Alfalfa

Medicago sativa

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Description

Alfalfa, Medicago sativa, is an herbaceous perennial in the plant family Fabaceae (peas and beans) which is primarily grown as a forage crop which can be grazed by animals or harvested as hay to be used as an animal feed. Alfalfa has a deeply penetrating taproot and the stems of the plant branch from a woody base, growing upright and erect or along the ground. The leaves of the plant are made up of three individual leaflets (trifoliate) which are are narrow and oval or oblong in shape with a smooth upper surface and slightly hairy lower surface. Alfalfa plants produce flowers on racemes (flower stalks) and each raceme possesses 10–35 densely packed purple flowers. Alfalfa produces spirally coiled seed pods each containing 2–6 seeds. Pods may have a smooth or hairy outer surface. Alfalfa plants can reach a height of 120 cm (47 in) and live for between 3 and 8 years. Alfalfa is also commonly referred to as lucerne and is believed to have originated in Caucasus area, north-western Iran and north-eastern Turkey.

Uses

Alfalfa leaves are edible and can be eaten as a leafy vegetable. Alfalfa is used primarily as forage for animals and is cut and stored as hay or silage. As a legume, the plant fixes nitrogen so can be used to increase nitrogen in the soil and is a commonly used cover crop.

Propagation


Basic requirements
Alfalfa is adapted to grow in a wide variety of environments but it requires careful management i fit is to be maximally productive. Alfalfa grows best in deep, fertile, well-draining soils with a pH between 6.0 and 7.0. Alfalfa has a deep and vigorous root system and quickly depletes nutrients from the soil. I grown in a soil that is poor in nutrients, the crop may require the addition of copious amounts of fertilizer. The vigorous root system means that alfalfa can tolerate dry periods and still give a good yield.

Seeding
Soil should be well prepared prior to planting alfalfa seeds. A firm seedbed is recommended to improve the stand by improving seed contact with the soil. This helps seeds to retain moisture and prevents new roots from drying out. The soil can be firmed prior to planting by using a roller. The seedbed should also be free of weeds and kept moist. Alfalfa fields should be sown 5 cm (2 in) deep in rows spaced 45 to 60 cm (18 to 24 in) apart. Seeds should be watered immediately and the bed kept moist while the seedlings emerge.

General care and maintenance
The alfalfa stand should be kept free from weeds. The easiest way to achieve this when the crop is being grown on a commercial scale is with the use of a preplant herbicide. There are several of these product available for use on alfalfa. Fertilizer should be applied in accordance with soil test results. Lime, phosphorus and potash are the most important nutrients for a healthy alfalfa stand.

Harvest
Alfalfa is harvested at different times depending on its intended use. Alfalfa which is cut between thee late bud and early bloom stage generally gives acceptable yields of high quality feed without reducing the quality of the stand. In contrast, repeated harvest of alfalfa which is till in the vegetative stage of growth leads to a reduced stand. In the first year, alfalfa can generally be harvested twice without any detrimental effect on winter survival; once in summer prior to the flowers emerging, and once later in the year with the date depending on location. Alfalfa is usually harvested by combine and baled as hay or cut for direct feeding to animals.

Diseases

Alfalfa caterpillar Insect Colias eurytheme

Symptoms

Defoliation of plants; entire leaf consumed, including midrib; adult insects are yellow-orange to white butterflies; larvae are bright green, velvety caterpillars which can reach 3.8 cm (1.5 in) in length; larvae have a white strip running down the side of their body.

Comments

Damage is most severe when eggs are laid in recently cut fields.

Management

Avoid unnecessary applications of insecticides to promote populations of natural enemies; organically grown alfalfa can be treated with Bacillus thuringiensis; harvest crop early to avoid serious damage.

Alfalfa mosaic Virus Alfalfa mosaic virus (AMV)

Symptoms

Yellow streaks parallel to leaf veins; yellow-green mottling of leaves; distorted leaves; stunted plant growth.

Comments

Transmitted by aphids; also spread via infected seed and pollen.

Management

Plant resistant cultivars; control aphid populations on plants; use virus free seed.

Alfalfa weevil Insect Hypera postica

Symptoms

Leaves skeletonized and appear bronzed; plants may be completely defoliated; adult insect is a dark gray beetle 0.5 cm (0.2 in) in length; larvae are pale green grubs with a thin white line down the center of their back and a brown head; larvae spin a cocoon and pupate on leaves or in soil.

Comments

Weevils overwinter in crop debris and emerges in Spring; both adult insects and larvae damage plants.

Management

Treatment of alfalfa weevils should be focused on the period before the first cutting; cutting the crop before budding is organically acceptable and can prevent serious damage and kill off most weevils; other control methods include the application of appropriate insecticide.

Anthracnose Fungus Colletotrichum trifolii

Symptoms

Large diamond shaped lesions with white centers on lower portion of stems; young dead shoots take on a characteristic "shepherds crook" appearance.

Comments

More common in warm weather and periods of high moisture.

Management

Cannot be managed in established alfalfa stands; cut crop before major losses occur.

Aphanomyces root rot Oomycete Aphanomyces euteiches

Symptoms

Infected seedlings have yellow cotyledons (seed leaves) with other leaflets beginning to turn yellow; seedlings dying back, seedlings with stunted growth; decaying roots in established plants leading to symptoms resembling nitrogen deficiency.

Comments

Disease is more easily spread in moist soils and over a wide range of temperatures.

Management

Grow varieties that have some resistance to the disease; only plant alfalfa in well draining soil.

Aphids (Cowpea aphid, Blue alfalfa aphid, Pea aphid, etc) Insect Aphis craccivora
Acyrthosiphon kondoi
Acyrthosiphon pisum

Symptoms

Small soft bodied insects on underside of leaves and/or stems; aphids are generally green in color but cowpea aphid is black and colonizes stems; aphids inject a powerful toxin into alfalfa which stunts plant growth and may kill the plant; aphids also secrete a sticky, sugary substance called honeydew which encourages the growth of sooty mold on the plants.

Comments

Blue alfalfa aphid and pea aphids prefer cooler temperatures and are most abundant in Spring and Fall; cowpea aphids are generally a sporadic pest but are most common in Spring.

Management

Insecticides are generally only required to treat aphids if the infestation is very high - plants generally tolerate low and medium level infestation; insecticidal soaps or oils such as neem or canola oil are usually the best method of control; always check the labels of the products for specific usage guidelines prior to use.

Bacterial alfalfa leaf spot Bacterium Xanthomonas axonopodis pv. alfalfae

Symptoms

Small water-soaked spots on chlorotic areas of leaves which grow into large irregularly shaped lesions with chlorotic margins; yellow to tan lesions with papery texture on leaves; premature defoliation; water-soaked lesions on stems.

Comments

Bacteria may enter the plants through wounds; bacteria can be spread by wind and water splash.

Management

Grow resistant cultivars; sow seed in Spring.

Bacterial wilt Bacterium Clavibacter michiganense subsp insidiosus

Symptoms

Dead plants scattered around field; stunted plants with small leaves and stems; bunchy appearance of plants; leaves curling upwards; plants wilting during day and recover at night; chlorotic leaflets; death of plants.

Comments

Occurs wherever alfalfa is grown, important disease in the US.

Management

Plant resistant cultivars.

Beet armyworm Insect Spodoptera exigua

Symptoms

Singular, or closely grouped circular to irregularly shaped holes in foliage; heavy feeding by young larvae leads to skeletonized leaves; shallow, dry wounds on fruit; egg clusters of 50-150 eggs may be present on the leaves; egg clusters are covered in a whitish scale which gives the cluster a cottony or fuzzy appearance; young larvae are pale green to yellow in color while older larvae are generally darker green with a dark and light line running along the side of their body and a pink or yellow underside.

Comments

Insect can go through 3–5 generations a year.

Management

Organic methods of controlling the beet armyworm include biological control by natural enemies which parasitize the larvae and the application of Bacillus thuringiensis; there are chemicals available for commercial control but many that are available for the home garden do not provide adequate control of the larvae.

Common leaf spot Fungus Pseudopeziza medicaginis

Symptoms

Small circular brow-black spots with uneven margins on leaves; leaves turning yellow and dropping from plant; raised brown fungal fruiting bodies may be visible in cool wet weather

Comments

More common during periods of high rainfall

Management

Harvest infected alfalfa early to avoid severe infections which reduce hay quality; rotating crops may reduce incidence of disease

Damping off Oomycete
Fungi
Pythium ultimum
Pythium irregulare
Pythium violae
Rhizoctonia solani
Phytophthora megasperma

Symptoms

Failure of seedling to emerge; light brown, seedlings with light brown water-soaked roots and stems; collapse of plants; plant dry up and die.

Comments

Occurs more often in cold temperatures when growth of seedlings is slow and in moist soil.

Management

Treat seeds with fungicide prior to planting.

Downy mildew Fungus Peronospora trifoliorum

Symptoms

Young leaflets may be dwarfed, twisted and cupped downward; light green or yellow blotches on leaves; a gray downy growth may be visible on infected leaves during cool, wet weather or during periods of high humidity.

Comments

Pathogen spread by wind or by splashing water.

Management

Grow resistant cultivars; cut alfalfa crop while still in prebloom stage; sow crop in Spring to reduce chance of seedlings becoming infected.

Fusarium wilt Fungus Fusarium oxysporum f. sp. medicaginis

Symptoms

Wilting shoots followed by bleaching of leaves and stem; rapid wilting of stems on only one side of plant; may be a reddish tinge to leaves; red streaks in root stele.

Comments

Disease emergence favored by high soil temperatures.

Management

No satisfactory method of management; plant resistant alfalfa varieties.

Lepto leaf spot Fungus Leptosphaerulina trifolii

Symptoms

Small red-brown flecks on leaves and petioles which develop into lesions with tan center and irregular brown margins; leaves become necrotic but remain attached to the plant.

Comments

Occurs wherever alfalfa is grown.

Management

Disease incidence and severity can be reduced by planting cultivars reported to have some resistance, using certified seed and rotating crop with a resistant plant such as soybean for at least 2 years.

Phytophthora root and stem rot Fungus Phytophthora megasperma

Symptoms

Chlorotic or reddish leaves which drop from plant; rotted roots which are yellow-brown in color; roots eventually turn black.

Comments

Disease more prevalent in water saturated soil.

Management

Grow resistant cultivars; try to improve drainage if soil has tendency to be waterlogged.

Threecornered alfalfa hopper Insect Spissistilus festinus

Symptoms

Stems girdled causing part of plant to above to break and turn red, purple or yellow; adult insect is green and wedge-shaped, tapering towards rear end; insect has a triangular area on back visible from above and piercing-sucking mouthparts; nymphs are soft bodied and gray-white in color.

Comments

Insect also a pest of other plants including soybean; may be 3-4 generations of insect per year.

Management

Applications of appropriate insecticide if insect becomes problematic.

References

CABI Crop Protection Compendium. 2014. Medicago sativa datasheet. Available at: http://www.cabi.org/cpc/datasheet/33040. [Accessed 05 November 14]. Paid subscription required

Frate, C.A. & Davis, R.M. (2007) Alfalfa diseases and management In C.G. Summers and D. H. Putnam (eds)., Irrigated alfalfa management for Mediterranean and desert zones. Chapter 10. Oakland: University of California Agriculture and Natural Resources Publication 8296. Available at: http://alfalfa.ucdavis.edu/IrrigatedA.... [Accessed 05 November 14]. Free to access

Lacefield, G., Henning, J.C., Rasnake, M. & Collins, M. (1997) University of Kentucky College of Agriculture Cooperative Extension Service Publication. Alfalfa: the Queen of Forage Crops. Available at: http://www2.ca.uky.edu/agc/pubs/agr/a.... [Accessed 05 November 14]. Free to access

Undersander, D., Gray, F., Kelling, K. & Rice, M. National Alfalfa and Forage Alliance Publication. Alfalfa Analyst. Available at: http://www.alfalfa.org/pdf/AlfalfaAna.... [Accessed 05 November 14]. Free to access