With heavy rains, soybean fields can experience flooded or saturated conditions. Here, we describe agronomic and disease issues for soybean exposed to prolonged periods of high soil moisture and cool temperatures.
Although soybean plants are generally sensitive to excess water, they can survive underwater for a week or more under ideal conditions. Generally, soybeans tolerate 48 hours under water quite well. However, flooding for four to six days can reduce stands, vigor and, eventually, yield.
Key factors that determine impact
Many factors determine how well a soybean crop will tolerate flooding, and the effect on the crop’s quality. Here are the most important factors that determine the fate of flooded soybean fields:
Duration of the flooding.
Temperature during the flood.
Rate of field drying and soil type
Growth stage of the crop during the flood.
Short-duration floods are less damaging to the crop than floods that last several days or more.
Producers rarely note yield losses in fields flooded for 48 hours or less. Four days or more of flooding stresses the crop, delays the plants’ growth and leads to shorter plants with fewer nodes.
Flooding for six days or more can significantly depress yields, while flooding for a week or more may result in significant – or entire – stand losses.
Temperature during flooding plays a large role in determining a submerged soybean field’s fate. Higher temperatures cause the soybean plant to more quickly deplete its stored energy.
Additionally, higher temperatures cause plants and soil microbes to respire at high rates that quickly deplete the water of oxygen and increase CO2 levels. Soybean plants seem very sensitive to high carbon dioxide (CO2) levels in the soil. Cool, cloudy days and cool, clear nights greatly increase a submerged soybean crop’s survivability.
The rate of field drying after a flood also plays a large role in soybean survival. Researchers have also found greater yield reductions on flooded clay soils than on flooded silt loam soils for the same time period.
At the V4 stage, researchers reported yield losses of 1.8 bushels per acre per day of flooding on a clay soil and 0.8 bushels per acre per day on a silt loam soil.
The effects of flooding are even more detrimental during the reproductive phases of development.
For example, flooding at the R1 stage caused yield losses of 2.3 and 1.5 bushels per acre per day on clay and silt loam soils, respectively. Even larger yield losses are expected in soybeans at the R3 to R5 stages.
Guide to soybean growth stages
While little is known about how late season flooding affects crop quality and losses, you can be sure that flooding of any duration on any soybean field in late September will damage the crop.
Potential flooding damage to soybean includes stem breakage and lodging, moisture-swelled seeds that can lead to pod splitting, seeds sprouting or rotting and contamination with mud.
Examples: 2009 and 2010 floods
Anecdotal information after a 2009 Mississippi flood indicates that soybean fields that had reached full maturity (R8) at the time of flooding had less damage than fields that were not yet fully mature (less than R7).
After flooding in 2010 in Minnesota, we noticed little loss from fields that were at R8 at the time of flooding and where ponded water receded in a few days. There was significant lodging and loss where heavy stream flooding occurred.
Effects of flooding
Some of flooding’s main indirect effects on soybean yields are:
Other plant nutrient imbalances.
Flooded and wet soil conditions increase the risk of seedling diseases (Figure 3), which may become a problem in flooded fields.
Favorable conditions for seedling diseases include wet and compacted soils, slow plant emergence and growth, crusted soil and poor seed quality.
Diseases can reduce plant and root growth, plant population and, ultimately, yield. Non-lethal infection at the seedling stage may cause damage that persists through the growing season.
The soilborne pathogens that cause diseases are widespread and persistent in Minnesota fields, and can cause damage when conditions develop that favor them. Several different pathogens and diseases can become problematic.
For example, wet and flooded soils are especially favorable for the soilborne, moisture-loving pathogens like Pythium, which appears to cause most damage to corn and soybean seedlings. Phytopthora can damage soybean seedlings or start infections in the early summer that can develop and kill soybean plants later in the season.
Caring for recovering plants
When caring for recuperating, younger soybean stands, focus on reducing crop stresses where possible. For example, consider cultivation to increase soil aeration and minimize or postpone herbicide stress where possible.
The only management considerations that are open to producers at a very late date (e.g., late September) may be harvest timing and logistics.
Harvest and storage
Rather than waiting for wet spots, harvesting the non-flooded portions of fields first will speed harvest, minimize wear and tear on equipment and keep water damaged soybeans separated from good-quality grain.
Harvest and store soybeans from flooded areas separately from areas that were not flooded.
Also, do not mix damaged soybeans with clean ones, as it’s difficult to estimate damage levels and know grain elevators’ thresholds and allowances. Avoid the temptation to blend a few bushels of damaged soybeans with a whole bin of good ones; the risks are too great.
Another reason to separately harvest flooded areas relates to crop insurance. It’s important to clearly document these flooded areas so you can make insurance or disaster relief assistance claims at a later date. Isolating these flooded areas is the best means to document losses from these heavy rains.
Again, harvest and store late-season flooded soybean acres separately. This is the best thing you can do to minimize risk.
Scott, H.D., DeAngulo, J., Daniels, M.B. & Wood, L.S. (1989). Flood duration effects on soybean growth and yield. Agronomy Journal, 81, 631-636. https://doi.org/10.2134/agronj1989.00021962008100040016x
Sullivan, M., VanTooai, T., Fausey, N., Beuerlein, J., Parkinson, J., & Soboyejo, A. (2001). Evaluating on-farm flooding impacts on soybean. Crop Science, 41, 93-100. https://doi.org/10.2135/cropsci2001.41193x
Reviewed in 2018