Kansas State University Extension
Well before reaching the end of the season, some are asking about the potential yield expected for their crops. In the August 1, 2014 Agronomy eUpdate, we discussed the calculation of the potential yield for corn prior to harvest and how to get a simple but fairly good yield estimate: https://webapp.agron.ksu.edu/agr_social/eu_article.throck?article_id=294
In this article, I will discuss how to estimate soybean yield potential. In contrast with corn, soybean can easily compensate for abiotic or biotic stresses. The final number of pods is not determined with finality until close to the end of the season (R5 stage). In corn, the final kernel number is established during the 2- to 3-week period after flowering. Thus, when estimating soybean yield potential, we have to keep in mind that the estimate could change depending on the growth stage at the time the estimate is made and weather conditions. For example, wet periods toward the end of the reproductive period can extend the seed-set period, promoting greater pod production and retention, with larger seed size and heavier seed weight.
From a physiological perspective, the main yield driving forces are: 1) plants per acre, 2) pods per area, 3) seeds per pod, and 4) seed size. Estimating final yield in soybean before harvest can be a very tedious task, but a simplified method can be used for just a basic yield estimate. This method is based on an article by Dr. Shaun Casteel (Soybean Specialist, Purdue University). For details, see:http://www.agry.purdue.edu/ext/soybean/News/2012/2012_0814SOYSimplifiedYieldEstimates.pdf
When can I start making soybean yield estimates?
There is not a precise time, but as the crop approaches the end of the season (R6, full seed or R7, beginning of maturity) the yield estimate will be more accurate. Still, you can start making soybean yield estimates as soon as end of the R4 stage, full pod (pods are 3/4 inch long on one of the top four nodes), or at the onset of the R5 stage, beginning seed (seeds are 1/8 inch long on one of the top four nodes), knowing that the yield prediction is less precise at those early stages.
Is plant variability within the field an issue in soybean?
Variability between plants relative to the final number of pods and seed size needs to be considered when trying to get an estimation of soybean yields. In addition, variability between areas within the same field needs also to be properly accounted for (e.g. low vs. high areas in the field). Yield estimations should be made in different areas of the field, at least 6 to 12 different areas. It is important to properly recognize and identify the variation within the field, and then take enough samples from the different areas to fairly represent the entire field. Within each sample section, take consecutive plants within the row so as to have a fairly good representation.
Conventional approach to estimating soybean yields
In the conventional approach, soybean yield estimates are based on the following components:
Total number of pods per acre [number of plants per acre x pods per plant] (1)
Total number of seeds per pod (2)
Number of seeds per pound (3)
Total pounds per bushel, or test weight, which for soybeans is 60 lbs/bu (4)
The final equation for the estimation of the potential soybean yield is:
[(1) x (2) / (3)] / (4) = Soybean yield in bushels/acre
Simplified approach to estimating soybean yields
The main difference between the “conventional” and “simplified” approaches is that the conventional approach uses the total number of plants per acre in its calculation; while in the simplified approach, a constant row length is utilized to represent 1/10,000th area of an acre (Figure 1).
For the simplified approach, sample 21 inches of row length in a single row if the soybean plants are spaced in 30-inch rows; in 2 rows if the row spacing is 15 inches; and in 4 rows if the row spacing is 7.5 inches.
Figure 1. In the “simplified” approach to estimating yields, sample 21 inches row length to equal 1/10,000th of an acre. The number of rows to sample will depend on the row spacing. With 30-inch row spacings, sample one row. With 15-inch row spacings, sample two rows. With 7.5-inch row spacings, sample four rows. Photo by Ignacio Ciampitti, K-State Research and Extension.
This procedure should be repeated in different sections of the field to properly account for the natural field variability.
Driving forces of soybean yield
1) Total number of pods per acre:
The total number of pods (Figure 2) within this constant row length should be counted. After counting all the plants within the 21-inch row sections that represent 1/10,000th of an acre, a final pod number per acre can be estimated. A similar procedure should be used in different areas of the field to get a good overall estimate at the field scale. One good criterion is to only consider pod sizes that are larger than ¾ or1 inch long. Smaller pods can be aborted from this time on in the growing season until harvest.
Figure 2. Total number of pods per plant (only consider the pod sizes larger than ¾ or 1 in). Photo by Ignacio Ciampitti, K-State Research and Extension.
2) Total number of seeds per pod:
Soybean plants will have, on average, 2.5 seeds per pod (ranging from 1 to 4 seeds per pod), primarily regulated by the interaction between the environment and the genotypes (Figure 3). Under severe drought and heat stress, a pessimistic approach would be to consider an average of 1-1.5 seeds per pod. This value is just an approximation of the final number of seeds per pod, and can change from the time the estimate is made until the end of the growing season.
Figure 3. The number of seeds per pod will vary somewhat, depending on the growing environment and genotype. Photo by Ignacio Ciampitti, K-State Research and Extension.
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