A formula to estimate corn yield

Wondering what your corn yields might look like in 2019? I know some folks who are pretty good at estimating yield who are really having troubling forecasting the crop this season.

The most popular predictor of corn yield is the yield component method, used when the ears get to be roasting ear size. To estimate yield, you need to measure ear number, kernel rows and kernels per row in multiple locations in a field.

At a minimum, I would suggest 5 sampling locations per 40 acres is a good start. Remember, however, that the more variation a field has, the more samples it will take to get a better estimate. And this year, the variations within fields seems to be much larger than normal.

Equipment you need to do this includes a tape measure, a pad of paper, a pencil and a calculator

Step 1. At each stop in a field, measure the length of a single row equal to 1/1000 of an acre. In the case of 30-inch rows, that is 17.4 feet. For other row widths, divide 43,560 by the row spacing (in feet) ad divide that result by 1000. For example, 40-inch rows equals 3.333 feet. The math works out to measure 13 feet of row on a 40-inch row spacing.

Step 2. Count and record the number of ears on the plants in the 1/1000 acre of row. Do not count ears that have dropped to the ground or are on stalks that are lodged and otherwise not harvestable.

Step 3. For every fifth ear in the sample row, record the number of complete kernel rows per ear and the number of kernels per row. Then multiply the number of kernels in an average row times the number of rows. This will give you the number of kernels per ear. Do not count nubbins or aborted kernels.

Step 4. Calculate the average number of kernels per ear by summing the values for all the sampled ears and dividing by the number of ears. For example, 5 sample ears with 480, 500, 450, 600, and 525 kernels per ear, the average number of kernels per ear would equal: (480 + 500 + 450 + 600 + 525) divided by 5 = 511.

Step 5. Estimate the yield for each site by multiplying the ear number (Step 2) by the average number of kernels per ear (Step 4) and then dividing that result by a kernel weight “fudge factor.” Unless your seed company can provide some insight into kernel weight values for their hybrids, I suggest simply performing separate calculations using “fudge factor” kernel weight values equal to 75, 85, and 95. This range of values probably represents that most commonly experienced in the central Corn Belt.

For example, if you counted 30 harvestable ears at the first thousandth-acre sampling site and the average number of kernels per ear, based on sampling every fifth ear in the sampling row, was 511. Using “fudge factor” values of 75, 85, and 95; the estimated range in yield for that sampled site would (30 x 511) divided by 75 = 204, or divided by 85 = 180, or divided by 95 = 161 bushels per acre.

That is a very big spread between the highest and lowest expected yield. If you believe that kernel weight will be below average this year, you would be more inclined to use the lower of the estimates. If kernel weight appears above average for the year, you would lean towards the higher side of the estimate.

It is important to know that these are only estimates, and are probably accurate to only 20 bushels per acre, plus or minus. The more spots you sample in the field, the more accurate the calculation is likely to be.

Jeff Burbrink is an extension educator, Purdue Extension Elkhart County. He can be reached at 574-533-0554 or jburbrink@purdue.edu.


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