A combine harvester includes threshing apparatus, separating apparatus, a grain cleaning system located downstream of the separating apparatus, and a material conveyance system arranged to convey crop material from the separating apparatus to the grain cleaning system. The grain cleaning system includes screening apparatus, a fan arranged to generate a cleaning airstream through the screening apparatus, and a fan control system configured to control a fan speed. The fan control system includes a proximity sensor mounted above the material conveyance system for sensing a material volume. The fan control system controls the fan speed in dependence upon the material volume.