Hens needing calcium will eat oyster shell to address their body's needs. Others will peck at it at first then leave it alone. Flock happy. The 'extra' protein in the grower will h3lp hens with molt and help roosters put on weight for table. None are harmed.
Putting all on layer forces all...
Like Nigel, I am making my own by mixing what's available fresh from a local mill - in my case 2 bags of 24% game bird to 1 bag of 16% layer. and of course free choice oyster shell. Protein is a bit high for developing ducks, and the final calcium content is closer to 1.5% than 1.0% where I...