This past spring I had to clean a 17 acre field of brush trees, shrubs, briars, fescue grass. It was a set aside field for 20+ years. A good 30 ounces of Round up, and the recommend amount of Cross Bow plus 2-3 onces extra per acre did the job. After 3 weeks everything was brown and ready to burn the field clean. This is the mixture I always use when clearing ground with a little of everything in it. Good luck.