Software tools and technologies, such as geographic information systems (GIS), are playing an important role in the field of agriculture. The capabilities of the open source GIS system GRASS are used to process LandSat imagery and calculate the Normalized Difference Vegetation Index (NDVI), as a measure of biomass, for a given agricultural field at times before and after early harvest activity. Areas for which the NDVI value decreases substantially between two images are assumed to have been harvested during that time. Knowledge of the harvested area, in conjunction with the delivery records from American Crystal Sugar, allows computing the yield per acre for a given field. The results are compared against the yield per acre for the complete field, which is available at the end of the year. The results correlate very well, demonstrating the value of the approach.