Herbicide Issues in Flood Damaged Corn Fields

Jim Martin and J. D. Green

The following are herbicide related issues involving flood damage corn fields:

1)    Previous Herbicides Applied May Affect What Crop to Plant:  The specific herbicide(s) used in the initial planting may determine what crop can be planted in flood damaged fields. See Corn & Soybean News April 2007, Vol 7 Issue 5 http://www.uky.edu/Ag/CornSoy/cornsoy7_5.htm#3

2)    Adding More Herbicide to Replant Corn: The need to add more herbicide will depend on specific field situations.  In some cases, where a previous soil-residual herbicide had been applied, relying on postemergence treatments following corn replanting may be the best option for weed control throughout the remainder of the season.  If additional soil-residual herbicide is desired the maximum amount that can be reapplied depends on the products used.  For example, if the previous amount of atrazine applied was 1.5 lb ai/A, then an additional amount of 1.0 lb ai/A can be applied up to a maximum total of 2.5 lb ai/A of atrazine per season.  For other herbicide active ingredients consult the label for maximum use rate guidelines. 

3)    Controlling Surviving Corn Plants to Replant: If there are surviving corn plants, it may be necessary to use a burndown herbicide program.  It will be more challenging to control surviving corn plants if the field is planted back to corn than if it is planted to soybeans.  For specific options for controlling corn prior to replanting back to corn see page 29 in the Corn Section of the Extension publication “Weed Control Recommendations for Kentucky Grain Crops 2010” AGR-6 http://www.ca.uky.edu/agc/pubs/agr/agr6/02.pdf 

There are several foliar-applied herbicides available to control corn in soybeans.  If corn plants have the Roundup Ready (glyphosate – tolerant) trait, consider such options as Asure II, Fusilade DX, Fusion, Poast,  Select and Select MAX. If corn plants do not have the glyhosate tolerant trait then glyphosate is an effective and economical option as a burndown treatment or as an “in crop” treatment in Roundup Ready soybean.

Flood Damage to Corn

The heavy rainfalls over the past weekend has many fields under water. According to the latest USDA Crop Progress & Condition report, about 80% of the corn crop in Kentucky is planted and almost 60% is emerged.

Two resources on assessing damaged corn include AGR-193: Assessing Flood Damage to Corn and AGR-195: Replanting Options for Corn.

The impact of flooding on corn depends on the depth of flooding, the soil temperature and the duration of flooding. When soil temperatures are 70 degrees F or higher, corn can withstand complete submersion for about 24 hours. Higher soil temperatures reduce that time. Corn that is not fully submerged can withstand longer periods of standing water. 

While you may want to walk through a field (or in some cases, take a boat through a field), assessment of corn can not occur until two or three days after the water has subsided. Plants will most likely look yellow, but if the growing point is white and turgid, the plants are alive. 

Nitrogen losses occur with submerged fields and they type of nitrogen and the duration of flooding affects how much N is lost. 

While losing corn and nitrogen are very frustrating, they don't compare to the loss of lives from this flooding. We hope that you and yours are staying safe. 

Flooded Corn

Chad Lee, Extension Agronomist
Dept. of Plant and Soil Sciences, University of Kentucky

Across Kentucky there are some fields of corn that have been flooded.

Generally, if soil temperatures are above 70 degrees Fahrenheit for more than 24 hours, then corn will not survive. Usually the status of the corn crop will be known about two or three days after the flood event.

Much more details about flooded corn are given in AGR-193: Evaluating Flood Damage to Corn.

Some of this corn was stacked with both RR2 and LL traits. In these cases, the two best options are Select Max or paraquat plus linuron. The Select Max will provide better control, but will require no re-planting of corn for about 6 days. The paraquat + linuron will not provide as good of control, but will allow immediate re-planting of corn. AGR-6: Chemical Control of Weeds for Kentucky Grain Crops, page 22 contains more details about removing corn for a re-planting situation.