Sweetclovers

Sweetclovers

 

Growing Period Type Annual or Perennial Drought Tolerance Shade Tolerance Salinity Tolerance
Cool Season Legume Perennial High Intolerant High

Common Name

Melilotus spp. are not true clovers, which are Trifolium spp. (Taylor, 1985). Bailey and Bailey (1976) in Hortus III listed the common names for Melilotus alba Desrousseaux as being white sweetclover, white melilot, and Bukhara clover and for Melilotus officinalis (L.) Pall., yellow sweetclover, melilot, yellow melilot, and melist. According to Duke (1981), the common names for Melilotus alba Desrousseaux are white sweetclover, Hubam, and white melilot; for Melilotus officinalis Lamarck, the common name is yellow sweetclover.

McLeod (1982) gave as common names for the white species, white sweet clover and Bokhara sweet clover, and for the yellow species yellow sweet clover, official melilot, and common sweet clover.

Metcalfe (1985) listed several common names under the general heading of sweetclover, including annual yellow sweetclover (Melilotus indica All.), Banat sweetclover (Melilotus dentata [W. & K.] Pers.), yellow sweetclover (Melilotus officinalis Lamarck or Melilotus officinalis [L.] Lamarck), white sweetclover (Melilotus alba Medik. or Melilotus alba Desrousseaux), Daghestan sweetclover (Melilotus suaveolens Ledeb.), Israel (Hubam) sweetclover (Melilotus alba var. annua Coe), and sourclover or sour sweetclover (Melilotus indica All.).

The approved common and scientific names for the two species treated here are white sweet clover (Melilotus alba Desrousseaux) and yellow sweetclover (Melilotus officinalis [L.] Lamarck) (Metcalfe, 1985).

Scientific Name

There is some confusion over the correct species names and authorities for both white and yellow sweetclovers. Duke (1981) termed white sweetclover Melilotus alba Medik., whereas Munz (1973) termed it Melilotus albus Desrousseaux. Bailey and Bailey (1976) in Hortus III listed the species names as Melilotus alba Desrousseaux for white sweetclover, and Melilotus officinalis (L.) Pall. for yellow sweetclover. The yellow species was termed Melilotus officinalis Lamarck by Duke (1981), but Munz (1973) called it Melilotus officinalis (L.) Lamarck. Metcalfe (1985) listed the approved names for white sweet clover as Melilotus alba Desrousseaux and for yellow sweetclover, Melilotus officinalis [L.] Lamarck. The annual form of white sweetclover was called Melilotus alba Desrousseaux var. annua Coe by Bugg and Ellis (1990).

Cultivar

White sweetclover occurs in both annual and biennial forms. 'Hubam,' 'Emerald,' 'Floranna,' and 'Israel' are cultivars of the annual form of white sweetclover (Madson, 1951). 'Denta' is a late-maturing, low-coumarin variety. 'Arctic,' an early-maturing winter-hardy variety (presumably biennial) and the low-coumarin cv 'Polara' are both adapted to Canada. The low-coumarin gene was transferred from Melilotus dentata (Waldst. and Kit.) Pers. to M. alba and then to M. officinalis (Duke, 1981).

According to Duke (1981), yellow sweetclover comprises 3 subspecies and about 16 ecotypes and forms. Recommended cultivars include 'Albotrea.' 'Common Yellow,' 'Erector,' 'Madrid,' 'Switzer,' and 'Goldtop.'

Seed Description

Seeds of white sweetclover have hard testa and must be scarified to speed germination (Duke, 1981).

Seeds of yellow sweetclover occur one or two to the pod and are smooth, ovoid-elliptical, 1.5-3 mm. long, ca 1.5 mm broad, and yellow or greenish-yellow, sometimes with purple spots. Seeds are said to poison horses (Duke, 1981).

Seedling Description

For yellow sweetclover, the seed leaves (cotyledons) are very small, twice as long as broad, and pale green. The first true leaf is heart shaped or roundish, and wavy around the edges. The second and subsequent true leaves are trifoliate (Fischer et al., 1978).

Mature Plant Description

According to the description by Duke (1981), white sweetclover is an annual or biennial, erect or decumbent herb, stem 1 m or more tall, stipules entire; leaves trifoliate, leaflets lanceolate or oblanceolate to narrowly oblong, rarely ovate, 1-2.5 cm long; rachis of larger leaves, excluding terminal petiolule, often prolonged more than 4 mm beyond lateral leaflets; racemes numerous, 5-20 cm long, on peduncles up to 4 cm long; pedicels 1-2 mm long; calyx about 2 mm long; corolla white, 4-6 mm long; style 1.7-2.3 mm long; pod with weak irregular network of veins.

Duke (1981) described yellow sweetclover as a biennial or annual herb with strong taproot; stems erect or ascending, 0.3-2.8 m tall, often tinged with red; leaves trifoliate, the leaflets of the lower and middle leaves broadly oval, lanceolate to rhomboid-ovate, 1-5 cm long, rounded at tip, those of upper leaves oblong-lanceolate, rounded or truncate at tip, irregularly dentate or entire; stipules entire, 8-12 mm long, lanceolate or subulate; racemes lax, 30-80 flowered, 4-15 cm long; pedicels 2-2.5 mm long; clayx 2-3 mm long, teeth as long as tube or shorter; corolla yellow, 4.5-7 mm long, standard and wings more or less equal to or up to 1 mm longer than keel; style 1.7-2.3 mm long, often persisting; ovules 4-6, rarely 3 or 8; pods ca, 3.4 mm long, 2-2.5 mm broad , 1.5 mm thick, compressed with strong rugose transverse veins or wrinkled gray to straw-colored, rarely black, obtuse at tip.

Temperature

Sweetclovers are considered resistant to cold (Madson, 1951).

Duke, (1981) detailed the thermal tolerances for the sweetclovers. White sweetclover tolerates an annual mean temperature range of 5.7-24.3 C, with the mean of 54 cases being 12.7; the corresponding figures for yellow white sweetclover are 4.9-21.8 C, with the mean of 47 cases being 10.9 C . White and yellow sweetclovers tolerate high and low temperatures and are seldom winter-killed. Both species are said to be tolerant of both heat and frost. There are intervarietal differences in winterhardiness for white sweetclover, with 'Arctic' and 'Polara' both tolerant of Canadian conditions. For yellow sweetclover, cv 'Madrid' was noted as relatively resistant to fall freezes.

Geographic Range

White sweetclover is native to Europe and western and central Asia, south to India; it is assigned to the Eurosiberian and Mediterranean Centers of Diversity, and is native to the Boreal Moist to Wet through Subtropical Dry to Moist Forest Life Zones, and persists up to an altitude of 2,000 m (Duke, 1981). As noted by Munz (1973), in California, the species is abundantly naturalized in waste places, especially in damp zones in southern California. It occurs from British Columbia to the East Coast of the United States.

Based on Duke's (1981) account, yellow sweetclover is assigned to the Eurosiberian and Mediterranean Centers of Diversity, and is native from Europe to central temperate Asia and western China, south to North Africa, Ethiopia, Eritrea, and Kenya, and has been introduced to various temperate reions, including North and South America. It is cultivated in India. It ranges form the Boreal Moist through Warm Temperate Thorn to Moist Forest Life Zones, and persists up to an altitude of 4,000 m. In California, Munz (1973) noted that yellow sweetclover is less commonly naturalized than white sweetclover or sourclover. According to McLeod (1982), sweetclovers will grow almost anywhere, given sufficient and suitably-distributed rainfall. They grow in southern Canada and throughout the United States, thriving in both semiarid and humid regions.

Water

White sweetclover tolerates from 0.9-16.0 dm of precipitation, with the mean of 54 cases being 7.8. The corresponding figures for yellow sweetclover are 3.1-16.0 dm of precipitation, with the mean of 54 cases being 7.8 (Duke, 1981).

Sweetclovers grow well where annual rainfall exceeds 17 inches, suitably distributed throughout the year, and often persist in areas too dry for alfalfa. Yellow sweetclover withstands drier conditions than white (McLeod, 1982).

Zachariassen and Power (1991) found that crimson clover showed a consistently-higher water use efficiency (g of dry matter produced per liter of water evapotranspired) than hairy vetch at 10, 20, and 30C. White sweetclover showed intermediate values.

Goldstein (1989) wrote that principal considerations in drought-proofing a farm on the northern Great Plains are: (1) improving soil structure through integrated use of grasses, (2) integrating animals with grain-farming operations, (3) effective management of yellow sweetclover, (4) stubble mulching to improve infiltration of water, and (5) timing tillage to have maximum positive impact on soil improvement. Yellow sweetclover is useful when grown in combination with a brome grass or a winter-hardy orchard grass like cv 'Pierre'. This is better than a solid stand of yellow sweetclover for both soil improvement and grazing. Such a mixture could be grazed or stubble mulched by undercutting the plants and leaving the residue on the surface. This approach to tillage leads to better water infiltration and efficiency of nitrogen use in the following grain crops than does plowing.

Nutrients

Sweetclovers have deep taproots and might be expected to mine nutrients from deeper soil strata (Bugg, pers. comm.).

McLeod (1982) ascribed to sweetclovers the ability to convert the insoluble nutrients phosphorus and potassium from forms unavailable to other plants.

Soil pH

White sweetclover tolerates pH of from 4.8-8.2, with the mean of 50 cases being 6.7, whereas the corresponding figures for yellow sweetclover tolerates pH of from 4.8-8.2, with the mean of 43 cases being 6.6. Both species are regarded as tolerant of alkali, high pH, and limestone (Duke, 1981).

Sweetclovers are intolerant of acid soils (Duke, 1981; Brinton, 1989). According to Johnny's Selected Seeds (1983), white sweetclover requires soil pH of at least 6.8. and best growth occurs on neutral or alkaline soil with abundant lime. If soil pH is less than 6, soil should be limed well in advance of seeding (McLeod, 1982).

Soil Type

Duke (1981) detailed the soil tolerances of the two main sweetclovers. White sweetclover tolerates heavy clays to light sands and does especially well on calcareous soils; it is said to tolerate alkali, limestone, high pH, mine sites, poor soil, slope, and waterlogging. Yellow sweetclover tolerates clays to light sands but does best on fertile, well-limed and -drained soils with good organic matter content. Yellow sweetclover has been said to tolerate alkali, high pH, limestone, slope, and both heavy and poor soils.

McLeod (1982) confirmed that sweetclovers tolerate cemented clays and gravels to poor sand and stated that the the deep roots open the subsoil. He also mentioned that the white species prefers clay loams and that the yellow does best on loam soils.

Madson (1951) stated that sweetclover tolerates loam to heavy soil types.

Schonbeck (1988) wrote that yellow sweetclover does well on soils that will not support true clovers (Trifolium spp.).

Shade Tolerance

Duke (1981) mentioned various tolerances of sweetclovers, but did not list shade tolerance.

Salinity Tolerance

Yellow sweetclover tolerates 3-6 mmhos of salinity (Duke, 1981).

Herbicide Sensitivity

White sweetclover is unusually susceptible to a number of herbicides, especially 2,4D (Duke, 1981).

Life Cycle

Duke (1981) listed both white and yellow sweetclovers as annual and biennial herbs. McLeod (1982) wrote that both species are summer growing, biennial legumes but that there are annual varieties of Melilotus alba. The latter point was confirmed by both Johnny's Selected Seeds (1983) and Madson (1951) who both cited cv 'Hubam' as an example of the annual form of the white species. 'Hubam' is usually seeded in the spring and tilled under in the fall (Johnny's Selected Seeds, 1983).

Seeding Rate

Seeding rate is 10 to 15 lbs/acre of scarified or 25 lbs/acre of unscarified seed (McLeod, 1982). Johnny's Selected Seeds (1983) suggested that white sweetclover be seeded at 15 lbs/acre. Madson (1951) recommended that cv 'Hubam' be seeded at 20 to 30 lbs/acre. Duke (1981) wrote that both white and yellow sweetclovers should be seeded at 11-17 or for the white species up to 25 kg/ha.

Seeding Depth

Sweetclovers should be sown at a depth of 1/2 inch (McLeod, 1982).

Seeding Method

The seed of white sweetclover has a hard testa, requiring scarification to ensure prompt germination (Duke, 1981). Seed of sweetclover should be covered lightly (Johnny's Selected Seeds, 1983). In Mendocino wine vineyards, double disking followed by broadcasting and cultipacking was used (Bugg, pers. comm.).

In New York state, Scott and Burt (1985) evaluated cover crops after overseeding into corn 6-18" high were medium red clover, mammoth red clover, alfalfa, yellow sweetclover, alsike clover, birdsfoot trefoil, Canada field peas, Austrian winter peas, cowpeas, perennial or annual ryegrass, medium red clover + ryegrass, or medium red clover + rye. Of these, alfalfa, medium red clover, yellow sweetclover, hairy vetch, ryegrass, and medium red clover + ryegrass have performed well.

Seeding Dates

Sweetclovers may be sown during the fall or spring (McLeod, 1982); Johnny's Selected Seeds (1983) of Maine wrote that biennial forms of both the white and yellow species can be sown in spring or late summer. Unscarified seed of both species germinates better when sown in the fall; white sweetclover also does better with fall sowing (McLeod, 1982). Madson (1951) suggested sowing from October to November. The annual form of white sweetclover (cv 'Hubam') did well in Massachusetts when sown in early May (Bugg and Ellis, 1990).

Inoculation

White and yellow sweetclovers require rhizobial strain "A" (Nitragin Co.), the same strain used for alfalfa (Burton and Martinez, 1980).

Seed Cost

Seed cost of sweetclovers is low (Madson, 1951).
 

Days to Flowering

Duke (1981) listed white and yellow sweetclovers as flowering during summer and fall. Munz (1973) listed white sweetclover as flowering from May through September, and yellow from May through August. McLeod (1982) stated that the yellow species blooms earlier than the white.

Annual white sweetclover (cv 'Hubam') sown on May 7 attained full flower during the first week of August (coastal Massachusetts) (Bugg and Ellis, 1990).

Days to Maturity

Duke (1981) listed maturation of yellow sweetclover as occurring in the autumn, and 10-14 days before biennial white sweetclover. Annual white sweetclover (cv 'Hubam') was avoided by one farmer because of its tendency to reseed (Schonbeck, 1988).

Seed Production

As related by Duke (1981), white and yellow sweetclovers are insect pollinated; pollination triples the seed set obtained for yellow sweetclover without pollination. For both species, 2-5 colonies of bees per acre are recommended, but 25 per acre gives maximum pollination. White sweetclover should be harvested for seed just as the pods turn black or dark brown.

Annual white sweetclover (cv 'Hubam') was avoided by one farmer because of its tendency to reseed (Schonbeck, 1988).

Seed Storage

Seed remains viable for a relatively long time (McLeod, 1982).

Growth Habit

White sweetclover shows erect or decumbent growth; yellow sweeclover stems are erect or ascending. Both are listed as annual or biennial herbs (Duke, 1981).

Maximum Height

White sweetclover attains a height of of 1 m or more, and yellow sweetclover attains heights of from 0.3 m to 2.8 m (Duke, 1981). Munz (1973) listed white sweetclover as attaining 1-2 m.

Annual white sweetclover (cv 'Hubam') slightly exceeded 1.00 m (Bugg and Ellis, 1990).

White sweetclover grows over 6 ft. high (Johnny's Selected Seeds, 1983). Sweetclovers can attain heights of 4-8 feet, and growth is vigorous (Schonbeck, 1988). In Mendocino County, height in May of October-sown mixed stands of white and yellow sweetclovers was 22.23+/-5.81 cm (Mean +/- S.E.M.) (Bugg et al., unpublished data).

Root System

The biennial form of white sweetclover has a deep taproot; however, in the north, mowing or grazing during the first year prevents good root growth (Duke, 1981). Johnny's Selected Seeds (1983) wrote that biennial sweetclover has a deep, strong root system that can penetrate through soil layers.

White and yellow sweetclovers have deep taproots, according to Bugg (pers. comm.).

Kutschera (1960) reported that yellow sweetclover (Melilotus officinalis) generally roots to a depth of 96 cm and white sweetclover (Melilotus albus) generally roots to a depth of 65-240 cm.

Establishment

White sweetclover is small seeded and rather slow to establish, and problems with weeds should be expected (Bugg, pers. comm.). This was confirmed by McLeod (1982) who commented that the plants grow slowly at first. A small-grain companion crop can reduce weed problems for either the white or yellow species (Duke, 1981).

Maintenance

Sweetclovers may benefit from being sown along with a small grain as companion crop (Duke, 1981). Sweetclovers tolerate heavy grazing (Duke, 1981), which may suggest that they tolerate close mowing as well.

Mowing

In the north, mowing or grazing biennial white sweetclover during the first year prevents good root growth (Duke, 1981). Grazing, and presumably, mowing can begin once plants have attained a height of 20-25 cm. For the biennial forms of the white species, mow during bud stage (before flowering). Hay yields the first year range from 2.2-3.5 Mg/ha, and in the second year 2.2-8.1 Mg/ha. In India, the annual form ('Hubam') can yield 9-10.5 Mg/ha in three cuttings.

Duke (1981) related that yellow sweetclover matures 10-14 days earlier than biennial white sweetclover. First-year top growth should be removed before August or after mid October, either by mowing or grazing. Grazing (or presumably, mowing) can begin when the stand height is from 20-25 cm, and frequent removal is needed to prevent forage from becoming coarse and unpalatable. Yields of cv 'Madrid' can be 4.2-4.5 Mg/ha in the first year, and 5.5-8.5 Mg/ha in the second.

Incorporation

Sweetclovers can be difficult to incorporate, and regrowth can be a problem (Schonbeck, 1988). White sweetclover may be turned under at any time, according to Duke (1981). However, McLeod (1982) advised cultivating in the early spring, rather than the fall, because biennial forms will resume growing the second year if turned under in the fall, but not in the spring. Duke (1981) recommended that yellow sweetclover is best incorporated during the second spring, after the stand is more that 15 cm tall.

Mouldboard plough, rotavator, tandem disc, offset disc, and heavy duty cultivator were evaluated as tools for incorporating yellow sweetclover prior to spring wheat planting. There were no significant differences due to implement for amount of soil nitrogen in the 0-60 cm profile 4285(Foster, 1990).

Foster (1990) also evaluated three dates of incorporation for incorporating yellow sweetclover prior to spring wheat planting: June 15, July 1, and July 15. June 15 incorporation gave the best result in terms of the amount of soil nitrogen in the 0-60 cm profile (59, 66, and 70 kg/ha in the three years of the study), and for grain yield of the wheat.

Harvesting

White sweetclover should be harvested for seed just as the pods turn black or dark brown (Duke, 1981). Yellow sweetclover is best harvested if windrowed when damp and threshed 4-5 days later with a combine having a pickup attachment. The binder or combine should be set high.

If the companion grain crop of sweetclover is to be harvested the first year, set the binder on the combine high (Duke, 1981).

Equipment

Mouldboard plough, rotavator, tandem disc, offset disc, and heavy duty cultivator were evaluated as tools for incorporating yellow sweetclover prior to spring wheat planting. There were no significant differences due to implement for amount of soil nitrogen in the 0-60 cm profile (Foster, 1990). If the companion grain crop of sweetclover is to be harvested the first year, set the binder on the combine high (Duke, 1981). Yellow sweetclover is best harvested if windrowed when damp and threshed with a combine having a pickup attachment 4-5 days later. The binder or combine should be set high (Duke, 1981).

Uses

Duke (1981) wrote that white sweetclover could be used as forage, hay, and ensilage, and for pasture and soil imporvement. It is also used as a green manure and cover crop and is a top honey plant. Cortical fiber can be used for paper manufacture, and seed oil is used in paint and varnish. Seed meal is detoxified and used as a protein supplement in cattle feed. Yellow sweetclover is used for forage, hay, pasture, and soil improvement and erosion prevention. As an herb, it is used to flavor cheeses and tobacco snuff, and seeds used as a substiture for Tonka beans. It is also a honey plant, and Kalmuks use the roots for food.

McLeod (1982) wrote that sweetclover can be used as hay, pasture, and green manure. Sweetclovers are recommended for renovation of very poor, thin soils. They fit well in a rotation with a cereal and corn. The sweetclover is sown with the cereal, wheat, oat or barley, for example. Even flax can be used in this manner. They act as nurse crops for the sweetclover.

Madson (1951) remarked that sweetclovers are little used as winter cover crops because growth occurs too late in the spring for them to fit into the usual cover crop program. Where they fit a short rotation program, they are excellent for opening up heavy soils.

Johnny's Selected Seeds (1983) stated that white sweetclover is grown for nitrogen fixation, organic matter, and soil life enhancement.

Mixtures

Sweetclover is often seeded together with a nurse crop of some small grain (Duke, 1981). During establishment, sweetclovers may benefit from a companion crop of oat (Schonbeck, 1988).

Goldstein (1989) wrote that in the northern Great Plains, yellow sweetclover is useful when grown in combination with a brome grass or a winter-hardy orchard grass like cv 'Pierre.' This is better than a solid stand of yellow sweetclover for both soil improvement and grazing. Such a mixture could be grazed or stubble mulched by undercutting the plants and leaving the residue on the surface. This approach to tillage leads to better water infiltration and efficiency of nitrogen use in the following grain crops than does plowing.

Sweetclovers may be interseeded into rows of squash or overseeded into brassicas, but they are not suitable for overseeding into corn (Schonbeck, 1988).

Biomass

As related by Duke (1981), for the biennial forms of the white species, hay yields the first year range from 2.2-3.5 Mg/ha, and in the second year 2.2-8.1 Mg/ha. In India, the annual form ('Hubam') can yield 9-10.5 Mg/ha in three cuttings. For yellow sweetclover, yields of cv 'Madrid' can be 4.2-4.5 Mg/ha in the first year, and 5.5-8.5 Mg/ha in the second.

Above-ground biomass yields for yellow sweetclover at 15 July (following establishment by overseeding into standing wheat during late April or early May the previous year) were 1,320, 2,117, and 3,238 kg/ha during a three-year study in Saskatchewan (Foster, 1990).

Mid-May harvest harvest of fall-sown mixed biennial sweetclovers yielded 1.656+/-0.626 Mg/ha, Mean +/-S.E.M. in an organic vineyard in Hopland, Mendocino County, California. When weeds were included, the biomass was 4.930+/-0.620 (Mg/ha, Mean +/- S.E.M.) (Bugg et al., unpublished data).

N Contribution

Duke (1981) wrote that 'Hubam' annual white sweetclover grown as a green manure could add about 80 kg N/ha. The review by Bugg et al. (1991c) listed white sweetclover as containing 80 - 180 kg N/ha. Bundy et al. (No date) wrote that sweetclover can contain from 80-120 lb N/acre.

During a three-year study in Saskatchewan (Foster, 1990), above-ground nitrogen yields for yellow sweetclover at 15 July (following establishment by overseeding into standing wheat during late April or early May the previous year) were 26, 52, and 65 kg/ha. Mouldboard plough, rotavator, tandem disc, offset disc, and heavy duty cultivator were evaluated as tools for incorporating yellow sweetclover prior to spring wheat planting. There were no significant differences due to implement for amount of soil nitrogen in the 0- 60 cm profile. Three dates of incorporation were evaluated for incorporating yellow sweetclover prior to spring wheat planting: June 15, July 1, and July 15. June 15 incorporation gave the best result in terms of the amount of soil nitrogen in the 0-60 cm profile (59, 66, and 70 kg/ha in the three years of the study), and for grain yield of the wheat (Foster, 1990).

Non-N Nutrient Contribution

Sweetclovers may facilitate P availability to subsequent crops (Schonbeck, 1988).

Effects on Water

Crimson clover showed a consistently-higher water use efficiency (g of dry matter produced per liter of water evapotranspired) than hairy vetch at 10, 20, and 30C. Sweetclover showed intermediate values (Zachariassen and Power, 1991).

Werenfels et al. (1963) conducted unreplicated field observations in long-term cover crop plots that had been established in 1924 and managed consistently until 1961 in an apricot orchard on a Yolo loam soil in Davis, California. The researchers suggested that infiltration rates (ring infiltrometers) were in descending order: alfalfa (Medicago sativa) > 'Hubam' white sweetclover (Melilotus alba) > sour clover (Melilotus indicus) > cereal rye (Secale cereale) > clean cultivation. The researchers stated that the alfalfa and white sweetclover alleviated a plow pan, but that cereal rye did not. Unfortunately, the unreplicated nature of the study severely limits interpretation.

Effects on Soil

Duke (1981) wrote that the biennial form of white sweetclover has a deep taproot and relieves soil compaction.

As reported by Wen-Cheh et al. (1989), leguminous cover crops cause acidification of soils. In the long run and in the absence of liming, this could lead to downward leaching of cationic nutrients and a decrease in base saturation. In terms of mg of H+ produced per g of nitrogen fixed, experiments in hydroponic tanks indicated a rating of red clover = crownvetch > alfalfa = white sweetclover. Extrapolations based on commonly-encountered field production values suggested that the amounts of acidity produced per year per hectare would be 4.2 to 9.4 kg for red clover, 5.2 to 14.0 kg for alfalfa, 4.5 kg for crownvetch, and 4.6 kg for white sweetclover.

Crimson clover showed a consistently-higher water use efficiency (g of dry matter produced per liter of water evapotranspired) than hairy vetch at 10, 20, and 30C. Sweetclover showed intermediate values (Zachariassen and Power, 1991).

Werenfels et al. (1963) conducted unreplicated field observations in long-term cover crop plots that had been established in 1924 and managed consistently until 1961 in an apricot orchard on a Yolo loam soil in Davis, California. The researchers suggested that infiltration rates (ring infiltrometers) were in descending order: alfalfa (Medicago sativa) > 'Hubam' white sweetclover (Melilotus alba) > sour clover (Melilotus indicus) > cereal rye (Secale cereale) > clean cultivation. The researchers stated that the alfalfa and white sweetclover alleviated a plow pan, but that cereal rye did not. Unfortunately, the unreplicated nature of the study severely limits interpretation.

Effects on Livestock

All Melilotus species contain a vanilla-smelling substance, coumarin, which embitters the older stems and leaves (McLeod, 1982). According to Duke (1981), coumarin is an anticoagulant. The following points are also based on accounts in Duke (1981): Yellow sweetclover seeds are poisonous to horses. 'Polara' and 'Denta' are low-coumarin varieties of white sweetclover; there are also low-coumarin forms of yellow sweetclover. Once plants have reached 20-25 in height, heavy grazing is essential to maintain palatability. Moldy hay of yellow sweetclover may poison cattle through a chemical bishyroxycoumarin.

Effects on Workers

Sweetclovers are tall and may interfere with workers (Bugg, pers. comm.).

Pest Effects, Insects

Duke's (1981) review indicated that sweetclovers are attacked by sweetclover weevil, Sitona cylindricollis, which can destroy a crop by defoliating seedlings. Sweetclover root borer, Walshi miscecolorella, and sweetclover aphid, Therioaphis riehmi (Boerner), damage sweetclover in the Great Plains. Some resistance is available for the latter. Blister beetles, Epicauta spp., feed on foliage of low-coumarin cultivars. Grasshoppers, leafhoppers, clover leaf weevil, cutworms, webworms, green cloverworm (Platyena scabra) and other caterpillars are seldom serious problems.

In a field trial in coastal Massachusetts, Bugg and Ellis (1990) found that high densities of nectarivorous predatory wasps (Hymenoptera: Vespidae and Sphecidae) were attracted to the flowers of annual white sweetclover (11 taxa). Buckwheat floral nectar also attracted several species of predatory wasps and appeared to be a more flexible means of attracting wasps, inasmuch as it grows rapidly and flowers sooner (30 days vs about 60 days). Of course, biennial forms of white sweetclover would have flowered still later than the annual form used here, i.e., during the subsequent year. Although several Fabaceae afford extrafloral nectar, white sweetclover appears to be among the few that provide floral nectar accessible and attractive to numerous predatory wasps. Buckwheat often ceases production of nectar during afternoon hours. Afternoon activity usually appeared low on buckwheat, whereas nectarivores continued to visit white sweetclover. No Ichneumonoidea were observed imbibing at the flowers of white sweetclover, and few were observed at blooms of buckwheat. Peak densities of sweetclover aphid, Therioaphis riehmi (Boerner), were observed on July 23rd. However, compared to what was observed in faba bean, sorghum, and hairy vetch, relatively few Coccinellidae were seen on white sweetclover. It is not clear whether sweetclover aphid was less palatable than aphids available on other cover crops.

Based on observations in New Mexico, flowers of yellow sweetclover attract predaceous wasps (e.g., Ammophila sp. [Sphecidae]) (Bugg, pers. comm.).

Pest Effects, Nematodes

Duke (1981) reported that the following nematodes have been isolated from white sweetclover: Belonolaimus gracilis, Ditylenchus dipsaci, Heterodera glycines, H. trifolii, Meloidogyne hapla M. javanica, Paratylenchus projectus Pratylenchus penetrans, P. pratensis, P. vulnus, and Tylenchorhynchus martini. On yellow sweetclover, the following nematodes were found: Heterodera glycines, H. trifolii, H. schachtii, and Meloidogyne hapla

Pest Effects, Diseases

Duke (1981) wrote that white sweetclover is attacked by 48 fungi, the bacterium Phytomonas lathyris, and cucumber mosaic, ringspot, and "Steinklee" viruses. Twenty-one species of fungi attack yellow sweetclover, as do 11 types of virus.

Pest Effects, Weeds

Sweetclovers as cover crops give poor weed control (Schonbeck, 1988). White sweetclover is small seeded and rather slow to establish. Problems with weeds should be expected (Bugg, pers. comm.). Sweetclovers may benefit during establishment from a companion crop of oat (Schonbeck, 1988) or some other small grain (Duke, 1981).

Annual white sweetclover (cv 'Hubam') was avoided by one farmer because of its tendency to reseed (Schonbeck, 1988).

In early May in an organic vineyard at Hopland, Mendocino County, California, fall-seeded mixed biennial white and yellow sweetclovers had a vegetational cover of 41.25+/-13.29 % (Mean +/- S.E.M.), which was significantly less than the cover afforded by most annual cover crops. A mid-May harvest of weeds (mainly chickweed, shepherds purse, rattail fescue, and annual ryegrass) from the sweetclover plots yielded a total weed biomass of 3.274+/-0.562 Mg/ha, Mean +/- S.E.M., which is 66.6% of the figure obtained from weedy control plots (Bugg et al., unpublished data).

Pest Effects, Vertebrates

The sweetclovers contain coumarin, an anticoagulant. (See Effects on Livestock.)