My definition of foreign material is anything that cannot outrun the baler. This would include paper, plastic, aluminum cans, insects, rodents, and snakes to name a few. Any of these materials would make the hay unacceptable for horses. Foreign material can also mean weeds or other plants that are not intended to be in a particular type of hay. For example, grass or weeds are not supposed to be in pure stands of alfalfa. Some weeds can be toxic if fed to horses. It is always a good idea to carefully inspect hay for foreign material when it is purchased and to examine it again when it is fed to horses.