The USDA-National Agricultural Statistics Service crop progress and condition report released Sept. 10, 2023, classified 52% of the soybean crop to be in fair (32%) or good (20%) condition. Overall, 37% of all soybeans in Kansas are dropping leaves, ahead of 25% from last year and the 19% average.

Soybean Seed Filling

Final maturity is defined as the formation of the black layer in the seeds. Soybeans will reach final maturity with high seed water content, moving from 75-80% (R6) to around 50% (R7) from the beginning of seed filling until final maturity (Figure 1). The process of seed dry matter accumulation and moisture changes will depend on the maturity group (affecting the length of the season), planting date, and weather conditions experienced during the latter part of the reproductive phase.

Soybean seed water loss can divided into two main phases: 1) before the “black layer” or maturity, and 2) after the black layer.

The overall contribution of seed weight to final yields can be studied by evaluating changes in seed weight during the seed-filling period (Figure 2). Overall, a seed-filling period of more than one month (37 days) duration until the black layer was achieved (this is the reference for changes in seed weight). The graph of seed filling provides a visual of the overall rate, increase in seed weight per day, and the duration of the seed filling.

From this example, we could observe that when the duration is reduced by one week (from 37 to 30 days) the attainable yield went from 61 to roughly 50 bushels per acre. Potential factors impacting the duration (beyond the current heat/drought) of leaf green area imposed by insects, diseases, hailstorms, and any other abiotic stress conditions such as cloudy days and early frost impacting the crop during the coming weeks will negatively affect the seed filling conditions for soybeans.

For this current season, we can largely expect negative yield impacts due to the last heat wave combined with drought conditions, which resulted in poor seed weight, and potentially increasing late pods and seeds’ abortion. Overall, seed weight contributes roughly 30-40% of the final total yield for this crop, emphasizing the impacts of the stress conditions on the attainable yields.

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Figure 1. Soybean seed filling process from full seed to full maturity. Photo and infographic prepared by Ignacio Ciampitti, K-State Research and Extension. Taken from Soybean Growth and Development, MF3339.

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Figure 2. Soybean seed weight changes from the beginning of seed filling (R5) to full maturity. Photo and infographic prepared by Ignacio Ciampitti and Santiago Tamagno, K-State Research and Extension.

Soybean dry down

Soybeans reach final maturity with high seed water content, moving from 90% to around 60% from the beginning of seed filling until final maturity. Final maturity is defined as the formation of the black layer in the seeds. The dry-down rate will depend on the maturity group (affecting the length of the season), planting date, and weather conditions experienced during the latter part of the reproductive phase.

To address the question related to the dry down rate for soybeans, a study was conducted to investigate the changes in water content from black layer formation (maturity) until harvest time (Figure 3). During the last days of September and mid-October 2016, the overall dry-down rate was around 3% per day (from 58% to 12% seed moisture) – taking an overall period of 15 days.

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Figure 3. Grain moisture dry-down (orange line) across three hybrids and different N rates near Manhattan, KS. Horizontal dashed lines marked the 58% seed moisture at black layer formation. * Graph prepared by Ignacio Ciampitti, K-State Research and Extension.

*Note: It is desired to reach harvest with 13% seed moisture to maximize the final seed volume to be sold, thus the importance of timing harvest with the right seed moisture content.

Soybean dry down rate was three times faster, at 3% per day, relative to corn at 1% per day. These dry-down rates for corn and soybeans are primarily affected by temperature, humidity, and overall water content at the point of black layer formation (maturity). These main factors should be considered when the time comes to schedule soybean harvest.

With the current weather conditions, soybeans entering to maturity could be ready to harvest in less than two weeks. Therefore, scout your fields to check for maturity and prioritize situations with lodging or other stress factor that could compromise plant standability, ultimately affecting seed quality.

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