Estimating Corn Grain Yield

This page was adapted from the article, "Estimating Corn Grain Yield," which appeared in Crop & Pest Report on August 26, 2021.

The most commonly used method to estimate corn grain yield in the field is the Yield Component Method, developed many years ago at the University of Illinois. The Yield Component Method can be used well in advance of harvest, as early as the milk stage (R3) of kernel development. Estimates made earlier than R3 risk being overly optimistic if later stresses cause kernel abortion or ear loss.

This method is based on the premise that yield can be estimated from the components that constitute grain yield, including ear number, number of kernel rows, and kernels per row. The final factor, weight per kernel, cannot be measured until maturity (black layer) and, even then, depends on grain moisture at harvest. So, the kernel weight used is somewhat of a “fudge factor” and can be adjusted based on the level of stress the crop endured during the growing season.

Crop uniformity also greatly influences the accuracy of any yield estimation technique. The less uniform the field, the greater the number of samples that should be taken to estimate yield. Attempt to sample locations within the field randomly so as to not bias the yield estimate up or down. In a fairly uniform field, calculate yield at five different locations.

1. At each location, measure off a length of row equal to 1/1000th of an acre. For 30-inch (2.5 ft) rows, this is 17 ft 5 in. For 36-inch rows, this is 14 ft 6 in. For other row spacings, divide 43560 by the row spacing in feet and then divide that result by 1000.
2. Count and record the number of harvestable ears on the plants in 1/1000th acre of row. Do not count dropped ears or ears on severely lodged plants unless you are confident that the combine header will be able to pick them up.
3. For every fifth ear in the sample row, count the number of complete kernel rows per ear. Do not sample nubbins or obviously deformed ears unless they are representative of the sampled area. If row number changes from butt to tip, which can happen due to drought stress, estimate an average kernel row number per ear. Don’t count the butt or tip ends, but rather choose an area in the middle of the cob where there is a complete ring of kernels. Do not count aborted kernels. The photo at right shows a cob with 20 kernel rows. Kernel row number is almost always even, but extreme drought or nutrient stress may result in an odd number. Photo credit: North Carolina Extension Service.
4. Using the same ears, determine the average number of kernels per row on each ear. This is done by counting the kernels along the length of the cob from butt to tip. If numbers of kernels per row are not equal among the rows of an ear, estimate an average value for the ear.
5. Estimate the yield for each location by multiplying the ear number by the average kernel row number by the average kernels/ row number, and then divide by 90. 90 is the “fudge factor” mentioned earlier and represents 90,000 kernels per 56 lb bushel. In growing seasons with stressful conditions, kernel size will likely be smaller than average and so a higher number, e.g. 100, can be used. In a year with excellent grain fill conditions and little stress, a lower number, e.g. 75, can be used, representing larger kernels. So, the equation looks like this:

                                     Ears     * Kernel Rows * Kernels

Corn Yield =        1000th ac              Ear                Row               

                                                                90

For example, say you counted 24 harvestable ears at the first location. Sampling every 5th ear resulted in an average kernel row number of 16 and an average number of kernels per row of 30. The estimated yield for this location would be (24 x 16 x 30)/90 = 128 bu/ ac.

Now, let’s say this field encountered moisture stress during pollination and silking. Using a higher denominator, that is, smaller kernel size and/or lighter kernel weight, would be appropriate. So, using the same numbers as before but a higher number of kernels/ bushel: (24 x 16 x 30)/100 = 115 bu/ ac.

Repeat these steps at as many sites within a field as you deem representative. Calculate the average yield for all sites to estimate the yield for the field.

Remember that this method for estimating corn grain yield is indeed only an estimate. Since kernel size and weight will vary depending on hybrid and environment, this method should only be used to determine ballpark grain yields. Yield will likely be overestimated in a year with poor conditions during grain fill (like our current drought) and underestimated in a year with excellent conditions throughout grain fill. The Yield Component Method for estimating corn grain yield tends to be accurate within +/- 20 bushels per acre. Use the yield estimates obtained by this method for general planning purposes only.

*This article was adapted from a previous Crop & Pest Report article by Dr. Duane Berglund*

Clair Keene
Extension Agronomist Small Grains and Corn
NDSU Williston Research Extension Center