It really depends on a compnay about by products even the FDA tells you By products are what people discard which generally has more protein sources...By products in quality foods are not the feathers becks and that kind of junk they are the cleaned innerards of animals
Now what really should be of concern is when a food does NOT label what kind of meat...you can be getting any striated muscle that was thrown in to a vat..which some companys back in 2001 showed the same chemicals which they euthansize dogs and cats with...these labels say Meat this is often confused as beef if it was only beef it could and would be labeled as such
now corn though yes as a ground corn though is a filler when processed can and does have some nutritional value ...you have to look at the food also about the ground corn many of your low budget dog foods do not ground down and process they grind down to sand size and this here is not digestable
..Whole ground corn this is literally the whole cob ground down this has roughage but no nutritional value at all
Next you need to look at the Kcal per cup value your food has protien and fat are also important but it is the caloric value where you will find how much your dog needs and burns for energy
here is a link to a comparison
http://www.kninebirddog.com/Information/So%20you%20want%20to%20Comparew.pdf
Also when a bag says Real Chicken or what ever meat source they are using...FYI for instance Chicken ...it takes 5 pounds of chicken to make 1 pound of chicken meal
also some companies have gone as far as labeling their By Products such as cartiledge saying it is a natural source of glucosamine thing its yes alot of your By Products are sources of Natural Glucosamime but they do not equate in to the amount worth beable to measure it is just a fancy way to get away with making people think they are getting more for their money