RANGE NUTRITION AND ITS
APPLICATION TO MANAGEMENT

-by C. Wayne Cook

COMPARATIVE NUTRITIVE VALUES OF
FORBS, GRASSES AND SHRUBS

The quality of a range plant is judged primarily on palatability or how readily it is eaten by animals, and its nutritive content with respect to its phenological development. Some plant species are eaten only during early growth and, in special cases, only certain portions of a particular species are consumed during the latter stages of development.

Of the three forage classes, shrubby species, in most cases, contain higher levels of carotene, lignin, phosphorus, calcium and protein

Grasses are higher in crude fiber, cellulose, and gross energy.

In most cases, forbs are intermediate between browse and grasses in chemical content.

The nutrient evaluation of most range areas can be based on how much protein, energy, phosphorus, and carotene are in the forage plants. Carotene, phosphorus, digestible protein,and energy content are the four main nutrients that are most important in evaluating the status of range nutrition.

The comparative nutritive value among the forage classes (browse, forbs, grasses) can best be discussed by measuring their apparent ability to meet the nutritional requirements of large herbivores for the more important physiological functions such as maintenance, gestation, growth, and lactation.

In addition to the influence of relative preference and floristic composition on nutritive content of range forage, there are other important factors such as stage of growth and variable site conditions.

For the most part, rangelands of the West can be evaluated for nutritional value on the basis of vitamin A (carotene), phosphorus, digestible protein, and digestible energy. There may be some local areas where other nutrients should be considered. During the period of initial growth, and for a time thereafter, all forage classes are high in nutrient content, and their nutritional contents are not materially different. However, as growth stages advance, the nutritional differences among forage classes become more evident. It is generally agreed that young growing animals and lactating animals have comparable requirements when expressed on a percentage basis.

It is a common belief that animals grazing green plant growth on spring and summer ranges receive adequate nutrients; however, on fall and winter ranges where plants are dry and mature, the diets of grazing animals are thought to be deficient in some nutrients.

Figure 1--Average carotene content of the three forage classes compared to phenological stage of development and requirements for lactation and gestation.

Browse species in all stages of growth furnish enough carotene to meet the vitamin A requirements for even lactating or growing animals. However, species of grass and forbs, when mature, fail to furnish even the minimum requirements of vitamin A for gestating animals. Vitamin A has body reserves for 90 to 120 days. Deficiencies cause abortion and night blindness.

There is a rather high correlation between total protein and digestible protein in the mixed ration normally used in farmstead feeding. In range forages the digestibility of protein may vary from 70 percent in early growth to as low as 10 to 15 percent in the quiescent stages. It thus appears more logical to evaluate grazing animals' diet on the basis of digestible protein rather than on total protein unless standards have considered the high variability in the digestibility of protein as plants mature.

Grass species decline in digestible protein rather rapidly and generally fail to meet the lactation requirements at about the time they come into full anthesis (Figure 2). This is the result of a rather rapid loss of total protein and a more rapid decrease in digestibility of protein that occurs with advanced growth. Grasses, in general, lose 75 percent of their protein during the period from early growth to seed formation; on the other hand, browse lose only about 40 percent of their protein content during a similar period. As a result, grasses that have matured are considered poor sources of digestible protein. Shrubs, however, are considered good sources of digestible protein during most of their active growing period; and even after they reach full maturity they continue to meet gestation requirements.

Figure 2--Average content of digestible protein for the three forage classes, compared to phenological development and herbivore requirements for lactation and gestation.

Even when mature, shrubs are generally considered good sources of phosphorus for general animal maintenance and gestation, unless they are deciduous. Even deciduous shrubs are perhaps only borderline if the young twigs are readily eaten (Figure 3). Most forbs have a phosphorus content that is only slightly lower than that of shrubs. Grasses, however, are low in phosphorus soon after they form seed; so they are considered poor sources of phosphorus during quiescence. Most grasses lose considerable phosphorus content when temporarily forced into dormancy by even brief periods of drought. However, when precipitation occurs and growth is renewed, the phosphorus content increases and lactation requirements are again met.

Figure 3--Average percent phosphorus for three forage classes compared to stage of growth and herbivore requirements for lactation and gestation.

In some cases, neither grass energy nor digestible energy in shrubs is considered a good index to the true energy values of forage because of the high content of essential oils, resins, or waxes that suggest high energy content; but these materials are not available for livestock metabolism. The digestible energy values for browse shown in figure 4 have been adjusted for species high in essential oils so that the trends are based on realistic digestible energy values. Shrubs are not considered good sources of energy after they reach the phenological stage of fruit development. Thereafter, they generally fail to meet the energy requirement for animals in gestation.

Figure 4--Average content of digestible energy for three forage classes compared to various stages of phenological development and requirements for lactation and gestation.

Grasses are generally considered good or excellent sources of energy, primarily because of their high content of cellulose. Even when they reach maturity, grasses seldom cease to furnish the energy requirements for lactation. Forbs are intermediate between shrubs and grasses in energy-furnishing constituents and, like shrubs, they generally fail to meet the energy requirements for gestation after reaching full maturity and dormancy.

In the intermountain area of the West, ranges are identified with the season that apparently best suits their forage producing qualities.

High mountain range is used during the summer;

foothills are used during the spring;

and the desert basin areas are used primarily during winter.

Normally, grazing animals bear their young and are in lactation during spring and summer. During the winter grazing period, range forage is expected to meet the gestation requirements (Table 1). Animals may be grazed year-round in essentially the same locality in the grassland types of the Southwest and the Great Plains where lactation begins with new growth in the spring and continues throughout the summer. On most grassland ranges, gestation requirements are met by intermittent farmstead feeding periods or by giving supplements to the grazing animals while they are on the dormant ranges.

The early growth of forage plants on spring ranges results in a high nutritive content; but as discussed previously, the percentage of nutrients decreases as the growth stages advance. Cool-weather grasses start growth early but lose nutrients rather rapidly, even during active growth. Warm-weather grasses start growth later in the season than cool-weather grasses, but the former retain their nutrients much better as the season advances.

Both forbs and shrubs may become decidedly deficient in energy-furnishing constituents for lactating animals when plants approach maturity during late spring, but grasses continue to furnish adequate energy for lactating animals during the entire spring grazing season and even later into summer and fall. Therefore, a stand of mixed vegetation that includes all forage classes generally meets lactating requirements for livestock throughout the entire spring and summer grazing seasons.

On mountain ranges where mountain brush, aspen, and coniferous types prevail, the vegetation is still in the initial growth stages even by late June or the first of July. Whereas, on the grassland plains used for summer grazing, many forage species are in rather advanced stages of growth by July 1. Intermittent precipitation, when it falls on the grassland plains area, produces renewed growth and thus increases the nutritional value. In both the mountainous areas and the grassland plains, drought periods may produce temporary dormancy in herbage and consequently produce reduced percentages of nutrient content. In some cases, serious nutrition deficiencies may occur.

On either mountain or grassland summer ranges, the plants may reach full maturity late in the season, and nutrient requirements may become borderline for growing and lactating animals.

As discussed previously, each of the three forage classes displays characteristic changes in nutrient content as advanced stages of growth occur. Late in the summer, grasses would be expected to be low in protein and phosphorus, but they would be low in energy. Forbs, as previously stated, would be intermediate between grasses and shrubs in these respects. During the summer grazing season, grasses may lose considerable quantities of their protein content and increase decidedly in lignin and cellulose. Protein content of forbs and shrubs generally decreases only slightly during the summer, while lignin and cellulose content increase only moderately as the season advances.

Shrubs and forbs furnish ample protein and phosphorus for growth and lactation until (at least) late in the summer grazing season or even into the fall unless drought conditions force plants to become dry and dormant before they normally complete their annual cycle. However, a diet composed largely of forbs and browse late in the summer might be deficient in the constituents that supply energy. Most grasses would furnish adequate energy for growing and lactating animals even late into the summer grazing season, but grasses would be deficient in both protein and phosphorus when they reached the advanced stages of growth. All three forage classes are comparatively high in carotene (vitamin A) during the entire summer grazing season unless the herbage becomes extremely dry as a result of an extended drought period.

Seasonal changes in nutrient content on summer ranges show generally that phosphorus, protein, gross energy, and carotene decrease in all three forage classes as the season advances. Grasses change the most as to nutrient content: shrubs change the least, except for changes in digestible energy. As would be expected, shrubs have the highest protein and phosphorus content throughout the summer grazing season, and grasses have the highest energy content. These changes that occur with advancement of the summer grazing season are affected by changes both in the stem-leaf ratio and in chemical content of the plant parts themselves. Leaves for all forage classes are higher in ether extract, protein, phosphorus, and calcium; and stems are higher in lignin and cellulose. The leaves of forage plants are also much higher in digestibility because of less fiber content compared to stems. Thus, leafiness leads to higher nutrition among range plants.

Thus, it is concluded that summer ranges, including both warm and cool weather grasses and more than one forage class, more nearly meet growth and lactation requirements than ranges on which there is a restricted diversity in forage species. Moving up in elevation on mountain rangelands as the summer season advances provides an increase in nutrient content because of delayed plant development.

During winter on desert ranges, shrubs generally meet the gestation requirements for livestock regarding protein and carotene, but they are borderline in phosphorus and are decidedly low in energy for animal metabolism. During winter, the grasses are decidedly deficient in protein, carotene, and phosphorus but are good sources of energy. The broad general class of shrubs on desert ranges can be divided into two groups. The first group would include species that possess a woody base but, for the most part, have herbaceous stem growth. These are known as "suffrutescent" (half shrub) species. The second group includes the true shrubs, or frutescent species. The species of the second group include wood stemmed plsnts of low stature.

As might be expected, the suffrutescent species are intermediate between shrubs (frutescent species) and grasses in nutrient content (Table 2). The suffrutescent species are slightly lower than the shrubs in these respects. In the suffrutescent species, energy-furnishing constituents are somewhat lower than in grasses but these species provide somewhat higher energy sources than shrubs. Again, it is suggested that a mix of forage classes in the diet is conducive to a balanced nutritional diet for grazing animals on desert ranges during the winter.

Sandstone and limestone breaks in the Great Plains are often excellent winter ranges because of the grass-shrub mixtures. Late in the winter when grass is the dominant forage, carotene will become deficient in the diets of grazing animals, but most herbivores store vitamin A in the liver as a reserve for 60-90 days.

During spring, while they are growing, desert shrubs are as high in nutrient content as mountainous shrubby species during comparable phenological growth stages. This is also true for grasses and forbs, but forbs are sparse on most desert ranges and, therefore, are not significant in the diet of grazing animals. Even though desert plants are high in nutrients during growth, they can withstand only about one-half the degree of defoliation that is tolerated by mountainous and plains plants during growth.

Predominantly grass ranges such as short grass or mixed grass are considered deficient in phosphorus, protein and vitamin A when the plants approach hard seed formation or maturity (See figures 1, 2 and 3). Therefore grass ranges are generally believed to be deficient in these three nutrients in late summer, fall and winter and should be supplemented somehow.

Planting agriculture farm lands to plants that can be used to supplement rangelands during the period that the native range forages are deficient in nutrients is useful. This practice, in most cases, is a real money saver for supplementing the diet of grazing animals. Examples of this are using grazing sorghums in late summer or fall to maintain or increase the gains of steers that will enter feed lots. Similarly, we plant domestic wheat or rye to be grazed in the late spring to enhance or extend the period of normal spring gains by steers that are to be placed in feed lots.

These are all examples for obtaining gain from grazing animals in the schemes for obtaining cheaper gain compared to the feed lot gains that would otherwise be employed prior to full feeding for desired finished fattening. Final finishing products for sale are shortened and total costs are materially reduced.

Recently we have discovered that our normal range forages from grass ranges are decidedly deficient in nutrients in late summer, fall, and winter. These deficiencies in grass range forage begin in late summer and continue to decrease in nutrients as the grazing forages mature and lose their ability to meet nutritional requirements.

During the last few years, there has been a move to add a special plant species to various portions of the native range by seeding in widely spaced contour furrows or by broadcasting seeds on the range here and there; then covering the seeds by dragging a spike-toothed harrow over the area. Only a relatively small portion of the entire range is needed,

These methods have been used in the grassland areas throughout the Plains states by interseeding portions of their large grassland areas to fourwing saltbush (atriplex canescens) so that this palatable, persistent-leaved shrub can be used to supplement their native range grasslands. This practice came about largely because suitable commercial supplements of phosphorus, vitamin A, and protein were costing well over $100 per ton.

Site conditions are important because they influence the growth characteristics of range plants and thus indirectly affect their nutritive value. Sites also indirectly affect the chemical content of plants and plant parts through soil and plant development, water runoff, intensity of shade, and other environmental factors.

Shrubs and forbs, when they approach maturity, are considerably more leafy on less favorable growing sites. Grasses in advanced stages of growth are, likewise, more leafy on poorer sites than on more favorable sites, but differences between sites are less conspicuous for grasses than for either shrubs or forbs. As a result, plants on unfavorable sites are more palatable, more digestible and, therefore, are more nutritious than plants on the unfavorable sites.

Nutrient content of plants on poor sites during advanced growth stages are generally considered more nutritious because of the finer material and the presence of more leafy material compared to stems. The differences in stem-to-leaf ratios would, to a large degree, account for chemical differences between plants growing on favorable and unfavorable sites since leaves are higher in ether extract, protein, ash, calcium, phosphorus, and nitrogen-free-extract; whereas stems are higher in lignin, crude fiber and cellulose.