Learn about a simplified approach to estimating soybean yields in the PSB Newsletter- Fall 2013.
Estimating Soybean Yields: A Simplified Approach: “Pod number, seeds per pod, and seed size are the driving forces of soybean yield,” says Dr. Shaun Casteel, a Purdue University soybean specialist and assistant agronomy professor. “The combination of these factors allows soybean to adapt to growing conditions over long periods (weeks and even months) and still yield well. These same factors can make it difficult to estimate soybean yields prior to harvest.”
Using these three factors, Dr. Casteel has simplified the process of estimating soybean yields so growers can scout multiple areas quickly while maintaining representative estimates.
The system for estimating soybean yield is based on 1/10,000th acre and the following formula:
Pods x Seeds per pod ÷Seed size factor = Bushels per acre
Step 1 – Pods
Count the number of pods in 1/10,000th of acre.
Most 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-inch width to equal 1/10,000th acre (Figure 1). Each one of these counts will be 21 inches in length.
Number of rows to count to equal 1/10,000th of an acre.
1 row at 30”
2 rows at 15”
4 rows at 7.5”
This simplified system can be adapted to other row widths as well. 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-inch row width would require 34.8 inches of 1 row to equal 1/10,000th acre (627.26 ÷ 18 inches =34.8 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 greater than 1”, 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).
A. Good soybean growth, good pod retention, and adequate late season moisture.
400 pods x 2.5 seeds per pod ÷ 18 = 55.5 bu/acre
B. Good early soybean growth, fair pod retention, BUT little late season moisture.
300 pods x 2.5 seeds per pod ÷ 21 = 35.7 bu/acre
C. Fair soybean growth, limited pod retention, BUT good late season moisture.
250 pods x 2.5 seeds per pod ÷ 15 = 41.7 bu/acre
A video describing the simplified approach to estimating soybean yields and additional information can be found at: http://www.youtube.com/watch?v=jPwPYwBy8k0
When should I take yield estimates?
“Any yield estimate improves as you get closer to harvest,” says Dr. Shaun Casteel. “The confidence level increases because the plants have responded to more of the growing season (pod retention, seeds per pod, and seed size). Soybean yield estimates can begin as soybeans enter into R5 (first seed). At this point, a fair portion of the pods have developed and seeds are filling throughout the whole plant. Flowering will continue at a limited rate and will soon cease.
“Pod development (retention and number of seeds per pod) will lag behind the pattern of the flowering. The yield potential at this point depends 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 or so and enter R6 (full seed), which lasts another approximately 20 days. Pod retention and seeds per pod will become clearer, and the potential for large, average, or small seeds will be more discernible.
“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. “