It's important to remember that poisonous plants or trees don't affect all horses alike, and that some plants or trees cause poisoning only during certain seasons of the year because they produce toxic agents only during particular stages of development. Poison hemlock, for example, is quite toxic and commonly grows in the Midwest along fence rows and other fertile, uncultivated areas. It affects the horse's with incoordination, muscle tremors, and usually leads to muscle paralysis and death. Whether green or dried, the wood, bark, leaves, and seeds of the Japanese yew are poisonous to horses as well as humans, dogs, cats, and other large animals. This ornamental bush is dangerous all year long.
Clippings are a common cause of poisoning in horses.
When consumed, it can cause sudden heart failure and death. In the Midwest, white snakeroot can be a problem in shaded, wooded areas. The toxin from this plant can be transmitted through a mare's milk. This in turn can affect the foal. When ingested, it can result in muscle tremors, incoordination, muscle weakness, heart damage, and death.