I read something in a magazine, Mother Earth News I believe, that addressed using chickens to weed a garden.
In between the crop rows they wired a large tube of chicken wire. The chickens could move back and forth between the rows while being confined to the chicken wire. The chickens could eat the weeds, insects and such while not being able to reach the plants. This allowed them to fertilize the garden at the same time.
I know I'm not describing this well, but let's say your garden is 50 ft long. Buy a 50ft roll of chicken wire and un-roll it. Then wire the long sides together so it looks like a chicken wire culvert. Set the wire tube between the crop rows and let the chicken have at it. One of these could be made for between each crop row, or you could move it as you wish. A fence around the garden with cut outs for the chicken wire tube would keep them out of the rest of the garden.
It sounds like an easy, fairly cheap way to let chickens do you weeding for you.