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Many producers like to estimate the yield potential of their soybeans well before reaching the end of the season. In contrast with corn, soybeans can easily compensate for abiotic (e.g., temperature, water) or biotic stresses (e.g., insects, diseases). The final number of pods is not determined near the end of the season (beginning of seed filling, R5 stage). For comparison, in corn, the final kernel number is attained during the 2-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.

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 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 those early stages.

Is plant variability within the field an issue in soybeans?

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). Make yield estimations 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 to have a 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 of 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 spacing, sample one row. With 15-inch row spacing, sample two rows. With 7.5-inch row spacing, sample four rows. Photo by Ignacio Ciampitti, K-State Research and Extension.

 

Repeat this procedure in different sections of the field to properly account for the natural field variability.

What are the driving forces of soybean yield?

1) Total number of pods per acre:

Count the total number of pods (Figure 2) within this constant row length. After counting all the plants within the 21-inch row sections that represent 1/10,000th of an acre, estimate a final pod number per acre. Use a similar procedure in different areas of the field to get a good overall estimate at the field scale. One good criterion is only to consider pod sizes that are larger than ¾ or 1 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 inch). 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 of estimation 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.

3) Seed size:

Seed size can range from 2,500 (normal to large seed weight) to 3,500 (small seed size) seeds per pound. This season, conditions are mostly favorable in Kansas for promoting large seed sizes. In more stressful years, such as 2012 and 2011, seed size is normally smaller, meaning a larger number for the seeds per pound (e.g. 3,500 seeds per pound). In the simplified estimation approach published by Dr. Casteel, you do not need to actually measure the number of seeds per pound in order to estimate yields, as is done in the conventional approach. Instead, a seed size conversion factor is used. If the conditions are favorable and large seed size is expected, the conversion is 15 units; while if abiotic or biotic stresses are present during the seed-filling period, a seed size factor of 21 units is used. Further details related to the seed size factor can be found in the link to the Purdue University extension article listed at the end of this article.

 

Example of the simplified approach for estimating soybean yields:

Say that we have 120,000 plants/acre in a 30-inch row. Then, we should have around 12 plants in 21 inches of row. In those 12 plants, we have measured on average 22 pods per plant, with a total number of 264 pods (22 x 12).

If we assume a “normal” growing season condition, then the final seeds per pod will be around 2.5, and for the seed size factor, we can assume large seeds, and will use a conversion factor of 15 units.

Equation for a “Favorable” Season:

264 pods x 2.5 seeds per pod / 15 = 44 bushels per acre

For a “droughty” (late reproductive, from R2 to R6 stages) growing season, the final seed number and size will be dramatically affected. Thus, even if the pod number is the same as in a normal season, the yield calculation could be:

Equation for a “Drought” or Short Seed Filling Season:

264 pods x 1.5 seeds per pod / 21 = 19 bushels per acre

Basically, this “simplified approach” relates the total number of pods in a “known” unit area (easily extrapolated to the acre unit), and is affected by the total number of seeds in the pod. This is adjusted by the estimated seed weight, which is affected by two main components: duration of seed fill and rate of dry mass allocation to the seeds.

Farmers across the state have access to many of the most cutting-edge wheat varieties ever bred. These varieties are all created with performance in mind, so how can producers gain that coveted yield bump when the dozens of varieties at their fingertips are all, by-and-large, on a fairly level playing field? According to Dr. Romulo Lollato, Wheat and Forage Extension Specialist at Kansas State University, the genetics of all the newest varieties have improved to the point where agronomic practices now have an even greater influence on yield than variety selection does.
“I think we are at the point where we have so many excellent varieties that we don’t have to be quite as picky. There are a lot of really good options, so we have to look at management, as well,” said Lollato. “That’s what the last 19 years of data that we have collected is telling us – that management practices are very, very important.”
According to this data, management accounts for 44-77% of yield variation. Because of this large yield gap, Lollato says, “It is time to manage wheat.”
To no one’s surprise, region and irrigation make the top of the list for yield producing management practices, but application of foliar fungicide and sowing date are also incredibly important for both irrigated and dryland farmers. The largest yield drag was dual-purpose wheat used for grazing.

Sowing dates can have a huge impact on final yields, but the optimal sowing date varies by region. Western Kansas farmers have an optimal date of October 1, north central’s optimal date is October 10 and south central’s is October 12. Planting after these optimal dates can mean substantial yield penalties. The South Central region loses about 1.1 bushels per day for around 20 days following October 12, but that loss increases to around 2.7 bushels per day after those initial 20. North Central Kansas consistently loses about 2.1 bushels per day, while the western region loses a whopping 3.5 bushels.

Lollato and his team have also found that seed treatments (like insecticides and fungicides) have a higher yield bump in good seasons, while foliar fungicides are beneficial in all seasons, but have more yield gain in those higher yielding seasons. Micronutrient applications have had a negligible bump during high performing years, while they have a monster gain of 9.7 bushels per acre during low performance years.

Lollato’s research has also focused on sulfur. Kansas has seen the removal of sulfur from the soil during wheat production exceeding the amount of atmospheric deposition since 2000, which he attributes as an effect of the Clean Air Act. This legislation has meant lower levels of air pollution, but less pollution means less sulfur coming in during rainfall. Sulfur application has a slight yield drag of -.6 bushels per acre during high performing years (like 2016 and 2017) but had a net gain of around 4.9 bushels per acre during drier years like 2018. He also advised that while sulfur and nitrogen deficiencies tend to have a similar yellowed appearance in plants, sulfur tends to express a brighter yellow discoloration in the upper plant canopy while nitrogen discolors the lower canopy.L
Although management practices are the most reliable source of yield gain, variety traits can have an effect. Stripe rust resistance was the trait with the highest yield gain, but there are others to keep an eye on depending on region and management practices. In the western region’s irrigated wheat, stripe rust, coleoptile length, straw strength and winterhardiness are the highest yielding traits. For dryland wheat, those traits are drought tolerance, coleoptile length, first hollow stem date and stripe rust.
In the central region, if you’re planning on applying fungicide, look for medium to late heading, drought tolerance, acid soil tolerance and medium to short height. With no fungicide application, those high yielding traits are stripe rust tolerance, leaf rust tolerance, fall grazing potential, early heading date and drought tolerance.
These projects were funded by the Kansas Wheat Commission and the Kansas Wheat Alliance. For more information on these research projects and others, please visit kansaswheat.org and kswheatalliance.org.