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Get most from yield maps

It’s that time of the year — map-reading time. As you sit down at your desk with a stack of GPS yield maps, remember that colorful maps are not the same as useful knowledge.

For these tools to be of real value, the information must be accurate. It must be organized and presented in a meaningful way. It must aid your management decisions.

Key Points

Yield map information must be accurate and well-presented.

The most useful types of maps are normalized yield and profit maps.

Use yield maps to identify large trends and management zones.


Here are a few suggestions for using this powerful technology.

Remove erroneous data

Accurate data collection begins with careful yield monitor calibration for each type of grain harvested. Don’t skimp on this step.

In addition, yield data often contains errors arising from field features or combine operations. For example, changes in combine speed or swath width can distort the data. If you don’t clean up these errors, you can end up with misleading maps. Data collected from field headlands, point rows, overlaps and outliers should be removed, as well as places where you had to stop the combine.

Most yield mapping software has a function that automatically cleans up your data by removing unrealistic values. These functions do a fairly good job, but on occasion they can mask problems, like insect or disease hot spots or wet spots. That’s why I usually clean up the yield data manually. This takes more time, but it will also give you deeper insight.

Map data

Think about what you want the map to show. How you group yield data has a big effect on the appearance of your yield map — and its usefulness.

Most mapping software offers several ways to aggregate data, such as grouping by quartiles, equal yield ranges or natural breaks. Each method has advantages and drawbacks. Try several methods to see what they reveal.

The number of yield ranges also affects your map’s appearance. There’s no magic number of ranges — it depends on your goals. As a general rule, using too few ranges will cause meaningful variations to disappear.

However, many growers err in the opposite direction, setting too many ranges, says Steve Tupa, a Minnesota-based business development specialist for Agco’s Advanced Technology Solutions. That makes it hard to see the whole picture clearly. “When I look at a yield map, I look for large zones. Don’t get too caught up in details,” he advises.

Choose the right map

There are many types of maps you can generate with your harvest data. One of the most useful is a normalized yield map, which expresses yields as a percentage of the field average.

These maps are especially good for comparing multiple years and different crops.You can see which parts of the field consistently yield above average and which are consistently below average. Normalized yield maps are also a good way to see if your management is reducing field variability.

Follow up with detective work

A yield map only documents variations — it doesn’t explain why they happened. Variability may be caused by any number or combination of management practices and natural variations. You have to play detective.

Bring in additional tools, such as your GPS-referenced planting, spraying and soil nutrient maps; scouting notes; soil surveys; topographic and drainage maps; weather records; and your own detailed knowledge of the field’s history.

“I look for areas that are consistently high-yielding, consistently low-yielding, and areas of variability,” Tupa says.

Each of these requires a different approach. In stable, low-yielding areas, for example, there might be an opportunity to correct a problem, such as poor drainage, or to reduce inputs and lower costs.

Consult an expert

Many growers don’t have the time or expertise to delve deeply into yield map analysis and interpretation. That’s why Tupa advises working with a precision agriculture consultant.

“There are many qualified agronomists who can help you,” he says.

Trcka is an agronomist with CHS Inc., Grand Meadow. Find information and links to Minnesota certified crop advisers on the Minnesota Crop Production Retailers website, www.mcpr-cca.org.

This article published in the January, 2011 edition of THE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.