Many cornfields across Wisconsin are underwater.
Recent rains have caused flooding, ponding and concerns for corn growers about growth, development and what effects short periods of flooding might have on yields.
In the June 2 issue of "Agronomy Advice", Dr. Joe Lauer, University of Wisconsin Extension corn agronomist, said the extent of injury corn will suffer by flooding is determined by when it hits during the plant's life cycle, the frequency and duration of flooding, and the temperature of the air and soil during the flooding.
Flooding affects respiration by reducing the exchange of oxygen between the soil and the atmosphere, Lauer explained, which eventually leads to decreased total root volume, less transport of water and nutrients through the roots to the shoot, and formation of plant-harming sulfides and butyric acid by microorganisms.
Waterlogged soil injures roots, while continued poor aeration causes cell death and will eventually kill roots. Research shows measurable short term reductions for root and leaf growth rates begins immediately within 1-12 hours, but tends to recover quickly within 2-3 days.
Leaf elongation ceases almost immediately and N, P and K concentrations decrease in leaves decrease, although the concentrations increase in roots.
Flooding restricts root growth in the upper 18 inches of soil, but roots continue to grow at deeper levels. Soil compaction and flooding will restrict root growth more severely than either factor separately, Lauer noted.
Temperature has an effect on the biological processes. Wet soils have a large heat capacity, and considerable amounts of heat are required to raise their temperature. That means wet soils are cold and corn growth is slower.
As drainage lowers the moisture content of the upper soil layers, air is able to penetrate more easily to the roots and carbon dioxide produced by the roots, microbes and chemical reactions can be transported to the atmosphere. Lower soil moisture content also means higher soil temperatures and faster growth.
The growing point of corn is near or below the soil surface prior to six visible leaf collars (V6). Research shows the oxygen supply in a flooded soil is depleted within about 48 hours. Without oxygen, the growing point of a corn plant cannot respire; critical functions are impaired.
If temperatures are greater than 77°F during flooding, plants may not survive 24 hours. Cooler temperatures prolong survival, so crop injury should be limited if the corn is flooded for less than 48 hours.
To check if plants survived flooding, look at the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually means plant death, Lauer said. In addition, look for new leaf growth 3 to 5 days after the water drains from the field.
Once its growing point is above the water level, a plant's chances of survival improve greatly, he noted.
If the corn escapes death, flooding may still have a long-term negative impact on crop performance. Data has shown that excess moisture during the early vegetative stages retards root development, Lauer said. As a result, plants may be subject to greater injury later during a dry summer because their root systems are not sufficiently developed to contact available subsoil water.
In addition, waterlogged soils can lead to nitrogen deficiency due to oxygen deficiencies, reduced microbe activity and decreased intakes of ammonia and nitrate.
Flooding can also reduce the activity of mycorrhizae needed for phosphorus uptake, and cause losses of nitrogen through denitrification and leaching. In fields where estimated nitrogen loss significant and yield potential is reasonable, corn may respond to the application of additional fertilizer, Lauer said.
Numerous studies have shown that crop yield losses are greater when flooding occurs early in the season. When six-inch corn was flooded for 24, 48 and 72 hours, corn yields were reduced by 18, 22 and 32 percent at a low nitrogen fertilizer level.
At a high nitrogen level, the reductions range from 19 to 14 percent one year and less than 5 percent in another year.
When 30-inch corn was flooded for 24 and 96 hours, yields were reduced 14-30 percent. However, when soil had a high level of nitrogen, there was very little reduction in yield, even with 96 hours of flooding.
Corn flooded near silking experienced no reduction in yield at high nitrogen levels, although yield reductions up to 16 percent were recorded when corn on soils with low levels of nitrogen was flooded for 96 hours.
Another issue with flooding is the mud and sediment that can cake leaves and stalks can also damage plant tissue and spark the development of non-typical fungal and bacterial diseases.
The early season stress may also predispose the plant to root and stalk rots later, so the timing of harvest may need to be adjusted accordingly, Lauer added.
Corn plants dealing with flooding and cold temperatures may also be more susceptible to "crazy top", a fungus that uses saturated soil conditions to infect corn seedlings. With warmer, wet or humid conditions, Pythium can reduce stands despite fungicide seed treatments.
There is limited to hybrid resistance to these diseases and predicting damage is difficult until later in the growing season, Lauer noted.