Sweet clover (Melilotus alba and M. officinalis) is grown as a green manure and hay crop in the northern U.S. and in Canada. Its coumarin content gives it a distinctive sweet odor similar to vanilla. Its use as hay was widespread in the 1920s. A series of wet summers led to an epidemic of “bleeding disease” in cattle. Use of the forage is less common now and low- melilotoside varieties are available. Sweet vernal grass (Anthoxanthum odoratum) is a coumarin-containing forage in England that also causes “bleeding disease”. Hay with >10 ppm dicoumarol should be viewed with caution.
How Poisoning Occurs:
Plant enzymes in sweet clover partition the aglycone coumarin from melilotoside. When sweet clover is put up as hay it is easy for it to be contaminated with molds because of its succulent stems. This is particularly likely when conditions are wet at cutting or curing or when foliage is lush. Molds such as Penicillium nigricans , P.jensi , and the Aspergillus metabolize the coumarin into dicoumarol. Dicoumarol is similar in structure to vitamin K. When consumed by livestock it inhibits vitamin K production. Vitamin K is necessary in the body to activate prothrombin. When tissue is damaged, thromboplastin is released and converts prothronin to thrombin. Thrombin alters the solubility of fibrinogen in blood and causes it to clot and seal the tissue damage. Dicoumarol prevents this process. Warfarin is a synthetic toxicant derived from coumarol. It is used in rat, gopher, and ground squirrel poisons and also acts as a vitamin K inhibitor to block the blood clotting process and provoke hemorrhaging. It is toxic to livestock that may accidently consume it in its pelleted form and to dogs that consume poisoned squirrels,etc.
Symptoms of Poisoning:
Bleeding disease “only in cattle?”
1. mild cases – extended blood-clotting time that can lead to hemorrhaging when livestock are castrated or dehorned.
2. severe cases – internal and/or external hemorrhaging cause pale mucous membranes, subcutaneous swellings of pooled blood, weakness, and death.
3. treatment – injections of vitamin K, whole blood transfusion.
Sweet Clover as a Weed
Check out this video from AgPhD to learn about how sweet clover can also be a weed in grain crops and how to control it.
To learn more about Sweet Clover, check out this online extension publication from Montana State University.