I have 9 11-12 week olds and they are still in the brooder with regular trips outside while I slowly finish and fox proof the coop. I have been giving them some food scraps and greens but as they only have limited access to the ground I have to give them some sort of grit. I can't find chick grit and the only grit I can find is labelled oyster shell grit. I have been giving them sand which they seem to go straight for. The risk that people quote for excess calcium is both renal damage and skeletal abnormalities. Does that risk continue until they start to lay or does it go when they reach skeletal maturity? In other words, when can I use oyster shell as grit?