I have been feeding my chicks (now 3 weeks old) Purina Start & Grow. I was just reading on the bag today to find out how much longer they should be on it, when I noticed it said Start & Grow is for small and medium poultry breeds. Large poultry should be getting Flock Starter. We have a mixture of different large breeds, no small or medium breeds, so I'm wondering now if I should be getting Flock Starter instead. Is it necessary?