The first thing that had to be tackled was figuring out just what ARE by-products in pet foods?
Well, the answer is two-fold. There are two different kinds. There are "meat by-products," and "XYZ by-product meals."
Searching, the first site I went to was about.com, because their articles about pet food usually aren't that bad. http://cats.about.com/od/catfoodglossary/g/meatbyprod.htm
What AAFCO doesn't mention is that meat byproducts may also legally contain: "4D animals (dead, dying, diseased, down), road kill, euthanized cats and dogs, including their collars. These source products are rendered, the fat is siphoned off to be used as "animal fat," and the remaining material is extruded to form "meat by-product meal."
"Meat," by definition, is limited to the meats of cows, swine, sheep, and goats."
Also Known As: Meat By-Product Meal
Examples: Meat by-products is a substandard form of protein, used by many popular pet food manufacturers because it is cheaper than using muscle meat
Wait - what? There's no difference between "meat by-products" and "meat by-products meal?" If both include 4D animals, and that's the issue, then I agree. I wouldn't want a food with by-products in it. And I feed the ferals Friskies. Those have by-products.
Well, it turns out the information in the about.com article is NOT correct. There IS a difference between by-products and by-product meal.
AAFCO definitions: http://www.braypets.com/FRR/aafcodef.htm
Common ingredient definitions: http://www.skaervet.com/documents/Common%20Pet%20Food%20Ingredients.pdf
AAFCO definition of meat by-product:
"the non rendered, clean parts, other than meat, derived from slaughtered mammals. It includes, but is not limited to, lungs, spleen, kidneys, brain, livers, blood, bone, partially defatted low-temperature fatty tissue and stomachs and intestines freed of their contents. It does not include hair, horns, teeth and hooves."
AAFCO definition of meat meal:
"the rendered product from mammal tissues, exclusive of blood, hair, hoof, horn, hide trimmings, manure, stomach and rumen contents except in such amounts as may occur unavoidably in good processing practices."
Note: no mention of "slaughtered" animals, but the product is rendered.
So" meat by-products" are NOT a rendered product. That's "meat meal," or "chicken by-product meal," or "XYZ protein meal" or "XYZ protein by-product meal."
"Meat by-products" are actually stuff that for the most part is actually good for cats - something they'd be eating if they were hunting, for sure, so at a minimum is a species-appropriate source of nutrition/protein. So why are they considered a sub-standard form of protein to be avoided?
Franny Syufy in the about.com article says they're potentially from 4D animals. But given that meat by-products are not rendered and ARE from slaughtered animals, they most likely can't be from 4D animals (dead, dying, diseased, disabled (downed)).* These 4D animals are considered unfit for human consumption and are not allowed in the human food chain, but do go to rendering plants, and they are allowed in pet food. I can totally understand why "xyz MEAL" is an ingredient to be avoided. (More on that and rendering later), but I'm still curious why meat by-products are considered an issue.
*I still haven't been able to determine if 4D animals can go to into non-rendered pet foods. Is "chicken" in a pet food by definition NOT a 4D animal? Is it just the rendered ingredients that have 4D animals in them? No answer to this question yet.
This paper on the AAFCO and the Pet Food Industry written in 2006 does a decent job of explaining. http://leda.law.harvard.edu/leda/data/784/Patrick06.html "Deconstructing the Regulatory Façade: Why Confused Consumers Feed their Pets Ring Dings and Krispy Kremes."
All ingredients must be listed according to AAFCO’s “common or usual” names; labels list “poultry meal,” “meat meal,” and “animal by-product meal” as ingredients, rather than poultry guts, feet and beaks. These common and usual names leave consumers asking: what the heck are meals and by-products? Are they good or bad for pets? Are some of them better than others?
However, before we blame the pet food industry for selling our pets these rejected parts in glossy packages adorned with pictures of wholesome chickens (which arguably constitutes misbranding under the FFDCA), consider that by-products might be good for your pet. The heart, liver, lung and brain of animals are considered high quality food ingredients by animal nutritionists. Furthermore, a cat or dog in the wild most certainly eats these “by-products” every time it consumes its meals. A wild cat doesn’t selectively remove the meaty muscle parts of the mouse while carefully discarding the bones and liver.
Unfortunately, also included in these “other parts” are the so-called 4D tissues, or “meat that came from animals that were dead, dying, diseased or disabled before they reached the packing plant.” Such animals are rejected for human consumption and shipped to rendering plants where they are made into bone and meat meals. More importantly, the inclusion of such tissues in pet foods violates the FFDCA. Such items are diseased and therefore “adulterated” under 21 U.S.C. § 342.  So why doesn’t the FDA bring an enforcement action for the industry’s blatant violation of the FFDCA? Most likely, the pet food industry uses such ingredients because they are cheap and while consumers remain oblivious to the inclusion of these diseased ingredients into their pets’ foods, the industry faces no opposition. Until the FDA feels external pressure, either from consumers or the industry itself, the FDA lacks incentive to enforce its own regulations. Comparatively, the FDA stringently enforces its human food regulations where it faces informed and vocal consumers and industries fearful of negative publicity.
Beyond the issue of the quality of the ingredients is the processing of the “meat and bone meals” themselves. Dr. Wysong, a veterinarian who has researched pet foods extensively and produces his own pet foods, notes that “processing is the wild card in nutritional value...” This is because the nutritional quality of meat and poultry by-products, meals and digests can vary from batch to batch due to the inconsistency of the raw materials used by rendering plants. Even assuming that the by-products possess nutrients pets need, there is no proof that modern pets are able to digest these ingredients after the harsh rendering and cooking processes. Thus, there is no proof that pets are able to obtain any nutrients from these cooked ingredients.
Furthermore, rendering does not necessarily destroy the hormones fed to livestock or the antibiotics, drugs, and even barbiturates used to euthanize animals. Over time defenseless Fido ingests a significant amount of antibiotics and euthanization drugs. It doesn’t take a veterinarian to conclude that the presence of such “extras” can’t be good for your pet. It is important to recognize that AAFCO’s “common or usual name” regulations hide the truly dangerous components of your pet’s diet in benign sounding ingredients such as “meat meal” and “animal by-product.”
OK - but even THIS discussion doesn't address the meat by-products, JUST the rendered stuff (meat by-product meal). But it provides a point that I haven't found discussed elsewhere as re: meat by-products: batch consistency. Whose to say that batch of by-products isn't mostly intestines or stomach as opposed to lung and liver? So far, that's really the only downside I'm finding to non-rendered by-products. Of course, feeding a food with non-rendered by-products over time means you get different batches, so you get different nutritional profiles of the differing ingredients.
IMO, so far I think the non-rendered meat by-products are species-appropriate for cats, and I'm not yet sure they're not as good a source of protein as "meat."
Meat meal, chicken meal, by-product meals, etc. are a different story. From the same paper,
Instead, United States rendering companies pick up 100 million pounds of “waste material” every single day. This “waste material” includes: heads, feet, stomachs, intestines, spinal cords, tails, restaurant grease, feathers, bones, and dead or diseased animals rejected from slaughterhouses. Remember that under FDA and USDA regulations half of every cow and at least one third of every swine is not consumable by humans. Cancerous tissue, tumors, contaminated blood, injection sites and any tissues treated with a substance not permitted by or in excess of FDA or EPA limits is also rendered. The inclusion of such items in pet food violates the FDA’s requirement regarding unadulterated food. Recall that foods containing “any part of a diseased animal” is deemed adulterated.  With an understanding of the rendering process and its ingredients, it is then unclear how AAFCO (and thereby the FDA) approves ingredients such as meat and bone meal for use in pet foods.
In addition to the “waste material,” six to seven million dogs and cats killed every year in animal shelters make their way into rendering vats. The city of Los Angeles alone sends 200 tons of dogs and cats to a local rendering firm every month. Road kill that is too large to be buried roadside, expired grocery store meats, and dead zoo animals are also thrown into the mix. Recall from the discussion of the AAFCO ingredient definitions that meat and bone meal must exclude hair and stomach contents “except as may occur in good manufacturing processes.” Considering that a 40 lb bag of dog food costs only $15-$17, that price cannot possibly cover the amount of time necessary to remove all the hair and stomach contents from the thousands of diseased and euthanized animals thrown into the rendering vats, not to mention the Styrofoam and saran wrap packaging from expired grocery store meats. In fact, it seems downright impossible. The rendering industry readily admits that meat wrappers are mixed in with its raw materials, their inclusion betrayed by the presence of polyethylene (used to make saran wrap) in rendered products.
Although pet food companies claim they don’t buy meat and bone meal from rendering plants that accept cats and dogs, the rendering industry acknowledges it would be impossible for a manufacturer purchasing products from rendering firms to know the exact raw materials of what they’re buying. An employee of the rendering industry points out that cats and dogs can easily be included in chicken by-product meals because of the similar protein content. Moreover, a rendering executive claims that Ralston purchased meat meal from his rendering facility for years, which included dogs and cats. Although somebody at the rendering plant finally revealed the true contents of the meat meal, the industry executive is quick to point out that only Ralston stopped purchasing from them, implying that the facility continues relationships with other pet product manufacturers.
The exact proportion of cats and dogs to cows and pigs in any given rendered production batch is difficult to determine. One rendering company estimated that it “rendered somewhere between 10,000 and 30,000 pounds of dogs and cats a day out of a total of 250,000 to 500,000 pounds of cattle, poultry, butcher scrap and other materials.” Some states have attempted to establish precautions against this quasi-cannibalism. For example, California law requires that rendered dogs and cats be labeled as “dry rendered tankage,” a product which is rarely used in pet food. However, due to the uncoordinated efforts of the pet food regulation system, such precautions are practically useless when pet food manufacturers operate on a national and often global scale. Consider that it is perfectly legal for tankage shipped outside of the state of California to be labeled as meat and bone meal. Moreover, California does not inspect meat and bone meals imported from outside the state.
OK. Even though the former AAFCO President is on video saying "Fluffy" may be in pet food, forget the dead cats and dogs being in XYZ meal (and their collars, tags, and flea collars, that are not removed prior to rendering). They're not in chicken meal.
But saran wrap, etc. is. And I question the nutritional value of what's left after the processing required to make "meal." And, of course, there's the same "batch consistency" problem. Is it mostly chicken backs or heads and feet? Does this batch have more saran wrap (expired meats from the supermarket/butcher) than another one?
Sadly, there isn't a dry food out there without some kind of meal in it. At least I haven't found it yet. But it seems to me that "meal" (rendered products) are the main non-grain ingredient that should be avoided in pet food, not the meat by-products (non-rendered) due to the toxin-potential, and because they contain animals not considered fit for human consumption. Why is it OK for our pets to eat diseased animals - because the disease is rendered somehow "inert" by the process it goes through before it becomes food for our cat or dog?
Just thinking out loud here....