Calibrating Your Yield Monitor

Maps that show the yields of your farm may be the most important asset in precision ag. The problem is that the maps are only as good as the monitor that took the data. Yield monitors in the combine need to be taught what's considered a high, low or medium yield--by grain type. Read on to get more tips about gleaning clean data.

By: Margy Eckelkamp, Farm Journal Media

With every combine pass, you have the opportunity to harvest more than grain. Farmers know there is variability across their fields, but having a GPS-referenced yield map provides a solid foundation for future zone management decisions.

"We’ve found that the most important data are the yield maps," explains Neal Wolheter, who farms near South Milford, Ind. "We’ve used soil maps for 12 years for zone management and a yield monitor for the past five to help refine those zones."

The key step to gathering usable data with a yield monitor takes place before harvest even begins—with proper calibration.

"Many yield maps have not been usable in zone management decisions because the calibration process was not done properly," explains Farm Journal Associate Field Agronomist Missy Bauer. "You have to teach the monitor what’s considered low, medium and high yields for each grain type."

zone maps
What a difference a year makes. The map on the left was made using an uncalibrated yield monitor. The map on the right was captured the next year using the correct calibration, providing spatial variability correlating to soil types.

Bauer and her crew have worked with farmers across Michigan, Indiana and Ohio to develop management zones. When preparing for harvest every year, she says the most common mistake is the size of calibration loads.

new zone map

This NDVI aerial map indicates several areas that need to be ground-truthed.

"Incorrectly, farmers will often run a calibration load as a whole semi load," she says. "We do 4,000-lb. to 8,000-lb. loads at the different speeds to simulate bushels per acre. That cleans up the data, but those calibrations are only good for +/- five moisture points in corn. So we run a high-moisture corn type and a lower moisture corn type when we calibrate."

For Wolheter, his motivation to properly calibrate yield monitors comes from the value of the data.

"We’ve seen how good-quality yield maps translate into better zone management," he says.

Following the calibration instructions changes the yield monitor from measuring averages to precisely mapping variability.

 "If a grower calibrates by the semi load, his maps could be accurate to the average of the field, but you can’t pick out the variations in yield. Those maps don’t show any defined patterns in yield," Bauer says.

Identifying that variation in a field is the difference between measuring accuracy and preciseness. This is the definition of spatial variability.

"If your yield monitor is accurate but not spatially accurate, it’s only good for managing inventory," says Farm Journal Field Agronomist Ken Ferrie. "It won’t be a good tool for zone management because it’s all about precision. We really need an accurate and spatially variable map. However, I can work with a spatially variable yield monitor that is 10 bu. off from the elevator better than one that only displays accuracy."

Ferrie encourages farmers to think of the yield map as their report card.

"Of all the layers we are going to look at, the yield maps show the history of the field. From there, we can make predictions on how different things will affect yield—nitrogen management, population, drainage—but it all comes down to what actually happens in the yield map," Ferrie says.

Focusing on a high-quality yield map is even more crucial in an extreme year.

"The years of extreme create some of the best zone maps," Bauer says. "It’s the very wet and very dry years that help us pick out the differences, which can be very valuable. Farmers should not hesitate spending the time to calibrate in those years."

Refine the lines. The data collected with a properly calibrated yield monitor is an irreplaceable component of zone management.

"When we talk about developing management zones, it’s about where to draw the lines," Bauer says. "We have to respect soil types, but yield can’t only reflect soil types; it also breaks down those soil types into finer zones."

For Wolheter, multiple years of calibrated yield maps showed that the zones formed by soil type needed further revision.

"Once we reviewed the yield maps, we saw water issues and misapplication of potash and lime that needed to be addressed," he explains. "In some cases that cut zones in half; in others, we just needed to move the lines."

Pairing yield results with soil types can help make variable-rate decisions for population and nitrogen rates.

"In the whole equation of zone management, you have to take out the emotion and put numbers to it," Ferrie says. "In a yield map, you can identify the lost potential."

zone maps 2

From year to year, yield maps might reveal new zones. By using other data layers and ground-truthing, several zones were broken out further (as indicated by the white circles) to create new zones.

The next level. Once you have a yield map showing variability in the field, it’s time to use other tools and layers of data to investigate.

"Our general process is to evaluate soil maps, overlay a yield map and factor in topography," Bauer explains. "That just helps define and refine the zones. Then we use additional layers to answer questions."

Using normalized difference vege­tation index (NDVI) aerial maps, Wolheter has been able to learn more about water management challenges.

"We saw that in some of our fields, there were specific areas that we needed to know more," Wolheter says. "So we prioritized the larger fields and starting capturing NDVI maps in-season. On our soils, parallel tiling can pay heavy dividends. Having an extra layer of data in-season supported the investment in some of our fields."

Both Farm Journal Field Agronomists stress the importance of using additional data layers, such as NDVI maps, in addition to a yield map; it is not a direct replacement.

"The two layers aren’t totally interchangeable," Bauer says. "If the NDVI is taken late season in August, for example, that map will reflect a yield map more closely. However, a late June NDVI is a better scouting tool."

It’s the detail and timing of an NDVI map that gives it value in the zone management process.

"NDVI maps can give us sub-meter resolution to the data," Ferrie says. "For example, even a well-calibrated yield map may not show a small area that needs to be managed. But a sub-meter NDVI map can show that area, and if it appears consistently year after year, it can be factored in."

If there are still unanswered questions, there are more tools available including in-season technologies, such as aerial photos and thermal imaging, and out-of-season agronomic measures such as soil analysis, tissue analysis and electroconductivity mapping.

Thermal imaging captures measurements every 10 days to closely monitor crop changes.


For five years, Neal Wolheter has overlaid yield maps and other data to refine zones.

"With any aerial service, you might want to ask for a high-resolution photo to be taken at the same time to ensure there weren’t obstructions from clouds," Ferrie cautions. "Even with all these maps, farmers need to put their boots on the ground. They look at each layer independently, separate each crop and look at each year differ­ently—their in-field experience is key."

Issues that show up on maps as poor but need further ground-truthing could range from a water-management concern to soil health.

"In a dry year like 2012, we saw that inside the same soil texture, problem areas ranged from an acre to the size of house. We looked below ground in those pockets and found perched water tables. That indicates a water supply problem, which has to be taken into consideration when we make the population map," Ferrie says.

Taking zone management layers one at a time helps farmers digest the massive amounts of information.

"No doubt, farmers themselves are in the driver’s seat to verify these zones," Bauer says. "We have yet to find a computer that can replace the farmer’s in-field knowledge. Really good management zones are achieved by layering multiple sources of information, including human experience."

Wolheter has a practical piece of advice—buy a good printer. While he has digital files on his iPad, nothing can replace the hard copies.

"We print out all of our maps and organize them by field and year," he says. "Being organized is the top way to not get overwhelmed. It’s also easier to share that information with the rest of our farm or landlords."

Wolheter also bought a small spiral binding machine to make the printouts into booklets for each field.

To ease into the process, Wolheter is varying population and nitrogen on the fields that will return the biggest payback. "For now, we only use VRT on fields larger than 15 acres," he says.

While the end goal in adopting zone management will be different from farm to farm, a calibrated yield map will pave the best path.

"It takes paying attention to a little bit of everything but not necessarily everything at once," Bauer says.

Ferrie likens managing data layers to writing an open book. "We learn a little more each year," he says. "The lesson along the way is that we don’t have all of the answers." 


soil management zones