While there are feedstock categories of Bakery Products, Distiller's grains, Brewers Grain and various by-products, these are not very common in feeds you'd get from your local feed mill, nor in quality bagged feed. They would more likely be used by large, commercial mega-operations that own their own mills.
I know what I get from our local feed mill. It is organic cereal grains and legumes that are grown all over the area. Corn, soy, wheat, barley, oats, field peas, etc. I recently purchased the neighbor's seven acre organic corn crop. It grows a stone's throw from my back door. It should be about 20 tons of grain that will go out to our feed mill and will be held on account for me to use against my feed bill.
As far as bagged feed goes, as with anything else, you get what you pay for. If you're reaching for the $8 bag of some brand you've never heard of, you'd better read the label. If it lists grains as the principal feedstock, good. If you can't make out what it contains or it lists very general descriptions that still leave you scratching your head, find a better brand.
Corn and soy and the like are not filler! Corn and soy, or wheat and peas are the basic feedstocks for making a nutritious layer ration. If they are the filler then you're going to have to tell me what the good part is...
