Agriculture : News
DATE: May 10, 2010
HARDIN COUNTY COOPERATIVE EXTENSION SERVICE
201 Peterson Drive
Elizabethtown, Kentucky 42701-9370
BY: Doug Shepherd
County Extension Agent for Agriculture and Natural Resources
Corn Crop Concerns:
The heavy rains and resulting flooding experienced across the county last week have given some corn growers cause to be concerned about the future status of their crops and what steps to take regarding replanting, fertilizer loss and herbicide control status. Many of the UK Extension Agronomy Specialists have done an outstanding job of providing growers with excellent, timely information that can be used to make some of these tough management decisions. As new information is provided, we’re forwarding it onto our webpage www.hardinext.org. Most of the information can easily be found under the Ag Enterprise Grain tab, and is listed under “Hot Topics”. Our total webpage has been recently updated so if you haven’t been there in awhile, check it out. You’ll note we now have a blog and Facebook presence.
The following are some exerts posted by various UK Extension Grain Specialists on the impact of the flooding on the corn crop across the state, and how producers can determine the route they should go concerning their crop. UK has two excellent resources on assessing damaged corn: AGR-193: Assessing Flood Damage to Corn and AGR-195: Replanting Options for Corn.
The latest USDA Crop Progress and Condition report, well over 80% of the corn crop is planted and over 60% of it has emerged. The heavy rains experienced the first weekend in May put many fields under water, some for several days, some up to a week or more. 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, 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 the 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.
Wet soils cause nitrogen losses. In cases where high intensity rain results in high runoff, leaching losses will probably be low. The primary nitrogen loss mechanism in saturated soils is denitrification, which occurs when soil nitrate nitrogen (NO3-N) is converted to nitrogen gas by soil bacteria. Two to three days of soil saturation is required for bacteria to begin the denitrification process. Well-drained upland soils that have been wet from a series of rains probably have not experienced much denitrification. Soils in lower landscape positions that stay saturated longer will likely lose more N. Losses can be calculated by estimating 3 to 4 percent loss of fertilizer NO3-N for each day of saturation. Tables are available to help you determine how much N was lost, and can be found on our website or the publications listed above.
An additional tool for determining NO3-N in the soil after flooding is a NO3-N test. The soil sample should be taken down to 12 inches deep, and several samples should be taken in each field of both the low and higher ground. The samples should be mixed well and a subsample sent for nitrate analysis. If the nitrate-N is less than 11 ppm, there is a low amount of plant-available N in the soil. Therefore, there is a good chance corn will respond to a sidedress application of N ranging from 100 to 150 lbs N/acre. If soil test nitrate-N is greater than 25 ppm, there is adequate plant-available N in the soil, which indicates corn will probably not respond to sidedress N application.
Farmers sometimes broadcast fertilizer nitrogen on a field within 24 hours of a heavy rain. In most cases, very little nitrogen is lost to runoff, especially if the field was under no-till soil management. The nitrogen fertilizer begins to dissolve almost immediately after being applied to the soil surface and will dissolve completely in a short period of time. As rain begins, the first water that falls moves into the soil, taking most of the fertilizer nitrogen with it. Once in the soil, most of the fertilizer nitrogen is protected from runoff. The only exception is a very intense rain soon after application that also erodes topsoil from sloping areas. Even in this situation, the loss would probably be less than one third of the fertilizer applied.
The following are herbicide related issues involving flood damage corn fields:
- 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, which is available either at the Hardin County Extension Office or online at http://www.uky.edu/Ag/CornSoy/cornsoy7_5.htm#3.
- 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.
- 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.
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