Duration, Temperature of Flood Waters Dictate Extent of Crop Damage

Duration, Temperature of Flood Waters Dictate Extent of Crop Damage

May 11, 2007

From our Archives

The following story was first published in CropWatch June 27, 2003. Although it was published six weeks later in the crop season, it may be helpful to keep in mind if more storms develop.

Some areas of central and eastern Nebraska received extensive precipitation over severalrelatively short periods this week, causing flooding in rural and urban areas. How much rain is too much and how long is too long when fields are flooded? The severity ofdamage from flooding and ponding will depend on the depth of the flooding, the length of timean area was flooded and the temperature of the flood water. Depth of flooding refers not only to the water visible above ground, but also to the level ofmoisture in the soil profile. In some situations, producers may want to dig ditches to help drain afield more quickly. Before doing so, check with the Natural Resource Conservation Service(NRCS) to ensure that you will not be committing a wetlands violation. Help facilitate fielddrainage by ensuring that drainage paths are clear of plant residue, brush and other cloggingmaterials. Even after the surface flood water has receded, the soil may remain saturated for sometime. While the shoot may appear healthy, the roots may be suffocating. Once the surface waterhas receded, the field's ability to drain internally will be important. Tiled fields will drain faster,providing quicker relief to the crop.

Understanding Types of Field Flooding

Field flooding may occur by either stream flooding or lowland flooding. The former results fromcreeks or rivers overflowing their banks onto a flood plain. In this case silt and sand is oftendeposited in the field and plant tissues are covered with soil material. The latter case results fromwater accumulation in depressed areas due to inadequate soil permeability or surface drainage.

These flooding types can be further divided into soil waterlogging, where the soil is merelysaturated, or complete submergence where the crop is entirely submerged. The type of floodingoccurring on a farm directly impacts growth and yield potential of [soybeans in] that field.

Although stream flooding can quickly recede, the resulting silt deposits can bury the crop andcover leaf tissue with thick layers of soil. Without rains to wash silt from soybean leaves,recovery is greatly slowed. Fortunately, soil types prone to stream flooding are oftencoarse-textured. These sandy soils drain well, allowing quick crop recovery or replanting.Lowland soils often retain water so that as the water disappears from the surface, the soil profilemay remain waterlogged for several more days.

Seth Naeve
Extension Soybean Specialist, University of Minnesota

From the University of Minnesota publication, Flooded Fields and Soybean Survival, MNCN80, published June 14, 2002.

The length of time a field is flooded also affects the rate of plant survival. Peter Thomison,Extension Corn Specialist at Ohio State University, notes that corn usually can survive two tofour days of ponding, depending on the temperature. If the air temperature is over 77°F, plantsmay not survive longer than 24 hours.

Early spring flooding is a lot less harmful than late spring flooding when the flood waters are aptto be warm. The temperature of the flood water is important for two reasons:

  1. there is lessoxygen available in warm water than in cool water; and
  2. microorganisms that use oxygen aremore active in warm water and will compete with roots for oxygen. After using the oxygen,microorganisms will begin to break down nitrate nitrogen, converting it to a gas throughdenitrification. (During flooding, nitrogen also is apt to be lost through leaching.)

Even if flooding doesn't kill plants outright, it may have a long-term negative impact on cropperformance and the potential for disease development. Thomison notes, "Excess moistureduring the early vegetative stages retards corn root development and can kill the deepest roots.As a result, plants may be subject to greater injury during a dry summer because root systems arenot sufficiently developed to access available subsoil water."

Seth Naeve, Extension Soybean Specialist at the University of Minnesota, notes several indirecteffects of flooding on soybeans, which producers also may need to be addressed. They include 1)root diseases, 2) nitrogen deficiency, 3) and other plant nutrient imbalances. Caring forrecuperating soybean stands should focus on reducing further plant stress where possible.

Cultivation should be considered to increase soil aeration, and post- emergence herbicides shouldonly be applied to conventional soybean crops judiciously. Herbicide stress should be minimizedand postponed where possible, Naeve writes in his University of Minnesota publication, FloodedFields and Soybean Survival, MNCN80, published June 14, 2002.

Producers may want to carefully weigh the costs and benefits of further inputs into some badlydamaged fields. To confirm plant survival, check the color of the growing point. It should bewhite and cream colored rather than dark and soft. New leaf growth should appear three to fivedays after water drains from the field.

Replanting may not be a very viable option in many fields where conditions are likely to staysoggy for several weeks, not allowing for timely field work and planting.

Soybean producers with field populations below 75,000 plants per acre may consider replantingif they can get back into the field relatively soon. Lenis Nelson, Extension Crops Specialist, didnot recommend changing the maturity class significantly when replanting soybeans. If a producerhad previously planted a mid-group II variety, he or she might consider now planting an earlygroup II variety.

Nelson also recommended planting in narrow rows so the canopy closes faster and weeds arebetter controlled.

Bob Caldwell
Extension Cropping Systems Specialist
Lisa Jasa
Crop Watch Editor

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