We offer free choice of both oyster shells (which are calcium the hens need for strong eggshells) and grit (which they need for digestion in they are eating treats or free ranging). They take what they need!
BOTH! unless they freerange then they only need oystershell.
SeaChick is absolutely correct....The best part about giving it to them free choice is they take what they need when they need it....so -for example- you don't have to worry about your roo getting too much calcium from eating layer feed. He won't touch the oyster shell!