Weed seeds are destroyed in the chaff from a combine harvester by repeated high speed impacts caused by a rotor mounted in one of a pair of side by side housings which accelerate the discarded seeds in a direction centrifugally away from the rotor onto a stator including angularly adjustable stator surfaces around the axis. Thus the discarded seeds rebound back and forth between the rotor and the stator to provide a plurality of impacts. The destructor is mounted on a suitable drive shaft within the straw path and the chaff and weed seed material is carried from the rear end of the sieve to the destructor on the straw chopper by one or more fans driving the material through a transfer duct.