Rule 5 doesn't apply to actual landings.
Stock is never good - if at all possible, avoid fields with livestock.
Crop fields are OK provided:
- you land parallel with the wheelings (deal with any crosswind)
- your wingtips & elevator are higher than the crop.
The idea is to round out & land on the top of the crop - it might end in a heavy landing but the undercarriage can cope with it as long as you don't go over any deep ruts during the ground run. Tall crops catching on the Grob's low wings can result in a damaging groundloop.
The only way to judge crop height is to assess how much of the ground you can see through the crop. If you can clearly see the earth, the crop is still small. If you can only see the ground in the tractor wheelings, the crop is maybe 2 or 3 feet high. Once the crop covers the wheelings, you should expect a low wing aircraft to be damaged during the landing, but the pilots should be OK.
A couple of years ago I saw a Cessna in an oilseed rape field. The crop must have been about 4 feet tall, almost to the top of the cowling. The clearly visible ground run was impressively short but the aircraft looked to be undamaged. That was early summer, when the crop was still green. Once oilseed rape turns brown it becomes a thick impenetrable mass that would probably flip any aircraft upside down.