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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MAR 14 2014 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

SUBJECT: 

FROM: 

THRU: 

BEAD Chemical Profile for Registration Review: Imazapyr (128821) and 

Imazapyr, Isopropylamine salt (128829) 

Biological 

Sunil Ratnayake, 

Analysis 

Biologist 

Branch C ~A? •· f'....IV 

·t 

\ 

~ 

1 

Cynthia 

Science Information 

Doucoure, Environmental 

and Analysis Branch 

Protection Specialist {! , ~ 

Derek Berwald, Economist .,fN <fl VA- I J 

Economic Analysis Branch \ ,J . ~ D 

Biological and Economic Analysis Division (7503P) 

Arnet Jones, Chief 

Biological Analysis Branch 

Diann Sims, Chief 

Science Information and Analysis Branch 

Economic 

Timothy Kiely, 

Analysis 

Chief 

Branch 

(;+_. 

~ j M~ 

Biological and Economic Analys1s Division 03P) 

TO: Joel Wolf, Chemical Review Manager 

Risk Management and Implementation Branch V 

Pesticide Re-evaluation Division (7508P) 

Product Review Panel date: February 5, 2014 

Page 1 ofll 
Page 1 of 11

OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

MEMORANDUM

SUBJECT: BEAD Chemical Profile for Registration Review: Imazapyr (128821) and

Imazapyr, Isopropylamine salt (128829)

FROM: Sunil Ratnayake, Biologist

Biological Analysis Branch

Cynthia Doucoure, Environmental Protection Specialist

Science Information and Analysis Branch

Derek Berwald, Economist

Economic Analysis Branch

Biological and Economic Analysis Division (7503P)

THRU: Arnet Jones, Chief

Biological Analysis Branch

Diann Sims, Chief

Science Information and Analysis Branch

Timothy Kiely, Chief 

Economic Analysis Branch

Biological and Economic Analysis Division (7503P)

TO: Joel Wolf, Chemical Review Manager

Risk Management and Implementation Branch V

Pesticide Re-evaluation Division (7508P)

Product Review Panel date: February 5, 2014
Page 2 of 11

SUMMARY

Imazapyr technical ingredient was first registered in the U.S. in 1985 (USEPA, 2006). Imazapyr

(imazapyr and the isopropylamine salt of imazapyr) is a non-selective systemic herbicide 

registered for use as pre and post-emergent treatments to control broad spectrum of terrestrial 

and aquatic weeds. This includes terrestrial annual and perennial grasses, broadleaf weeds, 

herbs, woody species, and riparian and emergent, aquatic weed species. In addition, imazapyr 

plays a major role on controlling grasses and broadleaf weeds in Clearfield® production systems

where the crop is tolerant to imidazolinone herbicides. Imazapyr belongs to the family of 

imidazolinone herbicides and this herbicide provides an effective and broad spectrum of weed 

control at lower rates than many other herbicides. Imazapyr is quickly absorbed through foliage, 

stems, and roots and rapidly translocated through the plants. The mode of action of this 

herbicide is inhibition of the acetolactate synthase (ALS) which is an enzyme involved with the 

biosynthesis of branched-chain amino acids such as leucine, isoleucine, and valine in plant 

species. Growth of treated plants stops within one to three weeks after the application. In the 

United States, 144 weed species have developed resistance to different modes of action of 

herbicides. Of these resistant weeds, biotypes of 46 weed species have developed resistance to 

the mechanism of action of imidazolinone herbicides. Of these weeds, only 4 weed species have 

shown resistance to imazapyr and cross resistance to other ALS inhibiting herbicides.

Based on private market usage data from 1998-2012, the highest usage of imazapyr was 

observed during the year 2002 and since then, a decline in usage is observed in all the states for 

which data are available. Over the last five years it has been applied to less than 300,000 acres 

annually and on less than 1% of corn and pasture acreage. Wider acceptance of glyphosate 

tolerant crops may have contributed to the reduction of usage of imazapyr in the United States. 

Imazapyr and imazapyr isopropylamine salt do not appear to have a major role in traditional 

agriculture, based on use data. Use data is unavailable on many other uses, however, especially 

in forestry sites, aquatic areas and non-agricultural uses. Imazapyr and imazapyr isopropylamine 

salt may be important for weed control in these situations.

INTRODUCTION

The purpose of this document is to convey usage information and a broad overview of the pest 

management roles of pesticides that are beginning to undergo registration review. The 

anticipated audience consists of the team of EPA staff currently evaluating the registration status 

of imazapyr and the isopropylamine salt of imazapyr, and eventually the general public. This 

document builds upon the data BEAD provides in the Label Use Information System (LUIS) 

reports and the Screening Level Usage Analysis (SLUA) to Registration Review teams. The 

document is based on information and data available to BEAD as of January 2014.
Page 3 of 11

HISTORY OF ASSESSMENTS FOR IMAZAPYR

The technical ingredient of imazapyr was first registered in the U.S. in 1985 (USEPA, 2006). 

However, a non-crop end use product had been previously registered in July 1984 and the first 

food use on corn was registered in 1997. Subsequently, the aquatic and grassland uses were 

registered in 2003(Imazapyr Chemical Fact Sheet, 2012). Currently there are 24 tolerances listed 

in 40 CFR § 180.500 for residues of the herbicide imazapyr, applied as the acid or 

isopropylamine salt.

Other than usage reports, BEAD has not assessed the importance or benefits of imazapyr. 

Although imazapyr was reregistered in 2006, BEAD did not evaluate the importance of imazapyr 

as part of that process.

BEAD prepared a Screening Level Usage Analysis (SLUA) in April 2013 that provided data on 

reported agricultural uses of imazapyr on corn and pastureland in the U.S. from 2004-2011. The 

SLUA reported that an average of 9,000 pounds of imazapyr active ingredient (a.i.) was applied

annually on corn and an average of 2,000 pounds a.i. was applied on pastureland. The average 

percent crop treated for both uses was <1%. 

COMMON FORMULATIONS AND APPLICATION METHODS 

Imazapyr is commonly formulated as a soluble and emulsifiable concentrates, wettable powder, 

and water dispersible granules. The isopropylamine salt of imazapyr is commonly formulated as 

aqueous and ready-to-use solutions. Imazapyr and isopropylamine salt of imazapyr are applied 

as a ground application using handheld equipment (backpack sprayers, sprinkling cans, handgun 

sprayers, high-pressure handwand sprayers) or granular broadcast applications using push-type 

or backpack spreaders. They can also be applied as an aerial spray. 

USE SITES 

Agricultural Use Sites:

Imazapyr is registered for use on field corn and imazapyr and the isopropylamine salt of 

imazapyr are registered for use on conifer plantations, forestry sites, and grass pastures and 

rangelands, as well as agricultural non-crop areas.

Non-Agricultural Use Sites:

Imazapyr and the isopropylamine salt of imazapyr are registered for use on wildlife openings, 

non-agricultural non-crop areas, animal housing premises (outdoor), outdoor residential areas 

(paved areas, driveways, patios, etc.), recreational areas, commercial/institutional/industrial 

areas, and in aquatic sites (ponds, lakes, streams, etc.). They are also used on established 

turfgrass on industrial grounds, athletic fields, cemeteries, parks, golf course roughs, and 

institutional grounds. 
Page 4 of 11

IMAZAPYR USAGE

Agricultural Usage Trend

According to proprietary market research surveys, reported use of imazapyr in the U.S. peaked in 

2002 and has decreased substantially in terms of pounds a.i. applied and total area treated since 

then, as shown in Figures 1 and 2. The overall average a.i. rate of application during this period 

was 0.015 lbs a.i. per acre. 

Figure 1. Imazapyr Agricultural Usage: Pounds AI Applied, 1998-2012

Source: EPA Proprietary Data, 2002-2012

0

40,000

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Pounds A.I. Applied

Year
Page 5 of 11

Figure 2: Imazapyr Agricultural Usage: Total Area Treated Applied, 1998-2012

Source: EPA Proprietary Data, 2002-2012

Agricultural Usage Trend: Top Crops, 2003-2012

Reported use of imazapyr decreased on corn and pastureland during the 2003-2012 time frame. 

As shown in Table 1, the average annual pounds a.i. applied on all crops decreased by 86% from 

19,600 lbs a.i during the earlier period (2003-2007) to 2,700 lbs a.i. in the more recent period 

(2008-2012). Application rates for pasture are higher than for corn, and the average a.i. rate 

across all crops is closer to corn, because corn is a much larger use (71% of the total pounds a.i. 

applied from 2008 – 2012). Between the two periods shown in Table 1, the application rate 

changed little for corn, but more than doubled for pasture uses, although this may be an artifact 

of small sample sizes in the use survey.

0

3,000,000

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Total Area Treated

Year
Page 6 of 11

Table 1. Imazapyr Top Crops: Comparison of Usage between the 2003-2007 and 2008-2012 

Timeframes.

2003-2007

Avg Annual Lbs A.I. Applied = 19,600

2008-2012

Avg Annual Lbs A.I. Applied = 2,700

Crop Proportion of 

Total Lbs A.I. 

of Imazapyr 

Applied

Avg. A.I. Rate 

of Application 

(lb/acre)

Crop Proportion of 

Total Lbs A.I. 

of Imazapyr 

Applied

Avg. A.I. Rate of 

Application 

(lb/acre)

Corn 83% 0.013 Corn 71% 0.012

Pastureland 16% 0.21 Pastureland 29% 0.471

Other* 0.2 0.050

*Other crop includes dry beans/peas; however, no active registration was found for this use.

Source: Proprietary Data, 2003-2012

Agricultural Usage Trend: Top States, 2003-2012

Reported use of imazapyr decreased in all states during this period. As shown in Table 2, the 

average annual pounds a.i. applied in all states decreased by 86% from 19,600 lbs a.i during the 

earlier period (2003-2007) to 2,700 lbs a.i. in the more recent period (2008-2012). The average 

a.i. rate of application in recent years was higher for Texas (0.50 lbs a.i./acre) than the overall 

national average a.i. rate (0.15 lbs a.i./acre), reflecting use on pastures only. Tennessee is also 

higher than the national average, but not dramatically so, reflecting use on corn and pastures. 

Indiana, Kentucky, and Illinois average application rates were lower than the national average, 

reflecting use on corn only.

Table 2. Comparison of States with Highest Use of Imazapyr – Percent of Total Lbs. Applied between 

the 2003-2007 and 2008-2012 Timeframes

2003-2007

Avg Annual Lbs A.I. Applied = 19,600

2008-2012

Avg Annual Lbs A.I. Applied = 2,700

State Proportion 

of Total 

Lbs A.I. of 

Imazapyr 

Applied

Avg. A.I. Rate of 

Application 

(lb/acre)

State Proportion 

of Total 

Lbs A.I. of 

Imazapyr 

Applied

Avg. A.I. Rate of 

Application 

(lb/acre)

Illinois 19 0.014 Indiana 26 0.012

Indiana 17 0.014 Kentucky 21 0.011

Arkansas 15 0.214 Texas 19 0.500

Kentucky 10 0.013 Illinois 9 0.014

Iowa 8 0.013 Tennessee 8 0.019

Kansas 7 0.014 Other** 17 0.025

Other* 24 0.014

*Other States in the 2003-2007 timeframe include AL, CO, MI, MN, MS, MO, MT, NE, NC, ND, OH, PA, SC, 

SD, TN, TX, UT, VA, WV, WI, and WY.

**Other States in the 2008-2012 timeframe include AL, AR, FL, IA, MI, MS, MO, NE, OH, SC, SD, and VA.

Source: Proprietary Data, 2003-2012
Page 7 of 11

Agricultural Use Intensity of Imazapyr Use in the U.S.

Another measure of usage is the agricultural use intensity. In this analysis, the use intensity is 

expressed as the average annual amount (in pounds) of the active ingredient applied per 1,000 

acres of crop acres grown. This differs from the application rate, which is expressed as the 

pounds a.i. applied per treated area. Figure 3 spatially represents imazapyr’s use intensity in the 

U.S. on corn and pastureland from 2008 - 2012, which is relatively greater in Kentucky and 

Indiana. This is probably due to relatively high share of total use that is on corn, and because 

these two states combine for 47% of the total agricultural imazapyr use. Note that use in 

Kentucky and Indiana are only high relative to imazapyr use in other states. The average 

application rate in Indiana is 0.012 lbs a.i. per acre (Table 2), which would imply 12 lbs a.i. per 

1,000 treated acres. Use over all cropped acres, however, is a small fraction of a pound. Viewed 

another way, this measure of intensity implies about three treated acres out of every 1,000 acres 

of cropland. For more information about this map, see Appendix A.

Figure 3. Annual Average Agricultural Use Intensity of Imazapyr (2008-2012).

(Source: Proprietary Data, 2008-2012)
Page 8 of 11

Non-Agricultural Usage

BEAD reviewed available data sources and found that imazapyr was reported to be used on 

institutional turf (1,800 lbs a.i. applied in 2011) and by Lawn Care Operators (2,430 lbs a.i. 

applied in 2011). These numbers seem small, but imazapyr is applied at low rates, and they 

represent roughly 50% more than the amount of imazapyr used for agriculture. BEAD has no 

data on what may be important uses of imazapyr to control woody plants in forest settings and in 

other non-agricultural area.

Imazapyr is reported to be widely used as an aquatic herbicide in Florida for control of aquatic 

weeds and woody species growing in water (Univ. of Florida, 2014). However, BEAD has no 

information about the amount of imazapyr applied for such uses.

BIOLOGICAL ASPECTS RELEVANT TO IMAZAPYR

Imazapyr is a non-selective systemic herbicide registered to use as a pre and post-emergent 

herbicide for control a broad spectrum of terrestrial and aquatic weeds, including terrestrial 

annual and perennial grasses, broadleaf weeds, herbs, woody species, and riparian and emergent, 

aquatic weed species. . In addition, imazapyr plays a major role on controlling grasses and 

broadleaf weeds in Clearfield® production systems where the crop is tolerant to imidazolinone 

herbicides. Clearfield® crop varieties were developed using traditional breeding techniques and 

they are not considered as genetically modified organisms or GMO products (Colquhoun, et al., 

2003). Imazapyr belongs to the family of imidazolinones or group 2 herbicides (ALS inhibitors)

(WSSA, Undated). Herbicides in this family are applied at very low rates and have a longer 

period of residual activity compared to many other herbicides. Imazapyr is absorbed through 

roots and foliage and rapidly translocated throughout the plants

The mechanism of action of imidazolinone herbicides is inhibition of the enzyme acetolactate

synthase (ALS) which is also called the acetohydroxyacid synthase (AHAS). It is an enzyme 

involved with the biosynthesis of branched chain amino acids such as leucine, isoleucine, and 

valine (WSSA, Undated). Other major classes of herbicides which inhibit this enzyme are 

sulfonylaminocarbonyltriazolinones, pyrimidinylthiobenzoates, sulfonylureas, and 

triazolopyrimidines (Colquhoun et al., 2013). Variations in residual activity, crop selectivity, 

and the spectrum of weeds controlled may be observed with the application of herbicides 

included in these classes (Univ. of Tennessee, Undated). Generally, injury symptoms due to 

applications of ALS inhibitors appear within a week after the herbicide treatment. Significant 

symptoms are chlorosis, purple leaf veins, and bottlebrush appearance of roots in some weed 

species. Complete death of susceptible weeds may occur within two to three weeks after the 

herbicide applications. Due to the activity of imazapyr, susceptible weeds stop growing and 

either die or become non-competitive with the crop planted in the fields. 

It has been reported that in the United States, 144 weed species have developed resistance to 

different modes of action of herbicides. Of these resistant weeds, biotypes of 46 weed species 

have developed resistance to the mechanism of action of imidazolinone herbicides. Of these 
Page 9 of 11

weeds, only 4 weed species have shown resistance to imazapyr and cross resistance to other ALS 

inhibiting herbicides (WSSA, Undated). This is primarily due to the extensive use of many ALS 

inhibiting herbicides for weed management in agricultural and non-agricultural sites in the 

United States. Weed species with resistance to imidazolinones can be managed by tank-mixing 

imidazolinones with other herbicides with different modes of action.

ROLE OF IMAZAPYR IN PEST MANAGEMENT

Imazapyr does not appear to have a major role in traditional agriculture. Imazapyr’s use on corn 

to control broadleaf and annual weeds, as well as sedge has decreased dramatically to the extent 

that in recent years, less than 1% of the corn acreage is treated with imazapyr. According to a 

USDA ERS publication, currently 85% of corn acreage in the U.S. is planted with Roundup 

ready corn varieties (USDA ERS, 2013). Adoption of glyphosate tolerant corn varieties has 

increased the use of glyphosate and reduced the use of other herbicides in corn production. This 

explains the little usage of imazapyr in the corn production. On the other hand, use of imazapyr

and imazapyr isopropylamine salt on rangeland and pastureland is increasing, especially in

Texas; however, the increase in pounds a.i. applied have not resulted in increases in the percent 

crop treated, which on average was also less than 1%.

Imazapyr is also reported in private market research to play a minor role in the management of 

broad spectrum weeds in institutional turf and lawns. The role of imazapyr in other registered

use sites such as agricultural and non-agricultural uncultivated areas, outdoor residential sites, 

animal housing premises, commercial/institutional/industrial, and recreational areas, as well as 

aquatic sites are not as clear as there is no available data. However, a literature search shows that 

imazapyr is used in the state of Florida to control aquatic weeds such as torpedograss (Panicum 

repens) and woody species growing in water, such as melaleuca (Melaleuca quinquenervia). 

Imazapyr also used for the control of floating-weed species where it is mixed with low

concentrations of glyphosate (Univ. of Florida, 2011). Currently, development of resistance in 

aquatic weeds for imazapyr is not reported in Florida. 

INFORMATION THAT MAY BE USEFUL FOR BEAD’S FUTURE WORK ON 

IMAZAPYR

The Agency currently does not have important information on imazapyr and imazapyr 

isopropylamine salt. Obtaining the following usage information may improve future assessments 

conducted by the Agency related to the use and importance of imazapyr and imazapyr 

isopropylamine salt:

 Information on uncultivated non-agricultural areas, outdoor residential areas (paved 

areas, driveways, patios, etc.), and aquatic sites (ponds, lakes, streams, etc.), including 

pounds a.i. applied, application timing, application rate, application method, and 

application frequency. 
Page 10 of 11

REFERENCES

Colquhoun, J., Mallory, C. S., and D. Ball. 2003. Weed Management in Clearfield Wheat with 

Imazamox. Oregon State Univ. Extension Service. Online. Available at:

http://cropandsoil.oregonstate.edu/wheat/reports/ORCF_weed_mgt.pdf .

Accessed on, Jan. 27, 2013.

EPA Proprietary Data, 1998-2012

EPA. Imazapyr Screening Level Usage Analysis (SLUA), January 2013

Heap, I. The International Survey of Herbicide Resistant Weeds. Undated. Online. Available at: 

 http://www.weedscience.org/summary/MOA.aspx. Accessed on, Jan. 17, 2014

University of Florida. 2011. Plant Management in Florida Waters: An Integrated Approach, 

Imazapyr Considerations. Available at: http://plants.ifas.ufl.edu/manage/developingmanagement-plans/herbicide-considerations/imazapyr-considerations-0

Accessed on, Jan. 27, 2014.

University of Tennessee. (Undated) . Mode of Action Definitions. University of Tennessee, 

Knoxville. Available at: http://hortweeds.tennessee.edu/webapp/test/moa.htm.

Accessed on, Nov. 18, 2013.

USDA ERS. 2013. Adoption of Genetically Engineered Crops in the U.S.

Available at: http://www.ers.usda.gov/data-products/adoption-of-genetically-engineeredcrops-in-the-us/recent-trends-in-ge-adoption.aspx. Online. Accessed on Feb. 6, 2014.

USEPA, 2006. Registration Eligibility Decision for Imazapyr. Online. Available at: 

http://www.epa.gov/oppsrrd1/REDs/imazapyr_red.pdf. Accessed on Jan. 24,2014.

Wisconsin Department of Natural Resources. 2012. Imazapyr Chemical Fact Sheet. Online. 

Available at: http://dnr.wi.gov/lakes/plants/factsheets/ImazapyrFactsheet.pdf. Accessed 

on Jan. 27, 2014.

WSSA (Weed Science Society of America). Undated. Summary of Herbicide Mechanism of 

Action. Weed Science Society of America. Online. Available at: http://wssa.net/wpcontent/uploads/WSSA-Mechanism-of-Action.pdf . Accessed on, Jan.17, 2014.

.
Page 11 of 11

APPENDIX A

The map included herein is primarily for the use of the risk assessors in the Environmental Fate 

and Effects Division. The map provides a very broad geographical view of the average annual 

amount (in pounds) of the active ingredient applied per 1,000 acres of crop acres grown. These 

data are included in the maps because risk assessors are interested in the amount of a pesticide 

used across agricultural land. The calculated values presented in the map are not equivalent to 

an application rate (lb a.i./A). 

The data used to make these maps have several limitations. Any interpretation of the maps 

should consider the underlying data and the associated limitations carefully.

The numerator (annual average pounds applied) is based on private market surveys of pesticide 

use in agriculture averaged over the last five years (Proprietary Data, 2008-2012). These surveys 

cover about 60 crops and are targeted in states that produce the majority of the crop. Although 

the surveys capture most of the use of a particular active ingredient in agriculture, there are 

several limitations to these surveys.

 States with minor production of a surveyed crop are not sampled

 Not all types of pesticides are surveyed in every crop in every year

 Many specialty crops with very small acreages are not included in the survey

The result of these limitations is that states that show no usage may actually have a small amount 

of the active ingredient being used. In some cases the displayed use intensity may be distorted 

because the surveyed crops and the reported pesticide usage may not accurately represent the 

actual pesticide usage on the crops produced in the state.

The denominator (1,000 crop acres grown) was also obtained from the same private market 

survey database. The “Crop Acres Grown” variable represents the total acres grown in a given 

state of all of the surveyed crops. This value is independent of pesticide usage and pesticide 

registration. It is important to note that the surveyed crops (about 60) are sampled from states 

that are major producers of each crop. Therefore, there are cases where the actual crop acreage in 

a state is higher than that reported by crop acres grown in the survey because either that state 

and/or crop was not included in the survey.

The reader should pay particular attention to the figure legends and realize that a map prepared 

for a particular chemical is not directly comparable to a map prepared for a different chemical as 

the legend bins will likely be different.