Judging by phone calls over the last week it seems there is a good amount of uncertainty about which weed control program to go with in corn. This is understandable with all the new corn acres this year and so many farmers planting a Roundup Ready hybrid for the first time.
The biggest confusion seems to be about all the premixes being marketed and how they best fit in a conventional or Roundup Ready corn program. Confusion surrounding the corn premixes is nothing new.
Marshal McGlamery, a well-known Extension weed scientist who retired recently, coined the term “can 'em and confuse 'em” to describe the corn premix phenomena decades ago.
So many premixes for corn have been marketed due to the simple fact that all herbicides work better with atrazine.
Typically the first part of a corn premix contains atrazine, which provides contact and residual control of most broadleaf weeds and some control of grassy weeds. The second part of a typical herbicide premix for corn enhances the residual control of grass weeds and small-seeded broadleaves.
The standard corn premix used for many years is Bicep (Dual II Magnum + atrazine). Others include Harness Xtra, Degree Xtra and Breakfree ATZ (acetochlor + atrazine) and Steadfast ATZ (Accent + Resolve + atrazine), one that has been very popular in Tennessee over the past several years.
A new premix, Lexar (Dual II Magnum + Callisto + atrazine), has three different modes of action to control pigweed. Lexar is a good example of the advantage these premixes provide — two and sometimes three modes of action which should help combat weed resistance.
In general, all these premixes have a few small differences in their strengths and weaknesses, but in the end they will all provide comparable weed control.
Whether the corn being managed is Roundup Ready or not, the best overall weed control will most often be achieved with some kind of two-pass program. One reason is being able to apply more atrazine in two passes (2.5 pounds per acre) compared to all in one application (2 pounds per acre).
A good approach is to either use Gramoxone Inteon or glyphosate mixed with 16 to 32 ounces per acre of atrazine for burndown. Then follow early postemergence (up to 12-inch corn) with glyphosate plus atrazine or (even better) with one of the premixes mentioned above.
Many folks do not realize that herbicides like Dual II Magnum and Harness can be applied postemergence to corn. They can, and I often recommend that method because those residuals are there when most needed.
The premixes mentioned above will mix very well with glyphosate postemergence in corn.
In non-Roundup Ready corn, you might need to add dicamba, Aim or Callisto to the post corn application to control any emerged Palmer pigweed.