Estimating Soybean Yields
Spending the time to count soybean pods can help to set end of season yield expectations and avoid surprises on the combine’s yield monitor or at drop-off. Yield estimates can be especially helpful for re-planted acres, double-cropped soybeans, and years with unusually severe pest pressure.
- [Liz] Hello, my name is Liz Bosak.
I am a field and forage crops educator based in Dauphin and Perry Counties.
Today, we will discuss how to estimate soybean yields on your farm.
There are a few different reasons that you may want to spend the time to calculate a soybean yield estimate before harvest.
Soybean yield estimates can help to set end-of-season expectations for your main crop beans and double-cropped beans after a small grain harvest.
In a drought situation, the growing season may have started off with adequate moisture, but mid-to-late-season drought will change those yield expectations.
Soybean plants are extremely responsive to their environment and will attempt to compensate for stresses.
But sometimes, the plant cannot overcome a stress such as prolonged drought.
Early-season pest pressure from insects, slugs, or diseases may trigger your decision to replant some acreage.
In this situation, you may want to get an idea of the potential yield off of those replanted acres.
If you have decided to estimate yield, when should you head out to the field?
At beginning pod stage, it is still too early to get an accurate estimate.
At R3, the pod length is about half the diameter of a dime.
Your target soybean stages for yield estimates begin at R5, when the seed is at least 1/8th of an inch long at the four uppermost nodes.
The full seed stage, R6, occurs when the seed at the top of the plant fills the pods.
At beginning maturity, R7, at least one pod has turned to its mature color.
At full maturity, it will be easiest to see the rows and make your counts, but you may be too busy to get out to the fields at that time.
Before heading out to the fields, you will need something to write on, a pencil, and a measuring tape or yard stick.
The first step is to count the number of pods in 1/10,000th of an acre.
At 30-inch row spacing, you will need to count one row for 21 inches of that row.
With 15-inch row spacing, you will need to count two rows.
And for 7.5-inch row spacing, you will need to count four rows.
To improve the accuracy of your estimate, it is best to have at least eight plants in that 21-inch row section.
Also, if you count more than one row section, then you will be able to account for some of the variability that occurs across a field.
After you finish counting the total number of pods in the 21-inch row section, take a look at the number of seeds per pod.
You are looking for the frequency of one-, two-, three-, and four-seeded pods.
If you forgot to observe the number of seeds per pod when you were in the field, then use two-and-a-half seeds per pod as a conservative starting point.
Don't forget to start recording your information.
It can be helpful to record the field name or GPS coordinates.
After counting three sections in different parts of the field, you can calculate the average pod number for the final calculation.
The next step is to make your best guess of the seed size or go back through your planting records to find the seeds per pound.
The number that you will need is the seed size factor, which ranges from 15 for very large seed to 21 for small seed.
To find the estimated bushels per acre, multiply the average number of pods by seeds per pod; in this example, 300 multiplied by 2.5 to give you 750.
Take that number and divide it by the seed size factor that you decided on in the previous step.
For this example, we will use an average seed size factor of 18.
After dividing by that factor, the estimate for this example is 42 bushels per acre.
And that's it!
You are finished with your first soybean yield estimate.
For more information about field crops and our educational events, please visit our website at www.extension.psu.edu, and thanks for listening.