This is essentially a reprint of last year’s article on this same subject.

In fields where the first corn planting is not viable and needs to be killed so that corn can be replanted, there are still no ideal herbicide options for control of glyphosate-resistant (Roundup Ready/Agrisure GT) corn. Current available options include the following:

1. Select Max in now labeled for control of failed stands of glyphosate-resistant corn (this could be used under a section 18 last year), and is probably the most consistently effective option. Apply Select Max at the rate of 6 oz/A with nonionic surfactant (0.25% v/v) and AMS (2.5 to 4 lbs/A) when corn is up to 12 inches tall. Do not replant corn until at least 6 days after Select Max application. The 6-day waiting period is necessary due to the short period of Select Max soil residual activity. Failure to allow 6 days between application and planting could result in injury and problems with establishment of a new corn stand.

2. The application of Liberty or a combination of Gramoxone Inteon plus metribuzin can control emerged glyphosate-resistant corn without the need for a waiting period between application and replanting. However, in university trials, control from these treatments has been more variable than with Select Max.

Ohio State University (OSU) and several other universities conducted studies in 2006 to determine the effectiveness of Liberty, Gramoxone, and Gramoxone plus metribuzin for control of glyphosate-resistant corn. The rate of metribuzin in these treatments was limited to no more than 3 oz/A, the maximum rate that can be applied preemergence to corn. The following is a summary of the results from selected studies:

• OSU – We were able to obtain near complete control of corn treated at the 3-inch stage (V1-V2) with Liberty (32 oz), Gramoxone Inteon (18 oz/A), or Gramoxone Inteon + Sencor (18 + 3 oz/A). Here and in the following statements, “near complete” indicates 97 to 100% control. We observed generally less effective control when treating 11-inch corn (V5), but still obtained near complete control with Gramoxone Inteon (36 oz/A) or Gramoxone Inteon + Sencor (18 + 3 oz/A).
• University of Illinois – the only treatment that provided over 80% control of 5-inch corn was Gramoxone plus Sencor (41 + 3 oz/A), at 82% control. Control of 3-inch corn did not exceed 65% for any treatment.
• Penn State University – Gramoxone Inteon (24 oz/A) and Gramoxone plus metribuzin (16 + 2 oz/A) provided 91 and 97% control of 6-inch corn, respectively, and 77 and 87% control of 9-inch corn.
• Purdue University – Control of 5-inch corn ranged from 80 to 84% for Gramoxone and Gramoxone plus metribuzin treatments. Gramoxone plus metribuzin and Liberty were the most effective treatments on 3-inch corn, but control was fair at best, ranging from 72 to 75%.

The variable control of glyphosate-resistant corn in university trials with Liberty, Gramoxone, and Gramoxone plus metribuzin makes it difficult for us to know exactly what to say about their use. Select Max is probably the most consistently effective of all the treatments discussed here, but requires a wait of 6 days until planting corn. Where it is not possible to wait 6 days to replant, the best option may be a mixture of Gramoxone plus metribuzin, which should generally control small corn (less than about 5 inches?). In the trial OSU conducted in 2006, Liberty effectively controlled 3-inch corn, but was much less effective at other universities. Consequently, we find it difficult to recommend Liberty to control a failed corn stand.

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