You cant bush hog it or it will ruin your tractor tires if it is extremely thick. Get a chain several in fact, leather gloves and start pulling out the larger clumps and then back blade them, until the heavy duty stuff is gone then bush hog them and then disc it with a heavy disc, if there is a farmer in the area hire him to disc it with an extremely heavy disc, then spray if they return and plant it and keep it mowed and under control. Might take a couple years to get it in shape like you want it.