A movable tractor-trailer combination (10) includes (a) a self-powered tractor (11) having a plurality of ground-engaging members (13, 14) and a steering mechanism (27, 28) for steering at least one said ground-engaging member (13, 14) so as to cause changes in a direction of movement of the tractor (11)； and (b) a trailer (12) that is towed behind the tractor (11) as the tractor moves forwardly and is connected to the tractor by a drawbar (32) that is pivotably connected at a tractor connection to the tractor (11) and at a location spaced along the drawbar to an attachment location on the trailer (12). One or more sensors (43) 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 (10) includes control apparatus (44) that operates in dependence on at least one output of the one or more sensors (44) to operate the steering mechanism (27, 28) of the tractor (12) in dependence on the at least one output such that the tractor (12) moves along the swath without any of the ground-engaging members (13, 14) running over the swath.
