Nutritional Value of Range Forage

October 02, 2012 Print Friendly and PDF

Written by Rachel Frost and Jeff Mosley, Montana State University Extension

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.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.