I was thinking the traditional riding lawnmower like this.
http://randsco.com/_img/blog/0904/rider-mower.jpg
Its not going to work with corn, but I was thinking it would work with lighter grains such as millet, amaranth, hulless oats . . .
The mower will pick up the stalk and a good bit of the seedhead. I'd probably just save it all - the
chickens can pick through and find the seeds and the rest of the material would become litter. Run the
chickens over the area after mowing, and what they do not find re-seeds for the next year.