California Code, Vehicle Code - VEH § 36300

Any person, while driving or operating an implement of husbandry incidentally operated or moved over a highway is not required to obtain a driver's license;  except that the driver of any farm tractor while being used to draw a farm trailer carrying farm produce between farms or from a farm to a processing or handling point and return, and the driver of an automatic bale wagon which is being operated as specified in Section 36102, but is not being operated as provided in subdivision (k) of Section 36005, shall be in possession of a driver's license of the appropriate class other than a junior permit.