Harvest rate, temperature, and conductivity are measured by a sensor mounted on the hull of each harvester. Harvest rate is estimated from the rate of change in the depth of the sensor. The depth trend is first smoothed to remove variation from waves and other harvester movements. Then the smoothed depth trend is translated to the wet weight of harvested plants using the hull geometry and the weight of displaced water. Harvest rates are only estimated for periods of at least 30 minutes. Sharp decreases in the sensor depth occur when the harvesters offload plant material.