Considerations for flooded corn and soybeans
Thousands of Minnesota crop acres succumb to stresses from excess water each year. This typically occurs in small, low-lying areas of fields that “drown out” frequently. This spring, several areas of the state have become waterlogged again. Although Iowa, Illinois, and Indiana are more affected than most of Minnesota, significant production losses will occur in our state due to excess rainfall this spring. This article discusses agronomic and disease risks for corn and soybean exposed to prolonged periods of high soil moisture and cool temperatures.
Agronomic considerations for corn
A comprehensive review of the effects of flooding on young corn has been prepared by Dr. Bob Nielsen at Purdue University. According to Dr. Nielsen, young corn can tolerate flooding up to about four days, with greater survivability when temperatures are cool. Thus, considerable stand loss can occur in areas where there has been flooding, leaving growers to consider their options for replanting.
Answers to questions on replanting corn can be found in a University of Minnesota Extension field guide for evaluating corn damage and replant options. Some key points from this publication are:
Corn grain yields in Minnesota drop rapidly when planting is delayed into June. For example, the yield potential of corn planted on June 14 is only about 65% when compared to corn planted in late April. If planting is delayed until June 19, the yield potential is reduced to 59%. Corn planted between June 11 and 15 should be 15 or more relative maturity units earlier than full-season hybrids adapted to the region. In addition, it is not recommend to plant corn after June 15 in southern Minnesota or after June 5 in central Minnesota.Additional considerations for replanting flooded corn in June are summarized in this article by Dr. Nielsen.
Nitrogen management considerations
With prolonged ponding or saturation in corn fields, nitrogen (N) loss can be important. Nitrogen will be primarily lost through leaching on coarser-textured soils and through denitrification on heavier soils. Denitrification occurs under oxygen-depleted conditions that result when all of the soil pore space is filled with water. Under these conditions, soil microbes utilize nitrate for respiration, and N is released as a bi-product in gaseous forms [dinitrogen (N2) and nitrous oxide (N2O)] that are lost to the atmosphere.
Nitrogen deficiency is characterized by yellowing (chlorosis) along the leaf midribs, starting at the leaf tip and moving toward the stalk (Fig. 1). Nitrogen deficiency will first be observed on the lowest leaves of the plant. The chlorotic areas will turn brown as the season progresses (Fig. 2). Severe N deficiency will result in a poorly-developed crop canopy that will be unable to intercept all of the sunlight during grain fill. Nitrogen-deficient leaves also have a lower capacity for photosynthesis, further limiting the potential for grain fill.
Fig. 1. Early N deficiency.
Fig. 2. Poorly developed crop canopy due to N deficiency.
Nitrogen uptake by corn from emergence through the V6 stage only represents about 5% of the total plant uptake (Ritchie et al., 1997). However, starting at about the V8 stage, there is rapid accumulation of N by the plant, with about 60% of the total N uptake occurring between V8 and silking. Thus, it is important that N-deficient areas are detected early, and that supplemental N is sidedressed on these areas as soon as possible. When determining whether supplemental N should be applied in corn, the University of Minnesota recommends the use of their supplemental N worksheet.
According to Dr. Gyles Randall, University of Minnesota soil scientist, this worksheet is more accurate when corn is at the V5 or V6 stage rather than when it is smaller. For entire fields or portions of fields meeting the requirements for supplemental N, this worksheet recommends a rate of 40 to 70 lb N/ac, depending on the situation. However, with the high cost of N fertilizer, producers planning to apply supplemental N based on this worksheet need to have a have a clear idea about how much N they should apply. According to Dr. Randall, research from a large number of trials in Minnesota indicates that supplemental N applied at 30 to 40 lb N/ac is sufficient for corn following soybean on most soils or corn following corn on silt loams or sands. In contrast, higher rates, possibly up to 60 to 70 lb N/ac may be needed on heavy soils where corn follows corn and high amounts of surface residue are present.
Agronomic considerations for soybean
Although soybean is generally sensitive to excess water, soybeans can survive underwater for a week or more under ideal conditions. Generally, soybeans tolerate 48 hours under water quite well, but flooding for 4 to 6 days can reduce stands, vigor, and eventually yield. Many factors determine how well a soybean crop will tolerate flooding. The most important factors that determine the fate of flooded soybean fields are: 1) duration of the flooding, 2) temperature during the flood, 3) rate of drying after the flooding event, and 4) growth stage of the crop during the flood. Yield losses are seldom noted in fields flooded for 48 hours or less. Four days or more of flooding stresses the crop, delays the plants’ growth, and causes the plants be shorter with fewer nodes. Flooding for 6 days or more can depress yields significantly, while flooding for a week or more may result in significant (or entire) losses of stand.
Temperature during the flooding event plays a large role in determining the fate of a submerged soybean field. Higher temperatures cause the soybean plant to more quickly deplete its stored energy. Additionally, soybean plants appear to be very sensitive to high carbon dioxide (CO2) levels in the soil. Higher temperatures cause plants and soil microbes to respire at high rates that quickly deplete the water of oxygen and increase CO2 levels. Cool, cloudy days and cool clear nights greatly increase the survivability of a submerged soybean crop.
The rate of field drying after a flooding event also plays a large role in soybean survival (Sullivan et al, 2001). Also, researchers have found yield reductions to be much greater on flooded clay soils than on silt loam soils when flooded for the same period of time (Scott et al, 1989). At the V4 stage, these researchers reported yield losses of 1.8 bu/ac per day of flooding on a clay soil and 0.8 bu/ac 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 bu/ac per day on clay and silt loam soils, respectively. Even larger yield losses would be expected in soybeans at the R3 to R5 stages.
Some of the main indirect effects of flooding on soybean yields are: 1) root diseases, 2) N deficiency, 3) and other plant nutrient imbalances. Caring for recuperating soybean stands should focus on reducing crop stresses where possible. For example, cultivation should be considered to increase soil aeration and herbicide stress should be minimized or postponed where possible.
Wet soil conditions increase risk of cor and soybean seedling diseases
Fig. 3. Corn and soybean seedling diseases
Seedling diseases of soybean and corn may become significant problems in parts of Minnesota due to flooding, prolonged wet soils, and slow crop growth. Seedling diseases can affect plant population and ultimately yield. Non-lethal infection at the seedling stage may also cause damage that persists through the growing season. The pathogens that cause these problems are widespread and persistent in Minnesota fields, and essentially “wait” until conditions develop that favor them and cause plant stress. Favorable conditions include cool, wet, and compacted soils, and poor seed quality. However, slow plant emergence and growth, crusted soil, and fertilizer or herbicide injury may also enhance the problems.
How can you recognize corn seedling diseases?
Corn seedling disease damage may look similar to damage caused by other environmental stresses. General symptoms include reduced emergence, slow growth and stunting in a random or circular pattern, wilting, chlorosis, and post-emergence damping-off. Specific symptoms include rotted seedlings before or after emergence; red/yellow discoloration of leaves; complete or partially rotted roots with firm or soft, brown/reddish/gray lesions; discolored and soft coeleoptile; death of leaf tips; wilting; and discolored lesions on the mesocotyl. The most common pathogens appear to be Fusarium and Pythium, but Rhizoctonia may also be present. Leaf spots can be caused by Pseudomonas (holcus spot) and Colletotrichum (anthracnose). Nematodes can also damage corn seedlings.
What should you look for in soybean fields?
Seedling diseases can begin soon after the seed is planted and can continue for several weeks. The first clue is reduced emergence and the second clue is seedling death after emergence. These problems are usually associated with wet soil, although they may not be seen until a week or more after wet conditions occurred. Infected plants often show symptoms that can be characteristic of multiple pathogens, but cannot be accurately diagnosed without laboratory testing. For example, tan/brown, soft-rot symptoms on roots and stems caused by Pythiumand Phytophthora are very similar. In contrast, the reddish/dark brown, often sunken lesions caused by Rhizoctoniaare easier to recognize. The symptoms associated with Fusarium damping-off and root-rot are light to dark brown lesions.
If significant diseases problems appear, what should be done?
Because seedling diseases often develop quickly, it is important to scout fields to determine if and when problems begin. Timely scouting will also enable collection of plants before they completely decompose, and will allow for diagnosis by the University of Minnesota Plant Disease Clinic (http://pdc.umn.edu/). Proper diagnosis of the major problem can be useful because management recommendations differ for different pathogens. Curative management approaches are not possible this season, but identifying when, where, and which diseases occur can be useful for improving management of them in the future.
Ritchie, S.W., J.J. Hanway, and G.O. Benson. 1997. How a corn plant develops. Iowa State Univ. Coop Ext. Serv. Spec. Rep. 48. Iowa State Univ., Ames.
Scott, H.D., J. DeAngulo, M.B. Daniels, L.S. Wood. 1989. Flood duration effects on soybean growth and yield. Agron J. 81:631-636.Sullivan, M., T VanTooai, N. Fausey, J. Beuerlein, J. Parkinson, A. Soboyejo. 2001. Evaluating on-farm flooding impacts on soybean. Crop Sci. 41:93-100.