Source: Magness et al. 1971
The sweet clovers are native to temperate Europe and Asia. Although reported as found in Virginia as early as 1739, it was not until the present century that their great value for soil improvement, pasture, hay and silage became recognized. During the decade 1948-57, seed production in the United States averaged about 46,000,000 pounds annually with near 16,000,000 additional pounds imported - sufficient to seed more than 5,000,000 acres. Since 1960, however, seed production has declined in this country to an annual average of about 21,600,000 pounds. Most of the sweet clovers grown here are biennial, although some annual kinds are grown. They are adapted only to soils that are near neutral in acidity. Major production is in a belt from the Great Lakes west to Montana and south to the Gulf of Mexico in areas baving 17 inches or more annual precipitation. All the biennial sweet clovers fix large amounts of atmospheric nitrogen. The deep-penetrating tap roots decompose after the second year, so the crops are very useful for opening up subsoil. They are therefore probably the best of the crops for soil improvement.
The sweet clovers have a high content of coumarin which reduces palatability. More important, in hay spoiled due to excess moisture when stored or in improperly prepared silage, dicoumarol, which reduces the clotting of blood, is formed. Animals fed such hay or silage are subject to excessive external or internal bleeding.
The species of Melilotus of most value agriculturally are described as
White sweet clover
Yellow-flowered sweet clover