Phillip Roberts, University of Georgia, Bugwood.org
Special consideration must be taken when the corn is herbicide resistant, such as Roundup Ready and Liberty Link hybrids. As usual, the timing of applications is also important.
With a spring like this, we are starting to get questions about poor corn emergence and replanting issues. There are times when corn has to be removed from a field with the intention of replanting a corn crop. Tillage is one effective method, but it is not appropriate in no-tillage situations. Use of glyphosate is highly effective for non-Roundup Ready corn. But, the challenge is in removing Roundup Ready hybrids. There are limited herbicides to consistently kill small corn plants. Gramoxone SL, Liberty, and Select Max are three products that have shown the most activity.
Several years ago we conducted a trial that simulated volunteer Roundup Ready corn control in a replant Liberty Link corn scenario. Roundup Ready corn was planted in mid-May. On May 31, preplant herbicides were applied and Liberty Link corn was replanted about 7 days later. The 6 fl. oz of Select Max provided 96% volunteer corn control. Control with Gramoxone ranged from 76 to 99%. The addition of a photosystem II inhibitors (group 5 or 7) herbicide to Gramoxone as well as increasing the rate generally improved performance - Gramoxone plus 2 oz of metribuzin had the highest numerical value (99% control). Liberty applied POST on June 20 provided at least 91% control. Below is the data from the study.
- Select Max (6 fl oz): 96% control
- Gramoxone (24 fl oz): 76%
- Gramoxone (32 fl oz): 84%
- Gramoxone (48 fl oz): 90%
- Gramoxone (24 fl oz) + metribuzin 75WG (2 oz): 99%
- Gramoxone (24 fl oz) + Lorox (8 oz): 90%
- Gramoxone (24 fl oz) + atrazine (32 fl oz): 95%
- Liberty 280 (in-crop, 22 fl oz): 91%
- Liberty 280 (in-crop, 29 fl oz): 92%
Some recommendations include:
Select Max: up to 6 oz of Select Max with a nonionic surfactant at 0.25% v/v plus AMS at 2.5 to 4 lbs/A. Do not use a COC or MSO. Wait a minimum of 6 days from time of application until planting corn due to risk of crop injury. (Select Max will also control corn hybrids containing Roundup Ready and Liberty Link stacked traits.)
Gramoxone SL: 3 to 4.5 pt/A in combination with metribuzin (2 to 6 oz/A), Lorox (1 pt/A) or atrazine (1 lb/A). These photosystem II inhibitors (group 5) are not added to control the corn but are used to slow down the Gramoxone activity, which helps provide more consistent control. (Be aware that not all metribuzin products have a label for use in corn in Pennsylvania.)
Liberty 280: 22 to 29 oz Liberty has not been as consistent for control corn as Gramoxone. (Keep in mind, Liberty will not control hybrids that have stacked RR/LL traits.)