Herbicide Resistant Weeds

Factsheet - ISSN 1198-712X   -   Copyright Queen's Printer for Ontario
Agdex#: 641
Publication Date: 04/01
Order#: 01-023
Last Reviewed: 08/09
History: New Factsheet
Written by: Hugh Martin - Program Lead, Organic Crop Production/OMAFRA; François Tardif - University of Guelph; Gabrielle Ferguson - GABE Consulting.

Table of Contents

What is Herbicide Resistance?

Herbicide resistance is the genetic capacity of a weed population to survive a herbicide treatment that, under normal use conditions, would effectively control that weed population. Herbicide resistance is an example of evolution happening at an accelerated pace and an illustration of the "survival of the fittest" principle. A herbicide may kill all the weeds in a population of a particular weed species except for a few individuals with the genetic capacity to survive the herbicide. Herbicide resistant weeds are normally very rare in a weed population. Applying the same herbicide in the same field year after year will select for resistant plants. The resistant weeds set seed and may eventually dominate the population. This population is then not effectively controlled by the selecting herbicide.

Resistant weeds may be resistant to only 1 herbicide group or to 2 or more herbicide groups. Also, resistant weeds may be resistant to 1 herbicide class within a herbicide group or all of the herbicide classes within 1 herbicide group. This is best illustrated with the example below.

If a population of a certain weed species is resistant to Group 2 herbicides (ALS inhibitors) it may be resistant to one, several or all of the herbicides that inhibit ALS (see Table 3). This is known as cross-resistance. Multiple resistance is when the weed population is resistant to not only Group 2 herbicides but is also resistant to Group 5 (triazines) herbicides. To control this population of weeds it would be necessary to select herbicides that are not included in Group 2 or in Group 5 (see Table 3).


A pigweed population resistant to atrazine may also be resistant to metribuzin (Sencor/Lexone) and simazine (Princep/Simadex) (cross-resistance). If the same pigweed population is resistant to imazethapyr (Pursuit - Group 2) it has multiple-resistance. [This weed population may also be resistant to thifensulfuron-methyl (Pinnacle), nicosulfuron or rimsulfuron (Accent, Ultim, Elim) or other Group 2 herbicides (cross-resistance).] However, herbicides from groups other than Group 5 and Group 2, such as dicamba (Banvel, in Group 4) or bromoxynil (Pardner, in Group 6), will be as effective at controlling this population of weeds as they are at controlling other susceptible populations of the same weed species. Some herbicide products combine more than one active ingredient (from different groups) as a strategy to delay resistance and to control resistant weeds (for example: Broadstrike Dual Magnum, Fieldstar, PeakPlus, Summit).

In Ontario, pigweed that are resistant to both triazine (Group 5 - examples: atrazine, metribuzin) and to ALS herbicides (Group 2 - examples: Pursuit, Classic, Pinnacle) have been identified. Some populations of ryegrass in Australia are resistant to up to 10 different chemical groups. Multiple resistance significantly reduces the options farmers have for the control of these weeds.

Multiple-resistance can appear after sequential selection. Green foxtail from Manitoba is an example of sequential selection. Initially this weed developed resistance to Treflan. Treflan resistance prompted farmers to use postemergence grass herbicides (Group 1) as an alternative. After 4-5 years of post-emergence grass herbicide applications, some foxtail populations became resistant to these types of herbicides. The result was green foxtail resistant to both Group 3 and Group 1 herbicides.

How Does Resistance Develop?

Worldwide there were more than 249 herbicide resistant weedy biotypes in 47 countries. These numbers grow annually with new reports of new resistant weeds. Some management practices increase the likelihood of developing herbicide resistant weeds.

  • Resistance is more likely to occur when the same herbicide, or herbicides from the same groups, are used repeatedly.
  • Monoculture often encourages the use of the same herbicide.
  • Resistance is most likely to develop in annual weed species since they produce high numbers of seeds (pigweed, lamb's-quarters and foxtail are good examples of these types of weeds).
  • Resistance frequently occurs to herbicides with the greatest efficacy on a specific weed species. This is because they impose intense selection on the weed species they are very effective in controlling. The result is only the resistant individuals are allowed to pass their genes to the next generation.

How do you Prevent Resistance?

Resistance management involves preplanning your weed control program. Strategies include the following:

  • use herbicides only when necessary
  • use the recommended rate
  • use herbicide mixtures that include 2 or more herbicide groups
  • rotate herbicides between herbicide groups

Herbicide mixtures and herbicide rotation strategies work on the premise that if a weed carries the genes to resist 1 group of herbicides, an alternate herbicide group will kill it. The difference between the 2 approaches is that herbicide mixtures kill the resistant weed using many active ingredients in the same season. Rotating herbicides controls the resistant weeds in the years when effective herbicide groups are used with the goal of reducing the resistant weed population.

Not only must mixtures contain herbicides from different groups, but also each herbicide in the mixture must be acting on the same weed species to effectively provide multiple modes of action. This means that a mixture of a grass and broadleaf herbicide may make sense in terms of overall weed control, but it may not be effective as a resistance management tool. Some herbicides are a premix of several herbicide groups, in other cases it is up to the farmer and their advisors to select the best tank mix of products to provide an affective weed resistance management strategy.


For these tactics to be effective it is important to select herbicides with different plant-killing modes of action. Table 3 lists common herbicides used in Ontario grouped by their mode of action and gives each group a unique number. This number system is the same across North America. This list is used to help you select products from different groups for use in rotations or mixtures.

A Self Test

  • Pick any one of your fields and fill out the crop rotation and herbicide use history report below, Table 1.
  • Determine which herbicide groups were used in each year using Table 3.
  • Compare the group numbers from 1 year to the next. More frequent use of the same herbicide group may put your fields at higher risk of developing herbicide resistant weed populations. Some herbicides control weeds by attacking a single site of action, such as Group 2, and may develop resistance after being used 4-7 times. Others such as Group 5 do not seem to develop resistance until 10-15 years of repeated use.
  • Include the next 2 years of your proposed crop rotation. This allows you to evaluate future herbicide programs for resistance management, as well as for soil residues and cropping restrictions.
Table 1. Crop History for Field

Group #
example 1 soybeans Pursuit plus Basagran 2 and 6
example 2 corn Marksman 4 and 5

Weeds That are Currently Resistant

The following are examples of herbicide resistant weeds that are known to exist in Ontario. In each case, growers have learned how to adapt their weed management programs to include the resistant weed biotypes.

Table 2. Herbicide Resistant Weeds in Ontario
Weed species
2,4-D (Group 4)
wild carrot
Species variability creating tolerance and resistance were found and reported in 1956 and re-confirmed in 1998.
atrazine (Group 5)
common lamb's quarters
First confirmed in 1974 and now found in most counties of southern Ontario. Field histories in where triazine resistant weeds have been found generally indicated at least 6-10 consecutive years of triazine herbicide use.
atrazine (Group 5)
green pigweed (also known as Powell's amaranth), red root pigweed, common ragweed
These were confirmed in the late 1970's and are now found in numerous locations across the province. Resistant pigweed occurrences are not as prevalent as lamb's-quarters but in areas where they occur they can have similar populations. Resistant common ragweed is more localized but known to occur in several areas of the province.
atrazine (Group 5)
barnyard grass, yellow foxtail, old witch grass
Each confirmed in 1981, each species has been found in localized areas of southern Ontario. Distribution is not wide spread despite each being found nearly 20 years ago.
atrazine (Group 5)
late flowering goosefoot, wild mustard, common groundsel
Each species was found in the 1980's in localized areas of southern Ontario
paraquat (Group 22)
Canada fleabane (also known as mare's-tail)
First found in 1995 in two orchards in southwestern Ontario. Very localized
ALS (Group 2)
red root pigweed, green pigweed (also known as Powell's amaranth), Eastern black nightshade, ragweed

First confirmed in 1997, resistant pigweed are now found in at least 8 counties.

  • Field histories indicated 3-5 years using Group 2 herbicides, not necessarily in consecutive years.
  • Not all populations are equally cross-resistant to all Group 2 herbicides; however, it is not possible to predict to which Group 2 herbicides a population is resistant to without trial and error. It is prudent to expect less than satisfactory control from most Group 2 herbicides and have alternative treatments in your herbicide program.
  • At least one population of Group 2 resistant pigweed is multiple-resistant to Group 5 (atrazine) herbicides.

Eastern black nightshade was confirmed in 2000 and found to be resistant to imazethapyr and possibly to other group 2 herbicides

linuron (Group 7)
green pigweed (also known as Powell's amaranth)
First confirmed in 1999 after a long history of linuron use in carrot crops, it is believed to be localized.

Table 3. Herbicides Groupings in Ontario

The group numbers involved follows products with two or more sites of action.

Site of Action
Single modes of action
(alphabetic order)
Two or more modes of action
Inhibitors of acetyl CoA carboxylase; ACCase* Acclaim Super, Achieve, Assure II, Excel Super, Fusilade II, Hoe-grass, Poast Ultra, Select, Venture  
Inhibitors of acetolactate synthase (ALS) and also called acetohydroxyacid synthase (AHAS) Accent, Arsenal, Classic, Elim EP, First Rate, Muster, Pinnacle, Prism, Pursuit, Refine Extra, Reliance STS, Telar, Ultim, Upbeet Broadstrike Dual,Magnum(2,15),Broadstrike Treflan(2,3), Clean Sweep(2,6),Conquest(2,5),Fieldstar(2,4), Meridian Plus(2,6), Patriot(2,5),PeakPlus(2,4), Striker(2,4,4), Summit(2,4), Ultimax(2,4), Valor(2,3), Viper(2,14)
Microtubule assembly inhibitors Edge, Bonanza, Dimension, Prowl, Rival, Treflan Broadstrike Treflan(2,3), Valor(2,3)
Synthetic auxins Banvel II, Caliber, Cobutox, Compitox, Covitox Plus, Dichlorprop-D, Diphenoprop, Dycleer, Dyvel, Embutox, Estaprop, Estasol, Estamine 2,4-D, Garlon 4, Kil-Mor, Killex, Lontrel, MCPA, Mecoprop, Savage 'Mecoprop Plus 2,4-D, Mecoturf Plus 2,4-D, Meco-D, Par III, Premium 3-Way, Release, Target, Tordon 101 mix, Tricep, Turf Herbicide, Turf-Rite 2+2, Turboprop, Weedone CB Buctril M(4,6), Calmix Pellets(4,5), Distinct(4,19), Fieldstar(2,4), Marksman(4,5), PeakPlus(2,4), Shotgun(4,5), Stampede CM(4,7), Striker(2,4,4), Summit(2,4), Ultimax(2,4)
Inhibitors of photosynthesis at photosystem II, Site A Aatrex, Atrazine, Betamix, Betanex, Bladex, Gesagard, Hyvar X, Lexone, Princep Nine-T, Pronone, Pyramin, Sencor, Simadex, Simazine, Sinbar, Spin-Aid , Velpar Axiom(5,15), Boundary(5,15), Calmix Pellets(4,5), Conquest(2,5), Converge(5,28), Krovar(5,7), Laddok(5,6), Marksman(4,5), Patriot(2,5), Primextra II Magnum (5,15), Shotgun(4,5)
Inhibitors of photosynthesis at photosystem II, Site A Basagran, Basagran Forté, Lentagran, Pardner Clean Sweep(2,6), Buctril M(4,6), Laddok(5,6), Meridian Plus(2,6)
Inhibitors of photosynthesis at photosystem II, Site B Afesin, Afolan, Herbec 20P, Karmex, Linuron, Lorox, Patoran Krovar(5,7), Stampede CM(4,7)
Conjugation of acetyl
co-enzyme A
Avadex, Avenge, Betasan, Eradicane, Eptam, Ro-Neet  
Inhibitors of 5-enolpyruvylshikimimate-3-phosphate synthase (EPSP) Clear-It, Credit, Expedite Grass & Weed, E-Z-Ject, Glyfos, Laredo, Maverick, Renegade, Roundup, Touchdown, Vantage, Vision, Wrangler Roundup Fast Forward (9,10)
Inhibitors of glutamine synthetase Ignite, Liberty Roundup Fast Forward (9,10)
Inhibitors of carotenoid biosynthesis Amitrol  
Inhibitors of protoporphyrinogen oxidase (PPO) Blazer, Goal, Reflex, Ronstar  
Conjugation of acetyl
co-enzyme A
Devrinol, Dual Magnum, Dual II Magnum, Frontier Axiom(5,15), Boundary(5,15),
Broadstrike Dual Magnum(2,15), Primextra II Magnum(5,15)
Inhibitors of auxin transport system Alanap Distinct(4,19)
Inhibits cell wall synthesis,
Site A
Photo system I - electron diverters Gramoxone, Reglone, Reward,
Weed & Grass Killer
Inhibitors of mitosis CIPC  
others Basamid, Krenite, Vapam  
Inhibitors of p-hydroxyphenyl pyruvate dioxygenase (HPPD)   Converge(5,28)

Herbicide groupings for Ontario follow the Weed Science Society of America's nationally accepted grouping. Since groups 12, 13, 16, 17, 18, 21, 24, 25 and 26 are not available in Ontario, they have been removed to simplify the chart. Taken from Table 3 in OMAFRA Publication 75, Guide to Weed Control

What to do if You Find A Resistant Weed Population?

Before declaring the weed resistant make sure that other explanations for weed escapes and misses are investigated. Weeds that emerge after application with non-residual herbicides can confuse the diagnosis. Some species are naturally more tolerant to some herbicides. Improper equipment setup, poor spray pattern, canopy penetration, improper weed stage or weather issues can all lead to misses that can be misdiagnosed as weed resistance.

When you do find resistance, the immediate answer is to use an alternative herbicide for which there is no resistance. This may mean adding another different herbicide to the spray tank, making a second pass with an alternative product, or switching to a completely different herbicide program. The main concern with these responses to controlling a resistant weed population is that they may lead to multiple resistance - as happened in Ontario with pigweed and Manitoba with green foxtail.

Resistance is a consequence of relying too much on herbicides for weed control. Changing herbicides may not be viable in the long term if there is no change in the way weeds are managed globally. Resistance management depends on many factors such as the herbicide choice, the types of crops and the type of weed infestations.

When you have resistance, what do you do about it? If you don't have resistance, how do you prevent it? For the most part the answers to both questions are the same. Here are some ideas on managing weed resistance.

  • Avoid using the same herbicide or herbicides from the same grouping in the same field, in consecutive years.
  • Do not use the same mode of action more than once per season. (Rescue sprays should be from a different herbicide group.
  • Use tank mixes where 2 or more products give effective control against the target weed, and the products are from different mode of action groupings.
  • Use crop rotation, changing herbicides is not enough. Different crops allow for a broader spectrum of herbicide and tillage options to control weeds. Some crops are more aggressive than others in competing against weeds.
  • Manage weed escapes effectively. Note trends, successes, and failures.
  • Use clean seed. Clean equipment when moving field to field.
  • Where possible use cultivation, cover crops, or other practices to reduce viable weed seeds in the soil.

Integrated Weed Management is Resistance Management

Resistance delaying tactics such as mixtures and rotations are most useful if they are made part of an Integrated Weed Management (IWM) system. An IWM approach combines all available weed control tools in the best possible way to manage weed populations while maintaining economic crop production. In IWM, cultural and mechanical weed control methods are complemented with chemical weed control.

Mechanical weed control may include inter-row cultivation or other forms of tillage. Cultural control may be an important part of reducing the over reliance on herbicides. This includes using varieties or hybrids that are more competitive, seeding in narrow rows or planting cover crops. Rotation is a part of crop management that may help considerably in preventing resistance. Adding wheat to a corn/soybean rotation can increase the opportunity to challenge weeds. Having a crop that is planted at a different time and that is subject to totally different management than corn and soybeans may have the effect of destabilizing the weed populations. Weeds have a harder time to adapt when management practices are changing.

The Bottom Line

Herbicides are very important tools for weed management Herbicides provide efficient and cost effective weed control and should be seen as resources that need to be protected. If they are overused, resistance will make them obsolete. This is especially important with the Group 2 herbicides because of their many desirable features such as low use rates, low toxicity and high weed control efficacy. By adopting an IWM approach, herbicides along with other methods of control contribute to weed management. This may reduce the selection pressure herbicides apply to resistant weeds.

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca