There are ALL kinds of implements out there for such use. You aren't too far off with that. I expect with the newest technologies expanding so quickly that broadcast herbicide application will become a thing of the past. Eventually. There are spray drones already in use that will spray just the weeds and not the whole field. There are protypes of self driving machines with lasers that roam a field and laser fry the weeds. It's all about cost efficiency at this point. Broadcast applications are still the cheapest per acre. That will change though.