A combine harvester (1) with a monitoring system (10) is described, the monitoring system (10) comprising a sensor (10.1) configured to provide an ultrasonic measurement wave (210) to a flow of crop residue (200) on the harvester (1) and to receive a response wave from the flow of crop residue (200), a processor (230) having an input terminal (240) for receiving a response signal of the sensor (10.1) representative of the response wave; the processor (230) being configured to determine a crop parameter associated with the density of the flow of crop based on the response signal of the sensor (10.1), whereby the processor (230) further comprising an output terminal (250) for outputting a density signal (255) representing the crop parameter.