Hi Derek,
My name is James Tyree II and I own and operate Oregon Field and Brush Mowing. I have been doing this for over 10 years now. People have mentioned zero turn mowers to me also.I had a customer who said that his previous mowing contractor used one to mow his 4 acre property. Unfortunately, the guy was killed when it rolled over on him. I say, use the right machine for the job. Zero turn mowers are great for slopes less than 20 degrees and for turf. Maybe an occasional tall grass mowing, but definitely NOT field and brush mowing. That is NOT what they are designed for and it will wear out their PTO clutch and the machine. 2.5 mph seems to be about as fast as I want to go on most fields, just for the fact of the roughness of the terrain, and that @2.5 mph, the volume of grass doesn't tend to overwhelm the tractor. 27pto hp minimum is what I find is necessary to run a 60"rotary cutter (bush hog) and some of the 72" rotary cutters are doable with less. I once used a Kubota L3010 and an Agri Tec 6 ft rotary cutter in 3-4' tall grass no sweat. Usually though, I am working in 5'6ft tall thick wet grass and the Reed Canary grass is some of the most power robbing vegetation I mow. Scotch broom requires a good bit of power too, but it can explode apart and one can clean up with a second pass small spots as needed. I would follow these guidelines to start. Woods Cadet 72" rotary cutters from the 1990's are some of the best, as are the Rhino rotary cutters. Those would be what I would use. Otherwise, a John Deere 609 or 709 is THE BEST you'll find for quality of cut and durability from what videos I have seen and what used ones look like in terms of how they hold up. I hope this helps!
Oregon Field & Brush Mowing
"Short or Tall, We Mow It All!"