That I don't know for sure. The loading is more to do with the sidewalls, on cars and truck I think they are letter coded. But it does make sense that a 6 ply tire would probably have a higher load rating than a 2 ply.
I do know the snowblower I mount on the front of my Cub weighs close to 300 pounds, I need to keep a little extra air in the tires when the blower is on.
That's probably close to the tongue weight you're looking at?