We have a grass area in which I would like to graze a colt, but it has two cherry trees in it. I am having the trees removed, but there are still some old leaves from last Fall in the grass, and it would be nearly impossible to remove all of them. Since these leaves are from last Fall, has enough time passed to render them harmless? There is also a lot of bark that came off the trees when we removed them; is this part toxic? If so, what are the species of cherry trees that are toxic to horses?
All species of cherry trees are toxic to horses. The seeds, leaves, and bark are the most toxic parts of the plant; fruits are the only relatively safe part. As for the leaves, usually the toxicity is worst when they are in the stressed state prior to dying or are wilted (e.g. Leaves on a fallen tree limb lying in a pasture). This is the point when the leaves are not only toxic but very tasty because they have stored a lot of sugar and are sweet and palatable.
Once the leaves have fallen off the tree in the autumn they are dead and no longer palatable to horses. Dried leaves will also contain much less toxin than freshly wilted leaves. By this point, horses will typically not hunt out leaves and consume them because the remaining grasses are much more palatable to them. If a horse consumes a few leaves here and there it is not enough to be a problem. However, putting an exact amount on what they need to consume is not possible either. No research has been done on this. Clinical veterinary findings of poisoning cases usually directly involve downed tree limbs where the horses have cleared off all the leaves.
It is advisable to make sure your horses have plenty of grass, hay or other forage to deter them from eating leaves or anything else they typically would not consume. Cherry leaves contain a precursor to cyanide which binds and interferes with the cellular use of oxygen. If an animal does consume a large enough amount of this toxin, it will basically suffocate. Early signs of poisoning are hard to catch since they come on very rapidly. Death is usually the first sign that something is wrong. If anything is suspected a veterinarian should be called immediately. An antidote can be administered, but it needs to be done as soon as possible.