion in the rumen. It travels to the small intestine where it is broken down and used directly by the animal. Escape protein can be important to the ruminant because any rumen degradable protein that is not consumed by the microbes can be lost through the urine. However, when animals are consuming low-protein forages, as is often the case on rangeland, then a supplemental protein source is required to stimulate rumen microbial activity and encourage forage intake. For cattle consuming low protein forage, 60 to 70% of the supplemental protein should be ruminally degradable.