Roundup or equivalant

I fully appreciate everyone's concern about the environment BUT in this case the lot is almost two acres and has a nice stand of "horse weeds" 6 to 7 feet tall. Trying digging but they have a tap root that goes WAY DOWN---been there, done that. I need the Roundup for a one time application to get the tap roots; after that, controlling the weeds with cutting and digging won't be a problem.