I'm going to step into this just to provide some information -- I don't have a dog in this fight, so to speak! I avoid by-products in the foods I buy my own dog and cat, because I prefer for ingredients to be named, and although I think dogs do very well as omnivores, I strongly advocate a low-carb, high protein/fat diet for cats who are obligate carnivores. I think diet is important in an animal's health, and post the following info because I think that the more factual arguments are, the more convincing they are. Please be sure to click the hyperlinks for more info.
has no corn (filler, is undigestible to dogs)
Corn is not a filler, and it is
highly digestible. It's also a source of protein.
Brewer's rice and corn are both nutritionally useful to dogs. Whether or not there are better ingredients out there is another question!
no byproducts (gross non meat parts of animals, like hooves and beaks)
This is a common misconception. The legal definition of by-products:
- MEAT BY-PRODUCTS (page 369, AAFCO 2011): "meat by-products is the non-rendered, clean parts, other than meat [which is basically muscle], 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 hoofs. It shall be suitable for use in animal food. If it bears a name descriptive of its kind, it must correpond therein."
- ANIMAL BY-PRODUCT MEAL (page 371, AAFCO 2011): "animal by-product meal is the rendered product from animal tissues,
exclusive of any added hair, hoof, horn, hid trimmings, manure, stomach and rumen contents, except in such amounts as may occur unavoidably in good processing practices. It shall not contain added extraneous materials not provided for by this definition. This ingredient definition is intended to cover those individual rendered animal tissue products that cannot meet the criteria as set forth elsewhere in this section. This ingredient is not intended to be used to label a mixture of animal tissue products"
Basically, in the US, by-products are the stuff that we won't eat, though many people in other countries eat this. My Italian grandfather happily ate kidneys, and "giblets" etc. By-products, by the above definition, are the exact parts that animals would eat first in a carcass. The guts, the blood, the organs. It's always the biggest baddest lion or wolf who gets the belly seat on a freshly killed animal, and this is because organ meat is highly nutritious (and probably tastes good). It's common in pet foods because it's a by-product in this country due to our aversion for liver, brains, and kidneys. People who feed their animals livers, bones, hearts, and tripe are feeding by-products, legally defined.
Now as I said, I do avoid buying foods with by-products because I prefer to have exactly identifiable ingredients. I think it's probably impossible for the companies to list it, since the composition would change (more tripe in one batch, more lungs and hearts in another). But there's no feathers, hooves, or poop in by-products and it's probably a very nutritious part of the food.
Cellulose is a source of insoluble fiber in mammalian diets. It's not generally harmful, and will help animals with certain digestive issues.
no animal digest (this can be anything from dead dogs who were euthanized, to actual poop)
Animal digest is defined by AAFCO as: "Material which results from chemical and/or enzymatic hydrolysis of clean and un-decomposed animal tissue. The animal tissues used shall be exclusive of hair, horns, teeth, hooves and feathers, except in such trace amounts as might occur unavoidably in good factory practice and shall be suitable for animal feed."
So, stuff like broth. It's not a good sign in a food because it's what some cheapies use to make the label able to say "chicken flavored" rather than use a quantity of chicken meat to earn the label "chicken." If that makes sense.
I looked into this stuff quite extensively a few years ago when my late dog needed (gasp!) a prescription diet, and the entire internet said the sky would fall if I fed my dog one of the evil rx diets full of toxic, undigestible crap. So I investigated each claim, and felt a lot better about the rx diet in the end. I fed it because he needed it, and I would have fed something else if he didn't, but it wasn't the end of the world and he did very well on it, actually.