1. Principles of Managing Herbicide Resistance
Lesson 5
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Welcome to Lesson 5 of the herbicide-resistant weeds training lessons, provided by the Weed Science Society of America. Prior to this lesson, we covered the growing problem of herbicide-resistant weeds, including the need for better management strategies, terms related to herbicide use, the process of selection by herbicides, and procedures for scouting and confirming herbicide resistance. It is now time to pull the information together into a lesson that will describe broad strategies and specific tactics for field management of herbicide-resistant weeds. Lesson 5 is the “Principles of Managing Herbicide Resistance.”
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2. Objectives By the end of this lesson, you will:
Understand that diversity is an important concept in the management of herbicide-resistant weeds.
Identify the broad strategies and specific tactics for managing herbicide-resistant weeds.
Above: Seedling stage of waterhemp, a weed that is known to be resistant to several herbicides.
Image number K at the USDA-ARS online image gallery.
This lesson presents several practical concepts related to the management of herbicide-resistant weeds in the field. We will emphasize the differences between two key management approaches relative to the presence of herbicide-resistant weeds: proactive management, or management before a herbicide-resistant population evolves, and reactive management, or management after the herbicide-resistant population is confirmed.
By the end of this lesson, you will <mouse click> understand that diversity is an important concept in the management of herbicide-resistant weeds; <mouse click> identify the broad strategies and specific tactics for managing herbicide-resistant weeds; and <mouse click> compare the value of proactive versus reactive management for herbicide-resistant weeds.
Before we begin, the reader is advised that this lesson, by design, is intended to provide a description of the most commonly used management options. Consult your local extension specialist for management recommendations that are best suited to your specific situation.
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Compare the value of proactive versus reactive management for herbicide-resistant weeds.

3. Diversity of Practices
Mechanical, Chemical, Biological and Cultural Management Options
Management
Diversity
Mechanical
Cultural
Biological
Chemical
Diversity of Practices
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 is a group 4 herbicide, and glyphosate is a group 9 herbicide.
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The best strategies to manage herbicide resistance in weeds are established on the concept of diversity. Diversity can be achieved by:
Using mechanical, cultural, and biological practices in addition to herbicides
Applying several herbicides with different mechanisms of action and overlapping control (each herbicide is active on the target weed or weeds)
and
The best strategies to manage herbicide resistance in weeds are established on the concept of diversity. Diversity in the management of herbicide-resistant weeds can be achieved by <mouse click> using mechanical, cultural, and biological tactics in addition to herbicides. Diverse herbicide use is achieved by applying several herbicides with different mechanisms of action, identified by their unique group numbers, and overlapping weed control spectrums. The phrase “overlapping weed control spectrums” refers to two or more herbicides that are effective on the same target weed or weeds. <mouse click> A combination of diverse management tactics reduces the selection pressure imposed by any single practice, such as the exclusive use of one herbicide, and reduces the risk of selecting for herbicide-resistant weeds.
Later in this lesson, details are provided around the use of mechanical, cultural and herbicide tactics in the context of herbicide-resistant weeds. Biological control tactics are generally not used as part of herbicide resistance management, but options are available in some environments. To learn more about mechanism-of-action, please click on the “For More Information” button. 
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A combination of tactics reduces the selection pressure imposed by any single practice.
Information
For More

4. Management Strategies
Proactive management is the implementation of tactics before herbicide-resistant weeds are apparent.
PROACTIVE: before confirmation
Reactive management is the implementation of tactics after herbicide resistance has been confirmed in the field.
In the broader scope of farming and land management, there are two general approaches to decision making. These same approaches can be applied to managing herbicide-resistant weeds. Decisions based on a proactive management approach implement tactics before herbicide-resistant weeds are apparent, while <mouse click> decisions based on a reactive management approach implement tactics after herbicide resistance has been confirmed in the field. Both forms of decision making have merit in today’s rapidly changing agricultural environment. We will now take a closer look at proactive management.
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REACTIVE: after confirmation 4

5. Proactive Management: Defined
Proactive management is a style of decision making that anticipates events or changes in the field, plans ahead for them, and should include weed management planning.
Examples of decisions to anticipate:
Seed varieties and supply
Equipment needs
Within a field season or across years, there may be decisions and events that you can anticipate prior to planting. Proactive management is a style of decision making that anticipates events or changes in the field, plans ahead for them, and should include weed management planning. Some common examples include <mouse click> choosing the best variety and planning for a possibility of a shortage in seed supply, <mouse click> ensuring that equipment needs are met, <mouse click> possible soil sampling to anticipate fertilizer needs and adjust for availability, and <mouse click> weed management. Weed management planning needs to include the potential for weeds to become resistant to herbicides within the current management program. Land managers that use proactive management understand that weeds and herbicide resistance are largely a consequence of management decisions. They plan for short- and long-term events, they work closely with local extension specialists to anticipate changes in the field, and they are willing to try new ideas.
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Fertilizer needs and availability
Above: Planning land management methods in a dryland cropping system.
Image number K at the USDA-ARS online image gallery.
Weed management

6. PROACTIVE: before confirmation
Proactive Management
PROACTIVE: before confirmation
Proactive management in the context of herbicide-resistant weeds is critical for the long term sustainability of effective herbicide options in all crops.
Advantages of proactive management:
Preserve crop yield potential
Save money compared to reactive methods
Proactive management of herbicide-resistant weeds is critical for the long term sustainability of effective herbicide options in all crops. There are several advantages of proactive management, including the abilities to: <mouse click> preserve crop yield potential; <mouse click> save money compared with reactive methods; <mouse click> prevent the need for dramatic, short-term shifts in farm practices; and <mouse click> protect herbicide options for the future.
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Prevent the need for dramatic, short- term shifts in farm practices
Common lambsquarters is found in many agricultural systems. It is known to be resistant to several herbicides.
Photo: Image number at image gallery
Protect herbicide options for future operations

7. Proactive Management: Saves Money
Weed management decisions based on proactive management can be more cost-effective over time compared to programs based on reactive management.
Example
PROACTIVE Management Program: Roundup Ready® soybeans without glyphosate-resistant pigweeds
Application
Product
Group Number(s)
PRE
metolachlor + metribuzin
15 + 5
POST
glyphosate 9
Relative Cost per Acre
POST
POST
Weed management decisions based on proactive management can be more cost-effective over time compared with programs based on reactive management. Let’s take a closer look at an example of glyphosate-resistant Amaranthus species, or pigweeds, in Roundup Ready soybeans. <mouse click> A possible proactive management program consists of metolachlor and metribuzin applied preemergence, followed by glyphosate applied postemergence. All three of these products have different mechanisms of action, and have overlapping activity on pigweeds. <mouse click> A reactive management program, implemented once glyphosate-resistant pigweeds are present in the field, requires an additional product, fomesafen, applied postemergence to control emerged pigweeds. <mouse click> This additional product results in the reactive program costing more than the proactive program.
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REACTIVE Management Program: Roundup Ready® soybeans with glyphosate-resistant pigweeds
Application
Product
Group Number(s)
PRE
metolachlor + metribuzin
15 + 5
POST
glyphosate + fomesafen
9 + 14
PRE
PRE
metolachlor
glyphosate
metribuzin
fomesafen

8. Progression of Weed Resistance
Year 0
Year 1
Year 2
Weed resistance progresses logarithmically
Credit: Mike DeFelice
Herbicide-resistant biotype
Treatment
% Resistant Weeds in Population
Weed
Control
0 Application
.0001
Excellent
1st Application
.00143
2nd Application
.0205
3rd Application
.294
4th Application
4.22
5th Application
60.5
Failure
Proactive management is important because herbicide-resistant populations generally evolve slowly.
By the time one recognizes the presence of a herbicide-resistant population in the field, the population is large. The following is an animation showing a typical progression of herbicide resistance in the field.
In year 0, a herbicide is applied to the base population. <mouse click> After the first application, the herbicide-resistant individual, shown in red, survives the treatment and produces seed. At the end of year 1, control overall is excellent.
<mouse click> In year 2, a second application of the same herbicide is applied to the field. Control is still excellent, but the number of herbicide-resistant seeds in the soil seed bank continues to increase.
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seed pool or seed bank in soil
After first application, the resistant individual survives the treatment and produces seed
First application of herbicide to the initial population
Control is still excellent, but
resistant seed continue to increase

9. Progression of Weed Resistance
Year 5
Year 4
Year 3
Weed resistance progresses logarithmically
Credit: Mike DeFelice
Treatment
% Resistant Weeds in Population
Weed
Control
0 Application
.0001
Excellent
1st Application
.00143
2nd Application
.0205
3rd Application
.294
4th Application
4.22
5th Application
60.5
Failure
By year 3, control is now very good. Herbicide-resistant seed make up less than one-half of one percent of the population, yet the number of herbicide-resistant seed in the soil seed bank continues to increase.
<mouse click> By year 4, control may still be in the acceptable range, but the soil seed bank is almost completely composed of the herbicide-resistant biotype.
<mouse click> By year 5, weed plants and the soil seed bank are composed of herbicide-resistant biotypes. With 60% of the weed population surviving the herbicide application, weed control has failed.
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Herbicide resistance cannot be reversed in a practical time frame. In many cases, the seed pool is unlikely to change back because there is no fitness penalty.
seed pool or seed bank in soil
Control may still appear acceptable, but the seed pool is almost completely composed of the resistant type
Control is still very good, but
resistant seed continue to increase
Weed plants and seed pool are now mostly herbicide-resistant

10. Proactive Management Tactics
Strategies to proactively delay herbicide resistance can include one or more of the following tactics:
Herbicide
Multiple herbicides with different mechanisms of action
Mixes
Sequence
Across seasons
Mechanical
Tillage
Pre-plant
In crop cultivation
Post harvest
Cultural
Crop rotation
Plant population
Row spacing
Planting date
Fertilizer placement
Cover crops
Diversified weed management is an important component of sustainable, long-term agricultural production. A combination of weed management tactics helps to reduce the risk of selecting difficult to control weeds, and specifically, herbicide-resistant weeds. Several crop management tactics, including <mouse click> herbicide tactics, <mouse click> mechanical tactics and <mouse click> cultural tactics are used to build a diversified weed management program.
<mouse click> Herbicide tactics include the use of several herbicides with different mechanisms of action, either within or across seasons, applying herbicides at the labeled rate at the correct timing, and proper application of a herbicide, including making adjustments for equipment and the environment.
<mouse click> Mechanical tactics include the use of tillage or cultivation to control weeds before, during, or after the crop season.
<mouse click> Cultural tactics include the practices of crop rotation, changes in crop plant populations, different row spacings, planting date, precision fertilizer placement to favor the crop and not the weeds, and the use of cover crops. Crop rotation has the added benefit that it may allow for easier inclusion of herbicides with different mechanisms of action within a rotation. We will now take a closer look at herbicide tactics. <mouse click>
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Photo credits from left to right: Flickr jwinfred; Deere Photo Library; Allianz 10

11. Proactive Management: Herbicide Tactics
Herbicide choice requires careful planning so that products with different mechanisms of action (MOA), or unique group numbers, and activity on the same target weeds, are intentionally combined with each other or other weed control practices.
Season 1: Group 9 plus 4
Season 2: Group 9 plus 14
Season 3 Group 1 plus 2
SUSTAINABLE
Repeated annual use of a herbicide with the same MOA in the absence of other MOAs or different management strategies can lead to resistance.
Season 1: Group 9
Season 2: Group 9
Season 3 Group 9
NOT SUSTAINABLE
Herbicides have been an important part of farming since their discovery more than 70 years ago. The choice of which herbicides to use, especially in the context of herbicide-resistant weeds, requires careful planning. Herbicides with different mechanisms of action, or unique group numbers, yet activity on the same target weeds, should be intentionally combined with each other and/or other weed control practices to minimize selection for herbicide-resistant weeds. <mouse click> The repeated annual use of herbicides with the same mechanism of action or group number in the absence of other herbicides and management strategies can lead to herbicide resistance in weed populations. In the illustration on the left, herbicides from groups 9, 4, 14, 1, and 2 are used over the course of three crop years. This is a sustainable approach. In contrast, repeatedly using a group 9 herbicide is not sustainable. For all herbicide applications, it is critical to apply the labeled rate at the correct time. Management strategies based only on a herbicide mechanism of action classification system, or herbicide group number, may not adequately address specific and local needs. Consult product labels and the assistance of your local extension specialist for more information.
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Note: For all herbicide applications, it is critical to apply the labeled rate at the correct time. Management strategies based only on a herbicide mechanism of action classification system, or herbicide group number, may not adequately address specific and local needs. Consult product labels and the assistance of your local extension specialist for more information. 11

12. Proactive Management: Herbicide Tactics
The main schemes for applying herbicides with different mechanism of action (MOA) to manage herbicide resistance are:
MOA #1
MOA #2
These options can provide the flexibility to choose the best fit or combinations of fit for local agronomic operations.
Mixture application
Sequentially throughout season
MOA #1
MOA #2
To minimize the risk of selecting herbicide-resistant weeds, herbicides with different mechanisms of action, or different group numbers, and activity on the same target weeds should be used within the cropping system. <mouse click> Herbicides may be combined as a mixture (for example, metolachlor or Dual, a group 15 herbicide, plus atrazine, a group 5 herbicide, in corn), or <mouse click> sequentially (for example, pendimethalin or Prowl, a group 3 herbicide, followed by glyphosate, a group 9 herbicide, in cotton). <mouse click> Alternatively, herbicides with different mechanisms of action may be used across seasons (for example, chloransulam-methyl, or FirstRate, a Group 2 herbicide in soybeans followed by isoxaflutole, or Balance, a Group 27 herbicide in the following corn crop, both with activity on major broadleaf weeds). <mouse click>
These options can provide the flexibility to choose the best fit for local and individual agronomic operations. When planning a herbicide program, consideration should be given to the planned crop rotation and the potential for herbicide carryover. Antagonism or antagonistic effects may occur from mixing herbicides, especially postemergence foliar herbicides. Consult product labels, extension bulletins, and the assistance of your local extension specialist for more information.
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Across multiple seasons
MOA #1
MOA #2
Note: For all herbicide applications, it is critical to apply the labeled rate at the correct time. Management strategies based only on a herbicide mechanism of action classification system, or herbicide group number, may not adequately address specific and local needs. Consult product labels and the assistance of your local extension specialist for more information. 12

13. Proactive Management: Herbicide Tactics
Tank mixing or the use of pre-mixed products with different mechanisms of action and activity on the same target weed or weeds can be effective at delaying the onset of herbicide-resistant weeds.
Herbicide mixtures contain more than one active ingredient.
Herbicide mixtures may be marketed as prepackaged formulations.
MOA #1
MOA #2
Soil residual
Soil residual
Post activity
Post activity
Tank mixing or the use of pre-mixed products with different mechanisms of action and activity on the same target weed or weeds can be effective at delaying the onset of herbicide-resistant weeds. <mouse click> Herbicide mixtures contain more than one active ingredient. Mixtures may include the use of <mouse click> two soil residual products, <mouse click> a soil residual product plus a foliar active product, or <mouse click> two foliar active products. <mouse click> Herbicide mixtures may be marketed as prepackaged formulations.
It is extremely important to follow all directions on the product labels when mixing and applying herbicides, and especially to use the full labeled rates for each product in a mixture. If you are unsure about mixing chemicals, please consult the chemical dealer, company representative, or local extension specialist for additional information.
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Note: For all herbicide applications, it is critical to apply the labeled rate at the correct time. Management strategies based only on a herbicide mechanism of action classification system, or herbicide group number, may not adequately address specific and local needs. Consult product labels and the assistance of your local extension specialist for more information. 13

14. Need for Full Label Rate
Proactive Management: Herbicide Tactics
Need for Full Label Rate
Definitions:
“Labeled rate” = A rate or range of rates set by herbicide manufacturers to consistently provide effective control of weed species across growth stages and site conditions.
“Low rate” = A rate applied below the labeled rate that may provide effective control at an individual location, but will not provide consistent control over a wide range of conditions.
Routine exposure to low herbicide rates can allow a portion of the weed population to survive, leading to the evolution of herbicide-resistant populations.
An important proactive management practice is to use the full labeled rates of each herbicide used. Research has demonstrated that cutting rates can lead to an increase in the evolution of herbicide resistance in certain weeds and to many different herbicides.
Labeled rates are defined as a rate or range of rates set by herbicide manufactures in cooperation with local weed control experts. These rates are established to provide control of a given weed species across several growth stages and site conditions.
Low rates are defined as those that may provide effective control at an individual location but will not provide consistent control over a wide range of conditions.
<mouse click> Routine exposure to low herbicide rates can allow a portion of the weed population to survive, leading to the evolution of herbicide-resistant populations.
<mouse click> There are many ways by which weeds are exposed to low rates that are not related to the intended application of low rates. These include: spraying plants that are larger or at a more advanced growth stage than recommended on herbicide label, inadequate coverage of weeds because of size, density and/or crop cover, or problems with sprayer calibration, faulty or ineffective equipment, and mixing.
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Weeds can be exposed to “low rates” due to:
Intended use of low rates
Spraying plants larger than those recommended on the label
Inadequate coverage of weeds because of size, density and/or crop cover
Inaccurate sprayer calibration, faulty or ineffective equipment, or mixing errors

15. Proactive Management: Mechanical Tactics
Mechanical tactics include techniques such as:
Pre-plant tillage
Strip or zone-tillage
In-crop cultivation
Post-harvest mowing and/or tillage
Mechanical tactics are an additional part of proactive management that can be used to delay the evolution of herbicide-resistant weeds. Advances in crop research and equipment design have led to several options and different tools for tillage. Mechanical weed control can be performed with <mouse click> pre-plant tillage, <mouse click> in the form of strip or zone-tillage, <mouse click> in season with an in-crop cultivator, or <mouse click> post-harvest in the form of mowing or tillage. If small patches of weeds are present in an area known to have herbicide resistance, <mouse click> hand-rogueing before seed set can prevent the build-up of herbicide-resistant weed populations.
<mouse click> Weeds and weed seeds can be spread from one field to another on equipment. Equipment sanitation is important to slow the spread of herbicide-resistant weeds and weed seeds.
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Hand-rogueing before seed set
Photo: Image number K at the USDA-ARS image gallery.
Equipment sanitation is also important to slow the spread of herbicide-resistant weeds and weed seeds. 15

16. Proactive Management: Cultural Tactics
Crop Management. Agronomic practices, such as choice of hybrid or variety, fertilizer management, row spacing, seeding rate and residue management can influence the growth cycle of weed species and therefore provide an advantage to the crop. For example, narrow crop row spacing can quickly shade sensitive weed species, while longer periods of weed control are generally required for wider row spacings.
Photo credits: Flickr Monsanto
Cultural practices greatly influence the composition of weed populations, and their germination and growth. These practices can also influence the amount of weed seed in the seed bank and the spread of herbicide-resistant weeds.
Cultural tactics involve the manipulation of agronomic practices to suppress weed growth while favoring growth of the crop. In practice, a vigorously growing crop can more successfully out-compete weeds compared to an unhealthy plant or stand. Choice of crop hybrid or variety, fertilizer placement, row spacing, seeding rate, residue management, and more can influence the competitiveness of crops and weeds. For example, narrow crop row spacing can quickly shade sensitive weed species, while longer periods of weed control are generally required for wider row spacings.
<mouse click> When crops are rotated, natural differences in the abilities of crops to compete with weeds are exploited. The greatest benefit in crop rotation comes as a result of the most diverse crop rotations, because they provide the greatest opportunities for exploiting differences in agronomic practices and herbicide choices.
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Crop Rotation. Natural differences exist among the abilities of crops to compete with weeds. The greatest benefit in crop rotation comes as a result of the most diverse crop rotations, because they provide the greatest opportunities for exploiting differences in tillage practices, competitiveness, and herbicide choices. 16

17. Proactive Management: Cultural Tactics
Cover Crops. Cover crops, sown prior to the primary crop, may suppress weed growth through their physical presence or through the release of substances that can affect the germination and growth of some weed species.
Slowing the Spread of Herbicide-Resistant Populations. Managing weeds in the borders around fields before flowering is important to prevent pollen movement between resistant and susceptible plants. Preventing the movement of seeds and vegetative propagules from field to field by cleaning equipment before it is moved can slow the spread of herbicide-resistant weeds.
Cover crops, sown prior to the primary crop, may suppress weed growth through their physical presence or through the release of substances that affect the germination and growth of some weed species.
<mouse click> The importance of preventing the movement of weed seeds and vegetative propagules, for example, johnsongrass and quackgrass roots, and managing weeds in field borders is often overlooked as a means of slowing the spread of herbicide-resistant weeds. Managing weeds of concern in the borders is important if it is done before flowering to prevent pollen movement between herbicide-resistant and susceptible plants. The inspection and cleaning of equipment between fields, has been shown to slow the spread of herbicide-resistant weeds.
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This concludes the section on proactive management. We’ll now switch gears and look at reactive management.
In general, cultural practices greatly influence the composition of weeds, and their germination and growth. These practices can also influence the amount of weed seed in the seed bank and the spread of herbicide-resistant weeds. 17

18. Reactive Management: Defined
Reactive management is a style of decision making that acts in response to events or changes in the field when they occur with little to no expectation or anticipation of the events or changes.
A reactive action is a response to an unexpected problem.
Reactive management is a necessary part of land management.
There is no way to plan for everything that can possibly happen.
Reactive management is a style of decision making that acts in response to events or changes in the field when they occur with little to no expectation or anticipation of the events or changes. Ideally, we all start out trying to be proactive. We make plans and then somewhere we run into an unexpected situation. <mouse click> A reactive action is a response to an unexpected problem. <mouse click> Reactive management is a necessary part of land management. <mouse click> There is no way to plan for everything that can possibly happen. Ask yourself, then – are herbicide-resistant weeds truly an unexpected problem? Generally not, so <mouse click> the proactive style of decision making is preferred in the context of herbicide-resistant weeds.
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The proactive style of decision making is preferred in the context of herbicide-resistant weeds.
Above: Common ragweed, a weed that is known to be resistant to several herbicides. Photo: Image number at image gallery

19. REACTIVE: after confirmation
Reactive Management
REACTIVE: after confirmation
Reactive management can be utilized during two general times:
Tactics used within the same season a population has been identified as herbicide-resistant
Tactics used in seasons after a population has been identified as herbicide-resistant or
The timing of the first reactive management tactics may affect the intensity and number of options necessary to manage herbicide-resistant weeds in the future.
Reactive management can be deployed during two general action times: <mouse click>
tactics used within the same season a weed population has been identified as herbicide-resistant, or <mouse click> tactics used in seasons after the herbicide-resistant weed population has been identified.
<mouse click> The initial timing of reactive management tactics may affect the intensity and number of options necessary to manage the herbicide-resistant weeds in the future. For example, <mouse click> early implementation will reduce weed densities in subsequent years and can reduce weed management costs in later years.
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Left: A small patch of glyphosate-resistant Palmer amaranth. Photo courtesy of Alan York, NCSU.
Early detection and remediation prior to weed seed set within the same season (see above) will reduce weed density in subsequent years and can reduce weed management costs in later years.

20. Reactive Management: Same Season
Same season management tactics are generally limited and may not be effective when dealing with herbicide-resistant weed populations. Because situations can vary widely, consult your local extension specialist for advice.
Herbicide Options:
Apply the most effective postemergence herbicide with a different mechanism of action.
If low-level herbicide resistance has been identified, and no other options are available, apply the maximum labeled rate of the same postemergence foliar herbicide.
Mechanical Options:
Cultivation or hand-rogueing may be the primary options in some agronomic environments and geographies.
Same season management tactics are generally limited and may not be effective when dealing with herbicide-resistant weed populations. Because situations can vary widely, consult your local extension specialist for advice.
Some herbicide options may include: <mouse click> Apply the most effective postemergence foliar active herbicide with a different mechanism of action or group number, or, <mouse click> if low-level herbicide resistance has been identified and no other options are available, apply the maximum labeled rate of the same postemergence foliar herbicide. Re-spraying these weeds with the same herbicide may not result in effective control because the weeds may be too large and/or the level of herbicide resistance may be exceed the dose of the maximum labeled herbicide rate.
<mouse click> Cultivation or hand-rogueing may be the primary mechanical options in some environments and geographies or when the herbicide-resistant population is small.
<mouse click> In general, these options are limited in their effectiveness because of larger weeds and/or crop stage limitations later in the season.
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In general, these options are limited in their effectiveness because of larger weeds and/or crop stage limitations later in the season. 20

21. Potential Yield Impact of the Failure to Manage Herbicide-Resistant Weed Populations
As a herbicide-resistant weed population increases in density and area, yield potential decreases.
When herbicide-resistant weed management practices are implemented early, the risks associated
with these losses can be reduced.
Example
Year 1
Initial selection
Negligible impact
> 50% infestation
Later Years
50-90% yield loss
Population increases
20 to 30% infestation
A failure to manage a herbicide-resistant weed population can lead to large decreases in crop yield potential over time. Let’s look at an example.
In the first year of selection, a few herbicide-resistant survivors persist, reproduce, and contribute seed into the soil seed bank. The impact on yield may be negligible.
<mouse click> In a later year with continued selection, without reactive strategies in place, the population increases, several more survivors persist and contribute seed into the soil seed bank. The impact on yield can range from 5-10% loss.
<mouse click> Still later, as the cycle continues, more and more herbicide-resistant plants survive. The weed population may contain between 20 and 30% resistant individuals, and the impact on yield may range from 20-50% loss.
<mouse click> Eventually, the percentage of herbicide-resistant weeds in the population grows and yields can decline in the range of 50-90% depending upon the competitiveness of the species.
<mouse click> When herbicide-resistant weed management practices are implemented early, the risks associated with these losses can be reduced.
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5-10% yield loss
20-50% yield loss

22. Conclusions
Strategies to manage herbicide resistance in weeds are best established on the concept of diversity.
Management diversity can be achieved by using herbicides in mixtures, sequences, or rotation with and without the use of mechanical and cultural methods of weed management.
This concludes the lesson “Principles of Managing Herbicide Resistance.” As you prepare weed management plans for fields in your area, please remember these key points:
Strategies to manage herbicide resistance in weeds are best established on the concept of diversity.
<mouse click> Management diversity can be achieved by using herbicides in mixtures, sequences, or rotation with and without the use of mechanical and cultural methods of weed management.
<mouse click> Proactive management can be more cost effective and provide greater yield protection and income versus waiting to implement reactive strategies after herbicide-resistant weed populations are identified.
Thank you for participating in the herbicide-resistant weeds training lessons as provided by the Weed Science Society of America.
Proactive management can be more cost effective and provide greater yield protection and income versus waiting to implement reactive strategies after herbicide-resistant weed populations are identified.

23. 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.