If you have so many egg shells that some is always available to the chickens, so they can eat as much as they want and never run out, then you probably do not need to buy oyster shell.As we offer them crushed egg shells, and have plenty, why would we need to purchase oyster shell?
But if your chickens eat all the egg shell you give them, they may want more: buying oyster shell would give them the option to eat more when they feel the need. Chickens tend to be good at eating the right amount of calcium for their own needs, when they are given the opportunity to do so.
So that's a reason why some people would feel the need to buy oyster shell even when they provide eggshells to the chickens.
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