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 to 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 Fahrenheit) 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 percent, respectively, at a low nitrogen fertilizer level (50 lb N per acre). At a high level of nitrogen (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.
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; e.g., 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. Saturated soil conditions can also result in losses of nitrogen through denitrification and leaching. Even if water drains quickly, there is the possibility of surface crusts forming as the soil dries that can impact the emergence of recently planted crops. Growers should be prepared to rotary hoe to break up the crust to promote emergence.
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 to 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 to 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.
Since late season flooding is an uncommon event, little information is available on its effects on corn at this stage of kernel development, and how to best salvage damaged corn. A major concern is the impact of flooding on grain and silage quality. In past reports, when corn in the dent stage was covered by flood water for six hours or more and nearly completely caked with mud for up to two weeks, damage from ear rots and premature kernel sprouting was extensive in those areas of fields where water had covered the ears the longest. Although such damage may be negligible in fields where water never covered the ears, prolonged flooding may cause significant injury to the roots, if not premature root death. Such plants will be more vulnerable to stalk rots thereby increasing the likelihood of stalk lodging, especially if harvesting is delayed. Therefore, as soon as plants have dried, stalks should be inspected to determine the degree of rot. If rot is extensive, these affected fields should be harvested first to minimize further yield loss.
When dealing with flood damaged corn, a common suggestion is to allow rains to wash off as much soil as possible before harvesting. Another observation is that flooding often deposits considerable debris on fields making harvesting difficult, as will dust associated with soiled plants.
Nielsen, R.L. 2011. Effects of Flooding or Ponding on Corn Prior to Tasseling. Corny News Network, Purdue University. Available at: kingcorn.org/news/timeless/PondingYoungCorn.html (URL verified 1/19/17)