They don't re-seed enough to make it workable. Mice, Birds, etc get most any seeds that are leftover. I get my seed from the Birseed Bin at the local hardware store for $1.00/lb. They are Black Oil Sunflowers.
I often plant them for fun, and for the birds around my plots. Deer almost never eat them in my experience. But, its not fair because the field is full of soybeans & clover usually. I hear about guys that cannot get their sunflowers to grow (for doves usually) - because the deer nip them back. I can see it happening - but I think most times its because the sunflowers are the only young, succulent growth at the time, or other foods for deer are lacking. I would rate sunflowers as a 1 out of 10 as far as Deer Food plots go.
FH