Replant Options Following Corn Pre-Emergence Herbicide Applications

June 6, 2014

Recent hail storms have many growers facing replant decisions and asking about their options in fields where pre-emergence herbicides were applied to corn. There are two primary issues:

1. What do I do about the existing or surviving corn?
2. What crops can I replant at this point if I have already applied a pre-emergence corn herbicide?

Killing a Poor Corn Stand for Replant Purposes

If you decide to replant, you'll want to kill your previous corn stand. Having a crop at different growth stages is difficult to manage and yield will be affected. You have several options for killing the poor stand of corn.

Mechanical Control. Tillage is effective for any type of corn, but waiting for dry enough conditions to work the soil may not be desirable. Mowing or chopping the above ground foliage will not kill the damaged corn unless the growing point is destroyed. The growing point is below or at the soil surface until approximately the V6 growth stage.

Chemical Control. If the damaged corn stand is a conventional or LibertyLink hybrid, glyphosate is the best herbicide option. However, if the corn is a Roundup Ready hybrid or a stacked Roundup Ready and LibertyLink hybrid, this becomes more complicated.

There are three chemical options to control Roundup Ready corn, but each has some drawbacks. The options are Select Max (clethodim), Liberty (glufosinate). and Gramoxone SL (paraquat).

Select Max (clethodim) is slow acting but very effective at controlling a poor stand for a replant scenario (e.g., it will kill all the plants remaining). The drawback is a six-day waiting period between herbicide application and replanting. If the waiting period is ignored, the replanted corn may be stunted.

Liberty (glufosinate) and Gramoxone SL (paraquat) also can be used. These products have no residual activity that would delay planting; however, their effectiveness was inconsistent in UNL trials. Immediately after spraying these herbicides, the corn will look ill. Unless conditions are just right (warm, humid, and the plants are large enough for the growing point to be killed), often the plants will survive and send up new leaves.

Gramoxone SL activity may be enhanced by tank-mixing it with atrazine or metribuzin. If Liberty is used, add AMS and use the appropriate spray volume (15 to 20 GPA) to maximize effectiveness. An additional reminder, any corn hybrid containing a Herculex insect protection gene also contains the Liberty Link gene.  Liberty will not be effective on these hybrids.

Replant Crop Options When a Corn Pre-emergence Herbicide has been Applied

The logical replant options at this point in the growing season are:

1. a short season corn hybrid,
2. a slightly shorter maturity soybean variety, or
3. grain sorghum.

Depending on the pre-emergence herbicide used in corn, it may not be possible to replant to any crop except corn. Table 1 lists corn, grain sorghum and soybean replant intervals for some of the more common pre- and post-emergence corn herbicides.

Corn

Corn can be replanted at any point, unless Select Max has been used to kill the existing stand (see section 1). In most instances, producers can then use the post-emergence program they had initially intended for subsequent weed control this year.

Another logical question is "I've applied a PRE to my initial corn planting. How does this affect my POST program after replanting?" There are two important items to consider:

1. Given the calendar date, late emerging weeds are still of concern, primarily waterhemp, Palmer amaranth, and morningglories.  In the majority of instances your initially planned POST program should be fine.  The post program should have two effective modes-of-action on waterhemp or Palmer amaranth to help with resistance management.  Applying herbicides with residual activity would still be prudent, as waterhemp and Palmer amaranth can continue to emerge well into July.  If you're considering applying additional residual, be careful not to exceed the seasonal total amount of any single active ingredient.  For example, if you applied 4 oz/ac of Corvus (active ingredients are isoxaflutole and thiencarbazone-methyl), you could still apply up to 2.75 oz/ of Balance Flexx (active ingredient is isoxaflutole) for additional residual weed control.  But if 5.6 oz of Corvus was applied, consider a residual herbicide other than Balance Flexx to avoid application that exceeds the total annual labeled allowance for isoxaflutole. 
    
2. It is currently early to mid-June. Consider the rotation interval if planning to plant soybeans next year.
   
   If the decision is to keep the initially planted corn crop, some effort should be made to determine the corn's growth stage when damaged.  Some POST herbicides have growth stage limits that should still be adhered to, particularly POST herbicides containing an ALS mode of action.   The injury from a late application of an ALS-containing product would be primarily to the eventual ear development.

Soybean

If atrazine has been applied pre-plant or pre-emergence, soybeans should not be planted in that field until next year. This is clearly stated on the label of any herbicide that contains atrazine, regardless of the rate used.

A consideration in soybeans:  Given the current date, if applying any fomesafen-containing product (Flexstar, Prefix, Marvel, etc.) for residual and/or POST control of waterhemp on retained or replanted soybean stands, consider the 10-month rotation interval to corn and your normal corn planting window to minimize potential issues with fomesafen carryover on corn in 2015.

Sorghum

There is more flexibility if deciding to replant sorghum where corn pre-emergence herbicides have been used.

Replanting sorghum where common corn pre-emergence herbicides containing atrazine, dimethenamid-P (Outlook), or S-metolachlor (Dual II Magnum, Bicep II Magnum, Cinch, etc) were applied should not be a problem.

The situation is more complicated where products containing acetochlor were applied. The formulation Degree Xtra is labeled for use in sorghum, so sorghum may be planted at any time. However, if other products containing acetochlor (Breakfree, Breakfree ATZ, Degree, Keystone, Keystone LA, SureStart, Tripleflex, etc) were applied, replanting sorghum is not allowed according to product labels. See Table 1 for details.

If products containing mesotrione (Callisto, Callisto GT, Callisto Xtra, Lumax EZ, Lexar EZ or Halex GT) were applied in corn, replanting sorghum is allowed. Lumax EZ, Lexar EZ, and Callisto are labeled for preplant applications in grain sorghum. Products containing mesotrione occasionally can cause some bleaching or whitening of the emerging and young sorghum seedlings. However, this generally occurs only when the products are applied within seven days of planting. Our experience has shown that bleached sorghum quickly grows out of these symptoms and develops normally with little to no impact on yield. If Lumax EZ, Lexar EZ, or Callisto were applied to the intended corn several weeks ago, there is little potential for injury to replanted sorghum.

Summary

This article has provided some general guidelines to consider. Consult the product label of any products applied this spring for specific details and directions.

Lowell Sandell
Extension Educator — Weed Science