Killing Briars and English Shrubbry on Farmland

What's the best Herbicide to kill wild briars and English shrubbry? I want to kill it dead and I don't want it to come back up again. I farm about 60 acres and mostly cut hay but around the edge of my fields are thick briar patches that I have to continuly bush hog down.
Also about 15 acres of my land is in large Red Oaks and this area gets over grown with English Shrubbry. If you are not familiar with it, it starts as a small woody plant and rapidly grows(over 1-2 seasons) to a small tree about 8-9ft tall with a trunk diameter od 2-3 in.
I've tried bush hogging it down and even plowing up the roots with a large disc and even a turning plow, but the stuff keeps coming back. Any suggestions as to a Herbicide to kill these two things would be GREATLY Appreciated. Thank You in Advance!! Belah:confused: