Apologies for the double post, but cannot edit the previous one anymore - I forgot to mention bread.
Old, dried bread - horses love it. At our stables there is always a big crate and whoever can get their hands on some bread - they throw it in there for shared use (some people bring in loads of it, much more than their horses would eat - likely left-overs from restaurants etc.).
Only concern that I am aware of is to make sure it does not have any mold - sometimes it is easily detected on the surface, but on other occasions it can develop inside larger pieces, especially if an air pocket has formed. Usually pieces that are larger than a fist and these would anyway need to be broken before feeding, so not too big of an issue with due care.