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New Information on Herbicide Labels  
by Bob Hartzler

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December 17, 2010 - Herbicide labels now include a standardized system to inform users of the product’s mechanism of action (MOA). A box labeled ‘Herbicide Group’ is present near the top of the label. The number in the box is the MOA of the active ingredient based on a system developed by the Weed Science Society of America. Premixes containing more than one mode of action will have multiple numbers listed. A similar system is also used on labels of insecticides and fungicides. The following is an example of the symbol:

The intent of this information is to simplify development of herbicide programs that reduce the likelihood of selecting herbicide resistant weeds. In production systems relying largely on herbicides for weed management, using herbicides with different MOA’s is the primary method of managing resistance.

Generally, the greater number of MOA’s used, the less selection pressure placed on weeds. However, designing an integrated program is not as simple as randomly adding MOA’s. The different MOA’s used in the program must have good activity on the important weeds in the field to successfully reduce selection pressure. Following are a few examples where the inclusion of an herbicide in a system relying on glyphosate in Roundup Ready crops would provide little benefit in terms of managing resistance for specific weeds.

The new labeling system eliminates the need for farmers, consultants and suppliers to learn the MOA of all the active ingredients used in Iowa agriculture. However, to use the information properly, users must still know the activity of the individual herbicides on the weeds present in the field to insure that the target weeds are being affected by multiple MOA’s.

Herbicide mechanism of action classification(1) with example herbicides

Group

Mechanism of Action

Family(2)

Common name

Trade name

1

ACC-ase inhibitor

‘fops’

quiazalofop

Assure II

 

 

‘dims’

clethodim

Select

2

ALS inhibitors

sulfonylurea

nicosulfuron

Accent

 

 

imidazolinone

imazethapyr

Pursuit

 

 

-

flumetsulam

 

3

Mitosis inhibitors

dinitroalinine

trifluralin

Treflan

4

Synthetic auxins

phenoxy

2,4-D

many

 

 

-

dicamba

Clarity

5

Photosystem II inhibitor(3)

triazine

atrazine

many

6

Photosystem II inhibitor (3)

-

bromoxynil

Buctril

9

EPSPS inhibitor

-

glyphosate

Roundup

10

Glutamine synthetase inhibitor

-

glufosinate

Ignite

13

Carotene inhibitor

-

clomozone

Command

14

PPO inhibitors

diphenylether

fomesafen

Reflex

 

 

-

flumioxazin

Valor

15

Lipid inhibitors

acetamide

metolachlor

Dual

 

 

-

dimethenamid-P

Outlook

19

Auxin transport inhibitor

-

diflufenzopyr

Status

22

Photosystem I inhibitor

bipyridilliums

paraquat

Gramoxone

27

HPPD inhibitor

triketones

tembotrione

Laudis

 

 

-

mesotrione

Callisto


(1) Weed Science Society of America Mechanism of Action. Only groups used in corn and soybean included in the table.
(2) Family name listed for groups with multiple herbicides.
(3) Group 5 and 6 herbicides bind at different locations of the same target site.

 

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
Submit questions or comments here.  

Copyright 1996-2006, Iowa State University, all rights reserved  

Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.