Black walnut (Juglans nigra)
is a large tree which is often planted as an ornamental or for its hardwood value. Wood shavings used for bedding for horses that contain as little as 20% of black walnut wood will cause laminitis
in horses. Close association with walnut trees while pollen is being shed (typically in May) also produce allergic symptoms in both horses and humans. There have been reports of horses experiencing colic and respiratory distress after chewing on the bark of black walnut trees.
The toxin responsible for black walnut toxicity in horses is juglone, a natural phenolic compound produced by the tree. The toxin is thought to be absorbed through the horse's coronary band and skin while they are standing on shavings. Horse owners also need to take caution when using wood chips in pastures, for drainage or landscaping, as these products can sometimes be produced by black walnut trees.
Clinical signs of black walnut toxicity (relating to laminitis) usually develop in affected horses within 8 to 18 hours after exposure to the toxin.