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Dr. Chad Lee

Extension Grain Crops

859.257.3203

cdlee2@uky.edu

Dr. Jim Herbek

Extension Grain Crops 270.365.7541 ext. 205

jherbek@uky.edu

Estimating Corn Yields

Chad Lee and Jim Herbek, Grain Crops Extension, University of Kentucky

August 22, 2005

Some producers and crop consultants will count kernels per ear and multiply that number by 0.300 to get a final yield estimate. While this method is a very fast way of estimating yield, it makes several assumptions and could be misleading. The multiplier of 0.300 assumes 27,000 corn ears per acre and an average seed size of 90,000 kernels per bushel. Changes in either ears per acre or kernel size changes this multiplier. Initial seeding rate, stress on developing plants and pests can all change the final number of ears per acre. Stressful weather conditions such as drought will reduce kernel size, while ideal growing conditions can increase kernel size.

The following information will help you to estimate corn yields. The procedures outlined here should start from the very simple but not very precise, to the more complicated but more precise methods for estimating corn yields per acre.

Simplest and Least Precise Method

The simplest and least precise method was described above. Select an ear or ears that represent the average ear size in the field. Count the number of kernels per ear and multiply by 0.300 to get a very rough yield estimate. To determine number of corn kernels per ear, multiply number of rows on an ear by number of kernels in a row. Do not count kernels near the tip that are less than half the size of kernels midway up the ear.

Example 1:

You count 12 rows per ear and 50 kernels per row to equal 600 kernels per ear.

600 x 0.300 = 180 bushels/acre.

To add a little more accuracy to the simplest method, you can count the kernels on each ear of 10 consecutive ears in the field. Then, you can average the counts from the 10 ears to have a better estimate of kernels per ear.

Adjust for Population and Seed Size

Not every field will have 27,000 ears per acre. If you wish to assume the number of ears per acre in a field, then assume 1,000 to 2,000 ears less than targeted plant population. If you want to adjust seed size based on the type of growing season, then you can use the multipliers in Table 1 to assist in making your yield estimate.

Table 1. Multipliers based on ears per acre and kernel size to calculate expected yield. Determine the number of kernels per ear and multiply that number by the correct multiplier to make a yield estimate.

Example 2:

You count 12 rows per ear and 50 kernels per row to equal 600 kernels per ear.

If you assume 25,000 ears per acre, then 

• In an average year, 600 x 0.278 = 167 bushels/acre 

• In a highly stressful year, kernel size will be smaller and 600 x 0.227 = 142 bushels/acre

• In a highly productive year, kernel size will be larger and 600 x 0.357 = 214 bushels/acre

If you assume 30,000 ears per acre, then

§         In an average year, 600 x 0.333 = 200 bushels/acre

If you assume 22,000 ears per acre, then

§         In an average year, 600 x 0.244 = 146 bushels/acre

So, in an average year, yield estimates for 600 kernels per acre can range from 146 bushels per acre to 200 bushels per acre by adjusting population from 22,000 to 30,000 ears per acre.

Use Ear Counts to Estimate Ears per Acre

Knowing ear number per acre is critical to estimating corn yield. Plant population is not a useful number since some plants may be barren and others may have two ears. There are several ways to determine ear population per acre. Many people use 1/1,000^th of an acre to do all calculations. This method is easy to follow since ear counts in 1/1,000^th of an acre can be multiplied by 1,000 to equal ears per acre. Table 2 provides the row width and length of row needed to equal 1/1,000^th acre.

Table 2. Row width and length of row needed to equal 1/1,000^th acre. Ear counts should be multiplied by 1,000 to equal ears per acre.

Example 3:

You count 12 rows per ear and 50 kernels per row to equal 600 kernels per ear. You count 26 ears in 1/1,1000^th acre to equal 26,000 ears per acre.

§         In an average year (medium kernel size), 600 x 0.289 = 173 bushels/acre

§         In a highly stressful year (smaller kernel size), 600 x 0.227 = 136 bushels/acre

§         In a highly productive year (larger kernel size), 600 x 0.357 = 223 bushels/acre

Improve Estimate of Ears per Acre

While counting number of ears in 1/1,000^th acre is a relatively easy way to calculate ears per acre, the length of a row counted is only 17 feet 5 inches in 30-inch row widths. Counting ears in longer sections likely will provide more accurate counts and more accurate estimates of ears per acre. Most farmers in Kentucky raise corn in 30-inch rows. Table 2 allows you to count the number of ears in either 50 or 100 feet of row to estimate the total number of ears per acre.

Table 2. Number of ears per acre based on the number of ears counted in either 50 or 100 feet of row in 30-inch row widths

When making ear counts from row lengths of 50 or 100 feet, the calculated number for ears per acre in Table 3 will be more precise than the numbers for ears per acre in Table 1. You can round to the nearest number in Table 1 or you can add more precision to the equation by using the actual population estimate in the calculations.

Example 4:  You count 12 rows per ear and 50 kernels per row to equal 600 kernels per ear. You count 145 plants in 100 feet of row, which equals 25,265 ears per acre. 

§         Option 1: Round 25,265 to 25,000 and use the multiplier in Table 1

§         In an average year, 600 x 0.278 = 167 bushels/acre

§         In a highly stressful year, 600 x 0.227 = 136 bushels/acre

§         In a highly productive year, 600 x 0.357 = 214 bushels/acre

§         Option 2: Use 25,265 ears per acre as part of the calculation.

§         kernels per ear x ears per acre / kernels per bushel = bushels/acre

§         In an average year, 600 x 25,265 / 90,000 = 168 bushels/acre

§         In a highly stressful year, 600 x 25,265 / 110,000 = 138 bushels/acre

§         In a highly productive year, 600 x 25,265 / 70,000 = 217 bushels/acre

As you can see from Example 4, the differences in yield estimates between Options 1 and 2 are only about 3 bushels per acre. For most yield estimates, the additional precision in Option 2 probably does not warrant the additional effort.

While most farmers currently raise corn in 30-inch rows, some farmers are using different row widths. Table 4 outlines the multipliers needed for 50 and 100 feet of row at various row widths.

Table 4. Multiplier needed to equal ears per acre for specified row widths and row lengths.

Keep Yield Estimates in Perspective

Remember that yield estimates are only as accurate as the field area that was sampled. The yield calculations mean little if you have selected the best or worst area of the field to estimate yield. Repeating yield estimates in several areas of a field will improve the accuracy of the yield estimate. Making ear counts from larger areas (for example, counting ears in 100 feet of row instead of 17 feet 5 inches) will improve the estimate of ears per acre. Counting kernels per ear from ten consecutive ears will improve accuracy of the kernels per ear.

Factors such as water availability, insects, weeds and diseases can affect seed fill and final yields. Yield estimates conducted in the dent stage close to blacklayer should be more accurate than yield estimates conducted during the milk and dough stages. If you are estimating yields in the milk or dough stage, then you may want to use the simpler and less precise method for estimating yield. If you are estimating yields in the dent stage or after blacklayer, you may want to use the more complicated and more precise methods.

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