Implement makers size their equipment in PTO horsepower, and hay tools show how fine the steps are. Papé Machinery's guide to John Deere's round balers gives a 55 PTO hp minimum for the 450E, 450M and 550M, 65 hp for the 460M and 460R, and 75 hp for the 560M and 560R, and says the entry balers pair with Deere's 5 and 6 Series utility tractors. The MegaWide HC versions want 125 PTO hp, beyond anything here; Papé points those buyers to row-crop or four-wheel-drive tractors. Six machines on this list clear the 75 hp mark, and most of the rest clear 65.
The PTO figure on a spec sheet is the maker's own claim. The independent check is the Nebraska Tractor Test Laboratory, set up in 1920 under a 1919 state law passed to protect Nebraska farmers at a time when more than 150 companies were selling tractors. Its reports open with PTO performance at rated engine speed, at standard PTO speed and at maximum power, all at full load, and it is worth looking up whether a model on this list has been through it before treating a two- or three-horsepower gap as real.
Power that has to pull, rather than spin a shaft, also needs weight to put it on the ground. Iowa State University extension guidance for fieldwork is 115 to 135 lbs of total tractor weight per PTO horsepower on a two-wheel-drive tractor and 90 to 105 lbs on a four-wheel drive, with ballast set so the drive wheels slip 6 to 12 percent on firm ground. By that rule a 65 PTO hp two-wheel-drive tractor wants roughly 7,500 to 8,800 lbs before it can use its rating on a disc or a chisel plow, so check each tractor's listed weight alongside its horsepower.