An agricultural machine includes a crop row sensor (32) which has a movable paddle (20) which interacts with plants in the crop row, and which periodically generates a sensor signal representing a position of the paddle. A signal processing system (40) includes a memory which stores a plurality of sensor signals. The system determines a minimum magnitude of the stored sensor signals, and generates a drop rate signal representing a change in the sensor signal in a period of time. A gap detection unit generates the gap signal if the minimum signal is less than a threshold and if the drop rate is greater than a drop rate threshold. Thus, the system generates a gap signal when the paddle encounters a gap in the crop row.
