An article in last week’s C.O.R.N. covered in brief the options for killing a first corn stand to make room for a replant, and referenced an article from the University of Illinois. We received a number of questions and comments after that relative to the use of Gramoxone + metribuzin to kill corn, some to the effect that we were too hard on it. Our comment last week was that Gramoxone or Gramoxone + metribuzin was not as consistently effective as Select Max, which is accurate. We also stated that the Gramoxone treatments could be variable in effectiveness, and we lumped it in with glufosinate (Liberty) in this regard, which is somewhat unfair to Gramoxone based on a review of data from our past studies. And Liberty will not have any activity on corn with the LibertyLink trait of course.
The summarized results of several OSU and Purdue studies follow. The V stage indicates the corn stage of growth at the time of herbicide application. Specific treatments and rates varied among studies. The “%” indicates final level of control at about 3 weeks after application.
Study 1 - 2006
Liberty (32 oz): V1-2 – 98% ; V4-5 – 73% SR
Gramoxone Inteon (18 oz): V1-2 – 96% SR; V4-5 – 83% SR
Gramoxone Inteon (36 oz): V1-2 – 100% SR; V4-5 – 97% SR
Gramoxone Inteon + metribuzin 75DF (18 + 3 oz): V1-2 – 97% SR; V4-5 – 96% SR
Study 2 – 2009 – all applied at V2
Liberty (22 oz): 7%
Select Max (8 oz): 100%
Gramoxone Inteon + metribuzin (40 + 3 oz): 93%
Gramoxone Inteon + atrazine 4L (40 + 16 oz): 88%
Study 3: 2009/2010 - OSU/Purdue Study – all applied at V3-4
Select Max (8 oz): 100%
Gramoxone Inteon (40 oz): 65% (Purdue only)
Gramoxone Inteon + metribuzin (40 + 3 oz): 97% or greater
Liberty (22 oz): 35% (OSU), 70% (Purdue)
Liberty (22 oz) followed by POST Liberty (22 oz): 97% or greater
For reference, both Gramoxone Inteon and Gramoxone SL (the current formulation) contain 2 lbs of paraquat per gallon. We suggest the following as the most effective approach to killing a stand of corn with Gramoxone: Gramoxone SL – at least 3 pt/A (or at least 2 pint of a 3L paraquat formulation); metribuzin 75DF – 3 to 5 oz /A (or equivalent amount of a 4L); plus a petroleum-based crop oil concentrate - 1 qt/A preferable but at least 1 gallon/100 gallons of mix. Apply in spray volume of at least 15 GPA with appropriate nozzles to maximize contact herbicide coverage. Keep in mind that this approach can have some variability in control, even given how good some of the data above appear, and not all weed scientists across the region are comfortable recommending it. Select Max or tillage will be more consistently effective, but there is a 6-day wait to plant corn after use of Select Max. Use of Gramoxone + metribuzin does allow immediate planting, or even planting first followed by herbicide application sometime prior to corn emergence.