I got a call today to come identify a maple tree to assist a vetranarian who was treating a dying horse. I had heard that "Red Maples" were toxic to horses but did not truly believe it. After identifying the Acer ruburm I now fully believe that "Rubrum's" are toxic. Research has now educated me to the fact that just three pounds of fall leaves can kill a large horse and just six pounds is sure death. "Rubrums" apparently don't affect any other animal than horses but horses I can attest they do. But please remember that 'Rubrums' are just one of the many maple species and have nothing to do with Japanese maples.