Nothing will happen, just provide a separate container of Oyster Shells at 16 weeks old for the Calcium.It states to feed up to 16 weeks. What happens, if they will eat that longer than 16 weeks? I was hoping to have them finish the bag.

It's a good idea to put out a separate container of Poultry Grit "Granite" so the Pullets not Laying won't substitute Shells for Grit.

I feed my Hens "22 months old" a Non-Medicated Starter-Grower 18% with Oyster Shells separately.
GC