My chickens are 17-18weeks old. The feed store has Purina and also Farmers Warehouse feed. The Farmers Warehouse has a 16% and also a 20% lay pellet. Is there one that is better then the other? Also, what is the difference between pellet and crumble, if they have been on crumble will it be hard to switch to pellet? Is scratch, BOSS, grit and oyster shell a necessity? My chickens are in there run everyday and we let them free range on our 1/4 acre twice a week.