I am providing grit and oyster shells both to my 9 pullets (20 weeks). So far I am keeping them confined to the coop and run and not free ranging them any for a few weeks until they get settled in. I was wandering if I did not provide the actual grit would they use the small bits of oyster shells as grit if it is needed? I didn't know if I needed to offer both or just the bag of oyster crushed oyster shells? They look to be about the same size.