A tractor comprises an electronic tractor controller. A baler comprises a crop receiver, a baling chamber, a rear door, a rear door actuator, a bale size sensor associated with the baling chamber, and an electronic baler controller. The baler controller is operable to submit a halt signal to the tractor controller when a bale size signal provided by the bale size sensor indicates that a bale has reached a size corresponding to a predetermined size. The tractor controller then halts the tractor and the bale is wrapped. The rear door is opened automatically after the bale has been wrapped, but only if an obstacle sensor detects no obstacle in a detection zone behind the baler.