Franklin Co. soybeans

Estimating corn and soybean yields

By Greg LaBarge

As the calendar turns to August, several regional and national crop tours lead up to the September NASS crop yield report. While crop tour reports are great conversation starters, yield estimates from your farm are the most meaningful to your decision-making. Yield estimate tools for corn and soybeans are widely published. An example is our Corn, Soybean, Wheat, and Forages Field Guide, Bulletin 827. So pick a representative corn and soybean field on your farm and join in on the crop tour fun. Here’s how to estimate corn and soybean yields.

Corn yield estimates are based on determining the number of kernels per acre and then using a standard kernel weight. A commonly used formula is found in the Corn, Soybean, Wheat, and Forages Field Guide, Bulletin 827, on page 14.

There are several techniques for estimating corn grain yield before harvest. The only equipment needed is a tape measure plus a pencil and paper.

Step 1. Count harvestable ears in 1/1000th acre. For example, the row length for 1/1,000 an acre in 30-inch rows is 17 ft, 5 inches, while 20-inch rows are 26 ft, 2 inches. And if you use a 15-inch row width, count the rows on both sides, down and back, of a 17 feet 5-inch length.

Step 2. Count the number of rows per ear on every 5th ear. Calculate an average.

Step 3. Count the number of kernels per row on those same ears. Do not count kernels on the butt or tip portion that are less than half-sized. Calculate an average.

Step 4 – Yield calculation. Yield (bushels per acre) equals ear number multiplied times the average row number times the average kernel number, then divided by 85.

• (ear # x average row # x average kernel per row #) / 85 = estimated yield in bu/A.

• Example: 33 harvestable ears and the five ears counted have an average of 18 rows and 28 kernels per row.

Calculation: (33 * 18 * 28) / 85 = 196 bushels per acre

This estimation method can be right on the money compared to harvest yields. One potential factor that will cause a difference in estimated versus measured yield is kernel weight. The equation uses a numerical constant of 85 for average kernel weight. The factor can be adjusted to 80 when a deep kernel exists or  90 if the kernel is shallow. Weight per kernel will vary depending on the hybrid and environment; yields are overestimated in a year with poor grain fill conditions and underestimated in a good year. Accuracy will increase if you check multiple locations in a field, especially when the field has different soil types or yield potentials.

Soybean yield estimates require determining seeds per acre, estimating seeds per pound then applying a standard seed weight. Corn, Soybean, Wheat, and Forages Field Guide, Bulletin 827, page 139, has the soybean yield estimate formula.
Step 1. Calculate plants per acre. Count the number of pod-bearing plants in 1/1,000th of an acre. The row length for 1/1,000 an acre in 30-inch rows is 17 feet, 5 inches, 15-inch rows is 34 feet, 10 inches, and 10-inch rows is 52 feet, 3 inches.

Step 2. Estimate pods per plant. Count the number of pods (containing one or more seeds) from 10 randomly selected plants. Then, divide the total number of pods by 10 to get the average number of pods per plant.

Step 3. Estimate the number of seeds per pod. Count the number of seeds from 10 pods selected at random. Generally, the number of seeds per pod is 2.5, but this number can be less in stressful environmental conditions. Divide the total number of seeds by 10 to get the average number of seeds per pod.

Step 4. Estimate the number of seeds per pound (seed size), and assume 3,000 seeds per pound. If the soybean plants are stressed, seed size will be reduced, and it will take more seeds to make one pound. Use 3,500 seeds per pound if you expect smaller seeds due to late-season stress.

Step 5. Use numbers from Steps 1-4 in the formula below to estimate soybean yield in bushels per acre:

• [(plants/1,000th acre) x (pods/plant) x (seeds/pod)] ÷ [(seeds/pound) x 0.06]. bushels per acre =soybean bushels per acre

• Example:110 plants per 1/1,000th acre, 30 pods per plant, 2.5 seeds per pod, 3,000 seeds per pound. [110 * 33 * 2.5] / [3,000 * 0.06] = 50.4 bushel per acre.

Greg LaBarge, OSU Extension Field Specialist, or 740-956-5047

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