1. Current Status of Herbicide Resistance in Weeds
Lesson 1
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Welcome to the herbicide-resistant weeds training lessons, provided by the Weed Science Society of America. Herbicide resistance education and training are critical for advancing the adoption of diverse weed management programs to delay and mitigate the evolution of herbicide-resistant weeds. These training lessons were developed by a team of weed scientists in an effort to provide to you, the agronomist, consultant, retailer or distributor, and interested grower, the most current information on herbicide resistance in weeds.
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2. Objectives By the end of this lesson, you will:
Understand the need for managing herbicide resistance in weeds.
Know the current status of herbicide resistance in weeds.
Above: Seedling stage of kochia, a weed that is known to be resistant to several herbicides.
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This lesson presents an introduction to herbicide resistance in weeds. By the end of this lesson, you will <mouse click> understand the need for managing herbicide resistance, and <mouse click> know the current status of herbicide resistance in weeds within the USA and around the world.
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3. Introduction
Evolution is a term used by weed scientists to refer to the process of herbicide-resistant weeds becoming dominant in a field over time. This process is a result of a herbicide selecting individuals that occur naturally and are not controlled by the herbicide.
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Due to the extensive use of herbicides to control weeds, populations of weeds with resistance to one or more herbicides continue to increase within the USA. The entire agricultural community must make an effort to understand herbicide resistance, learn to identify it early, and implement management tactics to reduce the evolution of herbicide-resistant weeds.
Above: Adult stage of waterhemp, a weed that is known to be resistant to several herbicides.
Photo credit: Aaron Hager, University of Illinois.
Herbicides are effective and economical tools for weed management. Did you know that nearly 90% of the cropland in the USA has received an application of a herbicide every year for the last 30 years? <mouse click> Due to this extensive herbicide use, populations of weeds with resistance to one or more herbicides continue to increase within the USA.
To combat the further selection of herbicide-resistant weeds, <mouse click> the entire agricultural community must make an effort to understand herbicide resistance, learn to identify it early, and implement management tactics to delay and mitigate the evolution of herbicide-resistant weeds. Proactive management practices that are designed to prevent or slow the evolution of herbicide-resistant weeds have significant advantages over waiting until herbicide resistance is present in the field and then implementing reactive management strategies.
To learn more about evolution in weeds, please click on the For-More-Information button.
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4. Why Care About Herbicide Resistance?
Require changes in weed and crop management practices
Increase the cost of weed management
Reduce viable herbicide options
Loss of yield potential and income
Photo credit (far left): Cotton planted into ultra-narrow rows in rye residue.
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As you will learn in the next few slides, herbicide-resistant weeds are found in nearly every environment that is involved with industrialized agricultural production. As a result, the question is not so much “will I have to deal with herbicide-resistant weeds,” but rather, when. Herbicide-resistant weeds can dramatically affect crop production. Their presence in a field <mouse click> may require changes in weed and crop management practices; <mouse click> may increase the cost of weed management; <mouse click> will reduce viable herbicide options; and <mouse click> may lead to losses of yield potential and income.
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Herbicide

5. Current Status of Resistance
No herbicides with new mechanisms of action are in advanced development trials. The last new mechanism of action was introduced over 20 years ago; therefore, we will have to rely on currently available herbicides for the foreseeable future.
The number of weeds with herbicide resistance continues to increase within the United States and around the world.
The number of weeds with herbicide resistance to more than one herbicide continues to increase.
we will have to rely on currently available herbicides for the foreseeable future.
Herbicides are weed management resources that can and must be preserved. Currently, no herbicides with new mechanisms of action are in advanced research and development trials, and the last mechanism of action was introduced over 20 years ago. Therefore, <mouse click> we will have to rely on currently available herbicides for the foreseeable future.
Additionally, the evolution of herbicide-resistant weeds is currently on the upswing. <mouse click> The number of weeds with herbicide resistance continues to increase within the USA and around the world. <mouse click> The number of weeds with herbicide resistance to more than one herbicide continues to increase.
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6. Current Status of Resistance
The number of acres infested with weeds that are resistant to one or more herbicides continues to increase.
Resistance to glyphosate, today’s most widely used herbicide, continues to increase.
Herbicide resistance changes how a herbicide is used by a farmer and the company. Resistance has not led to the total loss of any one herbicide.
The number of acres infested with weeds that are resistant to one or more herbicides continues to increase. <mouse click> Resistance to glyphosate, today’s most widely used herbicide, continues to increase. <mouse click> Herbicide resistance changes how a herbicide is used by a farmer and the company. Resistance, however, has not led to the total loss of any one herbicide, because most herbicides control many different weeds.
<mouse click> Strategies that successfully delay and mitigate the evolution of herbicide-resistant weeds must be in place to preserve and sustain herbicides as resources in weed management.
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Strategies that successfully delay and mitigate the evolution of herbicide-resistant weeds must be in place to preserve and sustain herbicides as resources in weed management. 6 6

7. Current Status of Resistance
The legend identifies herbicides by both mechanism of action and class. Mechanism of action (MOA) is the biochemical site within a plant with which a herbicide directly interacts. Herbicide class refers to herbicide chemistry.
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ALS-inhibitors
Triazines
ACCase-inhibitors
As of March 2011, reports worldwide have confirmed herbicide resistance in more than 196 weed species.
This figure shows the increases in the number of herbicide-resistant weed species by herbicide class or mechanism of action over time. Herbicide resistance has been reported most often for the <mouse click> ALS-inhibitors (for example, Classic herbicide), <mouse click> the triazines (for example, atrazine), and <mouse click> the ACCase-inhibitors (for example, Fusilade and Select herbicides). These herbicide classes or mechanisms of action represent some of the most effective and frequently used herbicides. With the unprecedented adoption of glyphosate-resistant crops, the use of glyphosate, also an effective herbicide, has grown, leading to <mouse click> an increase in the number of weeds resistant to the glycine family (bottom blue).
To learn more about mechanism of action and herbicide class, please click on the For-More-Information button.
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Source: Ian Heap, March 2011
Glycines
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8. Current Status of Resistance
Mechanism of action (MOA) is the biochemical site within a plant with which a herbicide directly interacts. Herbicides with different MOAs are identified by different group numbers. For example, 2,4-D belongs to group 4 and glyphosate belongs to group 9. Herbicides are assigned into groups based on a WSSA approved system of categorizing based on mechanism of action.
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Herbicide
Mechanism of Action
Number of Resistant Species
Herbicide Group Number
ACCase-inhibitors (ex. Select herbicide) 40 1
ALS-inhibitors (ex. Classic herbicide)
108 2
Photosystem II-inhibitors (ex. atrazine) 68 5
Glycines (ex. glyphosate) 21 9
The number of herbicide-resistant weed species is always increasing. For the most up-to-date information, please see the website below.
Actual numbers for weed species that are resistant to four common herbicide mechanisms of action are shown in this table. Please take a moment to consider how frequently you encounter or use herbicide products from these mechanisms of action. Additional information on herbicide-resistant weeds can be obtained from the International Survey of Herbicide-Resistant Weeds, available online at The address is shown at the bottom of this slide.
To learn more about herbicide mechanism of action, click on the For-More-Information button.
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Source: Ian Heap, March 2011

9. Chronological Increase in Weeds with Resistance to Multiple Mechanisms of Action (MOA)
Mechanism of action (MOA) is the biochemical site within a plant with which a herbicide directly interacts. Herbicides with different MOAs are identified by different group numbers. For example, 2,4-D belongs to group 4 and glyphosate belongs to group 9. Herbicides are assigned into groups based on a WSSA approved system of categorizing based on mechanism of action.
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MOA
MOA
MOA
The number of weed species with resistance to more than one herbicide mechanism of action has increased drastically since Currently, 50 weed species with multiple forms of herbicide resistance have been confirmed.
MOA
6 MOA
Examples with more than one form of resistance
2 Mechanisms of Action
Giant ragweed
Palmer amaranth
Common ragweed
3 Mechanisms of Action
Waterhemp
The number of weed species with resistance to more than one herbicide mechanism of action has drastically increased since Currently, 50 weed species with multiple forms of herbicide resistance have been confirmed. The number of weed species with multiple forms of herbicide resistance is increasing as growers shift to other herbicide products without implementing complementary management strategies.
<mouse click> Since 1990, giant ragweed, Palmer amaranth, and common ragweed have been reported resistant to herbicides with two different mechanisms of action. Some populations of waterhemp are known to be resistant to three different herbicide mechanisms of action, and resistance to even greater numbers of herbicide mechanisms of action, up to six, has been reported in other species.
To learn more about mechanism of action, click on the For-More-Information button.
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Source: Ian Heap, March 2011

10. Global Distribution of Herbicide Resistance
This figure shows the number of herbicide-resistant biotypes around the world. Biotypes are unique cases of herbicide-resistance in weeds separated by geography. Because of this definition, the number reported by biotype will always be greater than the number reported by weed species.
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Source: Ian Heap, March 2011
The growing problem of herbicide-resistant weeds is not unique to the USA. This figure shows the reports of herbicide-resistant biotypes around the world. One biotype in this example represents one confirmed report of a herbicide-resistant weed by country. Herbicide-resistant weeds are reported in Australia, Canada, Europe, and South America, where growers, like those in the USA, have readily adopted herbicide technology. These growers also rely more heavily on herbicides for weed control compared with growers in other countries. A longer use over time and nearly sole reliance on herbicides for weed management have contributed to the evolution of herbicide-resistant weeds. There are also greater numbers of weed scientists studying herbicide resistance in these countries, and therefore, the likelihood of reporting resistance may be greater.
To learn more about biotypes, please click on the For-More-Information button.
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11. Conclusions
Herbicide-resistant weeds will require changes in weed and crop management tactics, increase the cost of weed management, reduce viable herbicide choices, and reduce yield potential and income.
Proactive herbicide resistance management strategies must be implemented before new populations of herbicide-resistant weeds evolve.
The number of weed species with one or more forms of herbicide resistance continues to increase.
This concludes the lesson “Current Status of Herbicide Resistance in Weeds.” The three most important take-home messages from this lesson are: herbicide-resistant weeds will require changes in weed and crop management, can increase the cost of weed management, will reduce viable herbicide choices, and can reduce yield and income potential ; <mouse click> the number of weed species with one or more forms of herbicide resistance continues to increase; and <mouse click> herbicide-resistant weeds are found throughout the world, but are concentrated mostly in agricultural areas that rely heavily on herbicides for weed control. <mouse click> All of these factors point to the urgent need to implement proactive herbicide resistance management strategies before new populations of herbicide-resistant weeds evolve.
Four additional training lessons follow. Topics to be covered in these lessons include: how herbicides work; what is herbicide resistance; scouting and confirming herbicide resistance; and how to manage herbicide resistance. You may continue with these lessons now, or save your work and return at a later time.
Herbicide-resistant weeds are found throughout the world, but are concentrated mostly in agricultural areas that rely heavily on herbicides for weed control.

12. Credits:
This lesson was developed by a WSSA sub-committee and reviewed by the WSSA Board of Directors and other WSSA members before being released. The sub-committee was composed of the following individuals.
Wes Everman, PhD (North Carolina State University)
Les Glasgow, PhD (Syngenta)
Lynn Ingegneri, PhD (Consultant)
Jill Schroeder, PhD (New Mexico State University)
David Shaw, PhD (Mississippi State University)
John Soteres, PhD (Monsanto Company) (Sub-committee chairman)
Jeff Stachler, PhD (North Dakota State University and University of Minnesota)
François Tardif, PhD (University of Guelph)
Financial support for this was provided by Global HRAC, North America HRAC, and WSSA.
Our thanks are extended to the National Corn Growers Association for allowing us to use training materials posted on their website as the starting point for these training lessons.