Bob Nielsen pays a lot of attention to yield monitors and the maps they produce. That's because the Purdue University Extension corn specialist has moved to doing a lot of testing either at Purdue ag centers or on private farms in large-scale plots. Typically, yields are recorded by yield monitors.
It's a given that the yield monitor must be calibrated correctly. However, Nielsen says that's not the only possible source of error.
He's coined what he calls "wandering swath error syndrome." It can occur if the yield monitor display is equipped with a feature that allows you to set it to automatically change the header width if the header is overlapping areas already harvested. The changes are included when the monitor calculates yield.
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It's a good feature if the field has lots of point rows, he says. However, he has discovered that sometimes the automatic feature makes an adjustment when an adjustment shouldn't be made.
If it narrows the header width to 35 feet when 40 feet is really coming in, the yield estimate will be the same yield measured by the sensors divided by a smaller area, giving an erroneous high yield estimate.
In corn, the difference can be 30 bushels or more over the spot where it occurs, he says. Normally it only occurs over a small area in the field.
Two solutions are possible, he notes. First, you could switch it to manual and adjust the header width setting yourself if you're not taking a full swath.
The other solution is to switch to a more accurate GPS differential correction signal, he says. Most of the errors occur when someone is using the free WAAS signal. Errors are far less likely if you're using a more accurate differential correction GPS signal, like RTK. The improved accuracy costs money, but may result in more accurate results in the long run, he says.