Soybean Seed Filling and Dry Down Rate Before Harvest

The latest USDA-National Agricultural Statistics Service crop progress and condition report, released September 8, 2026, classified 37% of the soybean crop as being in good or excellent condition and 30% in fair condition. Overall, 26% of soybeans in Kansas are dropping leaves, ahead of last year (13%) and the average at 18%.

Weather Outlook

The weather conditions expected for the last portion of September will be critical for soybeans, particularly for seed filling and final seed weight.

The past month has been very warm across Kansas. Only one of the last thirty days was below normal, and collectively, the past month has averaged around 7 degrees above normal. The forecast for next week calls for multiple cold fronts to affect the state, but the term “cold” does not accurately describe the air behind them. “Cool” or even “Cooler” is a better descriptor, as temperatures are likely to still be above normal, just not as extreme as in previous weeks. The second half of September is expected to be warmer than normal, but the heat will not be as intense as it has been in the last week. Frost and freezing conditions are highly unlikely during September. Normal dates for a first freeze range from early October in the northwest to late October in the southeast.

Precipitation has favored western and northern Kansas over the past 30 days, while southeast Kansas has endured not only the heat but also minimal precipitation. A few locations have had less than one-quarter inch over the last month, which created a precipitation deficit of 3 to 4 inches across a wide area from the Oklahoma border north to near the southern Kansas City suburbs. Forecasts are optimistic about rainfall chances, with systems over the next week potentially producing over 3 inches of rain in east central and southeast Kansas, the result of multiple rounds of showers and thunderstorms. The area targeted for the highest rainfall in the near term has not been consistent in recent forecasts; some runs have shifted the heavy rain as far north as the state of Iowa. The parade of fronts expected through the state should bring chances for rainfall with each of them, but timing, placement, and intensity of rainfall is still in question. Longer term forecasts lean towards above normal precipitation, but the probabilities are only slightly elevated above the 33% equal chances threshold amongst the three possible outcomes: above, below and near normal precipitation.

Soybean Seed-Filling

Final maturity is defined as the formation of the seed black layer. 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). Seed dry matter accumulation and moisture changes depend on the maturity group, planting date, and weather conditions during these stated reproductive phases.

Soybean seed water loss can be divided into two main phases: 1) before “black layer” or maturity, and 2) after 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 duration (37 days) until 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 can observe that when the duration is reduced by one week (from 37 to 30 days), the attainable yield goes from 61 to roughly 50 bushels per acre. Potential factors (beyond the current heat/dry conditions) impacting the duration 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 2026, we could expect negative yield impacts due to the extreme temperatures (both daytime and nighttime) and limited rainfall, which will likely result in poor seed weight and potentially increased late pods and seed abortion. Overall, seed weight accounts for roughly 30-40% of the final crop’s total yield, underscoring the negative weather impacts on attainable yields.

Figure 1. Soybean seed filling process from full seed to full maturity. Graphic from the K-State Research and Extension Soybean Growth and Development poster, MF3339.

Figure 2. Soybean seed weight changes from the beginning of seed filling (R5) to full maturity.

Soybean Dry Down Rate

Soybeans reach final maturity with high seed water content, moving from 90% at the beginning of seed fill to around 60% at final maturity. The dry down rate will depend on the maturity group (affecting the length of the season), planting date, and weather conditions experienced during the pod and seed set phases.

In a 2016 trial, K-State researchers investigated the water content changes from black layer formation until harvest time. The overall dry down rate was 3% per day over a 15-day period (late September to mid-October). The trial started with soybeans at 58% and ended at 12% to find the 3% per day. The 3% per day is three times faster compared to corn’s 1% moisture daily loss. However, corn and soybean dry down rates are primarily driven by temperature, humidity, and overall water content at the point of black layer formation. These main factors should be considered when scheduling the soybean harvest, where we are aiming for 13% moisture. When harvesting less than 13% moisture, losses to chattering can increase.

Some areas of soybean production have been faced with droughty conditions, with the combination of high temperatures and limited rainfall. Scouting your fields for maturity and prioritizing situations with lodging or other stress factors should be considered in harvestability assessments. Some soybeans entering maturity could be harvestable in two weeks due to the dry down speed.

 

Tina Sullivan, Northeast Area Agronomist
tsullivan@ksu.edu

Jeanne Falk Jones, Northwest Area Agronomist
jfalkjones@ksu.edu

Logan Simon, Southwest Area Agronomist
lsimon@ksu.edu

Matthew Sittel, Assistant State Climatologist
msittel@ksu.edu