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Bouton, J.H. 1996. New uses for alfalfa and other "old" forage legumes. p. 251-259. In: J. Janick (ed.), Progress in new crops. ASHS Press, Alexandria, VA.

New Uses for Alfalfa and Other "Old" Forage Legumes

Joe H. Bouton

  1. TRADITIONAL ROLE OF LEGUMES IN GRASSLAND FARMING
  2. NONTRADITIONAL ROLES FOR ALFALFA
    1. Direct Grazing
    2. Alfalfa Sprouts
    3. Production of Industrial Enzymes
    4. Pulp Production
    5. Fuel and Bioremediation
  3. NONTRADITIONAL ROLES FOR OTHER FORAGE LEGUMES
    1. Grain and Oil Seed Uses
    2. Ornamental Use
  4. CONCLUSION
  5. REFERENCES
  6. Table 1
  7. Table 2
  8. Table 3
  9. Fig. 1
  10. Fig. 2
  11. Fig. 3
Legumes are a large, diverse, and agriculturally important family of plants. Most legume species are trees and shrubs as were their ancestral forms which probably arose about 135 million years ago during the Cretaceous period (Heywood 1971). To humans, however, the most important legume species belong to a small group of herbaceous crop and forage species. The main trait common to these legumes, and the trait of most importance to us, is the ability to fix atmospheric nitrogen and convert it to a useable form for plant growth (Allen and Allen 1981). This fixed nitrogen leads to a higher protein concentration in its various plant parts which in turn enhances our diet and can also be recycled into the environment as a form of fertilizer (e.g. green manure).

The role of grasses as the agricultural centerpiece in the ascent of human civilization is well documented. However, Isely (1982) makes a compelling argument that the archeological records show the domestication of grasses and legumes took place together. His reasons for this conclusion are that species of grasses and legumes as important and currently used food crops are specific to the early agricultural centers: e.g. Glycine with Oryza in China; Pisum and Lens with Triticum and Hordeum in the Middle East; Phaseolus with Zea in the New World; and Sorghum with Vigna in Africa. The exchange of legumes between the Old and New Worlds is also important in the development of our current civilization. In this exchange, Phaseolus, Arachis, and others went from the New World to the Old while Pisum, Lens, Vigna, Glycine, and others went from the Old World to the New.

Although the first written records of legume rotations trace back to the Roman Empire, it was not until the European feudal times that the practice of legume rotation was practiced to a great extent. This practice of using grasses and legumes as both rotation crop and livestock fodder reached its zenith in the ley farming system of Europe (Heath and Kaiser 1985). This practice is still the most economical and environmentally friendly method of farming.

Ley farming, therefore, is simply grassland farming. In today's grassland farming operations, grasses are the dominant plant component with legumes usually comprising a small percentage (<25%) of the sward. There are many methods of grassland farming now practiced throughout the world, but each has common objectives of using grasses and legumes to stabilize land from erosion, improve soil tilth, and allow livestock production either as a rotation with grain crops or as permanent pasture. Whatever method is practiced, the main purposes of legumes are to provide an input of nitrogen through its N2-fixation ability and to upgrade the nutritive value of the grazed pasture.

TRADITIONAL ROLE OF LEGUMES IN GRASSLAND FARMING

The legume family, Fabaceae (Leguminosae), has three subfamilies, Caesalpinioideae, Mimosoideae, and Faboideae (or Papilionoideae), which may be considered confluent (Allen and Allen 1981; Heywood 1971). The Caesalpinioideae are primarily tropical trees and shrubs with some rare herbaceous forms and are probably the most primitive and diversified subfamily. They possess a well developed corolla which may be regular or irregular but not papilionoid. Cassia and Swartzia are representative of the Caesalpinioideae subfamily. The Mimosoideae are mainly trees, shrubs, and woody vines which probably evolved from the Caesalpinioideae. It is characterized by a reduction of the individual flowers and their congregation in compact heads or spikes. These infloresences are their most noticeable feature with the reduced corolla and long, filiform stamens providing it with a showy presence. Mimosa tree, Acacia, and Leucaena are representative types. The Papilionoideae possess the most familiar and important forage and crop species with trees, shrubs, and herbaceous types being commonly found. They are intergradient with the Caesalpinioideae and possess a distinct, papilionoid flowers. Representative genera are Trifolium (clovers), Medicago (medics), Arachis (peanut), Glycine (soybeans), Phaseolus (beans), Lupinus (lupine), and Pisum (pea).

Today, forage legume species are assigned different roles in grassland farming depending on their plant structures and abilities (Ball et al. 1993; Heath et al. 1985). The traditional roles of three major forage legume species along with the characteristics which allow them to assume a particular role are shown in Table 1. From this, one sees that low growing, stoloniferous, and competitive species such as white clover (Trifolium repens L.) are used as a component of grazed pasture while high yielding, upright species such as alfalfa (Medicago sativa L.) are best suited to produce hay and silage in a monocropping system (Blaser et al. 1973).

Another important role for forages is in soil conservation (Shiflet and Darby 1985). In these systems, forage grasses and legumes are used in watershed management and to reduce erosion runoff. This includes their use as cover crops and their role in conservation tillage systems. A legume, when used as a cover crop, can contribute a large amount of nitrogen to the rotation. This is possible because legumes fix nitrogen in symbiosis with Rhizobium bacteria. The different species of Rhizobium are very specific to the legume species they infect (Table 2). This specificity may also be used to classify the different Rhizobium species (Heichel 1985).

The amount of nitrogen produced by cover crop legumes depends on the species used, the success of seed inoculation at planting time, and the total biomass produced. In a study in Georgia, adapted winter cover crops such as hairy vetch (Vicia villosa Roth) and crimson clover (Trifolium incarnatum L.) were found to replace all of the fertilizer nitrogen needed to produce good non-irrigated yields of grain sorghum [Sorghum bicolor (L.) Moench] and almost two-thirds of the nitrogen required for good yields of maize, Zea mays L. (McVay et al. 1989). There was also an improvement in soil physical properties such as water soluble aggregates and water infiltration rate when plots with forage legumes were compared with fallow areas.

What has emerged, therefore, is a traditional or "old" role for temperate forage legumes in grassland and conservation farming. Pertinent questions now being asked are: (1) can some of these species be modified to assume new roles within grassland agriculture, and (2) can forage legumes have major roles outside of traditional grassland agriculture?

The reasons for using old forage legumes in new roles are: (1) farmers and the lay public are already familiar with them, (2) each species is generally served by a successful seed industry, and (3) good cultivars and clear methods of successfully planting and managing them as crops are already available and practiced. These characteristics allow traditional forage legumes to assume any new role quicker and with less problems than a newly introduced species.

NONTRADITIONAL ROLES FOR ALFALFA

The forage legume with the greatest ability to assume many roles both within and outside of traditional grassland agriculture is alfalfa. Its main use is as a hay and silage crop, but when dried, ground, and pelleted, alfalfa is marketed as a dehydrated feed for many animals (Conrad and Klopfenstein 1988). Dehydrated or dehy alfalfa is used as either a protein supplement or the sole dietary ration for cattle, horses, and other small animals such as rabbits, mice, gerbils, etc. which are often used as pets. This dehy industry is now an important economic consideration in many areas of the United States.

Direct Grazing

Since tall, erect species like alfalfa require strict and intense grazing management to insure plant survival (Blaser et al. 1973), its widespread use for direct grazing was not practiced because of fear of losing stands. What was needed to broaden the pasture use of alfalfa was development of grazing-tolerant cultivars. These cultivars would give the livestock producers more flexibility for using alfalfa in many ways.

After World War II, development of grazing-tolerant alfalfa cultivars was a high priority for several alfalfa breeding programs (Heinrichs 1963). The approach for developing grazing cultivars was to identify a trait, which if enhanced in the germplasm, should lead to grazing persistence. In other words, find morphological features which should enhance grazing survival and incorporate these traits into alfalfa cultivars useful for grazing. The main trait identified and researched during that time was creeping rootedness (Heinrichs 1963). This trait is not really a true rhizome, but since a plant possessing it can originate a shoot from the root system, it was felt to provide the plant with a rhizome-like ability to regenerate in stands. However, creeping rootedness was found to be associated with low forage regrowth (Busbice and Hanson 1969), was erratic in its ability to truly creep and regenerate in pastures (Leach 1978), and most important of all, germplasms selected for the trait were not found to be any more grazing persistent than adaptive, hay-type checks (Gramshaw et al. 1982; Smith et al. 1989). Others traits thought to be useful in providing grazing persistence in alfalfa are deep-set crowns (Piskovatski and Stepanova 1980), subsurface budding (Lorenz et al. 1982), broad crowns (Kehr et al. 1963), prolific and nonsynchronous budding (Leach 1977), maintenance of root total nonstructural carbohydrates (Counce et al. 1984), and pest resistance (Leach and Clements 1984; Lodge 1991). However, no data was available which demonstrated that enhancement of any of these traits alone would increase grazing persistence.

Our approach at the Univ. of Georgia to develop and test grazing persistent alfalfa cultivars has been not to concentrate on individual trait enhancement, but to use the grazing animal during development and testing. Early screening of elite, multiple-pest resistant parental germplasms in grazed paddocks (sometimes with grass competition) was found to be important. During each cycle of selection, all germplasm is placed in replicated small plots within grazing paddocks and subjected to intensive, continuous stocking with beef cattle (Bouton et al. 1992; Smith and Bouton 1993). After this initial selection has eliminated material not adapted to the pasture environment, then reselection based on other specific limiting traits can be practiced. The importance of this approach of initially bringing all stresses to bear at the same time was seen with the success of 'Alfagraze' alfalfa (Bouton et al. 1991). 'Alfagraze' was found to possess excellent grazing tolerance (Fig. 1) and yield (Smith et al. 1989) and to be very competitive with grasses when grazed (Smith et al. 1992). Further work showed this same selection approach of achieving grazing tolerance was successful across a range of cultivars and also indicated that resistance to major alfalfa diseases played more of a secondary role in enhancing grazing persistence (Smith and Bouton 1993).

The importance and farmer interest of using alfalfa for grazing was recently seen at a research and industry conference held in Nashville, Tennessee and dedicated solely to grazing alfalfa (Certified Alfalfa Seed Council 1994). That an entire conference would be conducted and so well attended on this specific subject was noteworthy. The common message from both the scientists and producers speaking at the conference was using alfalfa as a grazing crop represented a very economical and efficient system for production of meat, milk, and wool. The current and future development of grazing tolerant cultivars was also highlighted as being instrumental in driving this interest and removing the central management problem of stand loss.

Alfalfa Sprouts

Beginning in the 1970s, alfalfa sprouts found a major use in human diets. When placed on salads or sandwiches, it supplies significant quantities of minerals, protein, and vitamins. In 1988, sprout production represented about 7% of the total alfalfa seed produced in the U.S. (Bass et al. 1988).

Production of Industrial Enzymes

A very important consideration for alfalfa is its adaptability in many areas of biotechnology research. It is easily grown and regenerated in cell culture and is receptive to transformation vectors such as the Ti plasmid (Austin et al. 1993). These traits allow it to be considered as a good plant for engineering with unique genes (e.g. trangenes). In this regard, using transgenic alfalfa containing genes which code for production of industrial enzymes has been proposed by a research group at the Univ. of Wisconsin (Austin et al. 1993, 1995). Most industrial enzymes are produced by microbes in large fermentation vessels, but an alternative system would be to produce enzymes such as alpha-amylase and manganese dependent lignin peroxidase (Mn-P) in alfalfa plants where they could be recovered by extraction. Since alpha-amylase is used in industrial starch processing and Mn-P has potential for lignin degradation and as a biopulping bleaching agent, this approach has great industrial potential. These investigators found that production of these enzymes in transgenic alfalfa was possible and strategies for extraction and processing of "juice" containing these enzymes from the alfalfa plant were also reported. There was a range of expression among transgenic genotypes for Mn-P (Fig. 2A), but in most cases, production of Mn-P suppressed plant yield (Fig. 2B) (Austin et al. 1995). Mn-P was also shown to segregate in sexual progeny from the transgenic genotypes. However, transgenic plants expressing alpha-amylase showed no effect on plant performance. These researchers concluded this technology could be used for inserting genes and extracting products for other value added traits in alfalfa (Austin et al. 1993).

Pulp Production

Another potential use for alfalfa is as a source of pulp for paper manufacturing. In Sweden, a basic feasibility study for an "Agro-Fibre" project designed to investigate the use of high yielding grasses and alfalfa instead of trees for production of pulp for paper was recently reported (Berggren 1993). Since the fiber yield per unit of land of alfalfa and grasses such as elephant grass (Miscanthus) was so much higher than the traditional birch trees, this report was fairly optimistic about the future of Agro-Fibre production. However, it was noted that two major limitations would need to be overcome before their use in production of pulp fiber becomes a reality: (1) the lack of a chemical process capable of economically handling these forages on a large scale and (2) risk factors. Since the investment in a new chemical pulp mill is always high, it is economically safer for an individual company to use well-know processes and raw materials. Therefore, more time and research will be needed.

Fuel and Bioremediation

Finally, new projects are being investigated by USDA and Univ. of Minnesota scientists to use alfalfa as a fuel for generating electricity and as a bioremediation system to remove leached nitrates and prevent nitrate contamination (J. Lamb 1995, pers. commun.). The Minnesota Agri-Power project estimates a potential for converting 700,000 t of alfalfa per year into enough electricity to power a city the size of St. Cloud, for a year. In the proposed process, the alfalfa stems would be separated and used to create gas for fueling a 75-megawatt power plant while the leaves would be processed into dehydrated animal feed.

For the nitrate bioremediation to be successful, it was important to develop an alfalfa which possessed a more extensive rooting pattern yet was ineffective in establishing the nodulation and nitrogen fixation process with Rhizobium bacteria. This extensive rooting, ineffective alfalfa was developed, and when tested, was found to remove 140 and 700 kg ha-1 more nitrogen than reed canarygrass (Phalaris arundinacea L.) and switchgrass (Panicum virgatum L.), respectively (Fig. 3).

NONTRADITIONAL ROLES FOR OTHER FORAGE LEGUMES

Grain and Oil Seed Uses

When first introduced into the United States, soybean (Glycine max Jacq.) was used as a forage crop mainly being grown for hay, silage, fattening pasture for hogs and sheep, and green manure (Probst and Judd 1973). In fact, as late as 1939, approximately 60% of the acreage planted to soybeans in the United States was for forage and 43 of the 108 cultivars available were forage types (Hartwig 1973). However, due to intensive research efforts in breeding and management, soybean is now one of the most important oil seed crops in the United States and the world.

In the United States, lupines (Lupinus spp.) were traditionally grown alone or in mixtures with cereals for grazing and crop rotation until cheap nitrogen and diseases reduced their importance (Hoveland and Townsend 1985). With the advent of "sweet" lupines which show reduced alkaloids in the seed, these species have assumed a new role as a grain legume in Australia, New Zealand, and Europe. Much like the above story for soybeans, white lupine (Lupinus albus L.) is currently being researched as a grain crop for the southern United States (Noffsinger and van Santen 1995). White lupine is reported to possess grain characteristics which compare very favorably with other grain legumes as shown in Table 3 (E.S. van Santen 1995, pers. commun.). Its yield and production also take place in the winter months allowing it to be produced during the "off-season" of a crop like soybean.

Ornamental Use

Due to their showy and colorful flowers, some forage legumes have potential for use as ornamentals. Recently, two white clover germplasms were described which possess genes having potential in this area. One germplasm showed corolla color change from white to red (Pederson and McLaughlin 1995) while another demonstrated expression of a large red leaf mark (Pederson 1995). Combining these two traits should give a red flowered, red leaved but still stoloniferous plant which has great potential for ornamental use as both hanging baskets and perennial ground cover.

The inherent colorful flowers of crimson clover, birdsfoot trefoil (Lotus corniculatus L.), and crown vetch (Coronilla varia L.) along with their ability to reseed and grow under stressful conditions has made them a natural for low-maintenance highway beautification (Grant and Marten 1985; Hoveland and Knight 1985; Hoveland and Townsend 1985). Other legume species with these same characteristics also have great potential as low maintenance ornamentals.

CONCLUSION

The forage legume with the greatest ability to assume many roles both within and outside of traditional grassland agriculture is alfalfa. It is the most widely planted and used forage legume species in the world, yet is an excellent model system for biotechnology especially in the area of genetic transformation. Its traditional use as hay, silage, and dehy crop has recently been expanded with the release of grazing tolerant cultivars to include a larger role as pasture for direct livestock consumption. Its use as a grazing crop has been one of the driving forces in moving some sectors of the United States livestock industry into more efficient pasture systems with a legume base. At the same time, nontraditional roles such as sprouts for salads and nutritional supplements for human diets have increased for alfalfa. It is also being investigated as a fuel for use in generating electricity, a bioremediaton system for removal of harmful nitrates, a source of pulp for paper manufacturing, and a "factory" for production of industrial enzymes.

Other forage legumes are finding a use as ornamentals. Recent successes with developing white clover to be more colorful and showy for perennial ground cover and captilizing on the inherent colorful flowers of several species for low-maintenance highway beautification demonstrate what can be accomplished.

Soybean was an important forage crop in the United States. However, due to intensive research efforts, soybean is now mainly used as an oil seed crop. Similarly, the recent development of lupines into a grain crop further demonstrates that traditional forage legumes can become important grain and oil seed crops.

The reasons for using old forage legumes in new roles are: (1) farmers and the general public are already familiar with them, (2) each species is generally served by a successful seed industry, and (3) clear methods of successfully planting and managing them as crops are already available and practiced. These characteristics allow old legumes to assume any new role quicker and with less problems than a newly introduced species.

REFERENCES

Table 1. Widely used forage legume species, their major role in forage production, and the characteristics which allow this role.

Species Role Characteristics
Alfalfa, Medicago sativa L. Hay and silage Crown former, high yield, upright growth for easy harvesting, difficult to establish in grasses and non-tolerant of intensive grazing.
Red clover, Trifolium pratense L. Hay and pasture Crown former, easy to establish in grasses, high yield, some tolerance to intensive grazing.
White clover, Trifolium repens L. Pasture Stoloniferous, easy to establish in grasses, competitive, tolerant of intensive grazing.

Table 2. Rhizobium species and their legume host species (from Heichel 1985).

Species Legume host
Rhizobium leguminosarum biovar trifolii Clovers
Rhizobium leguminosarum biovar phaseoli Bean
Rhizobium leguminosarum biovar viceae Vetch
Rhizobium meliloti Alfalfa, sweetclover
Rhizobium loti Trefoil, lupine, chickpea
Bradyrhizobium japonicum Soybean, cowpea, peanut

Table 3. Grain characteristics of white lupine compared with lima bean and soybean (E.S. van Santen 1995, pers. commun.).

Crop 1000 Seed wt. (g) Protein (%) Oil (%)
White lupine 245-609 34-45 10-15
Lima bean 450-2000 19-30 1-3
Soybean 120-180 39-42 20-22

Fig. 1. Survival of 'Alfagraze' alfalfa, a grazing tolerant cultivar, when compared to an intolerant cultivar after intensive grazing.

Fig. 2. (A) Manganese-dependent lignin peroxidase (Mn-P) activity of different alfalfa transgenic genotypic classes and (B) the relationship of Mn-P with plant yield (Austin et al. 1995).

Fig. 3. Cumulative nitrogen removal of an ineffectively nodulating alfalfa compared to reed canarygrass and switchgrass (J. Lamb 1995, pers. commun.).

Last update August 15, 1997 aw