Purina has Starter/Grower all in one, nearest as I can remember it is about 18% protien. Flock raiser 20% protien. Layena 16% protien.
In the past I fed starter/grower right up to the point of lay. This time around I swithed my birds to FR at 10 weeks. Then at 18 weeks I went to my 50/50 blend. When they started laying I started supplementin with oyster shell about 1oz mixed in the feed. Any difference? Maybe, but it can't be much.
Corn? Well it's kind of like giving my grand daughter a candy bar. It may not be the best nutrition but it sure makes her happy, and in moderation it will do no harm.
My chickens hatched May 24, were in a tractor from 8 weeks until Sept 19. I moved the tractor daily so I noticed how much they enjoyed the fresh grass. So when I designed my coop and run I made it with 3 run areas that the chickens could be rotated in. My mistake was building it way to small. Each area is 200 sq ft I think maybe I needed 3 times that much for my 10 eatin machines.
Solution: I planted a large area outside of the runs, grow turnip and rape specially for the chickens. They a loosely filled 5 gallon bucket every day.
What I can tell you: is almost any commercial feed, fed in adaquate quantity will keep your birds healthy. Starter/grower when starting and growing, Flock raiser if you want to feel good. Laying when they are laying. Unless you are giving extreme amounts you can also throw whatever else you want over the fence.
In generations past and presently in allot of cases chickens don't get much but a little corn and kitchen scraps. And while they maynot thrive they do survive.