Corn Replanting - Herbicide Replant Restrictions

As farmers consider replanting corn, the herbicides already applied may limit the options for replant. The following table is an updated version of the table 8 in AGR-195 Replanting Options for Corn. The table below lists corn herbicides and the potential risk associated with replanting corn, grain sorghum or soybean. This table is not exhaustive, and the herbicide label should be consulted for more details on crop rotation limitations.

Corn Replanting - Removing a Poor Stand

As farmers debate about destroying poor stands of corn and replanting, there are several herbicide guidelines to consider. There are several options to remove a poor stand, but those options depend on the type of hybrid currently growing in the field.
The following is an excerpt from AGR-6 2016 Weed Control Recommendations for Field Crops.

Focus on These Two Things, Cut Costs Elsewhere

Chad Lee, Extension Agronomist, University of Kentucky

Farmers are looking where to cut production budgets for 2015. Three of my colleagues and I were honored to speak at the National Commodity Classic this year. During our presentation, one of my colleagues closed out our session by saying that the two things farmers need to do is: 1) use appropriate soil-applied fertilizers based on soil tests; and 2) use highly effective weed control, which probably includes a soil residual herbicide. All of us in the room agreed that these are probably the two things many farmers will cut in 2015.

Wet Conditions Create Weed Management Challenges in Corn

 Jim Martin and J.D. Green, Extension Weed Specialists, University of Kentucky, April 27, 2011

The prolonged wet conditions this spring have created additional challenges in regards to weed control. The following are possible scenarios that corn farmers may encounter and factors to consider as we progress through the next several days.

Wheat Crop Condition, March 2011

Chad Lee, Extension Agronomist, University of Kentucky
email: Chad Lee

The cold weather and late emergence of some wheat across the state has many producers concerned about how their crop will do this spring. Much of the first application of fertilizer nitrogen has been applied as well. The warmer temperatures predicted for the week of March 14, 2011 should result in rapid wheat growth. There are several management decisions that must be made, including if the stand is suitable to keep.

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.

Corn, Late Planting and Management

Photo: Possible Mn defiency in corn, which is most likely due to weather and not lack of Mn in the soil.

Believe it or not, corn has emerged on 96% of the acres in Kentucky which is right in line with the five-year average, according to the Kentucky Crop and Weather Report. While emergence is on track, planting was not, and management decisions need to be adjusted.

Preplant nitrogen fertilizer and preplant herbicides may have been lost to the heavy rainfalls. Farmers may need to sidedress additional N and apply postemegernce herbicides. Because of the late planting, farmers will have a smaller window to make these postemergence applications.

Corn planted April 1 will likely reach V6 growth stage in 35 days, while the same hybrid planted May 15 will reach V6 in about 22 days. Corn planted even later will reach V6 earlier.

Many postemergence herbicide labels have restrictions for V6 corn. Corn will need about 20 to 40 lbs of N/acre through about V6. If additional N has not been applied by V6, then yield losses can be expected.

Determining how much N fertilizer or herbicide was lost from the heavy rains is not easy. A calculation for N fertilizer lost is available in the Corn and Soybean Newsletter from April. Assessment of herbicide losses are more of a case-by-case assessment.

In addition, the rapid growth and wet weather can result in some transient nutrient deficiency symptoms...symptoms where weather has more to do with nutrient deficiency that availability of the nutrient. Normally in these cases, the best cure is sunshine and warm weather. Often farmers will spray a foliar fertilizer over the top and a couple days later the crop turns greener, but it may not yield any more. We always suggest to leave a test strip, and take both the treated and non-treated areas to yield.

As always, consult your county agents for more information.