- Protologue: Gard. Dict., abr. ed. 4 (1754).
- Family: Leguminosae - Papilionoideae
- Chromosome number: x= 8;M. alba: 2n= 16, 24, 32;M. indica: 2n= 16;M. officinalis: 2n= 16, 32;M. suaveolens: 2n= 16
Major species and synonyms
- Melilotus alba Medikus, Vorles. Churpfälz. Phys.-Ökon. Ges. 2: 382 (1787), synonyms: M. alba Desr. (1796), Trifolium vulgare Hayne (1807).
- Melilotus indica (L.) All., Fl. Pedem. 1: 308 (1785), synonyms: Trifolium melilotus-indica L. (1753), Melilotus parviflora Desf. (1799).
- Melilotus officinalis (L.) Pallas, Reise russ. Reichs 3: 537 (1776), synonyms: Trifolium melilotus-officinalis L. (1753), Melilotus officinalis (L.) Lamk (1778).
- Melilotus suaveolens Ledeb., Ind. Sem. Hort. Dorpat, suppl. 2: 5 (1824), synonym: M. graveolens Bunge (1833).
- General: sweetclover (En). Mélilot (Fr).
- M. alba : White sweetclover, white melilot, bokhara clover (En). Mélilot blanc (Fr).
- M. indica : Sourclover, Indian clover, senji (India) (En).
- M. officinalis : Yellow sweetclover, yellow melilot (En). Mélilot officinal (Fr).
- M. suaveolens : Daghestan sweetclover (En)
- Vietnam: nhãn hương.
Origin and geographic distribution
Sweetclovers are widely distributed throughout Europe and Asia, mainly in temperate and subtropical areas, extending into North Africa, India ( M. indica ), Indo-China and Taiwan ( M. suaveolens ). M. alba and M. officinalis are cultivated extensively in North America, where they have attained their greatest importance in the Corn Belt and Great Plains of the United States and Canada, and in Eastern Europe and the Russian Federation. They have been introduced into Australia, South America and into eastern and southern Africa. M. indica is cultivated in India (mainly in the northern parts), in Pakistan and the United States. M. suaveolens is occasionally cultivated in China and North America.
Sweetclover is used for green manure, soil improvement and forage. It is said to have no equal as a soil-improving crop in the United States. As a forage crop, the use of sweetclover as a pasture crop far exceeds its uses for either hay or silage. Good quality hay can be made from the first-year growth. Because hay from the second-year growth is rather coarse, forage is often converted into silage. M. suaveolens is grown as a cover crop and forage plant on saline soils in China.
Seed oil is used in paint and varnish, and seed meal as protein supplement in cattle feed. Sweetclover is prized as a good honey plant, yielding large quantities of good quality, pale honey with a mild flavour. The honey has been used since the ancient Greeks for flavouring foods and for medicinal purposes, having astringent and narcotic properties. In Vietnam, M. suaveolens is used in lotions to treat eye diseases.
The composition of M. alba green fodder per 100 g is: moisture 79.2 g, protein 4.1 g, crude fibre 4-9 g, total digestible nutrients 12.8 g. Chemical analysis of green material showed per 100 g: N 0.83 g, P 0.07 g, K 0.67 g, and Ca 0.50 g. The silage is similar to maize silage in nutritive value, but is not as palatable, containing per 100 g: moisture 72 g, protein 4.5 g, crude fibre 9.6 g, total digestible nutrients 15.7 g. Good quality M. alba hay can approach the chemical composition and feeding value of lucerne hay, containing per 100 g: moisture 8.2 g, protein 16.5 g, crude fibre 24.6 g, total digestible nutrients 50.3 g. Sweetclover contains coumarin, hydrocyanic acid, malonic acid, and melilotin. Coumarin reduces the palatability to livestock. Feeding spoiled hay or silage from high-coumarin cultivars may lead to "sweetclover bleeding disease". Affected animals may bleed to death from small wounds or internal haemorrhages.
The weight of 1000 seeds is 150-200(-390) g.
Annual or biennial, sometimes scented herbs. Leaves trifoliolate; stipules adnate to the petiole, subulate; leaflets dentate. Inflorescence an axillary raceme; flowers small; bracts small; bracteoles absent; calyx campanulate with 5 subequal teeth; corolla yellow or white, rarely purple, glabrous, caducous, standard usually longer than keel and wings, keel shorter than wings, obtuse, not adnate to stamens; stamens diadelphous, anthers uniform. Fruit a small pod, straight, ovoid to nearly globose, with persistent pedicel and calyx, 1-4-seeded. Seed smooth or nearly so.
- M. alba . Erect, ascending or decumbent, sparsely branched, scented, annual herb, up to 1.5(-2.5) m tall, with long taproot. Stipules lanceolate to setaceous, 4-6 mm long; petiole 0.5-2 cm long, petiolule up to 5 mm; leaflets obovate to oblong-obovate, 1-2.5 cm × 5-12 mm, serrate-dentate almost to the base. Raceme 5-20 cm long, on an up to 4 cm long peduncle; flowers white, calyx about 2 mm long, corolla 4-6 mm long, style 1.7-2.3 mm long. Pod obovoid to ovoid, 4 mm long, reticulately veined, greyish to blackish-brown when ripe. Seed ovoid, about 2 mm long, yellow-brown.
- M. indica . Erect, annual herb up to 60 cm tall. Stem pubescent. Stipules lanceolate to setaceous, 5-8 mm long; petiole up to 4.5 cm long, petiolule up to 5 mm; leaflets oblong to obovate, 0.8-2.5 cm × 2-9 mm. Raceme 10-16-flowered; peduncle up to 3 cm long; flowers yellow; calyx about 1.5 mm long, teeth triangular-lanceolate; corolla 2-3 mm long; style 0.9-1.2 mm long. Pod 1-seeded, 1.5-4 mm long, prominently reticulately veined, olive-green. Seed ovoid, about 2 mm long, yellow-brown, finely verrucose.
- M. officinalis . An erect, much branched, scented annual or biennial with stout stem up to 1.5(-3) m tall and thickened roots. Stipules 3-6 mm long; petiole up to 3 cm long, petiolule up to 6 mm; leaflets obovate to oblong-lanceolate, 1-2.5 cm × 4-15 mm. Raceme up to 10 cm long, peduncle about 2 cm long; flowers 4.5-8(-10) mm long; calyx teeth equal, acute; corolla yellow; style 1.7-2.3 mm long. Pod ovoid, 3-6 mm long, glabrous, brown when ripe, transversely reticulate or irregularly rugose, usually 1-seeded. Seed ovoid, about 2 mm long, yellow-green.
- M. suaveolens . Annual to biennial herb, up to 1.5 m tall, pubescent to subglabrous, with thickened roots. Stem erect, angular, glabrous. Stipules 8-10 mm long; petiole up to 2 cm long, petiolule up to 1 mm; leaflets narrowly elliptical to obovate, 1-3 cm × 3-8 mm. Raceme 10-15 cm long, densely flowered, elongate after anthesis; peduncle 2-5 cm long; flowers pale yellow, 3-4 mm long; style 1.7-2.3 mm long. Pod ellipsoid, about 3 mm long, finely reticulate. Seed ellipsoid, 2 mm long, reddish.
Growth and development
Well-ripened, mature seed is hard. Seed can be stored for long periods. After storage for 40 years in stoppered glass bottles, 60% of the seed still germinated in a trial in the United States. During the first year, biennial cultivars form a primary stem which becomes much branched under favourable conditions, a deeply penetrating taproot and, as the season progresses, a crown. When sweetclover is cut early, regrowth is from buds higher up the stem. Top growth reaches maximum development during late summer when a rapid increase in the size of the taproot begins which continues during autumn. Growth in the second year starts quickly and largely consists of rather coarse stems, which may reach to nearly 3 m in M. officinalis . Root thickening does not occur in annual cultivars. Control of flowering and taproot thickening is not fully understood. The flowering of biennial cultivars is initiated by long days. Under a daylength of 18 hours, flowering starts within 3 months after sowing. Vernalization seems to be of only minor importance.
Sweetclover fixes atmospheric nitrogen and is an aggressive colonizer, quickly invading roadsides, railways and fence lines. Around 1900 it was listed as a noxious weed, but by 1910 its value as a cover crop and green manure plant was well established in North America.
Pollination is by insects, mostly honey bees. Flowers of M. alba and M. officinalis only set seed when tripped by visiting insects. M. alba and M. indica are self-fertile; self-incompatibility is common in M. officinalis .
Other botanical information
Melilotus comprises about 25 species found chiefly in the Mediterranean region and central Asia. The following characteristics may be useful to easily distinguish between the 4 species described here: M. indica : style length 0.9-1.2 mm (other species 1.7-2.3 mm); M. alba : flowers white (other species yellow); M. officinalis : pod strongly transversely veined (other species irregularly veined); M. suaveolens : style 1.7-2.3 mm long, flowers yellow, pod irregularly veined, plant very fragrant.
M. alba and M. officinalis are closely related and sometimes hybridize naturally. Some authors prefer to write M. albus and M. indicus instead of M. alba and M. indica .
Well-known annual cultivars of M. alba are "Emerald", "Floranna", "Hubam" and "Israel". Biennial cultivars are "Arctic", an early maturing, winter-hardy cultivar, "Polara", which is low in coumarin, but produces lower yields, both from Canada, "Denta", from the United States, which is low in coumarin and late, "Chermasan" and "Medet" from Russia. "Goldtop", "Madrid" and "Norgold" are cultivars of M. officinalis commonly used in North America, "Katek" and "Omskii Skorospelyi" are used in the Russian Federation.
Sweetclover is adapted to a wide range of climatic and soil conditions. It occurs in grassland, arable fields, wasteland and along roadsides, especially in calcareous soils. It is frost-hardy and grows well from sea level up to 2000 m altitude in the United States and China. Sweetclover is drought tolerant, requiring enough moisture for germination, after which it will survive under dry conditions. It comes up well under irrigated conditions, but does not give a good ratoon. For optimal growth it requires a well drained, fertile soil of pH 6.5-7.5 and adequate moisture. Heavy clays and light sands will produce a successful crop. Acid soils are not tolerated. In China, M. suaveolens is grown on saline soils.
Propagation and planting
Propagation is by seed, but propagation by cuttings is also possible. Due to a hard testa, seed must be scarified. Seed rate varies from 11-17 kg/ha. Inoculation with effective Rhizobium strains is recommended in fields not previously sown with sweetclover. It has been suggested that it is worthwhile using inoculated seed at each planting. Seeds are small, thus the seedbed should be fine and seed placed no deeper than 1-2 cm in the soil.
Sweetclover is often sown in mixtures with cereals grown for forage or grain, with grasses, or grown in rotation with cereals. When grown in mixtures with cereals it is left to cover the soil in winter and is ploughed in early next spring. The strong taproot opens up the subsoil, providing favourable conditions for growth in succeeding crops. Roots break down and release nutrients rapidly at maturity. Sweetclover is particularly susceptible to injury from herbicides, especially 2,4-D.
Diseases and pests
Sweetclover is affected by several diseases in the more humid parts of its area of cultivation in the United States and Canada. Phytophthora cactorum causes root rot and crown injury in the spring of the second year. Ascochyta caulicola and Cercospora davisii cause "black stem", characterized by stunted, blackened stems, poor flowering and reduced seed set. The sweetclover weevil ( Sitona cylindricollis ) is the main insect pest in North America. Seedlings are most vulnerable, adult plants may be defoliated, but generally survive and outgrow the damage.
Timing and intensity of grazing or cutting are very important in sweetclover. Intensive grazing or mowing in the first season before the formation of a crown may lead to poor regrowth if there are a limited number of buds on the stem. Grazing or mowing in autumn can prevent adequate development and accumulation of assimilates in the root, leading to poor growth in the second year. Second-year growth is vigorous and heavy grazing is essential when tops are 20-25 cm tall, to keep plants palatable. Good quality hay, equal in palatability and feeding value to lucerne, may be produced from first-year growth. Second-year growth is less satisfactory for hay, as leaves become brittle and shatter in handling. The best quality of silage is obtained from crops cut prior to flowering.
Yield of hay varies from 2.2-5.5 t/ha during the first year to 2.2-8.1 t/ha during the second year in the United States. In India, M. alba "Hubam" has yielded 9.0-10.5 t/ha in 2 cuttings. As a green manure crop, "Hubam" adds about 80 kg/ha N to the soil. Seed yield averages about 225 kg/ha in the United States, but it has been estimated that about 40% of the seed is lost due to shattering.
Genetic resources and breeding
A germplasm collection of some 1300 accessions is maintained at the Canada Department of Agriculture, Brandon, Canada. The development of strains with large seed, resistance to seedling diseases, winter hardiness, drought resistance, tolerance to acid and saline soils, and a higher proportion of permeable seed are objectives for improving these crops. Breeding work is being done in Canada, the United States and the Russian Federation. Cultivars with low coumarin content have been bred, following the discovery of a low-coumarin gene in M. dentata (Waldst. & Kit.) Pers. and its transfer into M. alba and M. officinalis .
Sweetclover may regain its former importance in North America and Europe as a green manure and honey crop as more land is left fallow. These 4 Melilotus spp. all occur occasionally naturalized in the tropics, especially in highland areas. Their qualities as soil-improving crops in temperate areas warrant their testing in tropical highland areas.
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- Gorz, H.J. & Smith, W.K., 1973. Sweetclover. In: Heath, M.E., Metcalfe, D.S. & Barnes, R.F. (Editors): Forages, the science of grassland agriculture. 3th edition. Iowa State University Press, Ames, Iowa, United States. pp. 159-166.
- Gross, A.T.H. & Stevenson, G.A., 1964. Resistance in Melilotus species to the sweetclover weevil (Sitona cylindricollis). Canadian Journal of Plant Science 44: 487-488.
- Ha, S., 1993. Genetical studies on interspecific differentiation in the genus Melilotus. Memoirs of the Faculty of Agriculture, Hokkaido University 18: 67-107.
- Helm, J. & Meyer, D., 1993. Sweetclover production and management. NDSU Extension Service (Publication), North Dakota State University, United States.
- Sales, F., 1993. Melilotus Miller (Leguminosae): typification and nomenclature. Anales del Jardín Botánico de Madrid 51: 171-175.
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- Townsend, C.C. & Guest, E., 1974. Leguminales. Melilotus Mill. In: Townsend, C.C. & Guest, E. (Editors): Flora of Iraq. Vol. 3. Ministry of Agriculture and Agrarian Reform, Baghdad, Iraq. pp. 142-149.
R.K. Arora & P.N. Mathur