Just feed them a turkey finisher or unmedicated chick starter. I like the finisher as it's in pellet form, less waste than crumble. The entire flock will benefit from that feed not just the growing Cochin. For your free range layers you should provide a calcium supplement like crushed oyster shell free choice. Either in dish or tossed by handful on ground couple times a week. Birds can survive on free range and corn but you can't expect them to perform up to snuff in both laying and weights. If they are getting a lot of quality forage they wont be eating much of the feed so it wont hurt the wallet. If they are currently eating a lot of corn then the forage quality isn't up to par. That's to be expected being they come back to the coop so aren't ranging far and wide like a wild turkey flock does. Wild birds have to cover a lot of terrain to get the forage they need, your free range birds are limited unless they abandon the coop and go rouge.
