According to eXtension.org, “by-product (often also called co-product) feeds are often used in animal diets.
These are by-products of other industries, such as the production of distilled spirits, ethanol, wheat processing, milling, and soy oil among others.
These co-products, such as brewer’s grains, distiller’s grains, soy hulls, and others can make excellent animal feedstuffs.”
It says farmers can also take advantage “of by-products from the wheat milling industry, such as wheat bran, middlings, red-dog, shorts, etc.
By-products from wet corn milling give us high fructose corn syrup and a variety of other corn products including corn gluten feeds and meals.
In addition, there are products such as citrus pulp, beet pulp, and whole cotton seed. Some farms even feed expired foods that have been returned to distributors from grocery stores.”
The advantage are that they are often very cheap compared to the traditional feeds and utilize a material that might otherwise become a part of the waste stream.