What to Watch For
Soybeans can be at a variety of growth stages during mid-season rainstorms. Partially submerged plants have an increased chance of survival compared to completely submerged soybean plants. Water-logging, where roots are flooded without plant submergence, is also common. Several factors influence potential soybean damage:
- Growth stage during flooding/ponding
- Length of time plants were flooded/ponded
- Temperature (soil and air)
- Drying rate and soil type
- Impact on Your Crop
Yield loss results from reduced root growth, shoot growth, nodulation, nitrogen (N) fixation, photosynthesis, biomass accumulation, stomatal conductance, and plant death from disease and physiological stress.
- Soybean plants completely submerged for 48 to 96 hours generally survive.1,2
- Flooding for four (or more) days delays plant growth, shortens plants, and reduces node number.
- Flooding for six days or more is expected to have a negative effect on yield.
- A week of flooding may result in significant or 100 percent stand loss.1
- Wide range of yield loss may be experienced depending on environmental conditions and soil types (Table 1). In cooler temperatures, submerged plants are expected to live longer with slowed respiration. Warm temperatures increase respiration and oxygen is consumed quickly.3
- Potential soybean yield loss from water-logging could be 17 to 43 percent during vegetative stages and 50 to 56 percent during reproductive stages.2
- Presence of soil borne diseases and differences in soil texture affect the response of plants to flooding.
Nitrogen. Soybean growth and development depends on N availability as the nutrient is in high demand, especially during seed development. The N2 fixation process is sensitive to lack of oxygen, and flooding can lead to reduced fixation. Fields with low soil organic matter experiencing dry conditions in August and September are expected to have a response to mid-season flooding. Regions in the United States with mid-season flooding and insufficient rainfall later in the season have developed low protein crops. Actual yield loss would be difficult to calculate; however, low protein levels are an indication of yield loss in fields.1
Scouting and Replanting Options
- Supplementing flooded soybean crops with N has not been shown to be an economical practice.
- Farmers with a soybean crop replanted after July 1st could experience a potential yield loss of 0.5 bu/acre/day.1 Therefore, a replant should be considered before mid-July to get a harvestable crop.
- Consider the remaining soybean stand. When 80,000 to 100,000 living plants/acre remain in uniform distribution, the field may approach full yield potential.2
- Living plants should be scouted when soils are getting dry enough to tolerate equipment. Darkened or soft growing points indicate a dead plant. Stems can be split lengthwise for inspection, as plants may have been simply covered with mud or residue from receding water.
- There may be genetic variability in soybean plants for flooding tolerance in maturity groups II and III.2 Soybean products that have demonstrated good yield potential during flooding could be selected for frequently water-logged low areas of fields.
- Disease tolerance should also be noted as Phytophthora, Pythium, Fusarium, and Rhizoctonia can infect surviving plants. Where certain diseases prevail, a new lineup of soybean products may be necessary.
- Weeds may take over a drowned-out area especially if a pre-emergence herbicide was washed away. Cover crops could be an alternative to replants after insurance dates have passed.
In summary, fields should be evaluated after water recedes for plant survival and disease. Fields with adequate stand counts may achieve near normal yield potential. Disease pressure and soybean product tolerance influences the health of plants for the remainder of the growing season.
1 Stahl, L., Coulter, J., Naeve, S., and Fernandez, F. 2018. Flooded fields and saturated conditions impact crops. University of Minnesota Extension. http://blog-crop-news.extension.umn.edu/2018/07/flooded-fields-and-saturated-conditions.html.
2 Conley, S. 2018. Assessing flood damage to soybean. University of Wisconsin. Integrated Pest and Crop Management.
3 Jean, M., Thelen, K., and Steinke, K. 2017. Managing corn and soybean fields submerged by recent, heavy rains. Michigan State University. Web sources verified 8/18/18