Most farmers wouldn't do that unless it was necessary. They might drive a tractor over the field to apply herbicide or something, but these days lots of them use control track farming (using GPS to ensure that the tractor tracks go in the same place each time), so you end up with only a single set of compacted ground from the tracks. That requires having implements that are all the same width though, which is not always practical. I really can't recall driving over a growing crop other than maybe a small section on the edge to get to a gate or something. Driving over corn would probably kill a reasonable portion of the corn that was directly under the wheels, but wouldn't cause extensive damage. Driving on a mature crop or stubble is a recipe for setting your vehicle on fire if you're not careful. Particularly if you have a catalytic converter or a vehicle with low clearance.