Fermented feed... gives a good dose of probiotics. Kind of like comparing yogurt to milk. They both start out being the same product, but all the happy little bacteria in the yogurt make it easier to digest, and yogurt infuses your gut with those good bacteria to boost your immune system. Supposedly, chickens who are on a FF diet have more villi per square inch according to one previous writer... Don't ask me for a reference, but you might want to do a little research on FF and compare it to articles written about the benefits of yogurt and probiotics. I can tell you that chicken poop is well formed, dryer, and no where near as stinky when the flock is on FF. They also don't drink as much water (most likely b/c they get a lot of hydration from the liquid in the FF) and a bag of feed will last a lot longer.
