A baler is described for forming square bales made up of several separately tied slices. The baler has a bale chamber and a plurality of needles movable in unison through the bale chamber to pass respective strands of twine from one side of the bale chamber to associated knotters located on the opposite side of the bale chamber. A first set of permanently operative knotters knot strands of twine tied around individual slices on each occasion that the needles are moved through the bale chamber. A second set of knitters configured to be selectively disabled by movement of a frame carrying their twine engaging components to a position out of reach of the tip of the needles, knot the remaining strands of twine only at the end of the formation of a complete bale.