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Know your yield gap

August is the time for estimating corn yields. By the end of September some of those estimates are being validated by yield monitors in combines harvesting this year’s bounty. What will you do with your yield maps? Yield monitors have been in use since the early 1990s. In today’s era of “big data” technology, yield maps are providing information to feed into the big data black box.

Know your yield gap


Corn Source

August is the time for estimating corn yields. By the end of September some of those estimates are being validated by yield monitors in combines harvesting this year’s bounty. What will you do with your yield maps? Yield monitors have been in use since the early 1990s. In today’s era of “big data” technology, yield maps are providing information to feed into the big data black box.

Understand the limitations of yield monitors. To effectively use yield monitors for big data applications, you first must understand the limitations. There are some precision and accuracy constraints due to improper calibration or adjustment of settings on the yield monitor. There is erroneous data from too great or too little impact on the flow sensors. Plus, there are several other factors that require yield monitor data to be cleaned up before a proper analysis can be done. Care should be taken to properly calibrate yield monitors for each field and even for each hybrid. Take steps to minimize extreme high or low grain flow across sensors where possible.

Another factor that is becoming more important in recent years is combine swath width (i.e., width of the combine head). A larger combine head means that grain yield represented by a single yield point represents a larger area. Sure, larger combine heads allow for harvest to proceed more quickly but from a yield data standpoint a small area with yield anomalies gets blended or averaged with neighboring areas. This makes it hard to truly “test” with yield monitor data how wheel compaction or malfunctions with manure application, for example, affect yield.

Transforming yield data. After collecting and cleaning yield data, start analyzing the yield maps to pinpoint areas where management can be changed. This step should also involve transforming the data. Transforming data is a systematic way of converting raw yield data into another meaningful matrices, such as relative yield (yield as percent of maximum), yield gap (yield as a percent of potential), or profitability mapping (yield times price minus cost of production).

Transforming yield monitor data may help you make better informed management decisions in the future.

Consider using relative yields that let you compare yields from field to field, hybrid to hybrid, crop to crop, or year to year. When making comparisons with relative yield data, don’t assume the comparisons are equal. Obviously, there are field, hybrid and year differences that are influenced by topography, soils and climate. But you need to ask yourself: Were there management influences such as hybrid type, fertility level, pest severity, etc.?

Determine and map yield gaps. A yield gap is the difference between actual yields and potential yields. What are your potential yields? This is a hard thing to determine; however, some assumptions can be made. Consider public yield information that represents your area or management. For instance, look at hybrid trials in your area or yield contest results. Compare those yields with your past production and yield maps.

Mapping your yield gaps may take some time, but the information will give you powerful insight into making future management decisions. Examining yield gap maps of your fields can help identify field areas that will respond to additional management. Yield gap maps will also help identify field areas that meet yield potentials and consequently will not respond to additional management. For instance, a field area with a 200-bushel-per-acre yield may only be at 75% of yield potential, whereas another area with 150-bushel yield may be at 100% yield potential. Which area needs more focus?

In addition, mapping yield gaps will bring economics into your mapping decisions. Explore the possibility of using your data to create profitability maps of fields. Are areas with low yield gaps making more money in addition to making more yield?

Attaining a zero yield gap may come with additional expenses that result in negative returns per acre. A profitability map can help determine what yield gap is acceptable to achieve the best financial return per acre. It is common knowledge that the first bushel produced has the greatest return to management, while the last bushel produced has the least return. Consequently, producing for maximum yield results in lost profitability.

Licht is an ISU Extension cropping systems agronomist. Contact him at

Estimating yield in the field

If you want to make a yield estimate of corn or soybeans in the field before harvest, what’s the best way to do it? Once you’re into September, you can make a pretty accurate estimate by walking out into the field and counting and evaluating the crop’s main yield components, or some variation thereof. In corn those components are ears per acre, kernels per ear and kernels per bushel. In soybean fields, those components are plant population, pods per plant, seeds per pod, seeds per pound and pounds per bushel. Yield estimates improve in accuracy the closer to crop maturity they are conducted.

Soybean yield estimation

When determining plant populations, it’s best to count plants per 1,000th of an acre at four to six locations in the field to define an average plant stand. If the population is calculated from beans per foot, a higher number of locations should be used to determine the average (maybe 10 locations). When counting pods per plant and estimating seeds per pod use 10 consecutive plants from each area where populations were determined. Generally, exclude pods less than 1 inch long.

To reduce variability and give a better estimate of yield, increase the number of locations and plants used for calculating the average population, pods per plant and seeds per pod. Seeds per pound (seed size) is more difficult to estimate. In normal-to-good growing conditions, there will be 2,500 to 3,000 seeds per pound. Under stressful conditions the number of seeds per pound will increase. The general rule of thumb is to use something between 2,600 to 2,700 seeds per pound. And lastly, soybean yield estimates are based off of 60 pounds per bushel.

Corn yield estimation

Like soybeans, determination of corn ears per acre should be done based off of counts from a 1,000th of an acre. Counting ears is a much better estimate of yield potential compared to counting plants, especially in stressful environments or where populations are too low. Generally, only the primary ears are used for making yield estimates.

Calculating kernels per ear should be done using five to 10 consecutive ears. In fields with great variability from ear to ear, using more ears improves the accuracy of the estimation.

When determining kernels per ear a general rule of thumb is to exclude kernels on the ear tip and butt that are less than 50% to 75% of normal size.

Kernels per bushel within a field can range from 70,000 to 110,000 or more. Generally, kernels per bushel decrease as yield potential increases.

A good rule of thumb for today’s hybrids under ideal conditions is between 75,000 and 90,000 kernels per bushel.

— Mark Licht

This article published in the September, 2014 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

Crop Yields

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