A combine harvester includes a straw chopper, spreading device and a weed seed destructor (WSD) which acts on the weed seeds in the chaff so as to cause devitalization. The WSD includes a discharge mouth where the WSD causes generation of an air flow which exits through the mouth. The system is operable in two modes where in both the straw is directed to the straw spreading device. In the first mode the chaff is directed into the WSD and in the second mode the chaff is carried past the WSD on a guide surface and is converged inwardly to drop into the air stream from the WSD. In both modes the chaff is spread using the air stream from the WSD at least to the sides either by feeding to the spreading device or by combining with the stream of straw after it leaves the spreading device.