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Classes of Forage: Legumes

Legumes are plant species of the Leguminosae family. Examples of legumes include alfalfa and clovers. Legumes are plant species characterized by the ability to fix nitrogen. Nitrogen fixation is a process in which rhizobia microorganisms in the nodules associated with the root system of the legume convert atmospheric nitrogen in the form of nitrogen gas (N2) to ammonia (NH3). Ammonia can be used by the plant to fulfill the nitrogen requirement of the legume plant. As a result, legumes do not require application of a source of nitrogen. As the relationship between rhizobia and legumes benefits both species, the relationship is described as symbiotic.

Figure 8.6 illustrates root nodules of a legume.

In general, the nutritional value of legumes varies from the nutritional value of grasses. In general, legumes are higher in crude protein, digestible energy, calcium, and magnesium compared to grasses. Legumes and grasses are often planted together in a forage system to improve the nutritional value of the forage and increase the productivity of the forage system. In addition, the legumes contribute nitrogen to the forage system and reduce the nitrogen requirement for the forage system. As with the grass species, there are legume species adapted to a range of environmental conditions. To learn more about individual legume species, click on the following link: http://www.agry.purdue.edu/ext/forages/forageid.htm.

In the world and in the U.S., the most popular forage legume is alfalfa.

Figure 8.7 and 8.8 illustrate an alfalfa field and an alfalfa plant, respectively.

In the U.S., in 1999, more than 84.3 million tons of alfalfa hay was produced. Varieties of alfalfa have been developed to grow in a variety of climatic and soil conditions given adequate water is provided to the plant. Alfalfa prefers well-drained, fertile, slightly alkaline, deep, loam soils. Alfalfa is a productive forage crop. Alfalfa is fed in the form of hay, silage, pasture, and dehydrated meal. Alfalfa may be included in the ration of all domestic species. Alfalfa is an especially important feedstuff for ruminants with high nutrient requirements such as lactating dairy cattle and late-gestation sheep. Alfalfa is a source of fibrous carbohydrates, protein, energy, minerals such as calcium, phosphorus, and magnesium, and vitamins. Alfalfa is a palatable feedstuff. Quality, yield, and longevity of stand are dependent on time of harvest.

Figure 8.9 illustrates the general relationship between forage quality and yield for forage grasses and legumes.

As alfalfa matures, quality of the forage decreases and yield of the forage increases. The change in nutritional value is a result of the changes in plant components. As the legume matures, a greater proportion of the plant is stems and a lesser proportion of the plant is leaves. In comparison to the stem, the leaves are higher in protein, lower in fiber, and higher in digestibility of nutrients. As a result, the earlier harvested (i.e. immature) alfalfa has a relatively higher crude protein content, a lower structural carbohydrate content, and a higher digestibility, and lower yield. Table 7-3 on page 121 of the text describes the effect of stage of maturity on the digestibility of alfalfa. In addition to the effect on yield, earlier harvest will decrease the longevity of a crop or stand of alfalfa. Other management practices that deplete the carbohydrate reserves in the root system such as overgrazing will also reduce the longevity of the stand. Varieties of alfalfa have been developed to allow earlier harvest of a higher quality forage and a reduction of the detrimental effect on crop longevity. Cutting frequency also has an impact on forage yield and quality, weeds, and longevity of stand. Table 7-4 on page 122 of the text describes the effect of different alfalfa cutting frequencies on forage yield and quality, weeds, and stand longevity.

The following is a brief discussion of the additional forage legumes used as feedstuffs. For more information, please refer to the following links: http://forages.orst.edu/main.cfm?PageID=191 and http://www.agry.purdue.edu/ext/forages/forageid.htm.

In North America, the most widely grown clover is red clover. The nutritional value of red clover is similar to alfalfa; however red clover is less productive than alfalfa. Red clover is suitable as a fresh or preserved forage. An important forage legume for pastures is white clover. White clover is relatively resistant to grazing and complements the nutritional composition of grass forages. Alsike clover is an important forage legume. It is an intermediate between red and white. Sweetclover is another variety of clover. Subterranean clover or sub clover is an annual adapted to milder climates. An additional annual clover is crimson clover.

Additional temperate forage legumes include trefoils, lespedezas, and vetch. The primary trefoil is birdsfoot trefoil. In comparison to alfalfa, birdsfoot trefoil is more tolerant to infertile, poor-draining, and acidic soils; however the productivity and quality of forage are also lower than alfalfa. Trefoil is moderate to high in tannins. At a high level, tannins are detrimental. However, at lower levels tannins increase the proportion of rumen by-pass protein and also reduce the incidence of bloat. Lespedezas are legume forages grown in the southeastern region of the country. The nutritional values of the annual species are greater than the values of the perennial species. Multiple vetch species are grown as forage legumes in the U.S.

The primary issue to address with legume species as feedstuffs is the potential for bloat. Bloat is a nutritional disorder characterized by the excess accumulation of gases in the tract. In a ruminant, bloat may be identified by a distention of the reticulorumen. Legumes are high in readily available carbohydrates and soluble proteins. Fermentation of the carbohydrates produces large quantities of gases. Ordinarily, ruminants will eructate the gases, however, with bloat conditions, ruminants are not able to eructate the gases because of the establishment of a stable foam in the rumen. The digestion of the soluble proteins is associated with the production of the stable foam. As a result, the gases accumulate and the condition described as bloat will occur. Bloat can lead to death. Treatment of bloat includes release of the gases and use of an antifoaming agent. The incidence of bloat is reduced by feeding legumes in the form of hay versus fresh forage. Reduction of the incidence of bloat may also be achieved by the use of low-bloat varieties of legumes, prevention of the initial overconsumption of legumes, and the use of an antifoaming agent such as poloxalene. The incidence of bloat varies by legume species.

Additional components that may cause issues are coumarin, tannins, alkaloids, saponins, and phytoestrogens. Coumarin is a component of some clovers, especially sweetclover. Tannins are present in lespedezas and trefoils. Alkaloids are present in lupines. Saponins are present in alfalfa. Phytoestrogens are present in a number of legumes; especially associated with sub clover. Varieties of these legumes and prevention techniques have been developed to reduce the impact of these compounds.

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