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Get That Yield Monitor Calibrated

If you like to use yield data to formulate prescriptions for next year's cropping strategies, you might want to take a few moments to calibrate your yield monitor, says Bob Nielsen, Purdue University corn specialist.

Yield estimates on a whole field or individual load basis made by a well-calibrated yield monitor are accurate in the sense that they often very closely match yield estimates calculated from weigh wagons or commercial weigh scales, Nielsen says. However, to achieve a satisfactory level of accuracy, yield monitors must be “trained” to correctly interpret the electrical signals generated by the impact sensor into estimates of grain flow rate.

Calibrating a yield monitor simply requires the harvest of individual “loads” of grain that represent a range of grain flow rates — for example, a range of yield levels — expected in the field to be harvested.The amount of grain required for each calibration “load” ranges from 3,000 to 6,000 pounds (50 to 100 bushels of grain) depending on the manufacturer’s recommendations for the specific model or make of yield monitor.

The grain weight of each “load” is estimated by the yield monitor as the grain is harvested. The grain for that specific “load” is then off-loaded from the combine hopper and weighed on weigh wagon or commercial scales. The actual weight is then entered into the yield monitor console and the yield monitor firmware makes adjustments to curve.

"Some manufacturers suggest that only one grain load is necessary to perform an accurate calibration," Nielsen says. "The need for only one grain load implies that the calibration response curve is a straight-line or near-linear relationship between grain-flow rates and flow sensor signals.

"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."

Nielsen says other manufacturers recommend between three and six grain loads to perform a satisfactory calibration of the yield monitor. This suggests that the calibration response curve for these yield monitors is not a straight-line, but is rather some sort of non-linear response curve that requires a number of calibration points to best “train” the yield monitor how to interpret the flow sensor signals.

"The goal here is to capture the full range of grain flow rates, or yield levels, you expect to encounter during the harvest of your fields," he says. "Capturing a range of grain flow rates 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."

This headache, plus the time it takes to off-load and weigh the individual grain loads, are among the most common reasons why growers do not faithfully calibrate their yield monitors, Nielsen adds.

"Yield monitor accuracy can be excellent if well-calibrated," Nielsen says. "Yield estimates by calibrated yield monitors that I use in my field-scale research trials are typically within 1% of the actual grain weight measured with a weigh wagon or farm scales.

"Conversely, yield estimates can be very poor if yield monitors are not well-calibrated. The error in accuracy can be as much as 100% if the yield monitor is taken “off the shelf” and put into service without any calibration. Errors in accuracy can easily range as high as 7% to 10% late in harvest season if the yield monitor was calibrated only at the beginning of the harvest season. Errors in yield estimates are especially likely if the full anticipated range of harvested grain flow rates are not included in the calibration of the yield monitor."

If you want to use the information that an accurate yield data set provides, then you should strive to ensure accuracy in the yield estimates made by your yield monitor, Nielsen urges. Common uses for yield-monitor data include comparisons of one field to another, one specific spot in a field to another, one hybrid’s performance to another, early versus late harvest season and experimental treatments in onfarm field trials.

Calibrating your yield monitor once a season will typically not be satisfactory," Nielsen says. "Check the accuracy of the yield-monitor calibration occasionally by harvesting and weighing additional calibration loads. Recalibrate the yield monitor when necessary to maintain an acceptable accuracy."

Don’t forget to...

• Also calibrate the combine’s grain moisture sensor.
• Also calibrate for the zero-flow combine vibration.
• Also calibrate the temperature sensor at the beginning of the season.
• Re-read the yield monitor operations manual prior to the harvest season.
• Create a preseason and inseason yield monitor checklist of all adjustments and settings.
• Go through the yield-monitor checklist every morning before beginning the day’s harvest.

"Yield monitor calibration, yield data processing and yield data 'cleaning' are necessary to ensure accurate yield data," Nielsen says. "Remember the old adage: Garbage in, garbage out."