It is approved and proven and may be a thing of the past

Written by Dr. Jason Weirich on .

Atrazine has been in the news lately. Given that the chemistry, ap­proved long ago, is under regulatory review yet again, I think it’s worth going over just how it works and why it is important.

Let’s start off by taking a look at the mode of action of atrazine. Atrazine is a photosynthetic inhib­itor—more specifically a Photosys­tem II inhibitor. As many as half of available herbicides have a mode of action that involves interaction with some component in the energy transfer chain of Photosystem II. In early plant science classes we learned that the transfer of elec­trons from PSII to PSI is essential for the production of photosyn­thetic energy. By interrupting the photosynthetic pathway, the plant becomes unable to fix CO2, which results in its inability to produce the nutrients the plant needs to survive. The mode of action is unique and efficient.

You may wonder why I launched this column with a chemistry lesson. I did it as an insight into the specter of weed resistance. You are well aware of weed resistance and the struggles it can bring to your operation. In previous columns, you’ve seen me write about herbi­cide modes of action and the fact that effective modes of action make for effective weed control programs. In fact, you’ve probably grown tired of me writing about it. But, resis­tance is something to think about as we wait to see if atrazine stays on the market. If we lose atrazine, we will lose one of those effective modes of action—one of the critical tools in our tool box.

If EPA pulls the registration for atrazine, there will be significant agronomic ramifications. In the short term, you can expect to see an increase in the density of broadleaf weeds—not only in cornfields but subsequent soybeans as well. You know that weed density is greatly influenced by the previous year’s production practices. There is al­ways someone in the neighborhood with a persistent weed problem, right? It’s that grower who can’t quite get ahead of a growing weed population. The weed seed bank on his place just increases every year.

Another perspective is all those prevented-planting acres from the summer of 2015. The weeds that went uncontrolled will cause trouble for several years to come. The point is, the weed seed bank in your soil is something you have to manage with the tools available on the market. We’re deep into a period with no real new chemistries headed for the market. We can’t afford to lose any herbicide tools that we have.

In fact, we haven’t had a new mode of action introduced since the 1990s, and that release was HPPD herbicides. The HPPDs con­sist of herbicides such as Callisto, Balance, Impact/Armezon, and premixes such as Lexar, Corvus, Capreno and Resicore, just to name a few. Think about your weed con­trol program. Did it include any of these? Quite a few premixes on the market today contain atrazine. For example, Halex GT, BiCep II Mag, FulTime NXT, Keystone NXT and plenty of others. If the label allows, applications of atrazine tank mixed with HPPD herbicides deliver a couple of benefits. First, when atrazine is used in combination or rotation with other products that have different modes of action, it lowers the risk of weeds developing resistance to those products. For example, weed resistance has developed in 10 ALS inhibitor herbicides used in corn. On average, 65 percent of acres treated with these herbicides also receive atrazine either in a tank mix or sequentially. The atrazine is used to slow the spread weeds resistant to the ALS class of herbicides.

Atrazine is also a useful rotational mode of action to delay the spread­ing problem of glyphosate resis­tance. For particularly hard-to-con­trol weeds like common cocklebur, Palmer amaranth, waterhemp and wild sunflower already known to be resistant to other herbicides, atra­zine is the only product which can be applied either pre- or postemer­gence that provides effective control.

Second, atrazine in combina­tion with Callisto can significantly improve weed management and has demonstrated better efficacy when these herbicides are applied postemergence for control of water­hemp, lambsquarter, giant rag­weed and Palmer amaranth. With the combination of atrazine and Callisto, there is improved control of weeds resistant to Photosystem II and ALS-inhibiting herbicides. In Missouri and other parts of the MFA trade territory, there are a lot of weeds that fit into that category. In general HPPD herbicides alone do not have exceptional levels of efficacy on many agronomically im­portant weeds without the inclusion of atrazine.

With the recent reports about HPPD-resistant waterhemp from University of Missouri Weed Scientist Dr. Kevin Bradley, it seems inevitable that HPPD resistance will increase unless atrazine is available to mitigate resistant populations. Thus, atrazine is a must-have tank mix companion to HPPD herbicides, especially because a high percentage of corn acres in Missouri receive an HPPD herbicide.

Another implication comes to mind concerning the tools we need for weed control: no-till or conservation tillage. Without the herbicides that we have today, no-till wouldn’t be an option. Weed control technology affords us the ability to use herbicides instead of cultivation for many years. And that saves soil. Keeping soil in fields is important not only from an environmental standpoint but for long term yield— and to your bottom line. When you lose topsoil, you lose nutrients and productivity. Without atrazine, no-till operations will face immense challenges, and fewer acres will enter into no-till production.

Perhaps more sobering than the agronomic challenges that would arise if atrazine is removed from the market are the bottom-line ramifications for crop production. Recent studies show that the loss of atrazine could cost an average corn grower 6.4 to 7.7 bushels per acre. If corn is $3.84 per bushel, that would cost the grower $24.50 to $29.50 per acre. I think this is a conservative estimate—and it doesn’t include the additional costs to the grower for controlling escaped weeds through additional herbicides, additional applications or tillage passes.

You’ll need more of three things if atrazine is pulled from the mar­ket: management effort, money and time. Here is why. Over the past couple of years, I have encouraged the use of overlapping residuals. This management technique is im­portant in soybeans and becoming more important in corn produc­tion. I believe that this will be the only method for effective weed control going forward. I would love to tell you that a Silver Bullet is coming down the product pipeline, but that is not the case. Atrazine is a crucial tool for managing weeds. In fact, it is often the building block for a sound management plan against weed resistance. Losing it will be costly.

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