The practice of feeding animal wastes and by-products back to animals is an age-old recycling strategy.  Hair, bones, blood, feathers, offal, mortalities, and manures were once routinely reprocessed and used as ingredients in commercial feed for pets and livestock.

Generally, non-ruminant by-products were fed to ruminants, and vice versa.

However, the U.S. Food and Drug Administration (FDA) now bans most  mammalian protein sources from ruminant (cattle, goats, sheep) feeds.  Poultry litter is also banned as a feed ingredient for ruminant animals, since poultry feeds may contain ruminant by-products.