We use 2-3lbs per year in two applications, behind the planter and post.  I believe it helps prevent more chemicals being necessary in corn and gets resistant weeds knocked down and a lot of weeds seeds out of the seed bank for beans the next year.  It's cheap, effective, and we have no known resistance issues with it so it stays as long as they'll let it.  Behind the planter with UAN anything green, especially broadleaf will be fried and post with Roundup, Callisto, and Status leaves clean middles at harvest as long as we can get and keep a canopy.