Sweetclover

Sweetclover (Melilotus spp.)

Plant Species
From Montana Interagency Plant Materials Handbook *
By S. Smoliak, R.L. Ditterline, J.D. Scheetz, L.K. Holzworth, J.R. Sims, L.E. Wiesner, D.E. Baldridge, and G.L. Tibke

Sweetclover is a legume that did not become of major economic importance until the last 50 to 60 years. It appears to have originated in the same general area in southwestern Asia as alfalfa and red clover. It is widely distributed over the world, and was used as a green manure and honey plant years ago along the Mediterranean. Even so, the economic importance of sweetclover today is limited almost entirely to the United States and Canada.

Sweetclover came into the United States during the colonial period, probably as an impurity in other forage seed. It grew mostly as a weed along fence lines and similar areas. Around 1856 it was distributed in Alabama, and was grown as a hay crop for a number of years on the black, limestone soil of Alabama and Mississippi. It also has been used for a long time by beekeepers as a source of nectar and pollen. However, it was considered mostly as a weed until about 1910.

Description

Sweetclover is a sweet-scented, upright, broad-leaved legume. The plants have many stems and branches. The leaves are 1/2 to 1 inch long and consist of three leaflets, which are mostly saw-toothed along the edges. In the seedling year, plants develop to a height of 12 to 36 inches. In the second year, growth develops from the base, and at maturity, plants average 4 to 5 feet in height. Also in the second year, flower racemes grow from the axils of the leaves and produce many florets, each of which may develop a seed pod containing one and sometimes two seeds.

The leaves and their leaflets are rather like those of alfalfa, but in contrast with alfalfa, the leaflets are toothed around the whole of their margins. The sweetclover plant is many branched and becomes woody as it matures. It may be grown as an annual, but is usually a biennial.

There are two common types of sweetclover, white-flowered and yellow-flowered. The white-flowered sweetclover, Melilotus alba Dser., is taller, coarser-stemmed and has coarser leaves than the yellow-flowered type, M. officinalis L. Yellow sweetclover is more drought tolerant, more vigorous as a seedling, flowers earlier and has spreading growth and purple flecks on the seed coat.

Growth in the first season consists of one central, much branched stem. Toward the end of the first year, several buds form at the crown, usually just below the ground surface. In the spring of the second year, the crown buds start growth quickly with vigorous, rapidly-growing stems that are much taller and coarser than in alfalfa.

Annual or sour sweetclover, Melilotus indica, is an annual type of yellow sweetclover. It may be grown for green manure on irrigated land.

Adaptation

Sweetclover is adapted to most of the environmental conditions found in North America. It is extremely drought resistant and grows well on those parts of prairies and the Great Plains. It has shown itself to be very winterhardy.

Sweetclover is winterhardy and productive, especially on fertile, well-drained clay and clay-loam soils. It can also be grown successfully on sandy loams and heavy, clay loams. It grows best on neutral or alkaline soils, and is one of the best legumes to grow on highly-alkaline soils. Sweetclover can also be used in the revegetation of mechanically-disturbed lands, but it should not be planted on land that is subject to flooding or on acid soils.

Sweetclover is an excellent source of nectar and pollen for honey bees. It produces good yields of high-quality honey.

Limitations

Seeds of sweetclover have a hard seed coat and should be scarified before seeding. Seed companies sell only scarified seed. Some seeds may remain in the soil for one to many years before germinating, so volunteer plants often develop on land sown originally to sweetclover. New seedings of sweetclover should be inoculated with the proper nitrogen-fixing bacteria.

Like alfalfa, it is very sensitive to acid soils, and growth is severely limited below about pH 6.0.

Although it tolerates a few days of flooding before growth begins in spring, it cannot endure flooding during the growing season. Like alfalfa, it is intolerant of waterlogged soil or poor internal soil drainage.

The high coumarin content of sweetclover makes it less palatable to livestock than many other legumes. However, low-coumarin varieties are now available.

Sweetclover plants decline in quality as they reach maturity and become weedy and stemmy.

Sweetclover weevils frequently damage seedlings and, in dry years, they can prevent establishment of the crop. Since infestation occurs from established stands, new seedings should be made as far from them as possible. Occasional damage to seedling stands may also occur from cutworms and grasshoppers.

Since sweetclover is very subject to injury from herbicides, particularly 2,4-D, weed control during the establishment year is more difficult than with other legumes.

Use for Hay

Sweetclover gives a high yield of hay, but it is difficult to harvest. Quality is high if the crop is cut in the bud stage. Delaying cutting until full bloom results in stemmy, low-quality hay since the large, moist stems take longer to dry than the leaves, which become brittle and shatter badly. The percentage of crude protein declines from 21 percent in young plants to 13 percent at the hay stages; at the same two stages, the percentage crude fiber is 19 percent and 36 percent, respectively. Coumarin, which gives this forage a sweet aroma, develops into dicoumarol, a potent anticoagulant, under poor curing conditions. This chemical causes "sweetclover disease" in livestock in which the blood loses its normal ability to clot and death results from internal bleeding or minor injuries.

Although most sweetclover hay is safe for feed, improperly cured or moldy forage should be used with caution. Samples from such forage should be sent to a feed testing laboratory for a chemical analysis of dicoumarol content. The danger of losing animals from bleeding can be lessened by feeding sweetclover for two weeks, followed by two weeks of feeding other forage. Spoiled sweetclover should be disposed of and not fed to animals.

As with seedlings, regrowth after hay cutting must come from stem buds. Therefore, hay should be cut at least 12 inches high if second growth is desired.

Use for Pasture

Sweetclover, particularly white sweetclover, produces good pasture yield. Growth begins early in spring with rapid production in June and July. A height of about 12 inches should be maintained throughout the summer to allow for rapid regrowth and good quality. This height is desirable in order to allow some green leaves to synthesize carbohydrates, while allowing light to get into the base of the stems so that active buds remain alive to recover growth. It is also necessary to prevent the forage from becoming tall and woody.

When growth has stopped in late autumn, seedling stands may be grazed without affecting yield the next year. Regrowth after hay harvesting can also be utilized by livestock.

Bloat can be a problem when grazing sweetclover, although it is less common than with alfalfa or true clovers. Scouring may also occur, especially when growth is young and succulent. If the animals have access to dry roughage, both conditions will be reduced.

Soil Improvement

Sweetclover is one of the best legumes for soil improvement. The widely-branched, deeply-penetrating tap roots open up the subsoil. The roots use nutrients not available to plants with shallow roots. When sweetclover plants decay, the nutrients in the roots are released for use by other crops.

When properly inoculated, sweetclover contributes considerable organic matter and nitrogen to the soil. When plowed under in the year after seeding, the succulent top growth and the fleshy, heavy roots decay rapidly. This improves aeration, drainage and the general physical condition of the soil.

Plow under when sweetclover plants are about 6 inches high in the year after seeding. Late plowing adds more organic matter, but if the growth is heavy, the surface soil may dry out and the organic matter may break down slowly. Release of nitrogen to the succeeding crop requires activity of the soil microflora, and late breaking will not allow adequate time for decomposition and nutrient release. Heavy growth of sweetclover, when plowed under, may form an insulation layer that prevents root penetration and good establishment of the crop following the sweetclover.

Silage

Sweetclover should be cut for silage at the 10 to 20 percent bloom stage of flowering. Moisture content should be from 65 to 72 percent. Sweetclover should be conditioned and lie in the swath for two or three hours on a sunny day, or longer on a dull day, before cutting with a forage harvester. When the silage pieces are 1/2 inch long, and a squeezed handful slowly breaks apart when hand pressure is released, it is ready for ensiling. If the squeezed handful falls apart when pressure is released it is too dry, and if it stays in a firm ball it is too wet. Silage needs firm packing as soon as it is placed in the silo to eliminate air, to assure rapid fermentation and to avoid spoilage. The silo should be filled as quickly as possible to limit spoilage. The surface layer should be of another forage that is less susceptible to mold than sweetclover. Place a plastic cover over the silo as soon as it is filled. Spoilage of sweetclover silage is dangerous because of the problem of sweetclover or bleeding disease.

Bee Pasture

Sweetclover is an excellent source of nectar and pollen for honey bees. It produces good yields of high-quality honey.

Seed Production

Sweetclover can be grown for seed on either dryland or irrigated land. Spaced rows or solid stands may be used. For pedigreed seed production, spaced rows are preferred to permit easier roguing of weeds and off-type plants.

Honey bees are essential for pollination to assure seed setting. One to two colonies are needed for each acre. If you depend on natural pollinators, the yields may be 50 to 100 pounds per acre, whereas, if you use honey bees the yields can be 500 to 700 pounds per acre.

Sweetclover is especially sensitive to some herbicides. Flower drop results from herbicide drift of spray or dust. Growing tips of branches become distorted at higher concentrations of herbicide drift. Plants may flower after such damage, but the seed crop may be too late to mature.

Seed pods turn brown, dark gray or white when mature. It is critical to swath when two-thirds of the seeds are mature. Within a week most of the immature pods ripen in the swath, so that the crop can be picked up with a combine. Seeds drop faster as they mature, and a crop may be lost by delayed swathing or by a wind storm. The swath is usually very fluffy, and may not feed well into the combine. Swath and pick up the seeds when they are damp such as in early morning, to lose fewer seeds from pod drop. Use a slow cylinder speed and wide clearance of concaves to avoid shelled or broken seed, and to obtain maximum seed yields. The hulls should also be left on because this makes cleaning the seeds easier.

* The Montana Interagency Plant Materials Handbook (EB69) is no longer in print, but is available for viewing in
Montana County Extension Service and National Resource Conservation Service Offices.