If a farmer is currently collecting crop yield data and fertilizing based on soil test recommendations, that individual is an excellent candidate for at least experimenting with the exciting new technologies that are being used for so called site-specific farming. Many early adopters have chosen grain yield monitors as the first component in a site-specific hardware complement. Some simply watch the monitor display during harvesting while others couple the monitor output with position information from combine-mounted global positioning systems (GPS) to create yield maps for post-harvest analysis. Mapping, recording, and subsequent data analysis require software, time, and patience. Regardless of how one chooses to begin the process of integrating site-specific farming techniques into existing operations, the following insights could ease the development of a management system.
Current yield monitors measure material flow through the combine clean grain system. Coupled with ground speed, swath width, and moisture content data, yields are reported on a per-acre basis at a standard moisture content. Accurate ground speed sensing is crucial to accurate point-to-point yield determination. Although GPS receivers can provide the data necessary for ground speed determination, ground-sensing radar can be used to ensure high-level accuracy. Speedometers that estimate ground speed based on transmission shaft speed are not recommended due to the errors that result from wheel slippage. Swath width, an input provided by the combine operator, should reflect actual cutting width, not nominal header width. When the full width of the header is not being used, on point rows for instance, the operator should adjust the swath width input accordingly.
Advanced yield monitoring systems can correct for the effects of combine dynamics (e.g. the lag time between material entering the header and being sensed at the top of the clean grain elevator) and grain characteristics (e.g. test weight). Yield monitor calibration is still required to ensure maximum accuracy.
At least one combine manufacturer has begun to offer grain yield monitors as a factory-installed option. Early models have been batch-type monitors which simply provide summary data for each grain tank load, not the continuous data stream necessary for creation of position-referenced yield maps.
If the farmer seeks to control field operations such as planting, fertilizing, or spraying according to map-based prescriptions, there must be the means of determining machine position as it moves across the field. A typical mobile, civilian GPS receiver by itself will provide positional accuracy no better than +/- 400 feet on occasion due to intervention by the Department of Defense (see the Macy article). It is clear that such accuracy is not sufficient to record, monitor, or control operations that are performed in 15- to 60-foot strips. Greater positional accuracy is available through the use of differential correction techniques.