UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MAR 10 ZC14 

MEMORANDUM 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

SUBJECT: BEAD Chemical Profile (BCP) for Registration Review Imazethapyr Chemical 

Case [Includes Imazethapyr (128922) and Imazethapyr, Ammonium Salt 

(128923)] 

FROM: 

Biological 

Bill Phillips, 

Analysis 

PhD, Agronomist Branch~V ~ 

Rafael A. Prieto. Environmental Protection Specialist /Jhh-t hf' c(' /Q .", r/7 

Science Information and Analysis Branch vvr v-L[ ""' r /W'f v 

THRU: 

Michelle Ranville, Economist ~ rri\) 

Economic Analysis Branch ~-J~J 

Biological and Economic Analysis tvision (7503P) 

Arnet Jones, Chief 

Biological Analysis Branch 

Diann Sims, Chief 

Science Information and Analysis Branch 

Timothy 

Economic 

Kiely, 

Analysis 

Chief 

Branch 

/J1~}(_a) ~ 

Biological and Economic Analysis Division (7503P) 

TO: Katherine St. Clair, Chemical Review Manager 

Risk Management and Implementation Branch 2 

Pesticide Re-evaluation Division (7508P) 

Peer Review Panel date: February 5, 2014 

1 
SUMMARY 

Imazethapyr is an imidazolinone herbicide that inhibits the production of the key plant enzyme 

acetolactate synthase (ALS). It is used preplant incorporated, preemergence, or postemergence 

to control annual broadleaf weeds and annual grasses. Based on the percent crop treated, the two 

crops with the highest percentage of use are green peas and rice. With over 40% and 30%, 

respectively, of the crops receiving applications of imazethapyr, it is clear that there is an 

important use for this herbicide in those crops. Although there are more than a dozen weed 

species that have biotypes which are resistant to imazethapyr, this herbicide, in combination with 

others, seems to be of particular importance for controlling some difficult to control broadleaf 

and grass weeds in green pea and rice production. In addition, imazethapyr may be an important 

tool for managing weeds in an integrated weed management program. 

INTRODUCTION 

The Biological and Economic Analysis Division (BEAD) Chemical Profile (BCP) provides an 

overview of the pest management roles and quantitative usage information for imazethapyr and 

its ammonium salt. The BCP includes summaries of previous BEAD products and available 

public and/or private use and usage data for imazethapyr to describe the role of this chemical in 

pest management. Additional information on use is available in the imazethapyr Label Data 

Report. Information provided in this BCP may be used by stakeholders (e.g., users and 

registrants), OPP risk assessors, OPP risk managers, and the general public during the 

Registration Review process for this pesticide. 

USE SITES 

Agricultural Use Sites 

Registered agricultural use sites for imazethapyr include: fallow/idleland /conservation reserve 

program (CRP), alfalfa, beans (dried-type, succulent, lima), clover, com (unspecified, field), 

garbanzos (including chick peas), lentils, lupine (grain), ornamental farms, peanuts, peas 

(unspecified, dried-type, southern, succulent), rice, and soybeans. 

Registered agricultural use sites for imazethapyr ammonium salt include: fallow/idleland I 

conservation reserve program (CRP), alfalfa, beans (dried-type, succulent, lima), clover, field 

com, endive (escarole), garbanzos (including chick peas), lentils, lettuce, lupine (grain), peanuts, 

peas (unspecified, dried-type, southern, succulent), rice, soybeans (edible, unspecified), and 

trefoil. 

Non-agricultural Use Sites 

Registered non-agricultural use sites for imazethapyr include: rights-of-way (railroad, other), 

fencerows, hedgerows, uncultivated areas/soils, and ornamental lawns and turf. 

Registered non-agricultural use sites for imazethapyr ammonium salt include: airports, landing 

fields, commercial/ institutionaV industrial premises and equipment (outdoor), industrial and 

2 
construction areas (outdoor), rights-of-way, fencerows, hedgerows, ornamental lawns and turf, 

and utility poles. 

FORMULATIONS AND APPLICATION METHODS 

Imazethapyr is formulated as an emulsifiable concentrate, a soluble liquid concentrate, water 

dispersible granules, and a wettable powder. 

Imazethapyr ammonium salt is formulated as soluble liquid concentrate. 

HISTORY OF IMAZETHAPYR and BEAD PRODUCTS 

The first herbicide containing imazethapyr was registered in 1990 and the first herbicide 

containing imazethapyr's ammonium salt was first registered in 1989. Both products were first 

labeled to control broadleaf weeds and grasses in soybeans. 

BEAD conducted assessments for two Section 18 emergency exemption requests for use of the 

ammonium salt of imazethapyr on Clearfield rice (genetically engineered to withstand 

imazethapyr) to control red rice in Louisiana and Texas for the 2001 growing season. In both 

cases, BEAD found that adequate alternatives existed, which included alternative chemicals and 

cultural/preventative controls (e.g., crop rotation, cleaning of machinery) (Deitrich & Halvorson, 

2001 a, 2001 b). In that same year, special local needs (SLN) registrations for imazethapyr were 

granted to several southern states (including Louisiana and Texas) to control red rice and other 

grasses and weeds in rice. By 2003, all of these SLNs had expired; in 2008, five states 

(Arkansas, Louisiana, Mississippi, Missouri, and Texas) declared a crisis exemption for the use 

of imazethapyr to control red rice on Clearfield rice. The crisis was approved in all five states, 

but Texas and Arkansas later withdrew their requests. No BEAD reviews were conducted for the 

crisis exemptions, but these requests and the earlier SLN registrations could indicate that the 

chemical is preferred by growers for the control of red rice despite no BEAD finding of 

emergency conditions for the 2001 Section 18 requests. The lack of an emergency situation does 

not preclude importance of the requested chemical. 

USAGE 

Usage Trends, Agricultural Uses 

Imazethapyr's annual agricultural usage averaged 441,000 pounds of active ingredient (a.i.) per 

year from 1998-2012. The average total area treated in that time frame was 8.4 million acres per 

year, and the overall average application rate was 0.053 pounds a.i. per acre. Figure 1 presents 

imazethapyr's usage in pounds a.i. over the 1998-2012 time frame. Figure 2 presents total area 

treated in acres over the same timeframe. These show that, after a deep decrease in usage, 

imazethapyr' s usage has been steadily increasing since 2007. (Proprietary data, 1998-20 12). 

3 
Fi re 1. Imazetha 

900,000 

0 ~ m o ~ N m ~ ~ ~ ~ ~ m o ~ N 

mmooooooooooooo 

mmoooooooooo~~~ 

~ ~ N N N N N N N N N N N N N 

Source: Proprietary Data, 1998-2012 

Fi ure 2. Imazetha r, Total Area Treated 

~mo~Nm~~~~~mo~N 

mmooooooooooooo 

mmoooooooooo~~~ 

~~NNNNNNNNNNNNN 

Source: Proprietary Data, 1998-2012 

lied, 1998-2012 

- Pounds AI 

- TAT (acres) 

As seen in Table 1, the average annual usage (lbs A. I. applied) across all crops increased by 

more than 100,000 lbs from the 2003-2007 time frame to the 2008-2012 time frame. 

Imazethapyr usage in com has decreased as a percent of total usage while usage in rice has 

increased its share. Usage in soybeans accounted for the largest proportion oftotallbs applied in 

both time periods. 

4 
T a e bl 1 P . ropo rti 000 fl maze th apyr p ouo d A s •PPJie r db ,y c rop, 2003 2012 - 2003-2007 2008-2012 

Average annuallbs AI applied = 286,000 Average annuallbs AI applied = 408,000 

Crop %Lbs Crop %Lbs 

Soybeans 49 Soybeans 58 

Com 19 Rice 32 

Rice 19 Alfalfa 3 

Alfalfa 7 Com 2 

Dry Beans/Peas 4 Dry Beans/Peas 2 

Other* 3 Other* 2 

*Other crops include peas (fresh/green/sweet), peanuts, beans (green, lima), sweet com, and fallow land (2008-

2012 onl 

Source: Proprietary Data, 2003-2012 

Table 2 shows the proportion of total usage in the top ten states. The top six states for 

imazethapyr usage are either top rice or soybean producing states. 

T a e bl 2 P . ropo rf 1000 fl maze th apyr p ouo d s Applied by State, 2008-2012 

Average annuallbs AI applied= 408,000 

State %Lbs 

Arkansas 18 

Iowa 11 

Nebraska 10 

Louisiana 9 

Indiana 8 

Illinois 6 

South Dakota 4 

North Dakota 4 

Kentucky 4 

Missouri 4 

Other* 23 

*Other states with usage in the 2008-2012 time frame 

are: South Dakota, North Dakota, Kentucky, Missouri, 

Minnesota, Wisconsin, Michigan, Mississippi, Texas, 

Kansas, Ohio, Pennsylvania, Washington, Utah, 

California, Maryland, Oklahoma, Montana, Virginia, 

New Mexico, Idaho, North Carolina, South Carolina, 

Arizona, Delaware, Tennessee, Oregon, Nevada, 

Georgia, Colorado, Wyoming, and New York. 

Source: Proprietary Data, 2008-2012 

Application Rates and Percent Crop Treated 

The highest use in terms of pounds a.i. applied and total area treated (TAT) was in soybeans. 

However, the highest application rates were on rice and peanuts, 0.068 and 0.067lbs a.i. per 

acre, respectively. (Table 3) 

5 
T a e bl 3 I . maze th apyr u se: p oun d s, TAT , an d A R vg a t e, 2008 -2012* 

Crop Average Annual Average Annual Average AI Rate 

Pounds AI Applied* Total Acres Treated** (Lbs Al/acre) 

Soybeans 238,000 4,610,000 0.052 

Rice 132,000 1,930,000 0.068 

Alfalfa 12,000 200,000 0.060 

Com 10,000 240,000 0.042 

Dry Beans/Peas 10,000 310,000 0.032 

Peas, green 3,000 70,000 0.043 

Peanuts 2,000 30,000 0.067 

*Table includes all crops with average annuallbs applied > 1 ,000 lbs; lbs AI applied is rounded to nearest thousand, 

TAT is rounded to n~arest ten thousand 

**Total Acres Treated (TAT) equals the sum of all acres treated for a crop; acres that receive multiple treatments 

are counted multiple times in TAT. 

Source: EPA Proprietary Data, 2008-2012 

Table 4 displays percent crop treated (PCT) estimates for eight crops for which data were 

available. Despite the highest proportion of usage in soybeans and rice, the crops with the 

highest PCT of imazethapyr are green peas, rice, dry beans and peas, and green beans. 

Tabl e 4 P . ercen tC rop T t d rea e WI 'th I maze th apyr, 2004 -2011 

Crop Avera2ePCT 

Peas, Green 40 

Rice 30 

Dry Beans/Peas 10 

Beans, Green 10 

Soybeans 5 

Peanuts 5 

Alfalfa <2.5 

Com <1.0 

Source: EPA, 2013, Screening Level Usage Analysts (SLUA) 

Geographic Distribution of Imazethapvr Use in Agriculture 

Another measure of usage is the 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 imazethapyr use intensity in the U.S. Use intensity 

is highest Louisiana and Arkansas, the two states with the highest rice production. For more 

information about this map, see Appendix A. 

6 
Figure 3. Imazethapyr Usage by State (2008-2012) 

Pounds Active Ingredient 

per 1,000 Crop Acres Grown 

c:::::J No Data 

0 <0.5 

c=J o.5-1.5 

. 1.5-3 ->3 

Source: Proprietary Data, 2008-2012. See Appendix A for additional details on this map 

Non-Agricultural Usage 

Available non-agricultural usage data for imazethapyr is limited and most are dated. Data 

available from the USDA NASS Chemical Use Survey shows that, in the year 2006, imazethapyr 

was applied to 1 percent of floriculture operations in Florida; there was also use in Michigan 

floriculture operations, but the percentage was not reported because it was less than 0.5 percent. 

The ammonium salt was applied to 12 percent of propagative material in floriculture operations 

in Michigan in the same year. (USDA NASS Chemical Use Survey, 2006) 

Older non-agricultural usage data for reports approximately 324lbs a.i. ofimazethapyr were 

applied in landscaping in 1999, which accounted for less than 1% of totall bs a.i. of herbicides 

applied in that sector. More recent data suggests showed that approximately 750 lbs a.i. of 

imazethapyr were applied by lawn care operators and another 440 lbs were applied to golf 

courses in 2011. Again, these amounts accounted for less than 1 percent of totallbs a.i. of 

herbicides applied in both sectors. (Proprietary data, 1999, 2012). Data between these years 

were not available to EPA. 

7 
CHEMICAL AND USE CHARACTERISTICS 

Imazethapyr is in the Weed Science Society of America (WSSA) herbicide mechanism of action 

group 2 which is a group of herbicides that inhibit the production of the key plant enzyme 

acetolactate synthase (ALS). The mode of action is characterized by growth inhibition, chlorosis 

and necrosis with death occurring over a period of weeks. 

Imazethapyr is an herbicide that is used preplant incorporated or broadcast preemergence and/or 

postemergence to control annual broadleaf and annual grass weeds. Imazethapyr is applied in 

many crops with the majority of the herbicide being used in soybeans, rice, alfalfa, com, and dry 

beans/peas (Table 3). However, it is important to note that the crops with the highest PCT of 

imazethapyr are green peas, rice, dry beans and peas, and green beans (Table 4). Rice, just as 

with com, is not naturally resistant to this herbicide therefore imazethapyr can only be used in 

genetically modified (GM) or imidazolinone tolerant varieties. 

ROLE OF IMAZETHAPYR IN PEST MANAGEMENT 

Imazethapyr is used to manage some difficult to control weeds including many annual broadleaf 

weeds in green pea production as well as grass weeds in rice production. The control of these 

two weed types is very important to sustaining the yield and quality of these crops. With over 

40% of the United States green pea crop and 30% of the rice crop being treated with this 

herbicide, imazethapyr would seem to have an important role in weed control in the production 

of these two crops. 

Green peas are highly sensitive to weed competition and as such aggressive weeds such as 

common lambsquarters (Chenopodium album) are considered serious impediments to yield 

quality and quantity (Krall et al., 2014). Because green peas are naturally resistant to the 

herbicide imazethapyr it is safe to use as a preplant incorporated or preemergence herbicide for 

the control of weeds. Efficacy for the control of broadleaf weeds in the production of green peas 

is sufficient enough that imazethapyr is considered in some states as " ... the most widely used 

herbicide to control broadleafweeds." (Dudley et al., 2000). 

In the case of rice production, red rice (Oryza sativa) is problematic in two ways. First, like 

many other crops, the production of rice has its problems with weeds which compete for 

resources. Second, similar to many other grain crops, the harvested portion can become 

contaminated with weed seeds. The red color of the seed of red rice is a problem in the 

harvested rice because it causes a red discoloration to the milled product when it gets harvested 

with the desired grain (Webster and Levy, 2012). 

While useful in controlling some difficult to control weeds, as stated above, there are more than a 

dozen weed species that have biotypes which are resistant to imazethapyr, some of the species 

include weeds that are difficult to control such as Palmer amaranth (Amaranthus palmeri), 

common waterhemp (Amaranthus tuberculatus) and Kochia (Kochia scoparia), among others. 

These weed species are among the most problematic and pervasive weeds in crop production and 

are prolific seed producers. However, despite the number of weed biotypes that are resistant to 

8 
this herbicide, there are some difficult to control broadleaf and grass weeds that are controlled by 

this herbicide. When used in combination with other herbicides, imazethapyr is an important 

tool in the production of some crops such as green peas and rice, which appears to be due, at 

least in part, to natural tolerance and engineered resistance in the host crops. 

DATA NEEDS 

The Agency currently does not have important information on imazethapyr. Obtaining usage 

information for the following sites may improve future assessments conducted by the Agency 

related to the use and importance of imazethapyr: 

Agricultural usage data for: endive, minor legume crops (e.g., lentils), clover, and lupine 

Non Agricultural usage data and use information for: industrial areas, rights-of-way, CRP 

land, and/or any registered use sites that are of importance to users 

Usage information on crops with low usage but important niche markets for imazethapyr. 

Usage data includes pounds applied, percent crop treated, application rates and timing, methods 

of application, target or niche pests. 

9 
REFERENCES 

Deitrich, V. and A. Halvorson. 2001a. Review of Request for Emergency Exemption for Use of 

Imazethapyr on Clearfield Rice to Control Red Rice in Louisiana (01-LA-02). 

Deitrich, V. and A. Halvorson. 2001b. Review of Request for Emergency Exemption for Use of 

Imazethapyr on Clearfield Rice to Control Red Rice in Texas (01-TX-04). 

Dudley, R., S.J. Fuchs, amd R. Hirnyck. 2000. Crop Profile for Peas (Green) in Idaho. Online. 

Internet. August 14th, 2013. 

EPA. 2013. Imazethapyr (128922) Screening Level Usage Analysis (SLUA). Biological and 

Economic Analysis Division, Office of Pesticide Programs, US Environmental Protection 

Agency. March 4, 2013. 

Gillespie, W.E., G.F. Czapar, and A.G. Hager. 2011. Pesticide fate in the environment: a guide 

for field inspectors (Contract Report 2011-07). Illinois State Water Survey, Institute of 

Natural Resource Sustainability, University of Illinois at Urbana-Champaign. Champaign 

Illinois. 

Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. Wednesday, 

August 14th, 2013. Available 

Krall, J.M., S.D. Miller, J.T. Cecil, C. Bastian, T. Foulke, D.D. Baltensperger, B.M. Harveson, 

P.A. Burgener, G.W. Hergert, G.L. Hein, D.J. Lyon, T. Nleya, J. Rickertsen and S. Blodgett. 

Pea production in the high plains. South Dakota State University Extension Fact Sheet 

FS932. Available at ianrpubs.unl.edullive/ec187/build/ec187.pdf, Accessed January 

Proprietary Data. 1998-2012. 

Senseman, S. ed. 2007. Herbicide Handbook Ninth Edition. Lawrence, KS: Weed Sci. Soc. of 

America. 

USDA NASS. Chemical Use Surveys, 2006 and 2009. 

http://www.nass.usda.gov/Surveys/Guide to NASS Surveys/Chemical Use/#description 

Webster and Levy 2012. Rice Production Handbook. Chapter 5 Weed Management. 

10 
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 amount (in pounds) ofthe 

active ingredient applied per 1,000 acres of"land in farms". These data are included in the maps because 

risk assessors are interested in the amount of a pesticide used across farmland. Because of the defmition 

of Land in Farms, as discussed below, the calculated values presented in the map are not equivalent to an 

application rate (lbs ail A). 

The data used to make these maps have many limitations. Any interpretation of the maps should consider 

the underlying data and the associated limitations carefully. 

The smallest areas shown on the maps within each state are Crop Reporting Districts (CRD). CRDs are 

defined by USDA and each consists of multiple counties that have similar soils, growing conditions, and 

farming practices. The CRDs within many states have roughly the same area. Because it is not feasible to 

conduct an agricultural survey that samples every county in the United States, the use of CRDs allow for 

a sample to be collected that represent a much larger area with relatively homogeneous agricultural 

characteristics. 

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, 2007-2011). 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 CRDs that show no usage may actually have a small amount of the 

active ingredient being used. 

The denominator (1 ,000 Acres of land in farms) was selected from the Census of Agriculture (USDA, 

2007) to represent agricultural acreage that is actively managed and includes most of the areas treated 

with pesticides. The category "Land in Farms" is defined by the Census of Agriculture as being made up 

of four components: 1) Total cropland, 2) Permanent pasture, 3) Total woodland, 4) Land in farmsteads, 

buildings, livestock facilities, ponds, roads, wasteland, etc. 

This category includes land in the various USDA conservation programs. It should be noted that the 

Census of Agriculture does not include grazing land rented (on a per-animal basis) from the 

Bureau of Land Management or Forest Service, thus it is an underestimate ofthe land being managed for 

agricultural purposes. 

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. Scale and units 

of measurement do matter and may be different between maps. 

11