University of FloridaSolutions for Your Life

Download PDF
Publication #SS-AGR-394

Turfgrass Herbicides: Mechanisms of Action and Resistance Management1

Ramon G. Leon and Bryan Unruh2

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).


[Click thumbnail to enlarge.]

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.


[Click thumbnail to enlarge.]

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.

MOA

Common Name

WSSA Code

HRAC Code

PRE/POST

Trade Name Examples

Acetolactate Synthase (ALS) Inhibitors

 

bispyribac-sodium

2

B

POST

Velocity

 

flazasulfuron

2

B

POST

Katana

 

florasulam

2

B

POST

Defendor

 

foramsulfuron

2

B

POST

Revolver

 

halosulfuron

2

B

POST

SedgeHammer

 

imazaquin

2

B

PRE/POST

Image

 

metsulfuron

2

B

POST

Manor, Blade

 

rimsulfuron

2

B

POST

TranXit

 

sulfometuron

2

B

POST

Oust

 

sulfosulfuron

2

B

POST

Outrider, Certainty

 

trifloxysulfuron

2

B

POST

Monument

Acetyl CoA Carboxylase

(ACCase) Inhibitors

 

clethodim

1

A

POST

Envoy

 

diclofop-methyl

1

A

POST

Illoxan

 

fenoxaprop

1

A

POST

Acclaim

 

fluazifop

1

A

POST

Fusilade

 

sethoxydim

1

A

POST

Segment

4-hydroxyphenyl-pyruvatedioxygenase

(4-HPPD) inhibitors

 

mesotrione

27

F2

PRE/POST

Tenacity

 

topramezone

27

F2

POST

Pylex

Cellulose Inhibitors

 

indaziflam

21

L

PRE

Specticle

 

isoxaben

21

L

PRE

Gallery

Fatty Acid and

Lipid Biosynthesis Inhibitors

 

bensulide

8

N

PRE

Betasan, Bensumec, Pro-San

 

ethofumesate

16

N

PRE/POST

Prograss

Mitosis Inhibitors

 

benefin

3

K1

PRE

Balan, Benefin

 

DCPA

3

K1

PRE

Dacthal

 

dimethenamid-P

15

K3

PRE

Tower

 

dithiopyr

3

K1

PRE

Dimension

 

napropamide

15

K3

PRE

Devrinol

 

oryzalin

3

K1

PRE

Surflan

 

pendimethalin

3

K1

PRE

Pendulum

 

prodiamine

3

K1

PRE

Barricade

 

pronamide

3

K1

PRE/POST

Kerb

 

S-metolachlor

15

K3

PRE

Pennant Magnum

Photosystem II Inhibitors

 

amicarbazone

5

C1

PRE/POST

Xonerate

 

atrazine

5

C1

PRE/POST

Aatrex

 

bromoxynil

6

C3

POST

Buctril

 

hexazinone

5

C1

POST

Velpar

 

metribuzin

5

C1

POST

Sencor

 

simazine

5

C1

PRE/POST

Princep

Protoporphyrinogen Oxidase

(Protox) Inhibitors

 

carfentrazone

14

E

POST

Quick Silver

 

oxadiazon

14

E

PRE

Ronstar

 

pyraflufen-ethyl

14

E

POST

Octane

 

sulfentrazone

14

E

POST

Dismiss

Synthetic Auxins

 

2,4-D amine

4

O

POST

2,4-D amine

 

clopyralid

4

O

POST

Lontrel, Confront

 

dicamba

4

O

POST

Banvel

 

fluroxypyr

4

O

POST

Spotlight

 

MCPA

4

O

POST

MCPA-amine, MCPA-ester, Rhonox, Shredder, Solve

 

MCPP

4

O

POST

Mecoprop-p, MCPP-p 4 amine

 

quinclorac

4

O

POST

Drive

 

triclopyr

4

O

POST

Confront

Dihydropteroate Synthetase Inhibitors

 

asulam

18

I

POST

Asulox

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 http://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.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.