Abstract: A baler for forming large round bales and drawn behind a tractor has an expandable bale forming chamber for receiving a crop windrow fed thereto by a pick-up device. Sensing means responsive to expansion of the chamber automatically steers the baler relative to the tractor in a sinusoidal path along the crop windrow to feed material over the length of the forming chamber. Turning movements of the baler are alternately in opposite directions transversely of the windrow at the same rate and like degree, and take place with an increasing frequency to a final expansion of the chamber to produce a cylindrical bale of uniform diameter.