A moveable tractor-baler combination (10) includes (a) a self-powered tractor (11) having a plurality of ground-engaging members (13, 14, 16, 17) and a steering mechanism for steering at least one said ground-engaging member so as to cause changes in a direction of movement of the tractor (11); (b) a baler (12) that is towed behind the tractor (11) as the tractor (11) moves forwardly and is pivotably connected to the tractor (11); (c) one or more sensors (36, 38) for sensing one or more lines (herein swath lines) of crop material corresponding to a maximal quantity of crop material per unit length of a swath (24); and (d) a control apparatus (41). The control apparatus (41) operates in dependence on at least one output of the one or more sensors (36, 38) to operate the steering mechanism of the tractor (11) such that the baler (12) follows a said swath line in a manner optimising the ingestion of crop material into the baler (12) for baling. One or more said sensors (36, 38) is at least temporarily operable t
