Moldy sweetclover poisoning

Attention! This is a potentially life-threatening condition for your horse. Time is of the essence, contact your veterinarian immediately.Find a Vet

Moldy Sweetclover Poisoning

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.

Factors That Effect Sweetclover Toxicity

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).

Effects of Poisoning in Horses

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.


Bleeding abnormalities
Pale mucous membranes
Difficulty swallowing
Spontaneous unilateral nosebleed
Black, tarry manure
Swellings over parts of the body


  • History
  • Clinical signs
  • Analysis of serum
  • Feed and hay analysis



Remove horse from suspected hay or pasture
Four daily intravenous injections of Vitamin K3
Administering fresh frozen plasma
Single whole blood transfusion


  • Always inspect hay for any signs of mold prior to feeding to horses. Dispose of any hay that shows signs of mold growth.
  • Ensure they hay is well cured before baling

Scientific Research

General Overviews

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Risk Factors

  • Feeding horses sweet clover hay or leading them graze on pastures containing these plants.