A combine harvester (500) comprises a threshing assembly (116), a cleaning shoe (117) having a chaffer (108) and a fan (113). A plurality of crop material sensors (104) are positioned on an underside of the chaffer (108). The fan (113) is arranged to force air up through the chaffer (108). A controller is configured to receive an input that is representative of chaffer material distribution (812) and decrease a fan speed (828) in response to the chaffer material distribution (812) being shifted toward the back of the chaffer (108).
