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Forage legumes belong to the legume or bean family (Fabaceae or Leguminosae). Members of this large family (more than 16,000 unique species) are characterized by having seeds born in pods, compound leaves with multiple leaflets, and root associations with bacteria that allow for symbiotic nitrogen fixation. Legumes produce seeds and foliage that are usually rich in protein with a desirable amino acid composition.
Legumes are second only to grasses in importance to human and livestock nutrition. The legume family includes well known, large seeded plants like beans, peas, peanut and soybean that are mostly used for seed (called a pulse) production, as well as small seeded legumes such as the clovers, birdsfoot trefoil and alfalfa that produce herbage for harvesting as forage.
Edible legume grains have been domesticated and used by civilizations for centuries as a dietary source of protein. For example, soybean, native to eastern China, was described more than 5,000 years ago as one of the five sacred grains vital to their civilization. Historical records indicate similar early importance for legume forage crops. Roman records note that alfalfa was introduced into Greece from Persia, by the Medes about 500 years before the common era, as a feed for chariot horses.
Forage legumes continue to be valuable crops throughout the world and especially in the north central region of the United States. They are integral components of sustainable agricultural systems, providing high quality livestock feed, nectar, seed, green manure, and soil cover.
In the north central region of the United States, forage legumes are mostly used for livestock feed and are an essential component of most cattle and sheep rations. They are biologically harvested by grazing livestock, or mechanically harvested following drying and stored as hay or silage.
Because of their growth habits, forage species vary in their suitability for use as grazing or hay. For example, the prostrate growth habits of kura clover and white clover make them especially adapted to grazing systems, whereas the upright growth habit of alfalfa makes it better suited as a hay crop. When used for grazing, forage legumes are often grown in mixture with perennial grasses such as smooth bromegrass, timothy, and Kentucky bluegrass.
Legumes are also well known as soil building plants, and their use for that extends far back in history. They enhance soil quality by adding organic matter, and improve soil structure and water infiltration. Earthworm populations are usually greater in fields planted to perennial forages than in fields planted to annual row crops.
Early Roman writers on agriculture, such as Pliny and Virgil, recognized the value of using legumes like peas and beans and vetches to replace manure. Later, in Colonial America, when farmers were exhausting their soils by continuously cropping tobacco, wheat and corn, Thomas Jefferson promoted the use of red clover in rotations for improving the soil.
The terms annual, biennial, or perennial are used to describe the time required for legumes to complete their life cycles. An annual legume like soybean germinates from seed, flowers, sets seed and dies within one growing season. In contrast, once established, perennial plants like alfalfa and kura clover live for three or more years, and have potential to set seed each year. An intermediate group of legumes, biennials such as sweetclover, live for two years. These grow vegetatively the first year, and flower and die in the second year.
Of the three life cycle types of legumes, perennials are considered to be the most valuable for the environment. They provide continuous ground cover, recycling of nutrients, and long-term carbon storage. The use of perennials also eliminates the need for annual reseeding.
Properly managed forage legumes are nitrogen self-sufficient. These plants can achieve vigorous growth without the nitrogen fertilizers that are required for grasses. This self-sufficiency is achieved through the process of biological nitrogen fixation.
Legumes convert otherwise unusable atmospheric nitrogen (N2, which is 78 percent of the earths atmospheric gases by volume) into ammonia and ultimately into nutritionally valuable plant protein. Symbiotic nitrogen fixation is a partnership between the legume plant and Rhizobia bacteria that actually perform the nitrogen conversion. In exchange, the legume plant supplies nutrients and energy the bacteria require for their growth and function.
Rhizobia that are present in the soil, or supplied with inoculum to the seed, infect plant root hairs and stimulate development of tumor-like nodules on the roots (figure A). Nodule shapes vary and can be elongated lobes as found in roots of alfalfa and the clovers, or round like those found on birdsfoot trefoil roots. Lobed nodules will overwinter and fix nitrogen for more than one growing season while round nodules die and reform on roots each year. A specific Rhizobium is required for a legume species. For example, bacteria infecting and nodulating kura clover will not effectively nodulate alfalfa. Upon dissection, active regions of nodules will be observed to contain a pink pigment, leghemoglobin, that is indicative of active nitrogen fixation.
Nitrogen fixation by healthy plants causes legume foliage and seed to both be consistently rich in protein. In addition to requiring no synthetic or organic nitrogen fertilizer for growth, protein and nitrogen rich legume plants can be plowed under to supply nitrogen to subsequent crops in rotations. Productive stands of legumes like alfalfa usually meet all the nitrogen requirements for a subsequent corn crop. In a traditional Midwestern crop rotation, a legume can be grown and harvested for two or more years, and then incorporated into the soil and followed by a corn crop.
In managing a legume like alfalfa, red clover or sweetclover as a green manure crop for plow down, the amount of fixed nitrogen contributed to the soil depends on the quantity of nitrogen-rich forage incorporated. Alfalfa herbage typically contains from 3 to 3½ percent nitrogen depending on its maturity, while roots contain only about 2½ percent nitrogen. That is why green manure systems usually allow a significant amount of herbage to accumulate in the fall before incorporation.
Nitrogen fixed by legumes also helps grasses growing in mixture with them. The nitrogen transferred represents, on average, about half (but ranges from 20 to 80 percent) of a grasses needs, but the amount can be greatly influenced by soil nitrogen status and the legume composition of the stand. Legume nitrogen is transferred by root exudation, decomposition of decaying leaves, roots, and nodules, and by mycorrhizal fungi growing in association with grass roots. Transfer of legume nitrogen to grasses also occurs when nitrogen excreted in the urine or feces of grazing livestock is taken up by pasture grasses.
Legumes vary in the amount of atmospheric nitrogen they can fix. This variation is in part due to the relative effectiveness of the symbiosis between plants and the bacteria. Efforts are underway to select bacteria that are more effective at nitrogen fixation. The range of fixation shown in table 1 for a given legume species can be due to variations in soil and environmental conditions where measurements were taken. For example, nitrogen fixation is likely to be less for a legume grown on soil naturally high in nitrogen because legumes will fix less atmospheric nitrogen if soil nitrogen is available.
Forage legume species also differ both in their adaptation to soil and climatic conditions and in their susceptibility to insect damage and diseases. As a result, each of the various types of legumes have characteristics that often suit it best for specific uses (tables 2 and 3). This is why being able to identify, use and manage the perennial forage legumes commonly grown in the north central region can be valuable to a livestock producer.
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