University of Illinois

No. 7/May 8, 1998

Weed Resistance to Herbicides:
The Problem Continues to Grow

The occurrence of weed biotypes resistant to particular herbicide familiescontinues to increase across much of Illinois. A few years ago, there didnot appear to be a great deal of interest among producers in the subjectof weed resistance, but the economic impact of resistant biotypes has contributedto a high degree of interest at many levels of production agriculture inIllinois.

 What exactly is a herbicide-resistant biotype? A herbicide-resistantplant may be defined as one that was once effectively controlled by a particularmode of herbicide action but which can no longer be controlled. For example,common lambsquarters usually can be effectively controlled with soil orfoliar applications of atrazine, but several areas of Illinois have biotypesof common lambsquarters that can no longer be controlled with atrazine.

 What are biotypes? Biotypes may be defined as "subpopulations"of a species that possess certain traits or characteristics not commonto the entire species. When a particular herbicide effectively controlsthe majority of susceptible members of a species, only those plants (biotypes)that possess a resistance trait can survive and produce seed for futuregenerations. This theory is often referred to as natural selection or survivalof the fittest. Biological organisms (people, plants, animals, etc.) exhibita wide range of diversity. The plants in a population that possess characteristicsenabling them to survive under a wide range of environmental and otheradverse conditions (such as herbicide applications) are able to produceseeds that maintain these survival characteristics. Plants less adaptedgenerally do not survive, and hence only the fittest plants reproduce.

 We should also review some definitions used in describing howherbicides work. Herbicide mode of action refers to the metabolicor physiological process within the plant that is impaired or inhibitedby the herbicide. In other words, mode of action describes how a particularherbicide controls a plant. Herbicide site of action is the actualphysical location within the plant where the herbicide binds or acts, inother words, the target site of the herbicide. Even though these two definitionsare often used as if they were synonymous, they actually describe two completelydifferent aspects of herbicide action. When dealing with herbicide-resistantbiotypes, it can be beneficial to understand the differences between howand where herbicides work.

 So what is the current status of herbicide-resistant biotypesin Illinois? There are confirmed cases of common lambsquarters, smoothpigweed, kochia, and some waterhemp biotypes that are resistant to triazineherbicides. Additionally, biotypes of waterhemp, kochia, and common cockleburhave developed resistance to ALS-inhibiting herbicides. Finally, therehave been reports, as yet unconfirmed, of other weed biotypes demonstratingresistance to these and other herbicide families.

 We are frequently asked about the potential for weed biotypesto develop resistance to glyphosate, the active ingredient in Roundup Ultra.With the widespread adoption of Roundup Ready soybeans and the potentialfor Roundup Ready corn hybrids, the selection pressure for glyphosate-resistantbiotypes will increase dramatically. However, confirmed instances of glyphosateresistantweed biotypes are rare. Even though the potential to select for biotypesresistant to glyphosate may be less than for other herbicide families,widespread use of one weed control technique carries the potential to causea shift in the weed spectrum to one that is not easily controlled. Froman applied viewpoint, weeds that are difficult to control, whether dueto resistance or simply due to a shift in spectrum, translate into manyof the same problems for the producer.

 What management steps can be implemented that could lower thepotential for developing herbicide-resistant biotypes or possibly delaya shift in the weed spectrum? The following list of strategies is offeredfor consideration. In most instances, incorporating as many of these strategiesas possible will prove more beneficial than using only one.


  1. Scout your fields to know what weed spectrum you are dealing with. If youhave been relying on one particular herbicide for several years and noticethat some weed species that was effectively controlled in past seasonsis now abundant, or some species are now present that you haven't everdealt with in a particular field, this could indicate that a herbicide-resistantbiotype or weed-species shift has developed.

  3. Rotate herbicides with different modes of action. Do not make more thantwo consecutive applications of herbicides with the same mode of actionagainst the same weed unless other effective control practices are includedin the management system. Consecutive applications can be single applicationsin 2 years or two split applications in one year.

  5. Apply herbicides in tank-mixed, prepackaged, or sequential mixtures thatinclude multiple modes of action. Both herbicides in the mixture must havesubstantial activity against potentially resistant weeds, as well as similarpersistence if they possess soil activity. For example, if you are concernedabout potentially ALS-resistant pigweed, a tank mixture of Basagran withan ALS-inhibitor would be a poor choice because Basagran has very littleactivity on pigweed. A couple of guidelines may help with tank-mix or premixselection: (a) When applied alone at the rate that will be used in thetank mixture, does the tank-mix or premix partner control the weed speciethat I am concerned may develop resistance? (b) If I apply the tank-mixor premix partner alone at the rate that will be used in the tank mix,will its residual activity be similar to the other component's?

  7. As new herbicide-tolerant/resistant crops become available, their use shouldstill not result in more than two consecutive applications of herbicideswith the same mode of action against the same weed species unless othereffective practices are included in the management system.

  9. Combine mechanical control practices (such as rotary hoeing and cultivation)with herbicide treatments whenever possible.

  11. Clean tillage and harvest equipment before moving from fields infestedwith resistant weeds to fields that are not infested. This may not alwaysbe practical, but it can help prevent the spread of resistant weed seedin soil that adheres to the equipment.
Implementing several of these management steps can help delay the developmentof herbicide-resistant weed biotypes; but, if you are dealing with a fieldthat already has a substantial population of a resistant biotype, strategiessuch as herbicide rotation and utilizing tank mixtures may need to be emphasizedfor a longer period of time. For example, if a resistant biotype has beenselected for with an ALS-inhibiting herbicide and is well established,it may take several years of using herbicides with some alternative modeof action to bring the problem to a manageable level before rotating backto the herbicide that selected for the resistant biotype.

 Tables 2 and 3 contain a list of triazine and ALS-inhibitingherbicides. Because weed biotypes resistant to these herbicide familieshave been identified in Illinois, producers may find it beneficial to knowwhich products on the market today have similar modes of action.

 For more information on weed resistance to herbicides, consultChapter 20 of the 1998 Illinois Agricultural Pest Management Handbook.

 Table 2. Herbicides and herbicide premixes containing a triazineherbicide

Trade nameCommon name
Bicep II, Bicep Lite II,
Bicep II Magnum, Bicep Lite II Magum
atrazine + metolachlor*
Buctril + atrazineatrazine + bromoxynil
Bladex, Cy-Procyanazine
Bulletatrazine + alachlor
Contouratrazine + imazethapyr
Extrazine II, Cy-Pro ATatrazine + cyanazine
Guardsmanatrazine + dimethenamid
FieldMasteratrazine + acetochlor + glyposate
Surpass 100, Harness Extra,
atrazine + acetochlor
Laddok S-12atrazine + bentazon
Marksmanatrazine + dicamba
Shotgunatrazine + 2,4-D
Sencor, Lexonemetribuzin
Turbometribuzin + metolachlor
Canopymetribuzin + chlorimuron
Axiommetribuzin + fluthiamide

 *Herbicides in italics have a different mode of action.

 Table 3. Herbicides and herbicide premixes containing ALS-inhibitors

Trade nameCommon name
Pursuit Plusimazethapyr + pendimethalin*
Steelimazethapyr + imazaquin + pendimethalin
Contourimazethapyr + atrazine
Resolveimazethapyr + dicamba
Lightningimazethapyr + imazapyr
Squadronimazaquin + pendimethalin
Scepter O.T.imazaquin + acifluorfen
Detailimazaquin + dimethenamid
Tri-Sceptimazaquin + trifluralin
Arsenal, Chopperimazapyr
Classic, Skirmishchlorimuron
Canopychlorimuron + metribuzin
Canopy XL, Authority Broadleafchlorimuron + sulfentrazone
Celebritynicosulfuron + dicamba
Synchrony STS, Reliancethifensulfuron + chlorimuron
Harmony Extrathifensulfuron + tribenuron
Basisthifensulfuron + rimsulfuron
Basis Goldnicosulfuron + rimsulfuron + atrazine
Accent Goldnicosulfuron + rimsulfuron
+ flumetsulam + clopyralid
Exceed, Spiritprosulfuron + primisulfuron
Peak prosulfuron
Glean, Telarchlorsulfuron
Broadstrike + Dualflumetsulam + metolachlor
Hornetflumetsulam + clopyralid
Scorpion IIIflumetsulam + clopyralid + 2,4-D
Broadstrike + Treflanflumetsulam + trifluralin
*Herbicides in italics have a different mode of action.

 Aaron Hager ( Marshal McGlamery (,Department of Crop Sciences, (217)333-4424