Chances are good that you have some of the more than 100 toxic plants in your horses' field. There's been some talk on the Lists of cocoa shell mulch poisoning. Do not burn the smoke is toxic, too. Believe it or not, one cup of seeds from these plants can kill a pony. The fleshy part of the fruit is fine for them to eat, but the seeds contain cyanide and the leaves contain cyanogenetic glycosides, which cause depression, colic, convulsions, and death.
Wilted leaves are especially toxic.
Ingestion of one pound will cause respiratory failure and eventual death. This weed is sometimes found in grain that is not well cleaned at the feed mill, and is also sometimes mixed in with hay. One mouthful for a horse equals stomach hemorrhages and a slow, painful death. Fortunately horses usually dislike the taste. Check all your feed for any questionable material it might contain.