I keep getting conflicting advice. My girls will be 18 weeks this week. I read someplace (maybe here) that I should switch them to layer feed at 18 weeks, but the people at the feed store were emphatic about not doing it until 20 weeks. I have 10 lbs of starter (unmedicated) left so should I just use that up and then switch over when it's done? Or will that keep them from laying? And I don't start giving them oyster shell until we have eggs, right?