A movable tractor-trailer combination includes a self-powered tractor having a plurality of ground-engaging members and a steering mechanism for steering at least one the ground-engaging member; and a trailer that is towed behind the tractor and that is connected to the tractor by a drawbar that is pivotably connected at a tractor connection to the tractor. One or more sensors sense a line of a swath of crop material corresponding to a maximal quantity of crop material per unit length of the swath. The tractor-trailer combination includes a control apparatus that operates in dependence on at least one output of the one or more sensors to operate the steering mechanism of the tractor in dependence on the at least one output such that the tractor moves along the swath without any of the ground-engaging members running over the swath.
