Topping them so that they cannot seed also helps as they grow weaker because they are not allowed to seed. I have sprayed a lot and then had my field topped recently and will do it again before autumn...but then I don't want a lot of grass as I have a native. Whatever you do, do not let them seed and shed because more will grow next year...even if it means you have to go around chopping them and bagging them up...I have done this before but it does help.