Ok - so for a rule of thumb this is what I'll throw out from experience... Applies to dry grass and dirt. If wet, if gravel, if ground rock hard, be more conservative. Requires good tires more aggressive R-4 (as opposed to the more construction oriented R-4 tires) or R-1 tires.
completely flat ground - tow as much as you want, just count on it taking as long to stop as did too to get to speed.
5% grades - 4/1 non braked vs braked wheels max. This is to hold load back in gear. Hit the brakes and you may slide.
10% grades - 2/1 non braked vs braked wheels max. This is to hold load back in gear. Hit the brakes and you may slide.
20% grades - 1/1 non braked vs braked wheels max. This is to hold load back in gear. Hit the brakes and you may slide.
beyond 20% grade - good luck!
This is what I've learned from pulling various loads in a hilly orchard. Make sure you start at the top of the hill in a gear that will hold the load back without using brakes. I like to use a gear that is on the low side and apply throttle to maintain speed.