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Flooding Effect on Crops
SATURATED SOILS AFFECT CROP GROWTH
Prolonged soil saturation affects crop growth and yield. Corn is very sensitive to flooding in the early vegetative stages (especially prior to the fifth or sixth leaf stage). In early growth stages, corn or 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.
The injury extent to seedlings is determined by the plant stage of development at ponding, duration of flooding and the air/soil temperatures as well as if axillary buds are present on damaged plants. 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. 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. Carefully assess damage before deciding to replant or before tearing up the existing stand.
Rotted seed or damped-off seedlings will reveal probable crop losses. Evaluate intended stand to the damaged stand, 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.
On surviving stands, remember that favorable weather for plants after ponding is important. Cultivation, once soils are dry enough, will open and aerate surface soil and promote root growth. Be careful working the soil. Working wet soil causes compaction that varies crop growth.
An additional nitrogen application in corn may be necessary in fields that show signs of yellowing or uneven growth. A late test for nitrate when corn plants are still six to twelve inches tall can determine if more nitrogen is needed.
Maintain a good weed control program so that crop plants are not robbed of nutrients and moisture later in the season.
FLOODING ON WHEAT AND BARLEY
What impact recent heavy flooding is having on crops?
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. The response to flooding will vary with duration, and temperature.
Obviously the longer the duration of the flooding the greater the injury. The injury results from plants inability to grow in an anaerobic environment. While specific information on how long small grains can survive under water has not been widely reported, most indications are that wheat can withstand water logged soils for up to 24 hours with out excessive damage, barley is less that this. Depending on the conditions wheat can probably survive saturated conditions for up to two days.
Conditions that influence this are temperature and stage of growth. Under cool conditions metabolic activity is less than at warm temperatures and the demand for oxygen is less. A younger crop has a smaller demand for oxygen and may be able to survive better. Even so, a small grain crop that is submerged in water for two days, or longer, has little chance for survival. When a small grain crop does survive flooding, recovery maybe very slow and yield will be dramatically impacted.
EFFECTS OF FLOODING ON CORN
Recent heavy rains in some eastern areas of North Dakota have promoted inquiries of flooding and ponding in corn and its effects.
The extent to which flooding injures corn is determined by several factors including plant stage of development when flooding occurs, the duration of flooding, and air/soil temperatures. Prior to the 6-leaf stage (when the growing point is near or at the soil surface), corn can survive only 2-4 days of flooded conditions. Once corn has reached the silking stage shallow depths of flooding will not cause any noticeable amounts of damage. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24 hours. Cooler temperatures prolong survival. 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%, 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% in one year to less than 5% the following year.
Research indicates that the oxygen concentration approaches zero after 24-hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions, such as nutrient and water uptake is impaired, root growth is inhibited, etc. Even if flooding doesn�t kill plants outright it may have a long term negative impact on crop performance. 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.
If flooding in corn is less than 48 hours, crop injury should be limited. 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 3 to 5 days after water drains from the field.
Cold, wet weather conditions also favor development of seed rots and seedling blights. Seed treatments are usually effective but can provide protection only so long; if seedling development is slowed or delayed 2-3 weeks, soil-borne pathogens have a much greater opportunity to cause damage. Other disease problems which may become greater risks due to flooding and cool temperatures are corn smut and crazy top. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage is difficult because disease symptoms do not appear until later in the growing season.
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