A combine harvester has a straw chopper attached to a rear hood behind the rear edge of the sieve assembly with a chaff guide system to bypass the chaff in front of the chopper and a straw guide system to bypass the straw above the chopper and over the tailboard. The chopper has a housing with a feed opening into which straw and/or chaff can be fed depending upon the positions of the chaff guide system and the straw guide system. A chopping assembly mounted in the housing discharges through a discharge opening through onto the tailboard having a plurality of guide fins for spreading. The housing bottom wall has a pivotal adjustment portion to allow chaff entering the feed opening and engaging the chopping assembly to discharge from the housing at a discharge position different from the discharge opening for spreading onto a supplementary spreading device.