Lets say a mill wants to make a batch of 18% poultry layer all they have to do is mix --
1200 lbs corn
700 lbs (40%) poultry concentrate
50 lbs ground oyster shells
50 lbs calcium carbonate
Want a 17 % grower ok, mix --
1400 lbs corn
600 lbs (40%) poultry concentrate
Exactly. And if you find smaller mills that don't use a base from a major feed mill, they are still adding a vitamin/mineral/amino acid "premix." This is actually a necessity, in part because beef scrap and other animal materials are no longer used in most feeds. Not too many years ago it was fairly common to find crab and fish meal in poultry feed; now only Scratch and Peck seems to use fish meal. The King feed non-GMO I am currently feeding is really big on wheat, rice products, ground sugar snap peas, ground peanuts and peanut shells, lima beans, and hydrolyzed yeast products, along with various herbs and probiotics. Note that it has significant premix and added assorted nutrients, in part because the proteins require far more careful balancing than the old soy/corn recipes.