Always make sure your enviroment is safe for livestock. Even an innocent looking apple tree can cuase problems. Horses love apples, but turn one into a yard or pasture with an apple tree in the late summer and you will have a colicky horse in less than 1 hour.
Horses usually know enough not to eat poisonous plants, but owners should be aware and keep there livestock safe.
Never feed livestock yard clippings. It is serious, a threat to your livestocks' life, and health.