linuron

The recommendation to rotate herbicide MOA and use two or more effective MOA in tank mixtures has been a common theme promoted by agricultural professionals to delay resistance. The probability that a weed will evolve resistance to two different herbicide MOA when used in combination is less likely than the weed evolving resistance to a single herbicide MOA. In addition, rotating MOA during and between seasons reduces exposure to the active ingredient and the potential selection pressure it imposes.  In the example that follows (Table 1), of the three weeds targeted, only Palmer amaranth is treated with two effective MOA.

Table 1.  Introduction to multiple effective modes of action.

Successful use of multiple effective MOA to help manage and delay herbicide resistance depends on the lack of weed population biotypes already resistant to herbicides from certain groups.  As an example, in Table 2, without resistance, several effective herbicide MOA can be used to control susceptible waterhemp.  However, as multiple resistance becomes more problematic, effective herbicide modes of action become limited. Without resistance, there are herbicides from seven mode of action groups that can target waterhemp.  However, if you encounter six-way resistant waterhemp which has been identified in at least two U.S. states, only glufosinate (Group 10) remains to help control the six-way-resistant biotype.  The presence of multiple resistance in some of our most problematic weed species has already greatly reduced our ability to use herbicide selection tactics effectively for resistance management.

Table 2.  Effectiveness of seven different MOA groups for control of herbicide-susceptible and herbicide-resistant waterhemp in corn or soybean.  Waterhemp ranges from susceptible to having six-way herbicide resistance.  Six-way multiple-resistant waterhemp has been identified in Missouri, Minnesota, and Illinois and five-way cross resistance in Ontario, Canada, and North Carolina. 

Recent research on tank-mixing versus rotating herbicide modes of action has shown new light on which strategy is most important for preventing herbicide resistance, including target-site and non-target-site resistance. For an overview of this, watch this GROW video with University of Illinois weed scientist Dr. Patrick Tranel:

There are many ways that weeds evolve resistance to herbicides, but the mechanisms fall into two main categories, 1.) target-site resistance (TSR) and 2.) non-target site resistance (NTSR).  TSR occurs when the herbicide can no longer bind to the site of action (Figure 10a), while NTSR occurs when a mechanism prevents lethal or toxic levels of the herbicide from reaching the intended site where the herbicide “acts.” Enhanced metabolism is one of the most concerning types of NTSR and leads to alteration and inactivation of herbicides (Figure 10b. (See Section 8 in the Basics of Herbicide Resistance – What are the mechanisms for herbicide resistance for more details). 

Figure 10.  Examples of (a) target-site vs. (b) non-target site resistance in plants.

In most cases, it can be difficult to know whether the resistance mechanism at the farm level is TSR or NTSR. Regardless of the mechanism, rotating herbicide modes of action and using multiple effective modes is always important for managing resistance evolution. In addition, effective non-chemical control strategies that emphasize prevention, cultural, mechanical, and biological tactics must be implemented.

There are many ways that weeds evolve resistance to herbicides, but the mechanisms fall into two main categories:

1. Target-site resistance (TSR)

2. Non-target-site resistance (NTSR)

Most plant target sites are enzymes — proteins that function as essential steps in a biological process. They catalyze (initiate) or direct critical reactions necessary for plant growth and development.

Target-site resistance was the first herbicide-resistance mechanism identified and continues to be a very widespread type of resistance. The most common type of TSR occurs when there is a change in the structure of the plant’s target site/protein, so the herbicide can no longer bind to the target site. As a result, the resistant plants survive the herbicide application and continue to grow (Figure 16). Using herbicides with different effective modes of action (by overlapping residuals, tank-mixing, and rotating herbicides) is widely promoted to help prevent this dominant type of resistance. Research conducted at the University of Illinois with waterhemp suggests that using multiple effective modes of action and tank-mixing was more effective than herbicide rotation in delaying TSR evolution.

Non-target site resistance occurs when a process within the plant prevents the herbicide from reaching the intended site where the herbicide “acts.” There are several mechanisms of NTSR including sequestration of the herbicide in the vacuole (a small, enclosed space in a cell) and enhanced metabolism. Enhanced metabolism, also called metabolic resistance, occurs when the herbicide enters the plant cell, is altered chemically and is ultimately rendered immobile, nontoxic, and inactive – in short, unable to do its work in the plant (Figure 17). This is the most concerning type of NTSR, because the mechanism can permit plants to develop resistance to specific or multiple herbicide groups without any physical exposure to them. For example, in 2021, a waterhemp population in Illinois was identified with metabolic resistance to dicamba, a Group 4 herbicide, yet the field had never been treated with dicamba. The dicamba-resistant biotype was also resistant to the Group 4 herbicide, 2,4-D.

Because NTSR and metabolic resistance are more recent discoveries, scientists are still evaluating their implications and looking at potential effective alternatives for managing these kinds of resistance. Unfortunately, NTSR is increasingly common in weed populations.

Figure 16. This is an example of classic target-site resistance with the Group 5 herbicides (Photosystem II inhibitors). The herbicide is (a) applied to the weed (common ragweed here), (b) enters the plant cell and (c) the chloroplast and (d) binds to a key protein that stops photosynthesis. With target-site resistance, the herbicide can no longer bind to the target site (d) in the resistant biotype, and photosynthesis continues. (Illustration adapted by William Curran from University of Minnesota Extension; Graphic credit: Lourdes Rubione)

Figure 17. Typically, in the first step in herbicide metabolism, the herbicide is altered and sometimes inactivated. In this example, an enzyme alters 2,4-D by cleaving (removing) a chloride ion and replacing it with an hydroxyl (OH). The herbicide is further metabolized by other enzymes and eventually either stored or incorporated into various plant parts. (Image credit: William Curran, Penn State University; Graphic credit: Lourdes Rubione)

Watch this GROW video from University of Illinois weed scientist Dr. Patrick Tranel, for an overview of the differences between target-site and non-target-site resistance:

Herbicides are important tools for weed management, but managing herbicide resistance requires knowing a little more about how active ingredients interact with weeds, as well as other integrated weed management strategies. To help users understand how herbicides control weeds, Herbicide Group Numbers are found on the herbicide labels and containers (See Figure 7).  These herbicide group numbers correspond with individual herbicide modes of action (MOA) (see The Basics of Herbicide Resistance for more details). Effective herbicide-resistance management must include rotating herbicide MOA, using multiple effective MOA, and reducing weed exposure to herbicides by integrating multiple management strategies that prevent weed-crop competition and weed seed production.  Here is a list of herbicide management considerations to help delay and manage herbicide resistance.

1. Know herbicide MOA group numbers when making herbicide selections, and use multiple effective MOA and rotate MOA within and between growing seasons. Cross resistance within a herbicide MOA is common, so the need to select alternative effective MOA is paramount  (See the Take Action Herbicide Classification Chart here).

2. Consider the potential for metabolic resistance in the weeds you are targeting along with the selected herbicide partners (see Sections 8 and 9 in the Basics of Herbicide Resistance).

3. Use full labeled rates with multiple effective herbicides (and MOA) and start clean (plant into a weed-free seedbed).

4. Include effective residual herbicides for soil applications.

5. Include overlapping residual herbicides with postemergence applications. Overlapping herbicides should target the same weeds with different MOA’s.

6. Make timely applications to small annual weeds using appropriate nozzles, spray volume, adjuvants, etc, once weeds are emerged.

7. Monitor fields regularly and scout 7-10 days after postemergence applications to evaluate weed control.

Herbicide-resistant weeds spread or invade new areas just as non-resistant weeds do. The mechanisms of new weed invasion and spread fall into two main categories: 

Mother Nature and Weed Biology. This includes spread by wind, water, and wildlife, and some weed species have specialized traits that help their seeds disperse naturally by these methods (Figure 5).  These specialized traits can include a feathery pappus attached to each seed giving flight (horseweed and dandelion are examples), and seeds covered in barbs or burs that easily attach to the fur of animals (examples include common cocklebur and common burdock).  Other examples include seeds that can pass through animals intact and potentially remain viable (includes many different species), and finally the tumbleweeds like kochia and Russian thistle that when mature break free traveling end over end spreading their mature seeds.

Human Activities. These include moving weed seeds or other plant parts with farm inputs like crop seeds, soil, manure, and compost, as well as using farm equipment or transporting freight that may be contaminated with herbicide-resistant weed seed.  Using certified crop seed, regularly cleaning farm equipment, and properly composting manure and other organic waste products helps avoid moving weed seeds with these activities.

Figure 5. Seeds dispersing and/or spreading via natural processes and human activities.

To learn more about how herbicide resistant weeds spread, visit this GROW webpage. 

Figure 3. Scouting is a key step in the prevention of herbicide resistance.

A number of approaches should be used when managing herbicide resistance. Prevention and control are key and aim to head off the introduction of resistant weeds in a field or reduce an already established problem. Monitoring crop fields and weed populations by regularly scouting should be implemented to help guide decisions (Figure 2).  

Preventative tactics for herbicide-resistant weeds should focus on avoiding weed seed introduction from contaminated crop seed, soil, manure, compost, hay, livestock feed, machinery, neighboring fields and field edges (Figure 3).  In addition to seeds from annual and biennial species, the vegetative structures such as rhizomes, stolons, tubers, and bulbs of some perennials can also help spread problem weed infestations.  In addition, prevention should go much further and include tactics that also slow the resistance evolution process by reducing weed exposure to herbicides within the same MOA group and integrating multiple management strategies that prevent selection and spread.  See this GROW webpage for detailed information on prevention.  

Figure 3. Weed seeds can spread across the farm many ways.

IWM is essential to help prevent yield and quality loss, halt the introduction of more weed seed, and reduce the problem over time.  Cultural, chemical, mechanical, and sometimes biological weed control tactics are critical for maintaining successful and profitable crop production (Figure 4). If you discover a new problematic weed on the farm that can still be contained, aggressively manage the infestation to control the population and prevent seed production.  As an example, if Palmer amaranth was recently introduced and only a few dozen plants are identified in a field, remove them by hand to prevent the weeds from reproducing and spreading. If the plants are flowering, carefully remove and bag the entire plant being careful not to spread seeds.  For these small infestations, burn or bury plants outside the field deep enough to prevent survival. If a resistant weed population has become more widely established on the farm, consider all appropriate options, based on the characteristics of the species, to drive down the weed infestation over time and maintain successful and profitable crop production (Figure 4).

Figure 4.  Examples of IWM practices to manage herbicide-resistant weeds. (Photo credits: Claudio Rubione, GROW)

GROW’s Farmer Case Studies page contains video interviews with farmers across the country who are experimenting with or have mastered the use of multiple integrated weed management tactics to control weeds on their operations. Visit the webpage to learn more. 

To develop an IWM strategy on your farm, consider using the GROW Weed Management Planner to help identify useful strategies for your weed spectrum. 

The Weed Management Planner was developed for the Mid-Atlantic region, but has relevance for growers across other parts of the U.S. Corn and Soybean Belt.  Additional versions that focus on other regions and cropping systems should be available in the future. The current tool will help you identify common crop management and weed control strategies that can have a positive effect in managing your biggest weed problems, as well as those that could worsen certain weed infestations.

A number of new technologies are under development that fit into the IWM toolbox of prevention, cultural, chemical, mechanical, and biological tools, along with innovations in plant genetics and plant breeding, precision agriculture equipment and machine learning technology, information technology including the development of new decision support tools, and federal, state and private third party incentives that promote diversity in agriculture (Figure 13). Finding new cost-effective tools for agronomic weed control are challenging, but many private companies and universities around the world are actively looking for methods to help farmers manage weeds.  

In addition, what can we expect from new herbicide MOA or other chemical-based strategies? Some of the major herbicide manufacturers have increased efforts to discover new herbicide molecules. Both private and public institutions are looking for naturally occurring or novel compounds that could be developed into effective commercial herbicides. Companies identify critical new enzymes and search for molecules that inhibit them, thus targeting enzymes that have not been targeted previously. See Section 10 in “The Basics of Herbicide Resistance” for more details.

Non-chemical strategies include prevention tactics, cultural production practices, mechanical systems, and biological tools. Limiting sunlight from reaching a weed is one of the most basic but important tools for weed control. To accomplish this:

1. Use cultural production practices that encourage the development of a quick, dense crop canopy or cover crop stand to help shade and suppress weeds. See this GROW Cultural Production Practices webpage for additional information on production practices that can aid weed management.  

2. Add crops with different planting dates to your rotation, particularly crops that have different life cycles (i.e. fall-seeded, early-spring seeded, summer seeded).  

3. If possible, selectively include tillage when you can target specific weed problems such as perennials or by burying the seeds of summer annuals. 

4. Be sure to control weed escapes prior to seed set to reduce future weed populations and prevent resistant weeds from spreading. This may require walking fields and using hand removal which was common before the introduction of herbicide-resistant crops.  If seed heads are present, remove plants by hand, bag them, and dispose of them outside the crop field by deep burial or burning. For larger infestations, consider adopting some newer technology such as weed electrocution and inter-row mowing (https://growiwm.org/mowing-zapping-and-outcompeting-weeds-opportunities-and-tradeoffs/) as well as harvest weed seed control tactics that include chaff lining or using seed impact mills.

5. Reduce the introduction of weed seed into crop fields by managing field edges with mowing or spot spraying and clean equipment between problematic fields and clean fields, so you don’t spread the problem.  

6. Harvest problem fields or weedy areas last to prevent the spread into other fields.  

7. Clean farm equipment, including the combine, regularly to reduce seed spread. 

For more information, visit the GROW Weed Management Toolbox, and see Section 4 in the Basics of Herbicide Resistance.

Integrated weed management (IWM) is a system that layers multiple weed control methods to suppress and manage weeds over time, by targeting weeds in diverse ways and at various stages of development. IWM requires expanding your weed management practices over more than one season. This requires a better understanding of the weed species present in the field and their biology, such as lifecycle, emergence timing, growth rate, and seed production. The result is a robust weed management program that doesn’t rely on a single herbicide mode of action or any other sole management tactic.  In general,  IWM techniques include Prevention, Cultural, Chemical, Mechanical, and Biological tactics (Figure 1). See more details on these GROW webpages on what IWM is and how you can combine weed control tactics effectively. 

Herbicide-resistant individuals initially may be present in a weed population at exceptionally low frequency. These “biotypes” differ from the majority population because they have mechanism(s) that allows them to survive exposure to the herbicide. Maybe only one individual out of 10 million plants carries the resistant trait. Often the frequency or rate at which this mutation occurs within a weed species is much lower than this. But, with repeated application of the same herbicide or herbicide group over time, without the use of other effective weed control strategies, the frequency of the resistant individuals or biotype within a population increases (Figure 3). Although the herbicide is still controlling susceptible individuals, the resistant biotype survives and produces seed. Using multiple effective modes of action (MOA), rotating modes of action and using non-chemical control tactics can help reduce selection for herbicide-resistant weeds.

Figure 3. After repeated use of the same herbicide group for multiple seasons, a resistant biotype may be selected that survives the application. As illustrated, the resistant individual (a) survives the herbicide and can (b) mature and produce seed. If the herbicide program continues, (c) more individuals survive the herbicide, they mature and produce seed and eventually, (d) the dominant biotype in the field is resistant. (Adapted by William Curran and Lourdes Rubione from University of Minnesota’s herbicide-resistance management webpage).

With few exceptions, resistant and susceptible weeds of the same species look the same (Figure 6). Resistant weeds are often identified by patterns in the field rather than by any differences in appearance. If weeds survive a herbicide application, investigate and rule out other factors affecting herbicide performance before concluding that resistance is responsible.

Good questions to ask are:

The Weed Science Society of America (WSSA) recommends exploring several other factors, including:

Figure 6. Whether herbicide-susceptible or resistant, these individuals look the same. (Photo credit: Claudio Rubione, GROW; graphic credit: Lourdes Rubione)

How do you confirm herbicide resistance? There are three primary ways to confirm herbicide resistance. These include field testing, greenhouse testing and more limited genetic testing for some types of resistance. Once you suspect herbicide resistance, quickly contact your local county Extension agent or ag consultant and seek advice.

Field testing, which involves treating some surviving plants with the same herbicide mode of action, is not very practical, but it may be the first reaction when weeds escape a soil or foliar-applied herbicide that should have been effective. More importantly, if the escaped weed(s) are suspected to be resistant, attempts should be made to prevent them from producing seed and furthering the problem. This will likely involve applying alternative effective herbicide groups or using mechanical control options.

A more accurate method to test for herbicide resistance is through greenhouse confirmation (Figure 7). This requires working closely with your local Extension service and weed science expert to allow some plants to mature for seed collection for a greenhouse assay (test) after the field season ends. Genetic testing for some types of resistance may also be available which can confirm resistance and provide the mechanism(s) at play. Because these types of tests are specialized and limited in availability, they are generally only sought and used for more unique types of resistance.

Plants surviving a herbicide application is not uncommon, since many issues can compromise weed control. Consider several factors when determining if resistance is the cause for lack of control (Figure 8).

Figure 7. Herbicide-susceptible horseweed (left) and resistant horseweed (right) are identified in a greenhouse glyphosate screening test. (Photo credit: Mark VanGessel, University Delaware)

Some herbicide mode of action groups are more prone to develop herbicide resistance from frequent use. Weeds emerged with resistance to Group 2 herbicides in the early- to mid-1980s, only a few years after their introduction (Figure 22). Since that time, more than 170 weed species have been identified with Group 2 resistance. The second most problematic herbicide mode of action is the Group 5 herbicides, although much of this resistance developed back in the 1970s and 1980s with atrazine use in corn (Figure 22). Glyphosate, a Group 9 herbicide, became the third most problematic mode of action, but only after the widespread and frequent use of glyphosate in Roundup Ready crops starting in the late 1990s accelerated resistance. The ACCase inhibitors or Group 1 herbicides are in fourth place, based on the number of resistant weed species, followed by the Group 4 synthetic auxins (Figure 22).

Figure 22. This chart shows the number of resistant species for several herbicide modes of action. These are the top five herbicide MOA’s based on the number of resistant weed species. (Chart credit: The International Herbicide-Resistant Weed Database, accessed online, May 20, 2024)

Some weedy plants are also more susceptible to resistance evolution (Figure 23). Within the grass family (Poaceae), rigid and Italian ryegrass, barnyardgrass, annual bluegrass, wild oats and goosegrass all have multiple resistance (Figure 24). Palmer amaranth, waterhemp, smooth and redroot pigweed, horseweed, and both common and giant ragweed all have biotypes resistant to multiple modes of action. Several weeds have evolved both target-site resistance and non-target-site resistance. The problem species discussed most often in both the scientific and ag communities include rigid ryegrass in Australia and Palmer amaranth and waterhemp in the U.S.

Figure 23. This graph shows the number of herbicide-resistant weed species by plant family. (Chart credit: The International Herbicide-Resistant Weed Database, accessed online, May 20, 2024)

Figure 24. This graph displays the top weed species resistant to multiple herbicide modes of action. (Chart credit: The International Herbicide-Resistant Weed Database, accessed online, May 20, 2024)

Once you have herbicide-resistant biotypes in your fields, it is unlikely that they will revert to susceptible biotypes within a farming generation, even if you stop using the herbicides that they resist. This is especially true if the resistant biotype becomes established for several years and is producing seed annually. Once resistance is established, fields typically will have a mixture of susceptible and resistant biotypes (we’ll refer to them as S and R biotypes). The ratio of these individuals could change over time depending on herbicide use. With repeated use of the same herbicide group over time, the population of a resistant biotype will increase and be the dominant biotype.

Without question, if you must manage a herbicide-resistant weed or weeds, it will require more time and money. With fewer effective herbicide options, you will likely need to use additional herbicides in your tank mixes or make more trips over the field to control the resistant biotypes. One of the most common recommendations for preventing selection of herbicide-resistant weeds is to add a second or even third effective mode of action to the spray program. This can prevent or delay selecting for resistant biotypes and is usually necessary after resistant weeds become established. However, rotating to another herbicide mode of action to control an already resistant population can lead to additional resistance. You will need to integrate more non-chemical control tactics such as mechanical control, changing cultural practices and using other strategies such as cover crops and harvest weed seed control. This multi-pronged approach is known as integrated weed management (Figure 5). If you are not successful, weed control will be inadequate and you could lose crop yield and quality.

Currently, managing a herbicide-resistant weed population may require use of a less effective herbicide, treating weeds when they are smaller, and ensuring the herbicide is applied at the right time. According to 2022-23 estimates of custom ag operator rates from university Extension publications in Illinois, Iowa, Ohio, Maryland, and New York, each spray pass (not including the individual cost of herbicides) can cost between $5 and $15 per acre for application. Custom rates for primary tillage average $17 per acre, while custom rates for row cultivation average almost $15 per acre across the Central and Eastern Corn Belt. Depending on the resistant target weed, a study conducted in Ontario, Canada showed that glyphosate-resistant weeds increased herbicide costs on average by $38 per acre for corn and $27 per acre for soybeans. The study reported that glyphosate-resistant weeds cost the province $28 million more annually in management, while causing $15 million in yield loss.

Figure 5. Various integrated weed management (IWM) tactics are employed to manage herbicide-resistant weeds. They can include using several types of preventive tactics, such as crop rotation, tillage, hand removal, integrating cover crops, using alternative herbicides, and adopting various harvest weed seed control tactics. (Photo credits: Claudio Rubione, GROW).

A herbicide mode of action describes the way a herbicide works to control weeds. In general, herbicides interrupt certain biological processes, often by disrupting enzyme activity or other plant functions. The numerical mode of action system groups herbicides and provides a reference for growers and ag professionals to rotate modes of action and help design integrated weed management programs (Figure 10). Currently, herbicides are grouped into 25 different modes of action (plus an additional group of herbicides with unknown modes of action for 26 total groups). Each class is identified by a Group Number.

The herbicide site of action, or target site, is the location of the primary interruption/disruption and is often a specific plant enzyme. Enzymes are proteins that function like stairsteps in a biological process required for plant growth. In most cases, herbicides within the same mode of action group have the same site of action. For example, all Group 1 herbicides inhibit the enzyme acetyl CoA carboxylase (ACCase inhibitors) and all the Group 2 herbicides inhibit the enzyme acetolactate synthase (ALS inhibitors) (Figure 10).

Figure 10. Take Action Herbicide Classification by Mode of Action Poster. In this chart, herbicides are grouped by mode of action. This visible portion shows three groups (1, 2, and 9), the chemical family, active ingredient, and branded product examples. Premix products and their active ingredients and trade names are also shown.

Discovering a new herbicide mode of action is a priority in both the private and public sectors. Some of the major herbicide manufacturers have increased efforts to discover new herbicide molecules. Both private and public institutions are looking for naturally occurring or novel compounds that could be developed into effective commercial herbicides (Figure 25). Several startup companies are also investing in the search. Newer herbicide discovery technologies should help with these efforts, including artificial intelligence, X-ray crystallography, and the use of DNA-encoded libraries.

One of the biggest differences in today’s search compared to past efforts is that companies now investigate the mode of action early in the discovery process. Companies identify critical new enzymes and search for molecules that inhibit them, thus targeting enzymes that have not been targeted previously.

If a new effective herbicide mode of action comes to the marketplace, what’s the chance for resistance evolution? Assuming the new mode of action targets a protein/enzyme that is an essential critical pathway for plant growth and development, the potential for resistance evolution remains high, if – as in the past – the new product is widely adopted and frequently used. More novel chemical solutions that do not rely on targeting single pathways in plants may have greater longevity, but they are much more difficult to discover and develop. The bottom line is that farmers will need to use a wider variety of weed management techniques now and into the future to prevent and manage herbicide-resistant weeds.

Figure 25. Herbicide discovery efforts seeking out new modes of action, such as in vitro assays to screen herbicide modes of action (middle), are on the rise from both major herbicide manufacturers and some newer startups, as well as the public sector. Private sector research and development teams include scientists and associates guiding robust discovery and development pipelines. (Photo credits: Top: Csail.mit.edu News, Feb. 20, 2020; Middle: Plant and Soil Sciences eLibrary, University of Nebraska, Lincoln; Bottom: https://www.fmc.com/en/innovation/research-development).

Herbicide resistance within or across chemical families with the same mode of action is called cross resistance. For example, cross resistance is common for Group 2 herbicides. Even though there are five different herbicide families within the Group 2 herbicides, they are all ALS inhibitors, so resistance to one herbicide may eliminate the effectiveness of them all (Figure 14).

Figure 14. Cross resistance occurs within or across chemical families with the same mode of action. Pursuit (imazethapyr) and Classic (chlorimuron) belong to different chemical families within the Group 2 herbicides. (Photo credit: Claudio Rubione, GROW; graphic credit: Lourdes Rubione)

Resistance across different modes of action is called multiple resistance. Multiple resistance can occur when resistance evolves after repeated use of a herbicide group followed by the repeated application of other herbicide groups. It seldom occurs simultaneously, although this is less certain with metabolic resistance. The evolution of non-target site resistance complicates both cross and multiple resistance since there is evidence that these can evolve without exposure to specific herbicide groups. This will be discussed more in Section 8.

Here’s an example of how multiple resistance evolves: Let’s say Group 2 and Group 5 herbicides are applied annually for several years for weed control in both corn and soybeans in rotation. Waterhemp control worsens as the waterhemp population first evolves Group 2 and then Group 5 herbicide resistance. In this scenario, the grower then adopts Roundup Ready corn and soybean technology and begins using glyphosate, a Group 9 herbicide, every year in both crops. After multiple years of using glyphosate and the Group 2 and 5 herbicides, the waterhemp population has evolved multiple resistance to three different herbicide groups (2, 5, and 9) (Figure 15).

Figure 15. Multiple resistance is resistance across different modes of action. When a weed is resistant to a Group 2 herbicide, Pursuit (imazethapyr), a Group 5 herbicide, Atrazine, and a Group 9 herbicide, Roundup (glyphosate), it has multiple resistance to three different modes of action. (Photo credit: Claudio Rubione, GROW; graphic credit: Lourdes Rubione)

Herbicide resistance is the naturally occurring, inheritable ability of an individual plant or population to survive a herbicide application that would kill a normal plant or population of the same species. These surviving plants can mature and produce seed, and the resistance trait is passed on to the next generation. Herbicide susceptibility means the weed or crop are killed by the normal use rate of the herbicide; as an example, the foxtails and other annual grasses are susceptible to the Group 15 herbicides such as s-metolachlor. In contrast, herbicide tolerance is the inherent ability of an entire plant species to survive and reproduce after herbicide treatment at a normal use rate. This term is generally used to describe both weedy plants and crops that were never susceptible to certain herbicides. For example, many broadleaf weeds are susceptible to the group 4 synthetic auxin herbicides, while grassy weeds like giant foxtail and johnsongrass are naturally tolerant. Most grassy weeds are susceptible to the group 1 grass-specific herbicides, while all broadleaf or dicot weeds are tolerant to those herbicides.

Figure 1. Examples of key herbicide-resistant weed species across the U.S.