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Act now or lose more yield, income to weeds, say agronomic experts

Article cover photo
Staying ahead of herbicide resistance generated plenty of conversation at the WMT Ag Expo and Crop Fair, Jan. 10 in Cedar Rapids. Iowa Soybean Association Sr. Director of Research Ed Anderson (right) stressed an integrated approach to managing weeds, including using pre- and post-emergence products, over-the-top biotech system solutions and cover crops. (Photo: Aaron Putze, APR)

By Aaron Putze, APR

(Editor’s note: Interested in learning more about herbicide resistance and improving weed management? The Iowa Soybean Association Farmer Research Conference to be held Feb. 6-7 in Des Moines will feature several presentations on the issues including a keynote by Kevin Bradley of the University of Missouri. Learn more and register now at

First in a three-part Series

Herbicide resistant weeds are appearing on more row crop acres in Iowa, reducing soybean yields and income potential for farmers already financially stressed by sinking commodity prices.

While eliminating resistance is impossible, experts agree the adoption of more effective management practices is critical to keep farmers one step ahead in the annual race against weed infestations and lost productivity.

The frank assessment received unanimous agreement during a panel discussion involving agronomists, scientists and farmers held January 10 at WMT Radio’s annual Ag Expo and Crop Fair in Cedar Rapids.

But opinions differed on the most effective techniques to win the escalating war against weeds.

Moderated by farm broadcaster Andy Petersen, the panel discussion challenged conventional wisdom about the scope of the problem and how best to combat it.

In this first of a three-part series, Iowa State University Extension Weed Scientist Bob Hartzler, Iowa Soybean Association Sr. Director of Research Ed Anderson and Mike Weber, Sr. Tech Services Representative for Bayer Crop Sciences, offer their perspectives of the proliferation of herbicide resistance, management resources and production techniques to mitigate yield and quality losses.

Bob Hartzler

I’ve been involved with the issue since 1992 and for the most part, we’ve stayed ahead of the problem. But the speed at which resistance is evolving is concerning. We still have tools that allow us to manage the situation but I’m concerned about continuing to use these tools and going down the path that we have in the past. We’re at a real turning point in which we must carefully evaluate our approach to weed management.

We need to focus on the biology of the weed. Glyphosate, likely the best herbicide I’ll see in my lifetime, allowed us to treat every weed the same. But it got us away from properly managing the weed seed bank. Obviously, we must protect yield performance but we can’t allow the weed seed bank to get out of control. When I first came to Iowa, I was surprised at how clean the fields were. It was because of good management and good soils. We can no longer accept those late season escapes of waterhemp. Those escapes build the seed bank which makes getting effective control the next year that much more difficult, allowing for resistant weed populations to gain a foothold. Each weed has different characteristics and the optimum management technique is unique for each one.

It’s true that we have a lot of options and products and I’ve given up trying to remember all of the trade names. While there are a lot of options, upon closer look, there are just a few in terms of what the weed sees. It doesn’t matter so much the product, they basically see the same relatively small set of chemistries but under different brands. You have several products to choose from, but there’s a limited number of herbicide groups. Keep records and look at how the product is killing the weed and know the herbicide group number. At the end of the year, evaluate how that herbicide worked. If the field wasn’t clean, find out why. Did it rain after application? Were the weeds too large when you sprayed? Then adjust.

The next silver bullet is not sitting on the shelf awaiting EPA’s approval. That’s why management is so important. We must protect the products we have because we’ll be relying on them for the next 10-15 years.

Ed Anderson

Herbicide resistance is real and can become insurmountable if we permit it to get out of control. We don’t want to have happen in Iowa what has occurred in southern states.

The ISA promotes an integrated approach to weed management. That includes maintaining a productive dialogue on this topic with everyone involved – farmers, agronomists, seed and chemical companies, academia and landowners. Successfully managing weeds will require scrutinizing all old and new technology and retaining as many tools in the tool box as we can.

I’m a strong proponent of this integrated approach. Leverage people in the industry, farm associations and at Iowa State University and put them to work on your farm. Also know that Iowa has taken a very proactive approach to managing herbicide resistance. The Iowa Pest Resistance Management Program co-led by ISU and the Iowa Department of Agriculture is one example. It’s been working for more than two years to integrate management strategies involving the many partners and stakeholders involved in this important issue.

I’m not very comfortable with more tillage to manage weeds, particularly when we’re trying to find ways to reduce it. I understand there are instances where it needs to be done but I’d prefer not to go back to running field cultivators over every acre. Let’s look at pre- and post-emergence products and over-the-top biotech system solutions. Also, consider the use of cover crops. They can provide a good defense against weeds, disease and pests. Look at row spacing and plant populations and select hybrids and traits that can grow fast, creating a canopy to mitigate weed growth.

I also encourage you to go to meetings that will help you understand the many options available in the short term while also seeing what’s on the horizon. ISA works to build private and public partnerships and while there may be new sites and modes of action coming along, we can rest assured that there are other considerations to keep in mind when managing weeds. Finally, be excited because there’s an incredible amount of cool research going on in soybeans and corn that will help you be more productive and profitable.

Mike Weber

Weed resistance is a hot topic. I covered the state of Missouri for 15-16 years up until about 2014. We focus on waterhemp and palmer while in eastern Iowa, giant ragweed and marestail are creating management issues. Starting with clean fields is getting to be a challenge given that most soybeans are grown with minimum or no till. Two topics I stress are mechanical and cultural.

Think about row spacing, mechanical tillage when we can, variety selection that will close the rows much quicker – these are management tools that farmers will need to consider moving forward as weed resistance continues to outpace herbicide technology.

Up until about five years ago, it was really all about post application. This approach is going to run through chemistry quickly. One of the biggest thing for producers is thinking back to the 1970s and 1980s; then it was all about residual herbicides and controlling your weed spectrum. It was all about residuals and maximizing the herbicides we had available. Waterhemp seed does best when it’s shallow. The challenge is holding residual herbicides high enough in the soil profile to prevent those shallow seeds from germinating. That can be difficult to do so these residuals can control weeds. It’s nice to have two effective sites of action. That’s our best weapon. You almost need to mix one of the oldies and goodies with the new technology. Sencor is a great herbicide and economic for foliar management of marestail.

Then there’s the cultural aspects of weed management. One example is row spacing. At one time, we used to have a lot of narrow row soybeans which helped suppress weeds. Then a lot of folks went to 30-inch rows so they could reduce cost and complexity by standardizing equipment and because white mold is sometimes a problem in narrow rows. Typically, it’s only one out of three or four years when white mold really emerges. Thanks to new options, row spacing isn’t the best way to control white mold. It’s variety selection. There are also fungicide options for white mold.

Many farmers are reluctant to go back to narrow rows because it takes work and costs money. You actually have to manage your fields. Therefore, like it or not, working with row crops is going to get more difficult moving forward. Chemical use is also going to get more complex. As we speak, we’re right there with herbicide resistant weeds. There are some new options out there but they will likely never get approval from the EPA. Managing the tools we have available right now is critical.

Time management is critical. There are many times we do meetings and rarely see farmers in attendance. As growers, you must absorb as much information as possible. When you learn, you can develop management ideas and then take them to your retailers and tell them what you have in mind instead of going to your retailer and opening your checkbook and asking them what their plan is. The time to make your plan is in the fall and early winter on the weed species that you have on your farm and what you can implement to control them.

There are some producers nervous with Liberty Link soybeans. I get it. It’s critical to communicate with your neighbors. Bayer encourages farmers to talk to each other. Sooner or later there will be some holes in glyphosate so let’s make the best use of it now. And that begins with stewardship, overcoming the mechanical and social hurdles and communicating with your neighbors.

Editor’s note: Part two of the three-part series to published in the next edition of ISA eWeekly will feature the observations of: Tony Pleggenkuhl, regional sales agronomist, Stine Seeds; John Cantwell, farmer and weed management technology development representative, Monsanto; and Kent Bennis, marketing specialist and farmer, Dow AgroSciences.

For media inquiries, please contact Katie Johnson, ISA Public Relations Manager at or Aaron Putze, ISA Communications Director at

For permission to republish articles or to request high-res photos contact Aaron Putze at Iowa Soybean Association | 1255 SW Prairie Trail Pkwy | Ankeny | IA | 50023 | US

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