How to make a preharvest estimate

UNIVERSITY PARK, Pa. — Everyone is wondering how their crops stack up this year. Did I get enough rain, too much rain, enough growing degree days?

Although he admits it’s not without problems (it’s difficult to estimate accurately), Penn State Extension educator Delbert Voight offers this simple procedure to help determine potential soybean yield prior to harvest.

1. Determine plant population

You need a reference row needed to equal 1/1000th acre. Voight uses this chart when it’s difficult to walk beans at later stages of growth.

Row Width (inches)           Length of Row Needed to Equal 1/1000th Acre
     6                                                 87 feet 1 inch
     7                                                 74 feet 8 inches
     7.5                                              69 feet 8 inches
     15                                               34 feet 10 inches
     30                                              17 feet 4 inches

Take several samples, calculate the average of these samples, then multiply this average number by 1,000 to get your per-acre population.

2. Determine the Pods Per Plant.

This will vary with weather, Voight cautions, and be sure to count the pods on the branches as well. Do this for at least 10 plants to get more accuracy.

3. Determine Seeds Per Pod.

Voight said you’ll typically find 2.5 seeds per pod. In ideal weather, 3 and 4 beans might be noted. In dry conditions this can drop to 1 or 2. Get a handle by counting a few and averaging what you think is the average seeds per pod.

4. Determine Seeds Per Pound.

This can be difficult and a weak link in this estimate, Voight cautions. Beans vary dramatically in weight, from 2,000 to 3,600 seeds per pound. The average is about 2,500 and this is what Penn State experts typically use. If a drought year exists during pod fill, then higher seeds per pound will exist, he said.

5. Do the math.

Enter Information into the following equation to determine estimated yield:

a. bushels per acre = (plants per acre) x (pods per plant) x (seeds per pod) ÷ (seeds per pound) ÷ (pound per bushel). [A bushel of soybean weighs 60 pounds.] Example: 121,968 x 32 x 2.5 ÷ 2,500 ÷ 60 = 65.05 or 65 bushels per acre.

Keep in mind that accuracy is difficult, Voight cautions. Harvest loss and other deviations can make the estimate inaccurate.

Estimating corn yields

The kernel count method is a quick yield estimate tool that can be used when harvesting corn either for grain or silage, according to Penn State Extension Ag Educator Andrew Frankenfield.

Here are his steps to make a preharvest estimate of corn yields:

  1. Measure off a length of row equal to 1/1000th of an acre (17.4’ for 30-inch rows)
  2. Count and record the number of harvestable ears.
  3. On every fifth ear, count number of kernel rows and number of kernels per row. Then multiply the number of kernel rows by the number of kernels per row to determine the number of kernels per ear.
  4. Calculate the average number of kernels per ear by adding up the values for all the sampled ears and divide by the number of ears sampled. Example: if you sampled 5 ears with 480, 500, 450, 600, and 525 kernels per ear, the average number of kernels will be (480 + 500 + 450 + 600 + 525) divided by 5 = 511.
  5. Estimate the yield by multiplying the ear count from step 2 by the average number of kernels per ear from step 4 and divide by 90,000 seeds per bushel (if you expect larger kernels than average or above average test weight because of excellent weather conditions at grain fill consider using 75,000 or 80,000 seeds per bushel).

Grain example. Let’s say you counted 29 harvestable ears with the average of 511 kernels per ear and growing conditions during grain fill were average. The estimated yield would be (29,000 x 511) divided by 90,000, which equals 165 bu/a.

Corn silage example. After you have estimated the corn grain yield for your field, divide that number by 6.5 to get the estimated tons per acre of corn silage. 165 bu per acre ÷ 6.5 = 25 tons per acre.

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