I have received a lot of calls lately about options for killing out poor stands of corn and planting a new stand of corn back into these areas. Most of these calls have to do with Roundup Ready corn and how to eliminate it and plant back into these fields. I don't have a lot of data on this topic, but we did conduct a few experiments in 2006 and 2007 to investigate several options and I have provided those results below in Tables 1 and 2.
First, one of the most obvious options for eliminating any type of corn is tillage. Many growers are not willing to resort to this, but this is a viable option and the costs of a tillage operation should be weighed against the cost of the available herbicide options before this possibility is ruled out.
Second, the type of herbicide resistance technology offered in the corn that was planted will be CRITICAL in determining which herbicide can be used for this purpose. If the corn was not a Roundup Ready variety, then any of the generic or brand name glyphosate products will provide good control. If the previously-planted corn was a Roundup Ready variety, then eliminating the corn stand will be more difficult. Obviously the problem is that there are few herbicides that provide consistent control of Roundup Ready corn and at the same time allow for corn replanting in a short period of time after application.
Towards the end of 2008, Select Max (clethodim) received a supplemental label for the control of poor stands of corn. This label allows for the application of Select Max at 6 fluid ounces per acre for the control of Roundup Ready corn and for replanting of the subsequent corn crop into these areas six days after application. Although we have not evaluated this exact rate, in our research with Select Max we have observed excellent control of small (V1-V2) corn stands with this product (Table 1). As the results in Table 1 indicate, taller and more mature (V4-V5) corn stands will be harder to control with the 6 or even 8 fluid ounce rate of Select Max.
Although Select Max is probably the cheapest option for eliminating poor stands of Roundup Ready corn, some growers are just not willing to wait six days before replanting corn back into their fields. Another option that allows for immediate corn replanting is Gramoxone plus Sencor or Gramoxone plus Linex (Tables 1 and 2). As shown in the tables below, these combinations can provide very good control of corn as well.
Tables 1 and 2 also show the inconsistency in corn control with a single application of either Liberty or Gramoxone. This is due to the contact nature of both of these herbicides and the likelihood that the growing point remained below ground at the time of these applications. Although Liberty or Ignite (the new formulation of Liberty) is certainly a better option than Gramoxone as long as the initial corn isn't a LibertyLink hybrid or hybrid stacked with LibertyLink, I still would not recommend applications of Ignite alone for this purpose. I believe you will obtain much more consistent control by combining Ignite with Linex or Sencor.
|Table 1. Influence of Select Max and comparison herbicide treatments on corn control at two application timings (Columbia, MO - 2007)|
|Treatments||Rate||V1-V2 Corn at Application||V4-V5 Corn at Application|
|Select Max||4 fl ozs/A||100||59|
|6 fl ozs/A
|Select Max||8 fl ozs/A||100||71|
|Liberty||34 fl ozs/A||100||88|
|* Indicates control of the initial corn stand 60 days after planting.|
|Table 2. Influence of Gramoxone combinations on corn control at two application timings (Columbia, MO - 2006)|
|Treatments||Rate||1-3 Inch Corn at Application||4-6 Inch Corn at Application|
|Gramoxone Max||1.5 pts/A||35||16|
|Liberty||32 fl ozs/A||2||90|
|* Indicates control of the initial corn stand 50 days after planting.|
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REVISED: March 6, 2012