Just a few sprigs can kill a horse. Often, illness and death result when a horse eats clippings thrown to it after the shrubs were trimmed. Oak, black locust, red maple, and black walnut trees can be toxic to horses. If these are in your pasture and your horse finds them attractive, then the trees should be fenced. Wild cherry trees, found in fence rows, are also toxic.
Horses are less likely to consume toxic quantities if suitable forage is available.
Buckwheat, St. Johnswort, and alsike clover can cause extreme sensitivity to sunlight. Skin without pigment, such as a blaze or white nose, may become swollen and eventually slough. People often mistakenly blame something else, such as fly spray. White snakeroot, another potentially poisonous plant for horses, grows in undisturbed shaded areas. In the muggy, wet weather of mid-summer, clover may get black patch disease, which is caused by a mold that also produces a potent agent that causes sudden salivation in horses.