Can't say I've ever seen a sprayed hay field. I've seen pastures but those are typically trampled and grazed by the livestock. You might plant something like alfalfa in a field, but I'm not sure I'd call that hay. For hay you want the most natural of grasses, cut in swaths, then baled. That tall grass in ditches and around marshland is perfect. The grass naturally chokes out most of the weeds. Weeds grow better in worked soil. But even most of the few weeds growing in the grass makes for perfect cattle feed. Once these cattle are sold and reach the feedlot, then they're fed all kinds of crap to fatten them up, shortly before slaughter. Once they're sold, it's not up to the farmers anymore.