I would not move round bales with a tractor that had less than about 75 hp. You can do it with a smaller hp tractor but not over the long haul, at least in my experience. .
I've found that the horsepower of a tractor has less to do with it than does the weight and hydraulic capacity. I have a neighbor who has a 45-50 HP Long tractor, an older one with a frontend loader, and he feeds a big herd of cattle. He handles several hundred round bales every year, not only hauling them out to feed, but moving them out of the field, loading on a trailer to move, and then unloading and stacking. His tractor has held up under all that work, but it's made of all metal, not part fiberglass like many tractors are today.
I used to use a 60HP Ford to feed with, and never saw a bale it wouldn't pick up. That tractor burned up, and before I bought another one, I used my in-laws 45HP Case-IH and it worked just fine also. But, these were older tractors that were heavier than the ones made today.
Having said all of that, I'll agree that the more HP you have is a definite plus. I currently use my 80HP NH tractor as my hay tractor, and wouldn't want anything smaller.
