If your birds will never be housed with roosters or younger non laying pullets you can switch to layer feed once they start to lay eggs. It contains the calcium needed for hard shells. However if you have a regular backyard flock that may have young pullets introduced over time (chicken math here!) then just feed the flock grower type feed (I’m assuming U2 is this type) and provide a separate feeder or bowl or whatever filled with oyster shell sold where you buy your feed. Laying birds can eat at will. Non laying birds won’t eat it other than a curious nibble.I feed them a feed called U2 which basically just means "after 7 week feed until they start laying"
Layer feed provides way too much calcium for males and non laying females. It adversely affects their livers (I think it’s the liver. Someone correct me if I’m wrong.)