The first major impact of soil completely covered with water is a rapid depletion of oxygen required for plant growth and development. The other major impact of flooding is change in nutrient status either by leaching or changing their availability to the plant, he says.
Corn is very sensitive to flooding in the early vegetative stages, especially before the fifth or sixth leaf stage, Berglund says. In early growth stages, corn and soybeans can survive for only two to four days under water in anaerobic conditions. Moderate water movement can reduce flood damage by allowing some oxygen to get to the plants, keeping them respiring and alive. Drainage within one to two days increases the chance of survival.
If temperatures are warm during flooding (greater than 77 F), plants may not survive 24 hours. Cool temperatures may prolong survival. However, cold, wet weather favors disease development. Seed treatments are effective, but limited in protection. Seedling development slowed or delayed two to three weeks allow soil-borne pathogens a greater opportunity to cause damage.
Iowa studies found that flooding when corn is about 6 inches in height for 72, 48, and 24 hours reduced corn yields by 32, 22, and 18 percent, respectively, at a low N fertilizer level (50 lb N per acre). At a high level of N (350 lb N per acre) these yield reductions ranged from 19 to 14 percent in one year to less than 5 percent the following year.
"Even if flooding doesn't kill plants outright it may have a long-term negative impact on crop performance," Berglund says. "If excess moisture in the early vegetative stages retards root development, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water.
"Seed rots, seedling blight, corn smut and crazy top affect corn plant development later even though ponding occurred earlier. Delayed soybean growth allows diseases such as Fusarium root rot, Phytophthora rot and Pythium rot to establish and weaken or destroy seedlings," Berglund says. "Carefully assess damage and check with your crop insurance agent before deciding to replant or before tearing up the existing stand."
At this late date, only a few crops should be considered for replanting. Early maturing sunflower, buckwheat, proso millet and very early group 00 soybeans may be options, Berglund says. Before replanting, make sure herbicides used on the prior planting will not injure the replanted crop. Also, don't seed into a wet seed bed as crops "mudded in" will result in poor stands and will have limited yield potential.
Look for rotted seed or damped-off seedlings to reveal probable crop losses, he advises. To confirm plant survival, check the color of the growing point (it should be white and cream colored, while a darkening or softening usually precedes plant death) and look for new leaf growth three to five days after water drains from the field. Compare the intended stand to the damaged stand and evaluate the uniformity of the stand, the original planting date versus a replant date, likely replant pest control and seed costs as well as projected crop prices.
"Weigh these costs and price projections against replanting yield gains to evaluate crop injury and replanting gains," he says.
"On surviving stands, remember that favorable weather for plants after ponding is important," Berglund says. "Cultivation, once soils are dry enough, will open and aerate surface soil and promote root growth. Be careful, however. Working wet soil causes compaction that reduces crop growth."
An additional nitrogen application in corn may be necessary in fields that show signs of yellowing or uneven growth, he notes. A late test for nitrate when corn plants are still 6 to 12 inches tall can determine if more nitrogen is needed. Similarly, maintain a good weed control program so that crop plants are not robbed of nutrients and moisture later in the season.
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