Alfalfa is a cool season perennial legume living from three to twelve years, depending on variety and climate. It resembles clover with clusters of small purple flowers. The plant grows to a height of up to , and has a deep root system sometimes stretching to . This makes it very resilient, especially to droughts. It has a tetraploid genome. The plant exhibits autotoxicity, which means that it is difficult for alfalfa seed to grow in existing stands of alfalfa. Therefore, it is recommended that alfalfa fields be rotated with other species (e.g. corn, wheat) before reseeding.
Like other legumes, its root nodules contain bacteria, Sinorhizobium meliloti, with the ability to fix nitrogen, producing a high-protein feed regardless of available nitrogen in the soil. Its nitrogen-fixing abilities (which increases soil nitrogen) and use as animal feed greatly improved agricultural efficiency. (The nitrogen comes from the air, which is 78 percent molecular nitrogen.)
Alfalfa is widely grown throughout the world as forage for cattle, and is most often harvested as hay, but can also be made into silage, grazed, or fed as greenchop. Alfalfa has the highest feeding value of all common hay crops, being used less frequently as pasture. When grown on soils where it is well-adapted, alfalfa is the highest yielding forage plant.
Alfalfa is one of the most important legumes used in agriculture. The US is the largest alfalfa producer in the world, but considerable area is found in Argentina (primarily grazed), Australia, South Africa, and the Middle East. Known as Kuthirai Masal in Tamil, alfalfa is mostly grown in the Coimbatore district of Tamil Nadu, southern India.
The leading alfalfa growing states (within the U.S.A.) are California, South Dakota, and Wisconsin. The upper Midwestern states account for about 50% of US production, the Northeastern states 10%, the Western states 40% and the Southeastern states almost none. Alfalfa has a wide range of adaptation and can be grown from very cold northern plains to high mountain valleys, from rich temperate agricultural regions to Mediterranean climates and searing hot deserts.
Its primary use is as feed for dairy cattle (Because of its high protein), and secondarily for beef cattle, horses, sheep, and goats. Humans also eat alfalfa sprouts, in salads and sandwiches, for example. Tender shoots are eaten in some places as a leaf vegetable. Human consumption of fresh mature plant parts is rare and limited primarily by alfalfa's high fiber content. Dehydrated alfalfa leaf is commercially available as a dietary supplement in several forms, such as tablets, powders and tea. Alfalfa is believed by some to be a galactagogue, a substance that induces lactation.
In most climates, alfalfa is cut three to four times a year but is harvested up to 12 times per year in Arizona and Southern California. Total yields are typically around 8 tonnes per hectare (4 short tons per acre) but yields have been recorded up to 20 t/ha (16 short tons per acre). Yields vary with region, weather, and the crop's stage of maturity when cut. Later cuttings improve yield but reduce nutritional content.
Alfalfa is considered an 'insectary' due to the large number of insects it attracts. Some pests such as Alfalfa weevil, aphids, armyworms, and the potato leafhopper can reduce alfalfa yields dramatically, particularly with the second cutting when weather is warmest. Chemical controls are sometimes used to prevent this. Alfalfa is also susceptible to root rots including phytophora, rhizoctonia, and Texas Root Rot.
Alfalfa seed production requires the presence of pollinators when the fields of alfalfa are in bloom. Alfalfa pollination is somewhat problematic, however, because the pollen-carrying keel of the flower trips and strikes pollinating bees on the head to help transfer the pollen to the foraging bee. Western honey bees do not like being struck in the head repeatedly and learn to defeat this action by drawing nectar from the side of the flower. The bees thus collect the nectar but carry no pollen and so do not pollenate the next flower they visit. Because older, experienced bees don't pollinate alfalfa well, most pollination is accomplished by young bees that have not yet learned the trick of robbing the flower without tripping the head-knocking keel. When western honey bees are used to pollinate alfalfa, the beekeeper stocks the field at a very high rate to maximize the number of young bees.
Today the alfalfa leafcutter bee is increasingly used to circumvent this problem. As a solitary but gregarious bee species, it does not build colonies or store honey, but is a very efficient pollinator of alfalfa flowers. Nesting is in individual tunnels in wooden or plastic material, supplied by the alfalfa seed growers. The leafcutter bees are used in the Pacific Northwest, while western honeybees dominate in California alfalfa seed production.
A smaller amount of alfalfa produced for seed is pollinated by the alkali bee, mostly in the northwestern USA. It is cultured in special beds near the fields. These bees also have their own problems. They are not portable like honey bees; and when fields are planted in new areas, the bees take several seasons to build up. Honey bees are still trucked to many of the fields at bloom time.
When alfalfa is to be used as hay, it is usually cut and baled. Loose haystacks are still used in some areas, but bales are easier to transport and store. Ideally, the hay is cut just as the field is beginning to flower. When using farm equipment rather than hand-harvesting, a swather cuts the alfalfa and arranges it in windrows. In areas where the alfalfa does not immediately dry out on its own, a machine know as a mower-conditioner is used to cut the hay. The mower-conditioner has a set of rollers or flails that crimp and break the stems as they pass through the mower, making the alfalfa dry faster. After the alfalfa has dried, a tractor pulling a baler collects the hay into bales.
There are several types of bales commonly used for alfalfa. For small animals and individual horses, the alfalfa is baled into small "square" bales — actually rectangular, and typically about 40 x 45 x 100 cm (14 in x 18 in x 38 in). Small square bales weigh from 25 – 30 kg (50 – 70 pounds) depending on moisture, and can be easily hand separated into "flakes". Cattle ranches use large round bales, typically 1.4 to 1.8 m (4 to 6 feet) in diameter and weighing from 500 to 1,000 kg, (1000 to 2000 lbs). These bales can be placed in stable stacks or in large feeders for herds of horses, or unrolled on the ground for large herds of cattle. The bales can be loaded and stacked with a tractor using a spike, known as a bale spear, that pierces the center of the bale. Or they can be handled with a grapple (claw) on the tractor's front-end loader. A more recent innovation is large "square" bales, roughly the same proportions as the small squares, but much larger. The bale size was set so that stacks would fit perfectly on a large flatbed truck. These are more common in western states.
When used as feed for dairy cattle alfalfa is often made into haylage by a process known as ensiling. Rather than drying it to make dry hay, the alfalfa is chopped finely and fermented in silos, trenches, or bags, anywhere where the oxygen supply can be limited to promote fermentation. Fermenting the alfalfa allows it to retain high nutrient levels similar to those of fresh forage, and is also more palatable to dairy cattle than dry hay.
Considerable research and development has been done with this important plant. Older cultivars such as 'Vernal' have been the standard for years, but many better public and private varieties are now available and better adapted to particular climates. Private companies release many new varieties each year in the US.
Most varieties go dormant in the fall, with reduced growth in response to low temperatures and shorter days. 'Non-dormant' varieties that grow through the winter are planted in long-seasoned environments such as Mexico, Arizona, and Southern California, whereas 'dormant' varieties are planted in the Upper Midwest, Canada, and the Northeast. 'Non-dormant' varieties can be higher yielding, but they are susceptible to winter-kill in cold climates and have poorer persistence.
Most alfalfa cultivars contain genetic material from Sickle Medick (M. falcata), a wild variety of alfalfa that naturally hybridizes with M. sativa to produce Sand Lucerne (M. sativa ssp. varia). This species may bear either the purple flowers of alfalfa or the yellow of sickle medick, and is so called for its ready growth in sandy soil.
Most of the improvements in alfalfa over the last decades have consisted of better disease resistance on poorly drained soils in wet years, better ability to overwinter in cold climates, and the production of more leaves. Multileaf alfalfa varieties have more than three leaflets per leaf, giving them greater nutritional content by weight because there is more leafy matter for the same amount of stem.
Modern alfalfa varieties have probably a wider range of insect, disease, and nematode resistance than many other agricultural species. The North American Alfalfa Improvement Conference records new varieties and encourages communication between breeders.
Roundup Ready alfalfa is a genetically modified variety, patented by Monsanto, that is resistant to Monsanto's glyphosate herbicide Roundup. Although most broadleaf plants, including ordinary alfalfa, are sensitive to Roundup, growers can spray fields of Roundup Ready alfalfa with Roundup, and so kill the weeds without harming the alfalfa crop. Roundup Ready alfalfa was sold in the United States from 2005-2007 and more than were planted with it, out of . However, in May 2007, the California Northern District Court issued an injunction order prohibiting farmers from planting Roundup Ready alfalfa until the US Department of Agriculture (USDA) completed a study on the genetically engineered crop's likely environmental impact. In response, the USDA put a hold on any further planting of Roundup Ready alfalfa. The key issues of the lawsuit were the possibility that Roundup Resistance could be transmitted to other plants, including both other crops and weeds, making major pest species resistant to an important herbicide, Roundup.
In early Chinese medicines, physicians used young alfalfa leaves to treat disorders related to the digestive tract and the kidneys. In Hindu societies, ayurvedic physicians used the leaves for treating poor digestion. They made a cooling poultice from the seeds for boils. At the time, alfalfa was also believed to be helpful towards people suffering from arthritis and water retention.