Sweetclover (Melilotus officinalis)
is a sweet-smelling, erect, annual or biennial herb that is native to Europe and Asia. It was cultivated as a forage crop and is now distributed throughout North America and Europe.
Under certain conditions, sweetclover can become toxic. Sweetclover contains a chemical called coumarol, which is normally nontoxic; however, if it becomes moldy, it causes coumarol to convert into dioumarol---which is toxic. This was first discovered in the 1940s, when cattle kept getting poisoned and dying from ingesting sweetclover silage and hay that had gone moldy. It was this discover that lead to the production of Warfarin, the first anticoagulant rodenticide put on the market. Besides poisoning rodents, Warfarin is also used in modern day medicine, as an anticoagulant (blood thinner).
If moldy sweetclover is eaten by horses, it can prevent normal blood clotting, resulting in hemorrhages and blood abnormalities such as severe hemorrhaging (bleeding), which is often internal. If eaten by pregnant mares, the toxin can cross with the placenta, adversely affecting the newborn foals and resulting in fetal re-absorption, stillbirths, or neonatal deaths.