A lot has been written and said over the last couple of months about management strategies for glyphosate-resistant (GR) Palmer amaranth. Of course, the one strategy many folks talk about is rotating out of cotton or soybeans into corn.
On the surface this would seem to be a solid recommendation to help reduce the Palmer amaranth soil seed bank. In fact, I made that very recommendation to some cotton growers in 2008 for a couple of fields that were heavily infested with GR Palmer amaranth that year.
However, in late August 2009, I stepped into a couple of these rotated fields and was very disappointed to find that as the corn was drying down, a large, late flush of GR Palmer amaranth was thriving. Much of this Palmer was as tall as I am, with a few plants even taller than the corn. The Palmer had flowered and was in the process of producing a large seed load.
I may not be the sharpest tool in the shed, but right then and there, it became apparent to me that rotating to a Roundup Ready corn hybrid and spraying glyphosate plus 2 quarts of atrazine was not going to cut it as a solid way to reduce the Palmer soil seed bank.
If not glyphosate plus atrazine, then what should be used in corn to combat GR Palmer amaranth? The traditional corn pre-mixes hold the best option, particularly if they are not used in a traditional fashion. What I mean by that is if the pre-mix is in part or totally applied postemergence instead of all pre-emergence, it has the best chance to curtail late GR Palmer flushes.
Once the herbicide that provides soil residual has been applied, the clock starts ticking on how long it will last and it is unknown when the clock will run out. Many factors come into play on how long a residual herbicide will last, including the rate of the herbicide applied, the soil type and most importantly, how much rain falls after application.
Therefore, if you can apply a residual herbicide as close to the time as possible when you need it, the more consistent the control will be. In our research we have put this strategy to the test and looked at most of the atrazine pre-mixes. We compared the given labeled rate for the soil type all applied pre- or in split-applications.
The split-applications consisted of one-third of the labeled rate being applied pre-emergence and the other two-thirds applied to V1 to V3 corn. We found we received more consistent late-season pigweed control with the split applications.
So which pre-mix should you use? Confusion surrounding which corn pre-mix to use is nothing new. Marshal McGlamery, a famous Extension weed scientist who retired a few years back, coined the term “can ’em and confuse ’em” to describe the pre-mixes decades ago.
The reason there have been so may pre-mixes marketed in corn is due to the simple fact that all herbicides work better with atrazine. Typically, the first part of a corn pre-mix is atrazine, which provides contact and residual control of most broadleaf weeds and some control of grass weeds. The second component of a herbicide pre-mix for corn enhances the residual activity on the grass and small-seeded broadleaf weeds.
Historically, the most popular corn pre-mix was Bicep II Magnum (Dual II Magnum + atrazine). But others like Harness Xtra and Degree Xtra (acetochlor + atrazine) have also been widely used. In our work, all these herbicides provided about the same level of residual pigweed control. In other words, it wasn’t so much which pre-mix we used, but how we used it that provided the most consistent late-season pigweed control.
Some more recently developed corn pre-mixes contain one of the newer bleaching-type herbicides (Callisto, Impact and Laudis). These herbicides have a lot of utility in corn.