It doesn't matter if cars, tractors or trucks are discussed, this break-in subject always pops up. An engine and drive train is made up from various metals and plastics and anything made by man is not perfect. The parts need time to conform to each other till the surfaces are better mated. The juvenile sayings like "run it like you stole it" is just one example of this nonsense. Generally speaking, piston rings like speed to polish the cylinder walls and the entire ring wears to conform. Now bearings can be damaged if they are too tight or too irregular, so they are best treated carefully. Heavy loads put on an engine will cause high pressures in the cylinder and on the bearings, so the system shouldn't be worked hard early on.
Since we are discussing slow turning diesel engines, in my opinion the RPM is not much of a problem, but heavy loads might be. A diesel has full compression pressures all the time since there is no throttle plate in most. I would simply let the engine run freely and not subject it to things like heavy weed cutting or plowing that makes the engine pull hard. For example, if you have high weeds and the job has to be done, just take a partial cut at a speed that doesn't labor the engine.
