Tom, I'm betting you have already figured this out...it's just common sense and a little bit of experience. It turns out that whether or not you can use a lower or higher HP range dirt moving implement sort of depends on why that implement was rated that way in the first place. Sometimes it is because it requires that much HP to operate it, sometimes it is because using full HP will tear up a lighter implement, and othertimes it is because most tractors of about the same HP do tend to weight about the same. Of course if it is truely HP that controls the situation then there is always the option of simply run at an idle. After all, HP is a direct function of RPM and half rpm is half as much HP. More often it is the rotational torque of the PTO that is actually setting the limit, and that is more constant. To control how much torque an implement sees your best bet is to limit it somehow by using either replaceable shear pins or some sort of torque-limiting clutch in the PTO shaft's driveline. Most things that rotate like Bush hogs will always have torque control built in....or at least they should. Often you can use lighter implements if you are careful. Buy a pocketfull of the right size shear pin and give it a try.
For brush hogs, they are torque limited as well as HP limited and are rated by the blade diameter or cutting width. The hog doesn't really care how much HP the tractor has.....as long as it has enough HP to rotate that size blade through your brush then the brush hog is built to handle the load. Overload is handled by shearing a drive pin. So you can probably use any size hog from a three foot diameter to a six or seven footer. Shear pins in the drive line are actually what controls and protects them from too much tractor torque from the PTO.
A back blade or box blade is also rated by hp but it's situation is slightly different. You want a back blade rated to got your hp or higher. If it's lessor then the tractor might bend it or pull it apart. There is no shear pin protection and I've done both. Of course various brands are built differently and with care you can get away with it. Some of the big wheel Ag tractors with lug tires may only have 40 hp, but they will pull like a hundred hp TBL. That's why good old 3 way back blades made for the older 9N Fords or JD As back in the 40s or 50s are built much heavier than you need on the
M59. The upper limit on back blades of any kind is to go as large in HP as you can as long as the tractor can lift the weight and can usefully pull it. I use an 8 foot blade rated at 120 hp because the tractor can easily handle it so why not get the advantage of the weight and width?
For a post hold digger it's different again. Lots of times the 20 HP tractor's 3pt hitch cannot lift as high as a larger tractor and so lower hp post hold diggers are likely to have a shorter digging depth and also lighter weight drive gears....both of which ends up putting a limit on the diameter and lenght of the auger that it can run. Still, there's no reason why you cannot try one. Just run it slow and make sure to use a shear pin or torque clutch in the driveline.
Real farm machinery like harvest or haying equipment has a PTO hp rating based on a more complex combination of requirements and you need to stay closer to the right numbers on that kind of machinery.
Good luck, rScotty
