Ok-new to chickens and only have 4. My girls have been eating Purina Start & Grow so far. Still on the same 50lb bag but almost done. They will be 12 wks on Wed 5/11. What do I put them on next? Flock raiser and then supplement oyster shells when they start to lay? Or go straight to Layer? Obviously a bag of feed lasts quite awhile here and don't like to waste stuff. I have been using Purina products so far cause they are easier for me to find. Any input would be appreciated. Thank you.