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Aminopyralid - New herbicide for pastures, roadsides, etc.  
Bob Hartzler

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 February 21, 2006 - The growth regulator herbicides, specifically 2,4-D and MCPA, ushered in the era of chemical weed control in the 1940's. More than 60 years later one might think this class of chemistry would have been milked dry, but the introduction of Milestone by DowAgrosciences shows there are still some interesting compounds to be found in this herbicide class.

Introduction - The growth regulator herbicides mimic the action of auxin (indole-3-acetic acid), a plant growth regulator found in all plants. Growth regulator herbicides are able to bind to auxin receptor proteins that trigger specific physiological responses involved in the growth and development of plant cells. Plants are able to closely regulate the concentration of auxin that binds to these receptors, and therefore control the hormone's activity. However, sensitive plants are unable to regulate the concentration of the growth regulator herbicides, and therefore physiological processes are disrupted that can lead to plant death. Table 1 lists several of the growth regulator herbicides that have been introduced since the discovery of 2,4-D and MCPA.

Table 1. Examples of growth regulator herbicides.


Common name


Year introduced






no longer marketed










Banvel, Clarity




Tordon, Grazon



Garlon, Remedy



Stinger, Transline, Hornet



Milestone, Forefront


Chemical characteristics Aminopyralid, like other growth regulator herbicides, selectively controls broadleaf weeds in grasses. In terms of environmental risks, aminopyralid has a few significant advantages over other products used in similar settings. First, aminopyralid has a higher specific activity than other growth regulator herbicides, thus it is used at lower rates (Table 2). Aminopyralid has a significantly shorter half-life than clopyralid and picloram, but is more persistent than 2,4-D or dicamba. It is relatively immobile in soil, with most of the chemical remaining within the upper 12" of the soil profile. Products containing aminopyralid can not be applied directly to water, but can be used to treat banks of ditches or other channels that do not carry water used for irrigation or drinking. Applications should be avoided to areas where movement into water used for irrigation or drinking could occur.

Table 2. Properties of several growth regulator herbicides.


Product rate /A

Lb ai/A

Half-life (Days)


4-7 oz Milestone




0.6-1.3 pt Stinger




2-4 pt Grazon P&D



1 qt 2,4-D 4 lb gal
1 lb


1 pt Banvel/Clarity







In terms of selectivity, aminopyralid falls between picloram and clopyralid. Clopyralid (Stinger/Transline) is highly selective, and provides some unique opportunities for weed control. Aminopyralid is active against many of our important pasture weeds, including weeds in the composite (sunflower), legume, and solanaceae (nightshade) family. It has poor activity on weeds in the carrot family, including wild carrot, wild parsnip, and poison hemlock. While the risk of injury to trees or other non-target plants in treated areas has not been fully clarified, data suggest that leguminous trees (black locust, honey locust, redbud, etc.) may be injured if aminopyralid is applied under their dripline.

Commercial formulations - DowAgroSciences recently introduced three products that contain aminopyralid (Table 3). Milestone and Milestone VM both contain 2 lb/gal aminopyralid, but have slightly different markets. Milestone and ForeFront are both registered for use in pastures; ForeFront will provide broader spectrum control due to the addition of 2,4-D to this formulation. Milestone does not have any grazing or hay restrictions, whereas forage treated with ForeFront can not be harvested for hay within 7 days of application. Aminopyralid may persist in the manure and urine of animals feeding on treated forage, thus animals should be allowed to graze on untreated pastures for 3 days before being transferred to areas where sensitive broadleaf crops occur.

Table 3. Commercial formulations containing aminopyralid.



Use rate

Application sites


2 lb/gal aminopyralid

3-7 oz/A

Pastures, CRP, non-cropland, natural areas

Milestone VM

2 lb/gal aminopyralid

3-7 oz/A

Industrial sites, rights-of-way, natural areas

ForeFront R&P

0.33 lb/gal aminopyralid +
2.67 lb 2,4-D amine

1.5 -2.6 pt/A

Pastures and CRP

A primary target for the aminopyralid products will be the thistles, both biennial (musk, bull, etc.) and Canada. Research across the Midwest has shown that Milestone provides equivalent or better control of these weeds than current standards (Grazon, Tordon, Transline, etc.). For Canada thistle, 5 to 7 oz of Milestone or 2 to 2.6 pt ForeFront is recommended. Applications can be made in the spring between bolting and bud formation, or in the fall to new regrowth. Addition of a non-ionic surfactant is recommended, and the use of higher spray volumes (>10 gal/A) will provide more consistent control. For musk and other biennial thistles, Milestone is recommended at 3 to 5 oz/A for fall applications or spring applications prior to bolting. After the stems of biennial thistles have begun to elongate, Milestone should be applied at a rate of 4 to 5 oz/A. Milestone and ForeFront will control any legumes present in treated pastures.

Aminopyralid is a new active ingredient from the pyridine class of compounds. While this compound has excellent activity on Canada and other weedy thistle species, it is important to realize than no single herbicide treatment can provide long-term control of an established infestation. An integrated program, in which the competitiveness of the desired vegetation is enhanced and appropriate follow-up tactics are implemented, will be required to provide long-term control of these and other weedy species.

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
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.