With most Kansas soybean fields already in reproductive stages, it is time to assess the yield potential. The latest USDA-NASS Kansas Crop Progress and Condition report (Aug 18) estimates that 90% of soybeans in Kansas are blooming. Around 67% of the soybeans in the state are setting pods, which is behind compared to 73% from last year (5-year average = 73%).
Soybean plants can more easily compensate for stress (relative to corn), more so if the timing of the stress is before seed filling. One of the key factors in the crop’s ability to compensate for stress situations is the significant overlapping of vegetative and reproductive stages that indeterminate soybeans express, allowing them to keep producing new leaves even toward the reproductive period. On the other hand, the production of flowers could normally last between 4-6 weeks, which potentially allows to replace flowers and small pods abortion.
Since the final number of pods is not determined until the beginning of seed filling, when estimating soybean yield potential, we must remember that the estimate could change based on the timing of this estimation 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 heavier seed weight.
Estimating the final yield before harvest can be a very tedious task, but a simplified method using yield components can be applied to start setting yield expectations.
From a physiological perspective, the main yield components to consider are:
- plants per acre,
- pods per area,
- seeds per pod, and
- seed size.
When can I start making soybean yield estimates?
There is no precise time, but we can start making soybean yield estimates as soon as the end of the R4 stage, full pod (pods are ¾-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). Keep in mind that yield prediction is less precise at these early reproductive stages since the seed number per area, as well as the seed weight, are not yet completely defined. At this early stage of seed development, it is important to only consider the pods that are at least ¾-inch long to avoid over-optimistic estimations since smaller pods can still abort under stress conditions.
As we move into the R6 stage (full seed), the seed number (main yield component) is majorly defined, yet the conditions during seed filling will determine the final seed number as well as the size and weight of the seed. The closer to maturity (R7 stage) we make the estimation, the more accurate the expectation and overall yield prediction.
Figure 1. Soybean phenological stages to start yield prediction using the yield components method.
How many samples are needed to account for field variation?
We should make yield estimations in about 10 (at least 6, ideally 12) different areas of the field. Of course, properly recognizing and identifying the variation within the field is important, and then taking enough samples from the different areas to fairly represent the entire field. Within each sample section, take consecutive plants within the row to have a good representation. The variability between plants in terms of the number of pods and seed size needs to be considered when trying to get an estimation of soybean yield. Variability between areas within the same field also needs to be properly accounted for (e.g., low vs. high areas in the field). The more variability to represent, the more samples we would need for a proper estimation.
The yield components equation
Soybean yield estimates following the conventional approach are based on the following components (Eq. 1):
Eq. 1
where,
- Plants/ac. A simplified approach can be applied by using samples covering 1/10,000th of an acre with sections of 30 inches width by 21 inches length (Figure 2, step 1.). Thus, the average number of plants in several sections multiplied by 10,000 will give us an estimation of the number of plants per acre. Following this simplified approach, if the soybean plants are arranged in 30-inch rows we just need to sample a single row; 2 rows if the row spacing is 15 inches; and 4 rows if the row spacing is 7.5 inches.
- Pods/plant. Once the samples have been obtained, we count and later average the number of pods per plant (Figure 2, step 2).
- Seeds/pod. Then, we proceed to count and later average the number of seeds per pod (Figure 2, step 3.). 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. Under severe drought and heat stress, a pessimistic expectation would be an average of 1-1.5 seeds per pod.
- Seeds/acre. Calculate the total number of plants/acre (1), pods/plant (2), and seeds/pod (3) to obtain the total number of seeds per unit area (Figure 2, step 4).
- lbs/bu. For this estimation, the “test weight” for soybeans could be considered here as a constant number at 60 lbs./bu.
- Seeds/lbs. The number of seeds per pound will vary depending on the seed-filling conditions, which will determine the seed size. Normally, this number could range somewhere between 2,500 (bigger seeds) to 3,500 (smaller seeds) seeds per pound for optimal to unfavorable seed filling conditions. Combined with a constant test weight of 60 lbs./bu, this will lead to a range of expectation of seeds per bushel between 150,000 seeds per bushel to 210,000 seeds per bushel, respectively (Figure 2, step 5).
The final step to estimating yield is dividing seeds/acre by seeds/bu to obtain the yield estimation in bushels per acre (Eq. 2).
Eq. 2
Figure 2. Example of yield estimation method using samples of 1/10,000th of an acre (21-inch x 30-inch sections) for a regular yield until seed filling, where the seed size is expected to be
reduced compared to favorable seed filling conditions, increasing the #seeds/bu component.
Example representing average conditions
In Figure 2, we have taken 10 samples of 30x21 inches sections, leading to an average number of 12.4 plants. Since these sections are 1/10,000th of an acre, our first component, the plant density, is 124,000 plants/acre. Then, in those 12 plants, we measured, on average, 27 pods per plant. Assuming a “normal” growing season condition, we could expect to count around 2.5 seeds per pod. Combining these components, we obtained an expectation of 8.370 M seeds per acre. Finally, we had to assume (if the estimation is early) or represent (if close to maturity) the seed-filling conditions. In this case, we used regular-to-poor seed filling due to the lack of precipitation combined with heat. Thus, we divided the seeds per acre by an expectation of 200,000 seeds per bu (small seed size), giving us an estimate of roughly 42 bu/a.
Adjusting for 2024 expectations
Soybeans have started well this season, especially in May. However, drought has created more challenging conditions in some parts of the state. Thus, even if the pod number is the same as in a normal season, the final seed number and weight will be negatively impacted for a “droughty” (from R2 to R6 stages) growing season, such as the present one in some specific parts of the state.
Estimating seed yields in soybeans will allow farmers and agronomists to obtain a more reliable prediction of yields and scout fields for associated issues before harvest, such as insects, diseases, and other potential production problems. In the coming weeks, the effects of the current weather conditions can be assessed to obtain a more precise estimate of the final yield expected on this crop.
Ignacio Ciampitti, Professor - Farming Systems
ciampitti@ksu.edu