My chickens are 14 weeks old and I feed them Purina Start and Grow. On the bag is says to switch them from the Start and Grow to Layena at 18 weeks old. So I thought I'd switch them when they are 18 weeks old. But I read in a magazine that you shouldn't switch them to layer until they actually start laying and that you should feed them grower. Should I switch my chickens from start and grow to layer when they are 18 weeks or when they start laying? Should I feed them grower, if so when and for how long? They are Buff Orpingtons and Brahmas.