Soybean yield potential is built on numerous factors including the genetics that have been selected, the management decisions during the season, and the weather. Yield components of soybeans are boiled down to pods, seed size, and number of seeds per pod.
When Should I Take Yield Estimates?
You need to use these factors and the understanding of your field conditions to estimate soybean yield potential prior to harvest. Soybean yield estimates can begin as soybeans enter into R5 (first seed, Figure 1). At this point, a fair portion of the pods have developed and seeds are filling throughout the whole plant. The yield potential at this point can be low or it can be high depending on the remaining 4 to 6 weeks of the growing season. Yield estimates will improve as the plants continue developing over the following ~15 days and enter R6 (full seed, Figure 2), which last another ~20 days.
Figure 1
Figure 1a - Soybean at R5 
Figure 1b - Seeds are 1/8 inch long in one of the pods at the top 4 nodes Figure 2
Figure 2a - Soybean at R6 
Figure 2b - Seeds filling the pod capacity in one pod at top 4 nodes Estimating Soybean Yields
Simplified Soybean Yield Estimation. Individual plant production will vary and we must take a representative sample without being extraneous. Every field will have variations based on soils, pests, fertility, and other factors. I have simplified the process of estimating soybean yields, so that you can scout multiple areas quickly while maintaining representative estimates.
The system is based on 1/10,000th acre and the following formula:
Pods X Seeds Per Pod divided by Seed Size Factor = Estimated Bushels Per Acre
Step 1 - Pods
Count the number of pods in 1/10,000th of acre. Yes, 1/10,000th of acre! Nearly 90% of our Indiana soybean acres are planted in 30-, 15-, or 7.5-in rows, so just remember 21. You will count the number of pods in 1 row for 30-in width, 2 rows for 15-in width, or 4 rows for 7.5-in width to equal 1/10,000th acre (Figure 3). Each one of these counts will be 21 inches in length.
We certainly have other row widths, and this simplified system can be adapted to your row width. If you have a different row width, divide 627.26 by your row width (inches) to calculate the linear length (inches) of 1 row to equal 1/10,000th acre. For example, an 18-in row width would require 34.8 inches of 1 row to equal 1/10,000th acre (627.26 18 inches).
This simplified system is more reliable when you have 8 or more plants in the sampled area, which translates to 80,000 plants per acre. If plant stands are less than 8, you should count additional areas to decrease the variability of the overall yield estimate for the field. If you want to have an idea of final plant stand, multiply the number of plants sampled by 10,000. However, you do not need plant population to estimate yield with this approach.
You will count the total number of pods in the 1/10,000th acre. You will need to use discretion to which pods you will include in the count. A good rule of thumb is to count the pods that are R5 or larger, with the knowledge that some of the smaller pods may or may not make it.
Step 2 - Seeds Per Pod
The starting point is an average of 2.5 seeds per pod, since there can be a range of 1-, 2-, 3-, and 4-seeded pods. This value is conservative since we do not know exactly how the rest of the season will finish. The soybean plants may arrest seed development on several 3-seeded pods or some pods are aborted completely. You can quickly increase or decrease the yield estimate by changing this one value. You can more confidently adjust this value because you are more likely to remember the frequency of 2- or 4-seeded pods within a few hundred pods.
Step 3 – Seed Size Factor
The starting point is seed size factor 18, which equals a fairly representative seed size of 3,000 seeds per pound. If you expect larger seeds (maybe from late season rains) you will use a smaller seed size factor such as 15 (2,500 seeds per pound). Similarly if seed fill will be limited (i.e., small seeds) due to lack of water or other late season stresses, you should use a larger seed size factor like 21 (3,500 seeds per pound).
[Table 1. Seed Size Factors.]
Examples:
- Good soybean growth, good pod retention, and adequate late season moisture.
- Good early soybean growth, fair pod retention, BUT little late season moisture.
- Fair soybean growth, limited pod retention, BUT good late season moisture.
