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Some areas in the state have received over 9 inches of rain in the last 7 days. These rains have left many crop fields flooded/ponded or fully saturated. Fully saturated and flooded soils are deficient of oxygen because the soil air spaces are displaced by water. Questions are being asked if the crops submerged or partially submerged in flood water will survive. The probability that such crops will survive depends on the type of the crop, crop growth stage, soil type, temperature, and the duration of the ponding/saturation.
Different crops differ in tolerating and adapting to waterlogging. According to Agronomists from the Canada-Manitoba Crop Diversification Centre (CMCDC), plant roots and shoots can adapt to short term reduction in oxygen levels by lowering respiration rates and slowing growth of shoots. Generally, most annual crops tolerate 3-7 days of excessive water stress, while forage legumes tolerate 9-14 days. However this will depend on other factors especially temperature and growth stage.
Temperature plays a big role on the ability of plants to survive excessive water stress; the higher the temperature, the faster the rate of oxygen depletion. When temperatures are lower, plants under flooding may survive longer than if temperatures are warmer. Temperatures in the next few days are expected to remain around 75°F, this is likely to help the plants submerged in water survive a little longer.
Another factor that affects survivability of submerged plants is soil type. Clay soils tend to hold water for a longer time partly due to small airspaces compared to sandy loam soils. Results summarized by CMCDC researchers indicate yield response of wheat under excessive rainfall was negative in clay soils when heavy rains fell in June, July or August but less detrimental in other soil types. The same report also indicated a yield loss of 80% in winter wheat when flooded for 6 days between planting and emergence. Winter wheat is further along than spring wheat but both are likely to be less damaged by excessive soil moisture at these growth stages.
Plants in flooded fields may also have the deposition of mud and old crop residues when water starts to recede. These residues may also have plant pathogens that might reduce survival of flooded plants (see Corn & Soybeans: High moisture may increase risk of some crop diseases). Mud on leaves reduces photosynthesis and heavy residue covered seedlings of young plants may fail to emerge. There is also likely to be soil crusting once the flooded parts of the field begin to dry out.
Extended periods under water may lead to loss in nitrogen due to denitrification and leaching, but this should affect only flooded parts of the field. Flood waters may also concentrate pre-emergence herbicides applied in the field to low spots of the field and this could injure young corn or soybean plants.
Managing flooded fields
There is little that can be done to help flooded plants. Assessment of damage should be done about 5 days after flooding. If signs of fresh shoot growth are seen, this may indicate that these plants will survive. Flooded corn and soybean fields should be assessed for plant stand to determine the level of damage. It may be late to consider replanting or crop switching but Delayed/Prevented Planting Provisions and South Dakota Final Planting Dates should be reviewed.
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