Turfgrass Herbicides: Mechanisms of Action and Resistance Management

Ramon G. Leon and Bryan Unruh

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TARGET AUDIENCE

The present document is a tool for turfgrass professionals, sod growers, landscape managers, and extension specialists to develop herbicide programs that reduce the risk of herbicide resistance (HR) evolution in turfgrass systems.

INTRODUCTION

Herbicides are the most common and effective tool for weed control in turfgrass. However, weed species are capable of adapting, and now almost all herbicide groups have confirmed cases of weed species with resistance. Although HR is always a concern, this problem is particularly serious in turfgrass because typical HR management practices such as crop rotation, tillage and cultivation, cover cropping, and fallow periods are generally not an option in turfgrass production. More importantly, there are considerably fewer products that can be used for herbicide rotation in turfgrass than there are for other agricultural systems.

REDUCING THE RISK OF HR

There are several things that can delay and perhaps prevent HR. The most practical and effective strategy is to rotate herbicides with different mechanisms of action (MOA). The MOA is the way the herbicide disrupts the metabolism of the weed, ultimately causing the weed’s death. Table 1 provides a comprehensive list of herbicides that are registered for use in turfgrass and their respective MOA and classification according to the Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (HRAC). Many herbicide labels include the herbicide group number (Figure 1) to help the user design herbicide rotations. Simply put, if two herbicides have the same MOA number or code, regardless of whether they have different names or active ingredients, they affect weeds in the same way. Thus, the frequent and repeated use of herbicides with the same MOA will increase the risk of weeds becoming resistant. Conversely, using a diverse herbicide program that either rotates or combines herbicides with different MOAs will help delay the appearance of resistant weeds.

Figure 1. Example of a label indicating the MOA group number (red arrow).
Figure 1.  Example of a label indicating the MOA group number (red arrow).

MOA rotation helps delay HR because changing the MOA reduces the chances of survival and reproduction of HR weeds. In simple terms, if a weed survives an herbicide application with one MOA because it is resistant, the problem can be controlled if the surviving weed is treated with a herbicide with a different MOA to which it has not become resistant.

As shown in Table 1, most herbicides for preemergence (PRE) control are mitosis inhibitors (Groups 3 and 15), while postemergence herbicides are predominantly ALS-inhibitors (Group 2). Although many turfgrass professionals base their herbicide programs solely on Group 3 and Group 2, it is critical that herbicides from other MOAs are included in herbicide programs. To ensure that the most frequently used herbicides in turfgrass will continue being effective for a long time, herbicides with different MOAs should be included in weed management programs even if they are not as effective or require repeat applications to provide the desired level of control.

Groups that are useful for MOA rotation in PRE programs in turfgrass:

  • Cellulose inhibitors (Group 21)
  • Fatty acid and lipid biosynthesis inhibitors (Groups 8 and 16)
  • PPO inhibitors (Group 14)

Groups that are useful for MOA rotation in POST programs in turfgrass:

  • Carotenoid biosynthesis inhibitors (Group 28)
  • Lipid biosynthesis inhibitors (Group 16)
  • Photosystem II inhibitors (Group 5)
  • PPO inhibitors (Group 14)
  • Synthetic auxins (Group 4)

There are two ways to rotate herbicides in turfgrass. The first way is to change the MOA from year to year (Figure 2). For example, one could use a Group 3 herbicide in the fall of year 1 and change to Group 21 in the fall of year 2. The second way is to rotate herbicides within a season. In this case, MOAs are rotated to control escapes from the previous application. For example, if a Group 16 herbicide was used as PRE or POST and some plants escaped, a Group 14 herbicide can be applied to kill the escapes. This is called the “double-knock down” strategy because weed control is based on two consecutive actions. Ideally, both approaches should be used within the same weed management program.

Figure 2. Example of MOA rotation within season and across years. MOAs change from the preemergence (PRE) to postemergence (POST) applications, and each year the MOAs of the PRE and POST herbicides change. MOA groups are specified within parentheses.
Figure 2.  Example of MOA rotation within season and across years. MOAs change from the preemergence (PRE) to postemergence (POST) applications, and each year the MOAs of the PRE and POST herbicides change. MOA groups are specified within parentheses.

Another strategy to delay HR is to apply herbicides with different MOA simultaneously. This can be done by tank-mixing herbicides with different MOAs or by using pre-mixed products with two or more herbicides with different MOAs. For this option to work for HR management, the herbicides with different MOA have to be effective when applied alone to control the target weed species.

TABLES

Table 1. 

Mechanism of action (MOA) classification according to the Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (HRAC) for preemergence (PRE) and postemergence (POST) herbicides registered for use in turfgrass.

MOACommon NameWSSA CodeHRAC CodePRE/POSTTrade Name Examples
Acetolactate Synthase (ALS) Inhibitors
 bispyribac-sodium2BPOSTVelocity
 flazasulfuron2BPOSTKatana
 florasulam2BPOSTDefendor
 foramsulfuron2BPOSTRevolver
 halosulfuron2BPOSTSedgeHammer
 imazaquin2BPRE/POSTImage
 metsulfuron2BPOSTManor, Blade
 rimsulfuron2BPOSTTranXit
 sulfometuron2BPOSTOust
 sulfosulfuron2BPOSTOutrider, Certainty
 trifloxysulfuron2BPOSTMonument
Acetyl CoA Carboxylase(ACCase) Inhibitors
 clethodim1APOSTEnvoy
 diclofop-methyl1APOSTIlloxan
 fenoxaprop1APOSTAcclaim
 fluazifop1APOSTFusilade
 sethoxydim1APOSTSegment
4-hydroxyphenyl-pyruvatedioxygenase(4-HPPD) inhibitors
 mesotrione27F2PRE/POSTTenacity
 topramezone27F2POSTPylex
Cellulose Inhibitors
 indaziflam21LPRESpecticle
 isoxaben21LPREGallery
Fatty Acid andLipid Biosynthesis Inhibitors
 bensulide8NPREBetasan, Bensumec, Pro-San
 ethofumesate16NPRE/POSTPrograss
Mitosis Inhibitors
 benefin3K1PREBalan, Benefin
 DCPA3K1PREDacthal
 dimethenamid-P15K3PRETower
 dithiopyr3K1PREDimension
 napropamide15K3PREDevrinol
 oryzalin3K1PRESurflan
 pendimethalin3K1PREPendulum
 prodiamine3K1PREBarricade
 pronamide3K1PRE/POSTKerb
 S-metolachlor15K3PREPennant Magnum
Photosystem II Inhibitors
 amicarbazone5C1PRE/POSTXonerate
 atrazine5C1PRE/POSTAatrex
 bromoxynil6C3POSTBuctril
 hexazinone5C1POSTVelpar
 metribuzin5C1POSTSencor
 simazine5C1PRE/POSTPrincep
Protoporphyrinogen Oxidase(Protox) Inhibitors
 carfentrazone14EPOSTQuick Silver
 oxadiazon14EPRERonstar
 pyraflufen-ethyl14EPOSTOctane
 sulfentrazone14EPOSTDismiss
Synthetic Auxins
 2,4-D amine4OPOST2,4-D amine
 clopyralid4OPOSTLontrel, Confront
 dicamba4OPOSTBanvel
 fluroxypyr4OPOSTSpotlight
 MCPA4OPOSTMCPA-amine, MCPA-ester, Rhonox, Shredder, Solve
 MCPP4OPOSTMecoprop-p, MCPP-p 4 amine
 quinclorac4OPOSTDrive
 triclopyr4OPOSTConfront
Dihydropteroate Synthetase Inhibitors
 asulam18IPOSTAsulox

FOOTNOTES

1. This document is SS-AGR-394, one of a series of the Agronomy Department, UF/IFAS Extension. Original publication date August 2015. Visit the EDIS website at https://edis.ifas.ufl.edu.2. Ramon G. Leon, assistant professor, UF/IFAS West Florida Research and Education Center, Jay, FL; and Bryan Unruh, professor, UF/IFAS West Florida REC, Jay, FL; UF/IFAS Extension, Gainesville, FL 32611.

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