I've recently switched my flocks to Purina Flock Raiser. The only ones I didn't switch were the chicks in the brooders because I had Purina Medicated Starter /Grower on hand and didn't want to waste the bag. While reading for something else, I came across the recommendation not to switch chicks from starter/grower until they lay their first egg. I've routinely switched them off of starter / grower when they are fully feathered and off heat and haven't seen any adverse effects. Is there a reason for this, I'm not be aware of?