Corn Corner: Yields may swing widely depending upon kernel size and fill.

Jul 30, 2019

With the corn crop maturing late, will yield estimate formulas still be accurate? Should I adjust the formula to account for the late crop?

The Indiana certified crop advisers panel answering this question includes Betsy Bower, Ceres Solutions, Terre Haute; Traci Bultemeier, Corteva/Pioneer, Fort Wayne; and Dan Ritter, Dairyland Seed, Wabash.

Bower: In any given year, the yield formula is often a bit off, since we need to assume or estimate how many kernels are in a bushel of corn. A typical estimate would be 90,000, but that will vary from year to year depending on the depth of the kernel. With late-planted corn, since it takes fewer growing degree days to mature, our depth of kernel may not reach what it would in a normal year. The 90,000 kernels per bushel may be low. Otherwise, you’ll be able to count number of rows per ear, number of kernels per row and number of ears in 1/1,000 of an acre, and then make calculations as you would in any other year.

Bultemeier: The simple answer is yes, you should adjust the formula. The long answer is filled with a lot of “what ifs” and “well, it depends.” The No. 1 piece of advice is to get a bird’s-eye view of the field, either with a drone or a Crop Health Index map, so you know how much of the field is “good” and how much is “questionable” or “poor.” Then sample several places from each of those areas. Then you can better decide what denominator to use for number of kernels per bushel. This number varies so greatly by test weight, hybrid and environmental conditions during grain fill that you can’t have an accurate estimation of yield until black layer and after.

If you Google any article on estimating yields, you’ll read that this number is often lovingly referred to as the “fudge factor.” Take that for what it’s worth, go fishing and enjoy the time before harvest hits!

Ritter: Look at the formula to estimate yield. It’s number of rows around the ear times number of kernels per row times ears per 1/1,000 of an acre. That number is divided by 90. This formula basically uses kernel count and weight. Basically, you’re estimating number of kernels per acre and a “factor” that considers weight per kernel.

Many times, 90 is used as that factor. So if due to late planting, grain weight or grain density is lighter, adjust that number to say 100. I will do this with timely planted corn, however, based on my knowledge of a hybrid and kernel weight.

Editor’s note: The Purdue University Corn and Soybean Field Guide contains information on estimating corn yields. One-one-thousandth of an acre in 30-inch rows is 17 feet, 5 inches. A table gives distances for other row widths. The guide also notes, based largely on Bob Nielsen’s work with modern hybrids, that kernel weight per 56-pound bushel can vary from 65,000 to 100,000. Instead of using 90 as the fudge factor, according to the guide, many agronomists use 75, 85 or 95. The guide also notes that kernel count per bushel for the same hybrid can vary by 20,000 kernels per acre year to year.