I picked a part on rendering for the board........ but there is an entire article on the site
http://www.critterchat.net/vet2.htm
Feeding Your Animals
Selecting a Commercial Food
Jean C. Hofve, DVM
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THE 4D'S. Animals that are dead, dying, diseased, or disabled prior to reaching the slaughterhouse are known as "downers" or "4D" animals. These are condemned for human consumption, and are generally sent for rendering along with other parts and items unsuitable for human use, such as out-of-date supermarket meats (along with their plastic wrappers), cut-away cancerous tissue, and fetal tissue (which is very high in hormones). Rendering produces two major items: animal fat or tallow, and a processed product usually called "meat meal" or "by-product meal." Rendered ingredients vary greatly in quality. Many rendering facilities are closely associated with slaughterhouses, which are in turn connected with feedlots or poultry farms. Such rendering facilities are more likely to produce a good quality, relatively pure meal. They are likely to be designated with the name of the source animal, such as "chicken meal" or "lamb meal." Virtually all "natural" and "superpremium" foods use meals as the first ingredient. Meals do contain higher proportions of protein that plain meat, since the fat and water have been removed.
Independent renderers are reported to accept for processing such items as road kill, euthanized shelter dogs and cats, and other unappetizing ingredients. These items are not supposed to find their way into the food chain but are theoretically converted for use in fertilizers and industrial applications. However, over the years there have been numerous unproven reports of this material being processed into dog and cat food.
The Center for Veterinary Medicine, a branch of the Food and Drug Administration, admits that dead dogs and cats are commonly rendered, and although there is no legal prohibition against it, they do not "condone" the practice. <SNIP>
READ ENTIRE ARTICLE:
http://www.critterchat.net/vet2.htm
