Talking with Bob Nielsen, Purdue University Extension corn specialist recently, the subject shifted to yield monitor calibration. He does on-farm trials on full-field scale these days more than he does small, typical research plots. One of the reasons is because farmers he works with typically have a yield monitor. It simplifies and speeds up harvesting.
He is a stickler for calibration, however. If you're combining a plot with Nielsen, you make sure you have the combine calibrated for that field and those conditions before you start collecting real trial data.
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It's all about making sure the monitor knows the parameters that are possible in terms of what type of load will hit the sensor that sends it information to determine yield, he says. He typically recommends running at varying speeds on different passes, weighing each pass with a weigh wagon, and using the information to dial in accuracy on a yield monitor.
Typically you can get to 1% or less, which he considers OK for using to measure yields. Sometimes it's possible to hone in to as closes as a 1% error, he notes.
The secret is getting different volumes of corn to hit the sensor so the computer in the monitor can figure out what low and high yields are like. Running at different speeds on different passes is a good way to simulate different flows of corn hitting the sensor. If the corn is consistent and the speed is consistent in each pass, then the computer knows what each type of load, from low to high and several points in between, is like.
Some people claim they can run corn, and at the end of the season compare to scale tickets from either their grain center or an elevator. Then to calibrate, they simply use software that adjusts the real weight and yield to what the combine yield monitor said, and goes back and aligns the data in each field based on the adjustment.
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The fallacy with that theory, Nielsen says, is so far he hasn't found any software that does it other than one way – if the true weight is higher, it adjusts all yield data from every spot higher. If it's lower, it adjusts it lower. What it doesn't account for is the fact that in some parts of the field, the monitor might have read low, and in other parts it might have read high, perhaps due to variations in flow rate on the sensor.
Nielsen concludes that calibration is time well spent.