A material chopper assembly (46) or construction that has a rotary chopper element (47) disposed across a passageway (70) for the material flow, comprising a chopper grate assembly (48), having a grate portion (66) with spaced generally transverse slots (68) therealong, positionable generally parallel to the rotary chopper element (47) of the residue chopper assembly; An active counter knife assembly (60) comprising: a rotatable shaft (76) positionable generally beneath and parallel to the grate portion of the chopper grate assembly, with a plurality of fixed or flail type knife elements (78) attached to such shaft and positioned thereon to be alignably rotatable through the spaced generally transverse slots of the grate portion of the chopper grate assembly; and a drive mechanism (88) for effecting rotational movement of the counter knife assembly, which drive mechanism is preferably controllable to allow an operator to vary the rotational speed and direction of the rotatable shaft so that the rotatable shaft may be selectably either maintained in an substantially fixed or locked condition or operated to rotate either counter to the rotation of the rotary chopper element of the chopper assembly or in the same tangential direction as the rotary chopper element thereby controlling to some extent both the throughput speed of the material at the chopper assembly and the quality of the chop by the chopper assembly.