By Kevin Bradley
I have already received a few calls about herbicide options for killing out poor stands of Roundup Ready and/or stacked Roundup Ready/Liberty Link corn, and I expect there will be more whenever things dry out. We conducted a few trials on this several years ago, and Dr. Larry Steckel has also published some data in the weed science literature from two years of research he conducted in Tennessee. I will attempt to briefly summarize the results from everything I've seen published or done ourselves in three points below.
First, Select Max (clethodim) has a label for this use, and it works. The label specifies a rate of 6 fluid ounces of Select Max per acre, and that you must replant corn in these areas no sooner than 6 days after application. In our research with Select Max we have observed excellent control of small (V1-V3, 4-8 inch) corn stands with this product. If the corn is taller and more mature (V4-V5, >8 inches), you more than likely will not get complete control of your failed corn stand.
A second option that allows for immediate corn replanting is paraquat (Gramoxone, etc.) plus a group 5 or 7 PSII-inhbitor like metribuzin, atrazine, or linuron. Paraquat should be applied at a rate of at least 2.5 pts/A, and it will not provide adequate control of the failed corn stand if applied alone. The addition of a metribuzin product (3-4 ozs/A), Linex (1 pt/A), or atrazine (1 pt/A) to the paraquat will increase the control of the failed corn stand substantially.
Third, even if your initial corn was not Liberty Link, we have not seen very good control of corn out of Liberty (glufosinate), or even Liberty plus metribuzin combinations. Many corn varieties have tolerance to glufosinate even if it isn't listed as Liberty Link on the tag, so for all these reasons I would move away from Liberty and stick with either of the other options described above.
Fields that have been flooded and have poor stands may need to be replanted. There are several herbicide options for killing out the failed corn stand and planting back into these areas.