By Darrell Smith, Farm Journal Conservation & Machinery Editor

Today’s yield monitors are incredible tools to ratchet up yields. Being able to gather data on the fly and produce maps that pinpoint yield victories and defeats is invaluable to producers who want to maximize yields. But don’t forget that the high-tech tools are only as accurate as you help them to be.

Yield monitors can’t magically calibrate themselves—at least not at this point. You have to follow the calibration procedure to dial in accuracy.

It hasn’t been long since collecting yield data with spatial accuracy capable of variable-rate techniques was a tedious process of running a measuring wheel, setting flags and weighing each individual area. By comparison, spending a bit of time with calibration is quick and easy.

“Even though it’s not hard to calibrate, half of the yield maps I see are of little value because the yield monitor had not been calibrated,” says Farm Journal Field Agronomist Ken Ferrie. “The data they generate is accurate enough for inventory purposes—telling a farmer how much total grain they harvested—but not for diagnosing any problems. Uncalibrated maps don’t provide the kind of spatial data you can use to identify problems or move up to variable-rate inputs, which is the next step on the road to increased efficiency.”

Knowing what a field yielded overall isn’t enough to ratchet yields to the next level. “To improve, you have to know where in the field that corn came from,” Ferrie says. “Some spots may have produced 250 bu. per acre, but other spots only produced 120. If you can’t identify those areas, trying to improve yields is like asking a doctor to diagnose a problem without telling him where it hurts.”

A good yield map has a lot of definition. Also, areas with high and low yield coordinate with factors in the field, such as soil types, topography and drainage.

“A poor map is the opposite,” Ferrie explains. “There’s no definition, and the yields don’t line up with any factors. On top of that, there is lots of variation from point to point—the yield may go from 200 bu. per acre down to 90 bu. per acre between two data points.”

Overlay a calibrated yield map with a soil map and you’ve created a road map to higher yields. “Most software makes that fairly easy to do,” Ferrie says. “If you’re not computer savvy, a seed company may print a map for you, or you can hire a crop consultant or mapping service to do it. A yield map can really come to life when you overlay different layers of information relevant to the field.”

With the soil map in place, identify varieties and see how each one responded to soil types. Then, look for drainage and pest problems, such as insects and nematodes. Finally, pick out your soils that yield the best and worst for a period of time. Look for repeatability; a true yield zone will repeat itself over time.

“When we analyze yield maps, we try to separate man-made problems, such as planting date, harvest date and moisture, from natural yield breaks,” Ferrie says. “Man-made problems usually show up on maps as straight lines.”

Analyzing maps needs to be a team effort. If different people did the planting, spraying and harvesting, all of them must be in the room. Your goal is to spot yield problems, and then see if anyone can figure out the cause.

Also, have your soil test information handy during the process. It may help identify the source of a problem.

Decide whether soil zones are accurate enough to serve as management zones or whether you must delineate yield zones. You’ll want to do that if you repeatedly see distinctly different yield zones within a soil type.

“The more years of data you accumulate, the more helpful your map will be,” Ferrie says. “The most important years of data are those reflecting dry and wet seasons because that’s when problems will show up more clearly.”

If you have RTK (real-time kinematics with sub-inch accuracy) GPS in your combine, your yield monitor can help you identify yield zones. Use your monitor to generate a topography map, and lay that over your regular yield map. 
     
NDVI mapping. To make your yield map even more useful, consider overlaying it with a Normalized Difference Vegetation Index (NDVI) map, Ferrie says. These maps—created by photographing fields with color and infrared film—cause problem areas and healthy areas to show up as different colors. The imagery can be obtained by satellite, airplane, drone or in-field technology such as GreenSeeker (www.ntechindustries.com).

GreenSeeker technology can map a field as you apply herbicide, fungicide or insecticide. So far, the GreenSeeker technology has been used mostly for sidedressing fertilizer—but the sensors could be used on other machines throughout the growing season.

“By taking NVDI readings through the season and overlaying the information on your yield map you may be able to document when a problem, such as nitrogen deficiency, showed up,” Ferrie says. “If you could mount a system like GreenSeeker on a sprayer, the turn-around time would be short enough that a farmer might be able to react to a problem and damage.”

NDVI maps will help define yield zones more precisely, which helps you create management zones, Ferrie explains. “You always have to ground-truth a situation to see why one area of the field isn’t keeping up,” he adds. “But two-thirds of the battle is won once you know where in the field to look.”
     
Mapping without monitors. What if you don’t have a yield monitor, if you grow crops for which yield monitors are not made, such as vegetables, silage and hay, or if your yield monitor does not collect enough data points (some record data only every eight seconds)? An NDVI map actually can substitute for a yield map, in some ways, by showing variance in the crop.

“Or you can hand-check yields at points in the field and let the software actually build a yield map,” Ferrie says.

Although the technology behind NDVI mapping has been around a long time, it is just starting to be used in agriculture. “As more companies begin offering the service and the price comes down, I think a lot of farmers will pick up on NDVI,” Ferrie says.

Fair warning. As you work with yield maps, be wary that they can be addictive for everyone involved.

“If you hire someone to drive your combine, getting him involved with the yield mapping process will change his whole outlook,” Ferrie says. “If he sees a 60-bu. yield drop, he’ll stop the combine, get out and do stand counts or collect some ears for analysis.

“Your behavior will change, too,” Ferrie continues. “You’ll find yourself taking time to recalibrate your yield monitor when corn moisture changes. Where once you would have continued harvesting even though your yield monitor had broken down, now you’ll shut down the combine and wait for a new part to arrive.” 

Snapshots of a Field

Picture time! NDVI (Normalized Difference Vegetation Index) mapping uses normal color photography and infrared photography to show differences in vegetation across a field. Because NDVI mapping is just starting to be used in agriculture, sources may be difficult to find in some areas.

“The market could open pretty wide in the next few years,” says Isaac Ferrie, who works with his father, Farm Journal Field Agronomist Ken Ferrie.

The following is a list of sources for NDVI mapping. Some of the companies also offer additional services, such as map interpretation and data storage.

• Focal Flight (http://focalflight.com), a company based in Ojai, Calif., that serves much of the western U.S., uses aircraft to photograph fields.
• Agri ImaGIS Technologies (www.satshot.com), a Fargo, N.D.,-based firm, provides satellite images.
• Calmar Mapping Services (www.calmarlabs.com) of Remington, Ind., uses airplanes and an unmanned drone aircraft to shoot NDVI images of farm fields.
• NTech Industries Inc., (www.ntechindustries.com), based in Ukiah, Calif., markets the GreenSeeker RT100 mapping system, which operates by collecting the light reflectance of plants in a field.

• Yield maps are a tool for obtaining higher yield and more efficient use of inputs.
• To create a useful yield map, yield monitors must be calibrated each harvest and recalibrated during the season if conditions change.
• Overlaying a yield map with a soil map and—if available—an NDVI map allows you to create management zones and vary the rate of inputs.
• NDVI maps can substitute for yield maps for crops that do not have yield monitors.
• The more years of data, the more useful the yield map.