Alfalfa (Medicago sativa L.) is grown on more than 15 million hectares of in North America (Barnes et al. 1988) and in excess of 32 million hectares world wide (Michaud et al. 1988). The true clovers (Trifolium species) are widely distributed and the total area on which they occur in the United States is believed to exceed that of alfalfa (Taylor 1985). These crops admirably meet the legume pasture, hay, and silage production requirements of temperate humid and subhumid regions. They are supported by smaller plantings of species of Coronilla, Onobrychis, and Lotus. Commercial cultivars of Indigofereae, Aeschynomeneae, Desmodieae, and Phaseoleae assume similar importance in tropical and subtropical America. A greater diversity of forage legume species is used and researched in southern than in northern North America. However, soil acidity, drought, cold, and deleterious diseases and insects restrict the productivity and distribution of many species.
Plant scientists are investigating the potential of previously unexploited legume species where environmental and biological stresses constrain the use of more conventional forage crops or where these species are better suited to the needs of sustainable agriculture. Alternative, more specialized crops and cultivars are being bred to meet very precise needs. The phrase "special purpose legumes" historically also has been used to indicate those commercially successful forage crops seeded on relatively small total land areas compared to alfalfa and the clovers. Special purpose forage legume research activities may be categorized as: domestication of previously unexploited species, or extension of the adaptive range or use of presently cultivated species. Previous reviews by Allen and Allen (1981), Duke (1981), Helyar (1984), Knight (1984a, 1987), Lancashire (1984), Pederson and Knight (1984), Rotar and Kretschmer (1985), Thro and Shock (1987), and A.M. Thro (1988, personal communication) were relied upon extensively in the development of this commentary
In cereal, pulse, and horticultural crops, domestication by neolithic plant breeders resulted in changes in gene frequencies which increased the average size of seeds, storage organs, or whole plants; loss of seed dormancy mechanisms; widened environmental adaptation; and shortened life cycles (Harlan 1975). Because pasture ecosystems are more similar to natural ecosystems than are monospecific crop production systems, such changes often are not pronounced in forage legumes and, as is the case for many summer annual species, may not be detectable. Pasture legume cultivars frequently are only a few generations removed from the wild populations from which they originated. A number simply are increases of ecotypes which best fit the agronomic requirements for stand establishment, survival, and production of seed stocks. Nevertheless, 48 cultivars of special purpose forage legumes were registered with the Crop Science Society of America between 1945 and 1988 (Fig. 1).
The domestication of cicer milkvetch (Astragalus cicer) essentially has been completed and improved germplasm and cultivars have been released for cool-season pasturage (Johnston et al. 1971, 1975; Kenno et al. 1987; Townsend 1980, 1985, 1987). A promising related species, sicklepod milkvetch (Astragalus falcatus), is being selected for reduced levels of nitro-toxins (Rumbaugh et al. 1985). Breeders in the People's Republic of China are domesticating A. adsurgens (Huang 1981) and A. melilotoides and limited germplasm is available in other countries. This genus, which includes more species than any other in the Leguminosae, encompasses much material with agronomic potential but none of the North American germplasm is seriously being investigated as a potential forage crop.
The needs of sustainable agriculture, coupled with a desire to reduce the amounts of fertilizers and herbicides currently applied for crop production, will lead to increased usage of species such as black medic (Medicago lupulina) in crop production systems (Cramer 1987). Spring wheat following black medic outyielded spring wheat receiving 168 kg ha of nitrogen per acre. Spring wheat following black medic outyielded wheat after chemical fallow by 92 percent. Seed of an improved cultivar is commercially available (Sims et al. 1985).
Goat's rue (Galega orientalis), an unselected forage legume originating in regions with a Mediterranean climate has been found to grow well and to persist in the northern European conditions of Finland and the Soviet Union (Varis 1986) and may have potential at similar latitudes in North America. Yield levels exceeding those of red clover have been measured. Mass selection for low alkaloid and fiber contents and for greater seed yield is in progress. It should be noted that a closely related species, G. officinalis, is unpalatable and has become a minor weed in one western state (Tingey 1971).
Societal requirements for the use of naive species in range improvement and reclamation seedings on public lands has stimulated research with previously underutilized legumes. Anticipated restrictions on the harvest of seeds from public wildlands will warrant the development of agronomic methods for seed production of selected ecotypes of these species by private firms. This should lead to more stable supplies of higher quality seed, lower prices, and more extensive planting of legumes such as Northern Sweetvetch (Hedysarum boreale) in the western United States (Rumbaugh and Townsend 1985) and of Glycyrrhiza lepidota (American licorice) in the northcentral region (Boe and Wynia 1985, Boe et al. 1988). An induced tetraploid of the native spanish clover (Lotus purshianus) may have similar value for California rangelands (Maclean and Grant 1986)
Sulla Clover (Hedysarum coronarium), a Mediterranean species, is a commercial success in Italy and has been evaluated in North America (Allen and Allen 1981). It has not become popular and a breeding effort apparently has not been initiated. Two Asiatic species, H. mongolicum and H. scoparium, used for soil conservation and reclamation in China (Li and Hovin 1981) are being evaluated in Utah. All species of this genus that have been tested contained condensed tannins and are believed to be bloat safe when grazed by ruminants (M.D. Rumbaugh unpublished).
Many leguminous shrubs have value as wildland browse species, especially in regions of consistent droughtstress. A few of these are being domesticated but breeding has been limited to ecotype evaluation and selection. Shrubby medic (Medicago arborea) plantations produced 3,500 kg of dry matter/ha-yr in northern Africa (Otsyina and McKell 1985). Evaluation of this Mediterranean region native has been initiated in the southwestern United States.
Ecotypes of a number of herbaceous species have been tested and selected specifically for soil conservation and reclamation purposes. Species indigenous to the southwestern United States are Indigofera leptosepala and Mimosa strigillosa. Lathyrus japonicus, L. latifolius, and Lotus crassifolius also are being examined critically by the Soil Conservation Service in the Pacific Northwest. In the same region, a naive lupine, Lupinus albicaulis, recently has been domesticated. The cultivar 'Hederma' has proved to be useful in reforestation, roadside, coastal, wildlife, and critical area plantings as well as for green manure on cropland (Oregon Agr. Expt. Sta. 1982). Lathyrus maritimus L. and Lupinus nootkatensis Donn. are being examined for the same usages in Alaska (Knight 1987).
Ruby Valley pointvetch (Oxytropis riparia) is an excellent example of a wild species being domesticated to meet very exacting edaphic requirements (Delaney et al. 1986, Hicks et al. 1987). It is an introduced species found on moist saline bottom lands with phosphorus deficient soils. Approximately 486,000 ha of such high elevation meadowlands are used for hay production in the western United States. This legume is nontoxic and compares favorably with alfalfa in nutritional value (Williams and Molyneux 1988).
Numerous Aeschynomene species are naive to tropical America (Allen and Allen 1981). American jointvetch or aeschynomene (A. americana) is a non-bloat-inducing, self-reseeding summer annual legume of value to both livestock and wildlife in the southern United States (Thro and Shock 1987). Seed of 'Florida Common' aeschynomene is an item of commerce and widely distributed. Evaluation and/or breeding programs have been initiated in several states and release of the first cultivar is projected to be within 1 to 3 years (e.g. Quesenberry et al. 1985, Thro et al. 1987). A number of Desmodium species have shown promise as suitable substitutes for alfalfa and clover in subtropical and tropical areas (Allen and Allen 1981). Desmodium barbatum, D. discolor, D. canum, D. heterocarpon, D. intortum, D. latifolium, are being investigated and are tolerant to acid soils and a warm climate. A cultivar of D. heterocarpon able to withstand heavy grazing in Florida has been released (Kretschmer et al. 1982). Resistance to root-knot nematode (Meloidogyne spp.) is being investigated (Quesenberry and Dunn 1987). The genetic bases for breeding programs to increase symbiotic dinitrogen fixation by at least one species, spanish clover (D. sandwicense), have been established (Pinchbeck et al. 1980).
The use of alyce clover (Alysicarpus vaginalis) for summer forage is increasing in the mid-south United States. It is a self-reseeding annual species. Another reseeding annual legume which is being adapted in this region is hairy indigo or Indigofera hirsuta (Baltensperger et al. 1985). A "soft" seeded line has been identified which should be less of a weed problem in rotations with row-crops. A breeding program is active in Florida (Baltensperger et al. 1987). Rhizoma peanut (Arachis glabrata) is a perennial forage legume having value as a hay, silage or pasture crop in the most southern part of the United States. Although this species is best adapted to humid tropical, subtropical or mild temperate climates with long wet, warm seasons, drought-resistant cultivars, 'Florigraze' (Prine et al. 1986) and 'Arbrook', have been released. Virginia Synthetic A crownvetch (Coronilla varia) germplasm was selected to be tolerant of acid soils (Miller 1980). Tifhardy-1 germplasm of Desmodium canum has proved to be much more winterhardy than most populations of this perennial legume from Brazil when grown at Tifton, Georgia (Miller and Wells 1981). Greenleaf desmodium (D. intortum) seed is imported from Australia for use in pasture experiments in the southern United States. It does not set seed at these latitudes. It and D. uncinatum over-wintered as far as 30°50' N and were compatible with bermudagrass (Cynodon dactylon) when tested for summer forage production in Louisiana (Thro and Shock 1987). A perennial tropical legume crop from Brazil and Australia, Lablab purpureus, has proved to be competitive as a warm-season annual forage species as far north as Tennessee (Fribourg et al. 1984, Miller and Wells 1983). Leucaena leucocephala shrubs or trees provide ruminant feedstuffs and are valuable for soil conservation and improvement in Hawaii and are being evaluated in several southern states (Allen and Allen 1981, Knight 1984a). Phasey bean (Macroptilium lathyroides), like aeschyromene, is wild or naturalized throughout the Caribbean. This short-lived perennial also occurs in Florida and Texas and is being tested in Louisiana (Thro and Shock 1987). It and a related species, M. atropurpureum, appear to merit more attention by breeders (Kretschmer et al. 1985). Stylo (Stylosanthes guianensis) and Caribbean stylo (S. hamata) are tropical and subtropical legumes for which the adaptive ranges have been somewhat expanded by selective breeding (Brolmann 1987a, 1987b). Climatic and edaphic factors critical in the persistence of stands of several annual legumes of tropical origin are being investigated at locations in peninsular Florida (Pitman et al. 1985). Bigflower vetch (Vicia grandiflora), a more temperate species, is being evaluated from Connecticut to Florida.
The very successful role of annual medics (Medicago spp.) in Australian agriculture has stimulated research with these species in regions which do not have a Mediterranean climate (Rumbaugh 1986). Some, primarily M. polymorpha, already are well established in certain localities with alkaline soils in south Texas (Ocumpaugh 1987). It is quite possible that the research in progress will prove to be successful and that medic usage will expand. Many of these species can be used in a manner similar to that for which annual and biennial sweetclovers (Melilotus spp.) historically have been used. Others may prove useful as reseeding legume components of permanent pastures (Rumbaugh and Johnson 1986).
Alfalfa cultivars have been developed especially for use in subhumid and semiarid pastures (Rumbaugh 1982, Rumbaugh et al. 1982) as well as for the humid subtropics and tropics (Horner and Ruelke 1981). 'Peace' alfalfa (M. sativa) was selected for northwestern Canadian conditions (McKenzie et al. 1981) and 'Anik' alfalfa (M. falcata) is particularly well adapted north of latitude 57°N (Pankiw and Siemens 1976). Because of its extensive lateral root system, the latter cultivar is suited for the ecological repair of damaged sites in the far north. These usages required breeding programs with very specific objectives and criteria and the resulting cultivars considerably extended both the adaptive range and the potential usage of this valuable forage crop. Similar results are anticipated from genetic selection of alfalfa for acid soil and aluminum tolerance (Bouton et al. 1986).
Special purpose cultivars of major forage species also have been bred for particular applications such as increasing soil fertility and tilth. 'Nitro' alfalfa (M. sativa) was designed to maximize symbiotic fixation of atmospheric nitrogen (Barnes et al. 1988). The demands of sustainable agriculture are expected to increase the prominence of such research. Selection of alfalfa for germination, establishment, and production under restricted moisture availability (Currier et al. 1987) and on saline sites (Dobrenz et al. 1983) has progressed to the point where improved germplasm has been released.
Low-coumarin cultivars of white- and yellow-flowered sweetclover (Melilotus alba and M. officinalis) have been available to farmers and ranchers for a number of years (Goplen 1971, 19811; Smith 1964). These eliminated the risk of toxicity to animals fed spoiled sweetclover hay or silage and increased palatability when used as a pasture legume.
The merit of faba bean (Vicia faba) as an irrigated silage crop is being evaluated in the Intermountain Region of the United States (Knight 1987) and as a component of binary mixtures with cereals for the same purpose in western Canada (Berkenkamp and Meeres 1987). Cultivars of field pea (Pisum sativum) developed specifically for forage production may be used for the same purposes as well as for silage (Robinson 1986). Canadian scientists are developing new cultural procedures for the use of annual legumes such as black lentil (Lens culinaris), Tangier flatpea (Lathyrus tingitanus), chickling vetch (L. sativus), and field pea (Pisum sativum) as an alternative to fallow in winter wheat production (Biederbeck and Slinkard 1988). Early seedings may be used to improve soil fertility or to control weeds. Late seedings may be used to trap snow and for control of soil erosion.
|Species||Common name||Use or potential use||Life formz|
|A: Wild species being domesticated|
|Aeschynomene americana L.||American jointvetch aeschynomene||Pasture||A|
|Astragalus adsurgens Pall.||Wind-resistant loco||Pasture, silage||P|
|Astralagus cicer L.||Cicer milkvetch||Semiarid and subhumid pasture||P|
|Astragalus falcatus Lam.||Sickle-pod milkvetch||Semiarid pasture||P|
|Astragalus melilotoides Pall.||Pasture, hay, silage||P|
|Desmodium heterocarpon (L.) DC.||Carpon desmodium||Pasture||P|
|Galega orientalis Lam.||Goat's rue||Pasture, hay, silage||P|
|Glycyrrhiza lepidota Pursh||American licorice||Pasture, soil conservation||P|
|Hedysarum boreale Nutt.||Northern sweetvetch||Semiarid foothill pasture||P|
|Hedysarum coronarium L.||Sulla clover||Soil fertility, hay||B, P|
|Hedysarum mongolicum Turcz.||Yang cai||Soil conservation||P|
|Hedysarum scoparium Fisch. et Mey||Sweetvetch||Desert dune stabilization||P|
|Indigofera leptosepala Nutt.||Western indigo||Soil conservation||P|
|Lathyrus japonicus Wild.||Purple beachpea||Beach dune stabilization||P|
|Lathyrus latifolius L.||Perennial pea||Soil reclamation||P|
|Lathyrus maritimus L.||Beach pea||Beach dune stabilization||P|
|Lotus crassifolius Pers.||Big deervetch||Soil conservation, reforestation|
|Lotus purshianus (Benth.) Clem. & Clem.||Spanish clover||Dryland pasture||A|
|Lupinus albicaulis Dougl.||Sickle-keeled lupine||Soil conservation||P|
|Lupinus nootkatensis Donn.||Nootka lupine||Upland pasture||P|
|Medicago arborea L.||Shrubby medic||Browse||P|
|Medicago lupulina L.||Black medic||Pasture, soil fertility||P|
|Mimosa strigillosa Torr. & Gray||Herbaceous mimosa||Soil conservation|
|Oxytropis riparia Litv.||Ruby Valley pointvetch||Pasture||P|
|B: Extension of adaptive range or use|
|Alysicarpus vaginalis DC.||Alyce clover||Southern summer pasture||A|
|Arachis glabrata Benth.||Rhizoma peanut||Hay, silage, pasture||P|
|Coronilla varia||Crownvetch||Pasture on acid soils||P|
|Desmodium canum (J.F. Gmel.) Shinz & Thell.||Kaimi clover||Pasture, reforestation||P|
|Desmodium intortum Urb.||Greenleaf desmodium||Pasture||P|
|Desmodium uncinatum (Jacq.) DC.||Silverleaf desmodium||Pasture||P|
|Indigofera hirsuta L.||Hairy indigo||Pasture, cover crop||A|
|Lablab purpureus (L.) Sweet||Hyacinth bean||Pasture||A|
|Lathyrus sativus L.||Chickling vetch||Green manure||A|
|Lathyrus tingitanus L.||Tangier flatpea||Green manure||A|
|Lens culinaris Medik.||Black lentil||Green manure||A|
|Leucaena leucocephala (Lam.) deWit.||Koa haole||Pasture, soil fertility||P|
|Macroptilium lathyroides (L.) Urb.||Phasey bean||Pasture, hay||A|
|Macroptilium atropurpureum (DC.) Urb.||Siratro||Pasture||P|
|Medicago spp.||Annual medics||Pasture, soil fertility||A|
|Medicago sativa L.||Alfalfa||Nitrogen fixation, grazing||P|
|Medicago facata L.||Alfalfa||Northern hay production||P|
|Melilotus alba Desr.||White sweetclover||Soil fertility, hay, pasture||B|
|Melilotus officinalis (L.) Lam.||Yellow sweetclover||Soil fertility, hay, pasture||B|
|Pisum sativum L.||Field pea||Forage, feed grain, green manure||A|
|Stylosanthes guianensis (Bubl.)||Sw. stylo, pencilflower||Pasture||P|
|Stylosanthes hamata (L.) Taub.||Carribbean stylo pencilflower||Pasture, dune control||P|
|Vicia faba L.||Faba bean||Silage||A|
|Vicia grandiflora Scop.||Bigflower vetch||Pasture, cover crop||A|
|Fig. 1. Frequencies of alfalfa, clover, and other forage legume cultivar registration articles published in the Agronomy Journal and Crop Science from 1940 to 1985.|