Row crop harvest has begun, or will soon begin, in many areas of the state. While we are focused on harvesting grain, it’s also important to remember the other harvest we have going on -- the harvest of yield data.
Why collect yield data?
Crop yield, or yield potential, is the basis of almost all agronomic decision-making. Everything from nitrogen rates to seeding rates are tied to yield potential, either explicitly or implicitly. Yield varies spatially within a field, and yield monitor data is the only way to truly quantify spatial variability in crop yield within a field.
Yield monitoring systems and calibration
Most commercial yield monitoring systems work by measuring mass (weight) over a given area, and assigning that value to a geographic location via GPS. Grain mass is measured by a mass flow sensor located at the top of the clean grain elevator. Grain moisture is obtained by a capacitance-type sensor that is placed in the flow of clean grain somewhere on the combine. The grain moisture sensor is calibrated by adjusting the average moisture content of a yield monitor load to the moisture of a sample. Ideally this is conducted while harvesting an area of the field that is relatively uniform in moisture content.
It is important to keep the top of the clean grain elevator chain and paddles at the proper distance to the mass flow sensor, as this affects the trajectory of grain into the sensor’s strike plate. Any change in this distance must be accompanied by recalibration of the monitor; distances outside of tolerance will result in reduced accuracy. It is important that proper calibration procedures are implemented so that the system can correlate a measured force to a known mass. The type of calibration to be performed depends on your yield monitoring system. A yield monitor can either be a single point (often 2 point) linear or a multi-point calibration. The differences between calibration methods are illustrated below:
Common single point systems on the market include Greenstar systems in John Deere combines prior to the S-series and Gleaner factory equipped combines. The multi-point method is used by Case-IH, AgLeader, and S series John Deere systems.
It is important to remember that a yield monitor is only as good as the calibration performed on it. A suggested flow calibration procedure for a multi-point type monitor is illustrated below. The full flow pass is done with full width and while “pushing” harvest speed. The resulting flows are then accomplished by traveling at a steady speed while using fractions of the available header width. For example, if running an 8-row head the operator would harvest 8, 6, 4, and 2 rows at the same ground speed to get the varying flow rates. Calibration loads should be between 3,000 and 6,000 lbs and taken from as uniform of an area in the field as possible. Using large calibration loads (i.e. truck loads) for a multi-point calibration will result in reduced accuracy.
¼ Full Flow
½ Full Flow
¾ Full Flow
Additional Pass As Needed
After completing calibration, errors should be in the range of 1.5 – 2.5% and must be less than 3% if the data is to be used for USDA Risk Management Agency (RMA) yield reporting purposes. Different calibrations are needed for each crop and multiple calibrations may be needed for extreme changes in crop type or condition (i.e. high moisture corn vs. drier corn).
In general, calibrations should be fairly stable across years unless changes are made to the clean grain elevator or yield monitor system components. Be aware that while some yield monitors will back-calculate and correct previous harvest data with the new calibration, not all will. For those yield monitors that cannot correct previous data it is important to perform calibration early in the harvest season.
Benefits of using properly calibrated yield monitors
Changes in RMA procedures regarding the use of precision agriculture technology have opened up many opportunities. Yield monitor data can be used to document production for APH (actual production history) reporting. Yield monitors data can also be used to report production separately for different practices in a given area even when the entire area is planted and harvested together (e.g. pivot corners and irrigated).
It is important to note than in order to meet RMA requirements the producer must have GPS technology integrated into the planter monitor, combine yield monitor, and yield mapping software. All three technologies are required if precision agriculture technology is to be used to provide information for RMA. Yield monitors used for RMA reporting must be calibrated to less than 3% error and the calibration procedure must be documented.
It is important to regularly transfer the data from your yield monitor to the datacard or flashdrive for transfer to your PC and to keep backup copies of the raw files on the card. It is best to evaluate yield data immediately after harvest while your observations from the operator’s seat are fresh in your mind.
Many opportunities exist to make decisions such as site-specific agronomics based on yield monitor data . Although you may not be ready to make a move into site-specific agronomic management today, having multiple years of high-quality yield monitor data will be invaluable when making this step in the future. Just make sure the yield monitors are properly calibrated so that the data is reliable. It’s better to have no data than bad data.
Lucas Haag, Northwest Area Crops and Soils Specialist