Spoiled Sweet Clover
Yellow sweet clover (Melilotus officinalis) and white sweet clover (M. alba) are legumes commonly grown in the northwestern United States and western Canada as a forage for livestock and they grow wild over the entire North American continent. As biennials, both species grow to 5 ft (1.5 m) in height and have compound leaves with three leaflets that have serrated edges, with the terminal leaflet on a stalk. The leguminous flowers are produced in axillary racemes up to 5 inches (12 to 13 cm) in length. The flowers are yellow or white, depending on the species.
Neither yellow nor white sweet clover is toxic, unless as hay or haylage they are not properly cured and become moldy. Mold, such as Penicillium spp, converts the nontoxic coumarin they contain to dicoumarol. Dicoumarol has strong anti-blood clotting properties that interfere with production of vitamin K-dependent clotting factors. The rate of depletion of these factors is directly related to the duration and amount of dicoumarol ingested.
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Moldy and improperly cured sweet clover hay or haylage, although not always toxic, should only be used for animal feed after they have been tested for the presence of dicoumarol. In one survey of 272 cured sweet clover hay samples, over one-third contained potentially harmful amounts of dicoumarol. The concentration of dicoumarol tends to be higher in large round bales where the hay is more likely to have a higher moisture content. Moldy dicoumarol-containing sweet clover may be fed to livestock as long as it does not constitute more than 25% of the animals' total diet, although feeding any moldy feed is not recommended. There are varieties of sweet clover that contain very low levels of coumarin; hay or haylage from these varieties will not cause decreased blood clotting, even if moldy. However, to prevent the risk of moldy sweet clover poisoning, sweet clover hay or haylage should not be fed for at least 2 to 3 weeks before foaling or elective surgery.
Generally, dicoumarol-containing sweet clover hay or haylage must be ingested for several weeks before clinical signs occur. The problem occurs most commonly in cattle, but horses and sheep may also be affected. Generally, only a few in a herd will be affected, but if untreated, mortality is high. Signs of poisoning include hemorrhaging from the nose, intestinal tract, or into body cavities and soft tissues. Hematomas may develop over areas of the body that are prone to trauma. Excessive hemorrhaging after any laceration is likely and can be life threatening. The degree of anemia depends on the amount of blood loss and is reflected by increased heart rate, pale mucous membranes, weakness, and other signs of shock. Blood clotting time is markedly increased from a normal of 3 to 15 minutes.
Affected horses should be treated with whole blood transfusions if necessary. Intravenous vitamin K1 administration controls hemorrhaging in 3 to 6 hours. If vitamin K3 is given by injection, therapeutic benefits may not occur for 1 to 2 days and excess may be harmful.
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