There’s a great article on providing oyster shell and grit to chickens in this month’s Backyard Poultry Magazine.
I have read and been given differing advice about using oyster shell and grit with my baby chicks. Many old hands at raising chickens said I did not need grit until I gave them larger grain and seeds. I did give small amounts of both oyster shell and grit to my first flock of biddies, but I did not begin with either with these new little chicks. The difference in the health of these two batches of chicks convince me that even with thoroughly ground grains and seeds, even some processed, baby chicks fed on a homemade mix need grit and oyster shell, too. Actually, after reading on this topic, I’ve come to believe that baby chicks fed commercial starter should be offered grit and oyster shell. There is some confusion that one serves as the other, but this is not so. Grit aids in digestion; oyster shell gives calcium.
You know, the bottom line is a bit of grit and oyster shell will not hurt a chicken, but lack of either could.