Animal by-products are often the parts of the animal that Americans don't like to eat—organ meats, blood and bone from mammals—and can also include necks, feet and underdeveloped eggs from poultry. (Horns, hair, teeth, hooves, intestinal contents and feathers are prohibited.) By-products can add flavor and nutrients to pet food. In fact, these ingredients often have higher levels of iron, copper, calcium, vitamin A, B12 and many other essential nutrients than muscle meat.
By-products' bad rep is likely due in part to marketing—companies advertising as having "no by-products" make these ingredients sound bad. Other concerns may stem from the fact that some (but not all) by-products are rendered into powders or "meals." Rendering is the process of cooking an ingredient to separate the fat and protein out, resulting in a dry, high-protein meal that is easily stored, transported and incorporated into dry pet foods. This process has gotten negative press as a way that "gross" things could be sneaked into your pets' food. But, while humans may find by-products unappetizing, they can be a healthy addition to pet food.