1) As someone has already pointed out, if you feed your birds on commercial feed they'll fatten up quicker, but then again, you could just as well buy your chicken from the stores, because commercially raised chickens are fed the same feed.
2) Although another member has stated meat animals can't be raised on grass alone, I beg to differ. For example, many farmers in the UK raise their cattle on grass only, and their cows thrive. Okay, I know a chicken is not a cow, so on to point No3.....
3) Apart from commercial poultry farms, I must be about the only person in Thailand who purchases feed for chickens. 99% of backyard chickens in this country free range without any supplemental food apart from kitchen scraps. In fact, the man who owns the shop where I buy feed has commented on it many times that I'm wasting so much money. Well, I started thinking about this, and thought that if I really want good quality free range chicken meat, then they birds should ideally be free ranging. So, as from week ago, the birds are put out to forage from morning to evening, although I do still provide feed for them in the coop/run. Interestingly enough, the coop stands open all day, and yet I'm yet to see one of the chickens going in to eat their feed. This tells me they actually prefer the diet nature provides. The two duck are a different story though, as they frequently wander into the coop during the day for a quick snack. I should also mention though, I grow a few vegetables as well such as Chinese Water Spinach, Chinese Kale, Pak Choy and etc. which is there for them if they want it.
So, in my opinion, you can raise chickens without any commercial feed providing they have enough space to forage for their food. I have often wandered about the way so many people regard their chickens as being "organic", and yet they feed them commercial food, they treat them for worms, and they give them vaccines. In my book, those chickens are a million miles away from being "organic", and to put in bluntly, they are hardly any different to the chickens one buys in the stores. Are my birds organic? No, not yet, but they will be soon enough. Give me another month to a month and a half, and I'll be there
TC