Soybeans compensate for changing conditions until the end of the growing season. Betsy Bower says that makes estimating soybean yields very challenging.
Bower, an Indiana Certified Crop Adviser, is also an agronomist for Ceres Solutions, based in Terre Haute. She knows that sometimes soybeans look like 65-bushel-per-acre beans from the road, but only put 50 bushels per acre in the combine hopper. Conversely, sometimes you’re hoping for 45 and wind up with 55 to 60 bushels per acre.
There is no foolproof way to judge soybean yields until you see the results filling in the map on your yield monitor screen. However, Bower says Shaun Casteel, the Purdue University Extension soybean specialist, has developed a method that is reasonably accurate. It involves time and a bit of math, but it can take you a lot farther than blind guessing. Here are six steps in the method. Find it at soybeanstation.org.
Step 1. Count the pods in 1/10,000 of an acre.
For 30-inch rows, that means counting all pods within one row for 21 inches of row length. If you’re in 15-inch rows, count all the pods in two rows for 21 inches. If you drill in 7-inch rows, count all pods in four rows that are 21 inches long.
Step 2. Determine if the sample area is representative.
If there aren’t eight or more plants in your sampling area, count in additional areas. This will decrease variability of the overall yield estimate.
Step 3. Decide which pods to count.
Casteel suggests discarding pods that are less than 1 inch long. They may not make it.
Step 4. Determine number of seeds per pod.
Use 2.5 seeds per pod as a starting point. That's conservative, but the further you are from the end of the growing season, the less confident you can be about how many seeds will actually make it to maturity. You have a better chance at accuracy if soybeans are at R5 compared to the R4 reproductive stage. You can quickly alter yield estimates by shifting this one value from 2.5 down to 2 or up to 3 beans per pod.
Step 5: Account for seed size factor.
The starting seed size is a relative number — 18. It roughly represents 3,000 seeds per pound. If you expect large seeds, use 15, for example. If you expect smaller-than-average seeds, increase the factor to 21.
Step 6. Calculate yield.
Suppose you count 400 pods in 1/10,000 acre as determined in Step 1. You estimate 2.5 seeds per pod with an average seed size figure of 18. The math is: 400 x 2.5 / 18 = 55.5 bushels per acre.
Suppose favorable weather late in the season results in 3 seeds per pod. Yield changes to: 400 x 3 / 18 = 66.7 bushels per acre.
Finally, suppose seed size is also above average. You decrease that factor to 16. The new estimated yield at 3 seeds per pod is 75 bushels per acre.