How about covering the beds with a run as darkbrowneggs suggests immediately after cropping - this gives them a chance to scratch over each bed thoroughly. Then when they've got all the pests and weed seeds, move hens on to next bed and apply manure and compost as a mulch to the cleared bed. If possible cover it with something - cardboard, thick straw, black polythene, for example - then when you are nearly ready to work it, pop the hens on again for a few days. By then the manure will have been pulled down and incorporated by the worms, so the hens will just be adding a little bit of fertiliser and scruffling up the top. Then sow/plant your crops, but be sure to keep the hens out from that point.