As others say, pulling a baler or mower/conditioner takes more than just horsepower. They are big heavy implements that pull to the side, so you need a tractor with some weight to it. The '60s JD & NH balers are very good, and really only need 15 hp to run the baler itself - the flywheel does the work. Many had Wisconsin engines on them, didn't need any tractor pto at all. But you need a tractor heavy enough to be a boat anchor for the things. The plunger going back & forth is what packs the hay into a bale, & it puts quite a rocking motion on a light tractor.
Round balers, you would want 50+ hp for a smaller 4' round baler (some are listed at 40 hp tho), and close to 100 hp for the big 6x5 balers. Again, you need the boat-anchor weight as you are pulling & rotating a lot of weight behind you.
--->Paul