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Forage Value of Range Plants ~ Grasses, Shrubs, and Forbs
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REM 456 –University of Idaho
Karen Launchbaugh
Based on the most limiting nutrients on rangelands in the western U.S.
 Energy –
o Structural Carbohydrates (CHO's)
o Starch & Sugars
o Fats - limited in range forage, but important to small herbivores (i.e., birds and rodents)
 Nutrients –
o Protein = Nitrogen-containing compounds
o Minerals = Phosphorus and to some degree Potassium, Calcium, Selenium, and others.
o Vitamins = Carotene or Vitamin A – Vitamin E, D, and others.
The 3 major factors determining nutritive value in plants:
1. Cell structure: (Cell Wall: Cell Contents)
2. Degree of Lignification
3. Secondary Compounds or “Anti-quality” factors
1) Cell Structure (Wall:Contents)
o
Most digestible compounds in cell contents
2) Degree of Lignification
 Lignin - indigestible portions of cell walls
that impregnates cellulose to form wood.
 Not digestible
 Becomes thicker as plant ages
3) Secondary Compounds or “Anti-quality” factors
 Plants may contain secondary compounds or toxins that reduce forage quality or adversely
affect the herbivore.
o Alkaloids
o Glycosides
o Terpenes
o Nitrates
o Tannins
o Soluble Oxalates
Comparative Nutritive Value of Plant Parts
 Fruits, seeds, root-crowns and flowers are more
nutritious than leaves or stems.
o Generally have higher levels of cell contents (cell
solubles)
o Seeds also can contain significant levels of fats
 Leaves are more nutritious than stems.
o
Why?
 More cell contents
 Less structural CHO's
1 Integrated Rangeland Management – University of Idaho
Plant Nutritive Value
Maturation Effects on Nutritive Quality of herbaceous plants
 Most range plants are highly nutritious when young.
 As plants mature, nutritive value decreases.
Why?
o Increased structural CHO's
o Lignification
o Increasing Stem:Leaf Ratio
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Leaching of nutrients by rain in dormancy.
o Leaching is when rain washes soluble nutrients out of the plant into the soil.
o Plants that resist leaching due to dry climate, morphology, or range site are said to “cure well.”
Comparative Nutritive Value of Grasses, Forbs, and Shrubs
 What is browse?
o The portion of shrubs used for forage.
o Generally, leaves & current season’s twigs.
 In shrubs, current season’s growth is generally more nutritious than old growth.
Why?
o as stem age they become LIGNIFIED
Seasonal Trends of Nutrients - graph adapted from Parker 1969
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Why are forbs more nutritious than
grasses and browse?
o Forbs have thinner cell walls than
Grasses
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During dormant season - browse are
important for nutrients.

The importance of shrubs for nutrient
value during growing season may be
more important that was once believed.
New research shows browse may be
more important than we once believed.
2 Integrated Rangeland Management – University of Idaho
Plant Nutritive Value
Seasonal Trend of Energy - graph revised from Parker 1969
o During growing season - grasses, forbs, and
shrubs all provide good amounts of energy.
o During dormant season - grasses provide a
stable source of energy.
Tropical (warm season) versus Temperate (Cool
Season) Grasses
Environmental Influences of Plant Nutritive Quality:
 Temperature:
o Higher temperatures - decrease water soluble CHO's and protein levels.
o Lignification and maturation occur more quickly at elevated temperatures.
 Moisture:
o Moderate moisture stress increases the nutritive value of plants by delaying maturation.
o Severe moisture stress hastens translocation of nutrients to the roots and senescence.
 Range Site:
o Fertile soils may delay maturity and increase leaf:stem ratios.
o High level of soil Nitrogen may increase protein content of plants.
Anti-Quality Agents:
 Most common in shrubs and forbs. (Rarely a problem in grasses).
 Resist Digestion
o Compounds that are not digestible.
o Reduce overall forage value.
 Inhibit Digestion
o Bind nutrients to make them indigestible.
o Kill rumen microbes to reduce digestive function.
o Toxins - cause illness or death (i.e. , alkaloids, tannins, terpenes).
Recap:
 Forbs are important sources of protein and carotene during the growing season.
 Shrubs are important to maintain phosphorus and protein levels in winter (dormant season).
 Grasses are important sources of energy (structural CHO's) throughout the year.
 Forbs and shrubs may contain anti-quality agents which decrease their nutritive value.
In Summary - Plant Management for Animal Nutrition
 Monocultures provide high quality at one time of year. This may be useful in management such as
spring grazing areas.
 A variety of forage classes on range provide nutrients throughout the year:
o Plants mature at different times during season.
o Seasonal supply of nutrients is improved by having different classes of forage (grasses, forbs & browse).
o Increased use of range by wildlife because not all animals eat the same thing.
3 Integrated Rangeland Management – University of Idaho
Plant Nutritive Value