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Weed control programs that maximize corn yield
Author: Tom Barber, Extension Weed Scientist

Recent heavy rainfall has left many Arkansas corn producers feeling behind all of a sudden on corn planting.  Last I checked the temperatures could get pretty cold this weekend so leaving that seed in the bag over the next week or so is probably not a bad idea.  I did receive a couple calls this week from an unfortunate few who had planted corn prior to the flooding and judging by the pictures they will probably have to call a mulligan on those acres.  Weed control in Arkansas has become more and more difficult over the last several years with the onset of glyphosate-resistant pigweed, marestail, and ryegrass.  To make matters worse, in 2015 the weed science team at the University of Arkansas identified over 35 populations of pigweed in the state that have become resistant to PPO herbicides such as Valor and Flexstar.  Fortunately for corn producers, there are a number of herbicide alternatives to control glyphosate and PPO- resistant pigweed in corn.

Atrazine and Dual II Magnum are still key players in corn weed control and have been forever.  To date we have not identified any populations of pigweed that are resistant to either one of these herbicides in Arkansas (knock on wood).  The use of atrazine in corn and grain sorghum is crucial for postemergence Palmer pigweed management and as an agriculture industry in Arkansas we need to protect its viability moving forward for future growing seasons.  Due to atrazine’s effectiveness, corn weed control programs are not very difficult to plan out.  However, this does not give us an excuse to be lazy or complacent with corn weed control planning.  The standard practice for many growers for the last several years is to plant as much corn as possible in between spring rains and then go back and spray all emerged weeds with one application somewhere around V3-V4.  However, once the second week of April rolls around, especially in south Arkansas weeds can begin to grow rapidly and compete at a high level.  Couple that with the wet springs we have experienced and  weeds are generally too big to completely control once the postemergence applications are made.  This sets up a situation that could lead to further resistance development in many weeds, most importantly pigweed.

The most popular corn herbicide program that I hear about is Halex GT plus atrazine POST.  Coming in a close second is Roundup plus Capreno plus atrazine POST.  Both of these are traditionally applied as one shot POST programs and for the most part they do a pretty good job of cleaning up the weedy mess left as the result of not applying a herbicide at planting.  We have conducted numerous weed control trials over the last 4 years looking at multiple products, premixes, and tankmixes and how they best fit into a program.  The goal of this research was to determine if we are leaving yield on the table due to the early season weed competition.  We evaluated numerous products in this study but basically I picked the most common programs to compare a one shot program either PRE or POST versus a two shot program which includes a PRE, followed by POST.  The figure below shows corn yields following no herbicide, Lexar 96oz/A PRE, Halex GT 96oz/A plus atrazine POST or a two shot program of Dual 1.3 pt/A PRE followed by Halex GT plus atrazine POST.  These data represent six site years of replicated trials which all seem to say the same thing.  We can continue to harvest high corn yields with a one shot program that includes a PRE or POST herbicide tankmix.  However, over the duration of this study we have increased our corn yields by at least 20 bushels with a two shot program that includes a preemerge herbicide.  I would not focus too much on the herbicides used PRE or POST in this study, the most important thing is that we use an effective premerge herbicide at planting followed by a timely postemergence application by V4 or 12 in corn.  I recommend a lot of Dual and Verdict PRE because they allow us to plant soybean if the corn stand is failed.  Halex GT is applied on a lot of acres but there are other herbicide premixes will work well too as long as atrazine is in the mix.  The main point here is to consider moving to a two application program for corn weed control, especially in years with wet springs that seem to push back the planting window into the middle of April or later.  Based on these data and conversations with growers, I think it is fair to say that we are leaving several bushels on the table by relying on a one-pass herbicide program in corn.

Weed Competition slide

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