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 into contact with stator bars at angularly spaced positions around the axis of the rotor. The stator bars are formed from bent sheet metal which are L-shaped or U-shaped in cross-section to define a first leg lying in a cylindrical surface surrounding the axis of the rotor and a second leg extending outwardly from the cylindrical surface connected to the first leg at an apex at a leading end of the first leg relative to the direction of rotation of the rotor. When U-shaped, the bars are symmetrical and can be reversed when the second leg is worn for extended life.
