The tread depth will vary depending on the tire type, for example, rear tractor, front tractor or implement tires will have different requirements for proper traction. During time of the field, if there is a loss of traction, and spinning of tire tread wear beyond standard look for liquid calcium leaks signs
At times, liquid calcium could leak at the valve stem, which would mean that the tire tube is failing at a drastic rate and could potentially be dangerous. If you detect rust, this it is most likely due to this reason, and you should consider it as a sign to repair the rim of the tire and quite possibly a new tube.