“Understand this one simple fact about grain yield monitors — they do not measure grain yield,” says R.L Nielsen, Purdue Extension agronomist, who writes a blog titled King Corn.
“Yield monitors estimate yield by converting electrical signals received from a mass impact or optical sensor, located somewhere in the clean grain elevator of the combine, into estimates of grain flow (pounds) per second or two of travel time,” he says. “Along with estimates of distance traveled (usually based on differentially corrected GPS signals), header width, and estimates of grain moisture content... the yield monitor's firmware / software then estimates ‘dry’ grain yield per acre, at a moisture content of your choice, and records those yield estimates, and their geographic location in the field, every second or two in the display's memory or uploaded by cellular connection to a Cloud-based web server.”
That’s why yield monitors need to be calibrated, and why calibration "loads" should be harvested in such a way as to mimic the range of grain flow rates (i.e., the range of yield) you expect to encounter when harvesting a field, Nielsen says.
Ideally, each calibration load is calibrated at a different, but uniform, grain flow rate. “Some folks harvest strips at different speeds to accomplish this. Some folks harvest strips of varying widths (full header, 3/4 header, 1/2 header, etc.),” he says. “In our field-scale nitrogen research trials where we have a wide range of N rate strips, our calibration loads typically come from five to six different N treatment rates because that often provides the greatest range of potential grain flow rates.”
The amount of grain required for each calibration "load" ranges from 3,000 to 6,000 pounds (50 to 100 bushels grain) depending on the manufacturer's recommendations for the specific model/make of yield monitor, according to Nielsen.
The grain weight of each "load" is estimated "on the go" by the yield monitor as the grain is harvested. The grain for that specific "load" is then offloaded from the combine hopper and weighed on calibrated or "known to be accurate" weigh wagon or commercial scales. The actual weight is then entered into the yield monitor console and the yield monitor firmware makes mathematical adjustments to the calibration response curve.
Some manufacturers suggest that only one grain load is necessary to perform an accurate calibration, Nielsen says. While the standard recommendation is for only one grain load, the "fine print" in the owners' manual suggests that additional calibration loads may be added to fine-tune the accuracy when necessary, he says.
Other manufacturers recommend between three and six grain loads are required
“The goal with multi-load calibration procedures is to ‘capture’ the full range of grain flow rates you expect to encounter during the harvest of your fields. Capturing a range of grain flow rates during calibration can be a nuisance because it typically requires harvesting individual full header-width ‘loads’ at different speeds or partial header-width ‘loads’ at a constant speed.”
Big errors possible
Estimates can be off as much as 100% if the yield monitor is taken off the shelf and put into service without any calibration, Nielsen says.
Also, errors can easily range as high as 7% to 10% late in harvest season due to changes in grain moisture content if the yield monitor was calibrated only at the beginning of the harvest season.
Yield estimates errors are especially likely if the full anticipated range of harvested grain flow rates are not included in the calibration of the yield monitor.
Accurate yield monitor estimates may not matter for simple farm record-keeping purposes, Nielsen says. In fact, you should use grain cart, truck or certified elevators scales to weigh all grain if you want an accurate measurement of yields for use in financial records, insurance reports and government programs.
If you want to use yield maps to make production decisions, then you should strive to make sure the yield estimates are accurate across the full range of grain flow, Nielsen says.Source: Purdue University, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.