I worked in a flour mill for 2 years,
anything that touched the floor or was contaminated in anyway,
sometimes as simple as to much or to little protein in testing, or cross contamination between flour/grain types, would all be mixed, packed and sold as animal feed.
there are also a lot that the farmers grow that doesn't meet the requirements,
lower quality or a level to high of foreign objects when harvested (weeds or other plant material)
that can only be sold as animal feed
funny enough though, the highest grade is always kept for export as it returns the best dollar value,