Tillage may help...It may hinder your cause. Depends on soil conditions, climate conditions, and type(s) of undesirable plants already established.
IMHO, the best way to deal with noxious weeds is to mow, mow, mow, and mow some more. Keep them from establishing a new seed crop. Fertilize (as needed, based on soil testing) Healthy stands of grass will fill in and crowd out weeds. Finally, spray. Do so AFTER fertilizing. Healthy weeds take up more herbicide, which will spell their doom.
2 years ago, I sold my farm and moved to a smaller place. The 45 acres we have now were once a mess of weeds. The place sat empty for 2 summers. Weeds had taken over. We moved here in the early fall of '05. I mowed everything, then treated with various products based on need. Mostly 2-4,D based products for control of broadleaf weeds. But by that point, the '05 seed crop was already planted. I sprayed everything again in mid spring of '06 to get newly emerged weeds, kept everything mowed all summer, then sprayed again in the fall. This year I sprayed in the spring and have no plans (or need) to spray in the fall. Pastures are almost weed free now. Doing it this way, I never lost grass cover. Being on hilly ground, that reduced erosion. I have a power seeder reserved (rental) for this fall, along with fall and next spring fertilizing. What was 2 years ago a mass of weeds now looks like a golf course.