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.