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Managing Weed Resistance
weeds among crops
NEW TOOLS: Weed resistance to glyphosate is leading many farmers to use new combinations of both preemerge and postemerge herbicides this year.

Are we losing the war on resistant weeds?

Iowa is home to 17 different species of herbicide-tolerant weeds, with the most troublesome probably waterhemp, giant ragweed and marestail.

We are “celebrating” a dubious anniversary as we enter the 2017 cropping season: In 1957 growers and researchers ran across the first known herbicide-resistant weeds.

While they aren’t weeds that we fight here, they were a spreading dayflower found in Hawaiian sugarcane and a wild carrot variety found in Ontario — both showed resistance to the same herbicide group. Records show that some of these weeds were able to survive up to five times the normal dose of “synthetic auxin” (Group 4) herbicides.

I shared this bit of trivia with some fellow growers and agronomists this winter. “No kidding?!” one of them said. A couple others expressed similar thoughts. He went on to say, “If flowers and carrots could do it 60 years ago, I wonder why it took so long to get soybeans that are resistant to synthetic auxins!”

We found that pretty humorous and insightful. More on synthetic auxins and soybeans.

In the 60 years since those first herbicide-resistant weeds were found, scientists have documented around 250 more weeds around the world with resistance to one or more of 23 of the 26 known herbicide modes of action.

According to information from EPA, the U.S. has at least 155 weeds resistant to one or more herbicide modes of action. Other estimates say acres with resistant weeds have more than doubled in the U.S. since 2009; some put us over the 70 million-acre mark. Economists say herbicide-resistant weeds cost U.S. farmers $2 billion annually, a number that keeps climbing as they spread across more acres.

Resistant weeds list is growing
So that’s a look at the “big picture,” but what about closer to home? According to, Iowa is home to 17 different species of herbicide-tolerant weeds, with the most troublesome probably being waterhemp, giant ragweed and marestail. With Palmer amaranth documented in at least 49 counties across Iowa now, my bet is at some point in time, we’ll be adding that to the list.

I had a front-row seat as an ag retailer to watch waterhemp evolve resistance to some herbicides that had been providing great control, and it happened quickly, seemingly in a season or two, for some.

Triazines (Group 5) and ALS inhibitors (Group 2) were the first victims, but luckily we had enough tools to get by until Roundup Ready beans launched. We are all familiar with what has happened with glyphosate resistance. Luckily we’ve had some other solid chemistry to help, but that picture is changing quickly.

A “go-to” set of herbicides we’ve used to fight glyphosate-resistant waterhemp in soybeans has been the group 14 PPO inhibitor products like Aim, Cadet, Cobra, Flexstar, Sharpen, Ultra Blazer, Valor, Authority and a slug of others.

A couple of years ago, the University of Illinois Plant Clinic started screening waterhemp for herbicide resistance to glyphosate (Group 9) and PPO inhibitors, as reports of escapes from both herbicide groups were an increasing problem in Illinois.

The clinic received samples from 10 states across the Midwest in 2016. In the 378 samples from Illinois, 48% were resistant to both glyphosate and PPO inhibitors. Out of the 87 samples sent from Iowa, resistance to both herbicides was detected in 75% of them.

These results are probably biased to the high side on resistance to both modes of action, since this wasn’t true “random” sampling, but rather samples sent in from clients. My guess is most were taken from fields where neither Group 9 nor Group 14 products did the job on the waterhemp, and folks sent the samples in wanting to know if they had true resistance or not. Biased high or not, the message is still alarming; resistance to group 14 herbicides is out there.

Indeed, we have a problem
In a project funded by the Iowa Soybean Association, over several recent seasons our ISU weed science team analyzed more than 900 samples of weeds (primarily waterhemp) from across Iowa.

Some of what they found was not a huge surprise; essentially all the fields had herbicide resistant waterhemp. More troubling was that a significant amount of the waterhemp samples had some level of resistance to five groups of herbicides. In addition to group 2, 5, 9 and 14 resistance, some also had resistance to another group of herbicides we often turn to in corn (and soon in HPPD-tolerant soybeans) to stop waterhemp: Group 27.

To borrow a line from the Apollo 13 crew, “Houston, we have a problem.” We are getting backed into a corner in the fight against herbicide-resistant weeds, especially waterhemp.

War strategy
So how do we start fighting back against these weeds? Unfortunately, we do have a fair amount of experience. With the help of weed scientists, agronomists and growers, I’ve summarized some of the tactics we’ll implement to manage our current resistant weeds and, hopefully, slow the evolution of new ones.

• Know what weeds you are facing; field histories and scouting are valuable tools.
• Start clean with effective tillage or burndowns.
• Stay clean with full rates of effective residual herbicides.
• Use multiple, effective herbicide modes of action.
• Rotate herbicide-tolerant traits.
• Rotate crops if possible.
• Proactively manage postemerge applications, i.e., small weeds, favorable growing conditions, quality additives.
• Recognize and manage weed escapes with zero tolerance for resistant or challenging weeds.
• Clean equipment as well as you reasonably can to prevent movement of seed from resistant weeds.

For a comprehensive look at management strategies, visit the Weed Science Society of America site at or the industry coalition site at

Handle herbicides right
The recent launch of soybeans tolerant to synthetic auxins (dicamba on Xtend beans and 2,4-D on Enlist beans) expands our options when it comes to management practices like using multiple herbicide modes of action and rotating herbicide tolerant traits.

Adding these and the HPPD tolerant soybeans to our herbicide-tolerant soybean lineup, which already includes a very effective LibertyLink soybean system, will be a big help against tough weeds, especially waterhemp. If we handle them right, they’ll be excellent tools; we’ve had good results with these herbicides for decades when we put them in the position to succeed.

Handling the herbicides right includes sticking tight to the stipulations in the seed technology agreements and the herbicide labels; I doubt there has ever been a product launch under as much scrutiny as synthetic auxin beans. The unfortunate and preventable series of events surrounding dicamba-resistant crops that happened in some Southern states last summer ensure that herbicide drift and off-target injury will be closely watched.

This brings it all full circle, where we try to tie weed management, weed resistance management and herbicide-resistant trait preservation together. As mentioned previously, we have weeds with resistance to one or more of 23 of the 26 herbicide modes of action. And we haven’t had a new mode of action come to market in about 30 years. A look at the product development pipeline doesn’t give us a lot of optimism that we’ll be getting any new modes of action soon.

So, our new tools to fight weeds will likely consist of developing, and then stacking, resistance traits to current herbicides and inserting these traits into corn and soybean genetics. We will need to always read and follow label directions, and use as many diverse management practices as possible. Or we’ll fight an increasing number of resistant weeds. Knowing that 60 years ago “flowers and carrots” became resistant to the same family of herbicides that are the cornerstone of new tech launched this year will help keep us at the top of our game.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University.

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