You need to just get the hand of bush hogging.
WD is 35 HP I have used one to brush hog , have not found too much I can not cut with in reason. Do not have to make a full cut. You cutting with a 45 HP. Make sure your blades are sharpened. Run tractor at about 3/4 of max., may run it faster if it is for just a little while, if not go to a lower gear. I cut allot of fields in 3rd gear. Using a 5 ft. woods with the side open.
Good Luck !!