A long time ago (4 years) we stopped giving our chickens oyster shell. For some reason our chickens kept getting them lodged in their throat and would die. I've been trying to find some that is more ground up but then I'm reading that the ground up oyster shell is bad for chickens. We are using bags of feed currently but are going to a bulk poultry ration from Purina this week, and I'd like to give them some free choice calcium but I don't want to lose any chickens. What alternative can I give?