July 12, 2017
The heavy rainfall encountered throughout the north-central portion of the state over the last two weeks has many growers wondering how to approach management for the remainder of the season. There are a couple of items to consider moving forward.
Assessing plant viability
The first step is to determine whether plants survived being under water or in saturated soil. Seed that has germinated but not emerged is also susceptible to excessive water damage. In corn, most of the plant mortality will be in localized low areas of the field that were subjected to standing water. The survivability of plants or seedlings under water is dependent on temperature, growth stage, variety, soil type, nutrient status and other factors, but it’s generally in 24 to 96 hours.
In cool temperatures, plants will survive longer as respiration slows. Conversely, when temperatures warm up, plants consume oxygen quicker, and submerged plants will not survive as long. In cool conditions, corn can survive four days submerged.
WAITING TO DRY OUT: This soybean field still has standing water after four days.
With soil temperatures warmed up in the upper 70s, submerged corn may survive for only 24 hours. In soybeans, soilborne disease such as pythium and phytophthora can result in plant mortality beyond those areas that were under water. In assessing a soybean stand, for more fertile soils, a minimum of 80,000 to 100,000 living plants per acre are needed to approach the full yield potential of the field. In poorer soils, 100,000 plants per acre are needed.
Generally, the time it takes for submerged soil to dry out enough to get equipment on for management is usually time enough to make a determination whether or not plants have survived. However, it can be difficult to determine survivability when plants are covered with soil and plant residue left behind by receding water. In these cases, split a plant lengthwise through the stem. The growing point should be relatively firm and white or cream colored. Darkening or softening of the growing point indicates a nonviable (dead) plant.
Unnecessary tillage operations on soils that may be marginally wet increase the risk of creating tire and tillage compaction layers that can haunt root development and corn health later if excessively dry conditions suddenly become the norm.
The presidedress nitrate test (PSNT) is an option for those growers that still have the opportunity to sidedress. PSNT results are more accurate if 40 pounds of nitrogen per acre or less was applied at or before planting. If PSNT results measure greater than 25 ppm, no additional N is suggested.
Soil temperatures are generally high across much of the state, so unless a nitrification inhibitor was used, much of the early-applied N likely has converted to nitrate. Denitrification may be an issue in those soils that remained ponded or saturated for an extended period of time. Fields with fine-textured soils and poor internal drainage will be at the greatest risk of denitrification. Estimates are that close to 5% of available nitrate may be lost per day of saturation.
Prior to the rainfall, many fields were heading into prime corn growing weather conditions and approaching peak stages of corn N uptake. The downward movement of nitrate through the soil does not necessarily mean that N is lost, as corn roots can still grow into and access N that hasn’t reached a tile drain.
ROTTED: Here’s what a rotten core in a corn plant looks like after rain damage.
Corn that appears chlorotic and N-short may still have sufficient N stored in the soil. Root growth has likely been limited during the saturated conditions, which limit plant growth and development. As soils begin to dry and plants resume growth, chlorotic-looking corn may begin to green. Corn that was planted late and experienced dry early-summer soil conditions may especially look N-deficient.
Although recent research in Michigan has shown that some yield potential may be sacrificed when the majority of N was applied at V10 or later, growers may benefit from a rescue N application, as yields greater than 187 bushels per acre have still been achieved with late-season N applications when little to no N was applied preplant or early sidedress.
In flooded-out areas, another problem is the potential for weed problems from weed seed brought in by encroaching waters, herbicide residue washed in from adjacent fields, loss of herbicide control from excessive leaching or erosion, loss of nitrogen through denitrification or leaching, and increased incidence of phytophthora and other root diseases. Nodule function in soybeans is also reduced in saturated soil. However, nodule activity resumes to normal levels once the soil dries out. Scout fields closely for these factors and use a PSNT soil test to determine nitrogen losses.
Source: MSU Extension
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