A combine harvester with a straw separating system includes a straw chopper and discharge assembly mounted for receiving the straw and having a rotor for chopping the straw and for generating a stream of chopped straw for spreading the chopped straw. A pneumatic air source is provided preferably separately driven from and remote from the rotor for generating a stream of air having an outlet duct forming a stream of air. The air is used either to assist the spreading action from the chopper and can be individually controlled by valves. If used to assist the air stream of the chopper it is fed at the outlet of a rear chopper or directed at the guide wall in an internal chopper arrangement for directing higher velocity air onto the guide fins assisting the spread of the chopped materials.