Abstract: A forage harvester including a chopping drum having a plurality of knife blades, a knife sharpening apparatus, a discharge spout, image capture apparatus for generating a series of captured images of processed crop in the discharge spout, an electronic control unit, and a memory. The electronic control unit determines data relating to aspects of crop flow from a first image or a first set of images in a series of captured images. The memory stores this data as control data. The electronic control unit then compares data relating to aspects of crop flow determined from subsequent captured images, with the control data. In the event of a predetermined deviation of the data relating to the aspects of crop flow, from the control data, the control unit generates a signal indicating that sharpening the knife blades is required.