I give mine double duty Oyster shells. (#2 grits with shells) They are available all the time free choice (during winter)
During the warm months, they are given just shells free choice all the time, and grits also still offered sometimes in separate feeder.
The also get some table scraps. Growing up cracked corn is what we always feed the chickens. These are the first I've had on my own. I have 8 hens and 4 roos. They are bantam mixed breeds. I want to start feeding them something better but not sure what. I also have 12 pullets but they are on chick starter.