I have a few answers on this:
1. Chickens are great 'cleaners' in the garden beds.
2. Wait until you inbetween an old planting and a new one - they will go in and scratch the hell out of everything, tearing up everything as they go. They can turn a lush patch of green into bare earth in a matter of a couple of days. I have fenced in small areas that were super thick with weeds and put a flock of 10 or 12. They stripped it bare. I have found chickens to be the most effective tool to clear property - have you ever seen a chicken coop with even the tiniest plant in it?
3. I used to turn my loose in an area of well established plants about an hour before sundown because that only gave them enough time to go scratch around the base of the plants and then with dark coming they would head for the coop before they did too much damage to the plants.
4. Never turn them loose around seedlings or fragile plants - they will be scratched into oblivion.
5. Their manure, even though it is fresh, is scattered here and there as they wander and I have found that it is a great way to get my beds fertilized with no work at all on my part. I like the no work part, it is the essence of permaculture