UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

MEMORANDUM 

DATE: February 28, 2013 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

SUBJECT: Registration Review - Preliminary Problem Formulation for the Ecological 

Exposure and Risk Assessment for Iodosulfuron-methyl-sodium (PC Code 

122021; DP Barcode 404946) 

FROM: 

THRU: 

~ Rochelle F. -H. Bohaty, Ph.D., Chemi 9 Katherine R. Stebbins, Biologist 

Rosanna Louie-Juzwiak, RAPL 

Environmental Risk Branch 3 

Environmental Fate and Effects Division (7507P) 

Dana Spatz, Chie~.....__............__ c:::_"" z__\ "2 Q) -~ \ 3 

Environmental RiAs~~ ~ 'I 

Environmental Fate and Effects Division (7507P) 

·- TO: Katherine St. Clair, Chemical Review Manager 

Kevin Costello, Branch Chief 

RMIB4 

Pesticide Reevaluation Division 

The Environmental Fate and Effects Division (EFED) has completed the preliminary problem 

formulation (attached) for the ecological risk, environmental fate, and endangered species 

assessments to be conducted as part ofthe Registration Review ofiodosulfuron-methyl-sodium 

(also referred to as iodosulfuron). This document is intended to provide an overview of what is 

currently known regarding the environmental fate and ecological effects associated with 

iodosulfuron and its degradation products, and outlines uncertainties regarding attributes of the 

parent compound and its transformation products. It describes the preliminary ecological risk 

hypothesis and the processes that will be used during the completion of the ecological risk 

assessment in support of Registration Review. 

' 1 
2

1. INTRODUCTION 

Iodosulfuron-methyl-sodium (iodosulfuron) is a sulfonylurea herbicide used to control annual 

and perennial grasses and broadleaf weeds. Iodosulfuron slows or stunts plant growth (or causes 

ultimate plant death) by limiting amino acid synthesis in plants through inhibition of acetolactate 

synthase (ALS). Iodosulfuron is absorbed through the leaves and roots of susceptible weed 

species. The structure of iodosulfuron, as well as the chemical name and other identifiers are 

provided in Table 1. 

Table 1. Iodosulfuron Chemical Identifiers

Parameter Identifier 

Chemical Structure 

CO2CH3

I S

O2

N

H

N

O N N

N

CH3

OCH3

Na

Chemical Name 4-Iodo-2-[[[[(4-methoxy-6-methyl,-3i, 5-triazin-2-yl)amino]carbonyl]amino]sulfonyl 

benzoic acid methyl ester, monosodium salt 

SMILES [Na+].COC(=O)C1=CC=C(I)C=C1S(=O)(=O)[N-]C(=O)NC1=NC(OC)=NC(C)=N1 

CAS Number 144550-36-7 

Company Code AE F115008 

In addition to the technical product (Reg. No.264-687), there are eight end-use products that 

contain iodosulfuron. Two different safener ingredients, isoxadifen-ethyl and mefenpyr-diethyl, 

are included in some of the formulations to protect the sprayed field corn (Reg. No. 264-686), 

triticale and wheat (Reg. Nos. 264-1103; 264-820) from damage. Several of the products contain 

additional active ingredients including: mesosulfuron-methyl (PC 122009), thiencarbazonemethyl (PC 015804), foramsulfuron (PC 122020), and dicamba (PC 029801). A summary of all 

the iodosulfuron formulated products is provided in Table 2. 

Table 2. Iodosulfuron Formulated Products

Product 

EPA 

Registration 

Number 

Active Ingredient 

(Product Code) Safener1 

Atlantis Herbicide 264-1103 

0.6 % Iodosulfuron-methyl-sodium 

(122021) 9.0% mefenpyr-diethyl 

3.0 % Mesosulfuron-methyl (122009) 

Autumn Super 51 

WDG Herbicide 264-1134 

6.0% Iodosulfuron-methyl-sodium 

(122021) 

45% Thiencarbazone-methyl (15804) 

Tribute Solo WG32 

(also known as 

Equip Corn 

Herbicide) 

264-686 

2.0% Iodosulfuron-methyl-sodium 

(122021) 30% isoxadifen-ethyl 

30 % Foramsulfuron (122020)

Hussar Herbicide 264-820 5.0% Iodosulfuron-methyl-sodium 

(122021) 15% mefenpyr-diethyl 
3

Product 

EPA 

Registration 

Number 

Active Ingredient 

(Product Code) Safener1 

Iodosulfuron 10 

WDG Herbicide 264-843 10 % Iodosulfuron-methyl-sodium (122021) 

Iodosulfuron 10 

WDG 

Herbicide 

264-856 10 % Iodosulfuron-methyl-sodium 

(122021) 

Iodosulfuron Turf 

Herbicide 432-1404 10 % Iodosulfuron-methyl-sodium (122021) 

Celsius WG 432-1507 

1.9% Iodosulfuron-methyl-sodium 

(122021) 

8.7% Thiencarbazone-methyl (15804) 

57.4% Dicamba (029801) 

1. Reported percentages are based on data provided on the material safety data sheet (MSDS) 

Iodosulfuron is registered for use on the following agricultural crops: cereal grains, corn (field, 

pop and sweet), cotton, sorghum, soybeans, triticale, and wheat. In addition, iodosulfuron is 

registered for use on lawns (residential and commercial), turf/sod farms, roadsides, and 

recreational areas including golf courses, sports fields, parks, campsites, and school grounds. All 

currently registered uses of iodosulfuron are listed in Table 3. 

Based on the BEAD Chemical Profile prepared by OPP’s Biological and Economic Analysis 

Division, from 2006-2010, an annual average of <500 pounds of iodosulfuron was used on corn 

and soybeans.1

 Pesticide use was surveyed but not reported during this time period for cotton, 

sorghum, and wheat. Usage data are not available for triticale and the non-agricultural use sites 

such as: lawns, turf/sod farms, rights of way, and recreational areas. 

 

1

 Ratnayake, S.; Prieto, R.; Smearman, S. BEAD Chemical Profile for Registration Review: Iodosulfuron-methylsodium (122021) August 28, 2012. 
4

Table 3. Currently Registered Iodosulfuron Uses

Crop/Site 

(Reg. Nos.) 

Residential 

Agricultural 

Forestry Timing; Application Type Method/ Equipment 

Maximum 

Single 

Application 

Rate by 

Formulation1 

(lb a.i./A) 

Maximum 

Application Rate 

Maximum 

Application 

Number 

3 

Per Year 

lb a.i./A 

Per 

CC2 

lb a.i./A 

Per 

Year 

Per 

CC2 

CEREAL GRAINS, SWEET, AND POP CORN COTTON, SORGHUM 264-856 

 Postharvest, burn 

down treatment ground boom 0.002 WDG 0.002 NA 1 NS N

FIELD, CORN 264-686 

 Foliar, 

broadcast, ground boom 0.002 WDG 0.002 NA NS 1 4

264-1134; 264-856  Postharvest, burn down treatment ground boom 0.002 WDG 0.002 NA 1 NS 3GOLF COURSE TURF/ ORNAMENTAL LAWNS AND TURF/ORNAMENTAL SOD FARM (TURF)/ RECREATION AREA LAWNS/ RESIDENTIAL LAWNS 432-1404; 432-1507

 

foliar, 

established 

plantings 

broadcast 

ground boom 0.009 WDG 0.009 NA NS NA N

432-1507   foliar, spot treatment handheld 0.0001 lb/gal WDG 0.009 NA NS NS N
5

432-1507   foliar, zone application 0.006 WDG 0.009 NA NS NA NRECREATIONAL AREAS 432-1404 

Weed 

postemergence, 

established 

plantings 

broadcast 

ground boom 0.009 WDG 0.009 NA NS NA N

SOYBEAN 264-1134; 264-856 

 Postharvest, burn 

down treatment ground boom 0.002 WDG 0.002 NA 1 NS N

TRITICALE 264-1103 

 Postemergence, 

broadcast ground boom 

0.002/ 

0.003 (ID, OR, 

WA) WDG 

0.009 

0.0023/ 

0.0026 

(ID, OR, 

WA) 

NS 1 N

WHEAT 264-1103 

 Postemergence, 

broadcast, spray ground boom 

0.002/ 0.003 

(ID, OR, WA) 

WDG 

0.009 

0.0023/ 

0.0026 

(ID, OR, 

WA) 

NS NS 3

264-820  Foliar, broadcast ground boom 0.009 WDG 0.009 NS 2 NS 2

1. WDG – water dispersible granular in water soluble packet 

2. Reported as per crop cycle or per season 

3. PHI – Preharvest interval; REI – reentry interval; MRI – Minimum retreatment interval 

4. Rotational crop rotation intervals are provided on product labels. 

NA not applicable; NS not specified 
6

2. PREVIOUS RISK ASSESSMENTS 

Three ecological risk assessments have been completed for iodosulfuron: the new chemical 

assessment (field corn; DP267567; March 2002), and two new use assessments (turf, soybeans; 

DP315491; March 2006) and (wheat; DP350049; March 2008). All three of the previous 

assessments assessed both iodosulfuron (parent) and an environmental transformation product, 

metsulfuron methyl. Metsulfuron methyl also referred to as metsulfuron and is a registered 

sulfonylurea herbicide (PC 122010). EFED evaluated these assessments in association with the 

current toxicity, exposure, and usage information to determine if sufficient data are available and 

if further updates are needed to support registration review. 

Potential direct risk to terrestrial and aquatic vascular and non-vascular plants from the use of 

iodosulfuron is expected. There is also the potential for various indirect effects on non-target 

animal species due to the effects on the plant community (for example, depletion of food sources 

and habitat). The major conclusions presented in the most recent assessments for iodosulfuron 

are summarized in Table 4. 

Table 4. Summary of Risk Concerns Identified for Iodosulfuron in Previous Assessments

Birds Mammals Terrestrial 

Plants 

Terrestrial 

Inverts Fish Aquatic Inverts 

Aquatic 

Plants 

Groundwater 

Contamination Persistence Degradates of Concern 

-- -- √ -- -- -- √ -- -- √

√ Risk/concern identified in previous assessment 

Previous Drinking Water Assessments 

A drinking water assessment for iodosulfuron was conducted at the time of the new chemical 

assessment, and updated with each subsequent new use, as highlighted in Table 5 and Table 6. 

The uses assessed include applications made to field, sweet and pop corn, soybean, turf, and 

wheat. These assessments included estimated drinking water concentrations for both 

iodosulfuron and metsulfuron, modeled using FIRST, PRZM-EXAMS, and SCI-GROW. The 

two residues were modeled independently. 

A drinking water assessment will be conducted to support future human health dietary risk 

assessments of iodosulfuron. Using EFED aquatic models, the drinking water assessment will 

incorporate model estimates of iodosulfuron in surface water (e.g., PRZM-EXAMS) and 

groundwater (e.g., (PRZM-GW). The major degradate, metsulfuron is currently considered a 

residue of concern in drinking water; however, all environmental transformation products of 

iodosulfuron will be reevaluated as potential residues of concern in drinking water as part of the 

registration review risk assessment process. Any additional residues identified as a potential 

toxicological concern with similar toxicity will also be considered using a total toxic residue 

(TTR) approach. 

 
7

Table 5. Summary of Prior Drinking Water Assessments for Iodosulfuron and Metsulfuron in Surface Wa

Assessment Crop 

Tier I Surface Water (FIRST) 

Iodosulfuron Metsulfuron 

1-in-10 Year Peak 

Concentration 

μg/L (ppb)

1-in-10 Year Annual 

Average 

Concentration μg/L 

(ppb) 

1-in-10 Year Peak 

Concentration 

μg/L (ppb)

1-in-10 Year An

Average Concentr

μg/L (ppb) 

D279400/DWA 

New Chemical Field Corn 0.056 0.0058 0.068 0.014 

D315491/ 

DWA Turf 0.6 0.06 0.53 0.11 D318084/ DWA 

Soybean 

(preplant) 

Tier II Surface Water (PRZM-EXAMS) 

1-in-10 Year 

Peak 

Concentration

μg/L (ppb)

1-in-10 Year 

Annual 

Average 

Concentration 

μg/L (ppb)

30 Year 

Average 

Yearly 

Concentration

μg/L (ppb)

1-in-10 Year 

Peak 

Concentration 

μg/L (ppb)

1-in-10 Year 

Annual Average 

Concentration 

μg/L (ppb)

30 Ye

Avera

Year

Concent

μg/L (p

D350049/ 

DWA Wheat 0.36 0.067 0.025 0.26 0.077 0.02

Table 6. Summary of Prior Drinking Water Assessments for Iodosulfuron and Metsulfuron in Groundwat

Assessment Crop 

Tier I Groundwater (SCI-GROW) 

Iodosulfuron Metsulfuron 

Peak Concentration 

μg/L (ppb) 

Yearly Mean 

Concentration 

μg/L (ppb) 

Peak 

Concentration 

μg/L (ppb) 

Yearly Mean 

Concentration 

μg/L (ppb) 

D279400/DWA 

New Chemical Field Corn 0.0006 0.0006 0.005 0.005 

D315491/ DWA Turf 

0.00004 0.00004 0.02 0.02 D318084/ DWA Soybean preplant 

D350049/DWA Wheat 0.00002 0.00002 0.02 0.02 
8

Clean Water Act Programs 

Iodosulfuron is not identified as a cause of impairment for any water bodies listed as impaired 

under section 303(d) of the Clean Water Act. No Total Maximum Daily Load (TMDL) criteria 

have been developed for iodosulfuron. Aquatic Life Ambient Water Quality Criteria documents 

or Aquatic life benchmarks have not been published for iodosulfuron. 

3. ENVIRONMENTAL FATE AND TRANSPORT 

Physical and Chemical Properties 

The physicochemical properties of iodosulfuron are shown in Table 7. Based on these properties 

alone, iodosulfuron is not expected to be volatile or to bioaccumulate. It is highly soluble in 

water. Solubility is pH dependent and has been shown to increase at higher pHs. 

Table 7. Selected Physical/Chemical Parameters of Iodosulfuron

Parameter Value Source/Classification

Molecular mass (molecular formula) 529.24 g/mol 

(C14H13IN5O6SNa) EPISuite v4.0 

Vapor pressure (20°C)

2.6 x 10-9 Pa; 1.95 x 10-11 mmHg (20 °C) 

6.7 x 10-9 Pa; 5.03 x 10-11 mmHg (25 °C) 

1.6 x 10-8 Pa; 1.2 x 10-10 mmHg (30 °C) 

MRID 45108609 

Aqueous solubility (20C)

170 mg/L (20 °C; pH 5) 

25,000 mg/L (20 °C; pH 7) 

65,000 mg/L (20 °C; pH 9) 

MRID 45108619 

MRID 45108620 

 (corrected values) 

pKa 3.22±0.06 (20 °C) MRID 45108626 

Log octanol-to-water partition 

coefficient

(log KOW)

3.96 (pH 4) 

-0.70 (pH 7) 

-1.22 (pH 9) 

MRID 45108622 

MRID 45108621 provides iodosulfuron solubility data for a number of common solvents. 

Environmental Fate Properties 

Environmental fate properties of iodosulfuron are summarized in Table 8. These half-life values 

presented in the table have been updated from previous assessments to reflect the NAFTA 

degradation kinetics guidance 2 for characterization purposes and will be used to calculate model 

input values. Updated model input values will be calculated based on current model input 

guidance for each respective exposure model at the time the assessment is completed. The 

updated kinetic analyses are presented in APPENDIX A. The results of the revised kinetic 

analyses are similar to those previously reported in the data evaluation documents. In general 

terms, iodosulfuron is susceptible to microbial mediated degradation in soil as well as in aquatic 

 

2 Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media, Health Canada, U.S. 

Environmental Protection Agency, December 2012. 
9

environments. Iodosulfuron does not persist long in soil and is only slightly more persistent in 

aquatic environments under both aerobic and anaerobic conditions. It forms several different 

environmental transformation products (discussed in the section below). Hydrolysis of 

iodosulfuron is pH dependent with increased hydrolysis occurring under acidic conditions. 

Photodegradation in aquatic environments is not expected for iodosulfuron; however, it is 

susceptible to indirect photolysis (primarily observed in soil). Like all of the sulfonylurea 

herbicides, iodosulfuron does not bind strongly to soils and sediments, and is expected to be 

mobile. 

Table 8. Environmental Fate Parameters of Iodosulfuron 

Parameter Conditions Value Source/Classification

Persistence1

Hydrolysis halflife 25C pH 4: 2.5 d2 pH 5: 18.4 d2

pH 7: >365 d2 pH 9: 167 d2 

MRID 45108623/ 

acceptable 

Aqueous 

photolysis halflife 

25C 

212 d 30 °N 

334 d 40 °N 

644 d 50 °N 

MRIDs 

45108624, 45108625/ 

Supplemental 

Soil photolysis 

half-life 

25C 

SFO 4.2 d 30 °N 

SFO 5.0 d 40 °N 

SFO 7.0 d 50 °N 

DFOP Slow t1/2 = 11.3 d 30 °N 

DFOP Slow t1/2 = 13.4 d 40 °N 

DFOP Slow t1/2 = 17.5 d 50 °N 

45109003 

Reclassified to 

supplemental3 

Phenyl label 

A clear pattern of 

formation and decline of 

the transformation product 

was not established. 

Aerobic soil 

metabolism halflife 

20°C, 40% 

MWRC 

IORE DT50= 2.9 d; DT90= 12 d; 

tR IORE = 3.6 d (Germany, silt loam) 

MRIDs 

45108934, 45109005/ 

Supplemental3 

Triazine label 

IORE DT50= 3.3 d; DT90= 13.7 d; 

tR IORE = 4.1 d (France, clay loam) 

IORE DT50= 2.9 d; DT90= 13 d; 

tR IORE = 3.9d (United Kingdom, loamy 

sand) 

MRIDs 45108935, 

45109006/ 

Supplemental4

Phenyl label 

IORE DT50= 4.3 d; DT90= 26.5 d; 

tR IORE = 8.0 d (United Kingdom, silt 

loam) 

MRIDs 45108936, 

45109007/ 

Supplemental4

Triazine label 

20°C, 30% 

MWRC 

IORE DT50= 6.0 d; DT90= 41.2 d; 

tR IORE = 12.4 d (United Kingdom, silt 

loam) 

10°C, 40% 

MWRC 

IORE DT50= 13.8 d; DT90= 104 d; 

tR IORE = 31.3 d (United Kingdom, silt 

loam) 

20°C, 25% 

MWRC 

DFOP DT50= 16.2 d; DT90=132 d; 

Slow t1/2 = 50.5 d (Germany, clay) 

MRIDs 45109001, 

45109008/ 

Supplemental4

Triazine label 

IORE DT50= 9.2 d; DT90= 51.4 d; 

tR IORE = 15.5 d (France, clay loam) 
10

Parameter Conditions Value Source/Classification

20°C, 50% 

MWRC 

IORE DT50= 2.3 d; DT90= 24 d; 

tR IORE = 7.3 d (Germany, clay) 

IORE DT50= 3 d; DT90= 11.9 d; 

tR IORE = 3.6 d (France, clay loam) 

20°C IORE DT50= 1.5 d; DT90= 7.43 d; 

tR IORE = 2.2 d (Germany, sandy loam) 

MRID 45108933/ 

45109004 

Supplemental5

 

triazine label 

20°C SFO DT50= 1.6 d; DT90= 5.2 d; 

 (Germany, loamy sandy) 

20°C 

DFOP DT50= 3.1 d; DT90= 10.2 d; 

DFOP Slow t1/2 = 3.1 d (USA, sand) 

Same as SFO 

20°C SFO DT50= <1 d; DT90= 2.5 d; 

 (Germany, silt loam) 

Aerobic aquatic 

metabolism halflife 

20°C, Nidda 

SFO t1/2 = 19.1 d; DT90= 63.3 d (water)

SFO t1/2 = 20.1 d; DT90= 66.9 d (total 

system) 

MRID 45109021 

Supplemental 

Storage stability data was 

not provided. 

Up to 30% of the applied 

radioactivity was not 

extracted. 

20°C, Rhine 

SFO t1/2 = 10.5 d; DT90= 34.9 d (water)

SFO t1/2 = 11 d; DT90= 36.5 d (total 

system) 

Anaerobic aquatic 

metabolism halflife 

20°C 

DFOP DT50= 21.3 d; DT90= 90.1 d; 

Slow t1/2 = 29.6 d (water; silt loam) 

IORE DT50= 25.6 d; DT90= 108 d; 

tR IORE = 32.6 d (total system; silt loam)

MRID 45109002, 

45109009/ 

Supplemental (model input 

value calculations will be 

based on total system)

SFO DT50= 15.1 d; DT90= 50.1 d; 

SFO t1/2 = 15.1 d (water; loamy sand) 

IORE DT50= 17 d; DT90= 45.8 d; 

tR IORE = 13.8 d (total system; loamy 

sand) 

SFO DT50= 15.6 d; DT90= 51.9 d; 

SFO t1/2 = 15.6 d (total system; loamy 

sand) 

IORE N value <1; SFO is an 

appropriately conservative model 
11

Parameter Conditions Value Source/Classification

20°C 

SFO t1/2 = 11.8 d; DT90= 39.1 d 

IORE N value <1; SFO is an 

appropriately conservative model 

MRID 45109023/ 

Supplemental 

A highly polar unknown 

was not identified. 

Up to 10% of the applied 

radioactivity was 

unextracted. 

The concentration of CO2

varied substantially 

between days 275 and 366.

Storage stability data are 

not adequate. 

Mobility

Freundlich soilwater partition 

coefficients (KF); 

organic carbonnormalized 

Freundlich 

coefficients 

(KFOC) 

Loam, Loivre, 

France; pH 7.3, 

3.1% OC 

0.694 L/kg; 

22.6 L/kgOC

MRID 45109015/ 

Acceptable Clay loam, 

Bourgogne, 

France; 7.2, 2.4% 

OC 

0.368 L/kg; 

15.5 L/kgOC

Sand, Germany; 

pH 5.62, 1.3% OC 

0.12 L/kg; 

10 L/kgOC

MRID 45109014/ 

Supplemental

The soils used in this 

study were autoclaved. 

Generally this would 

invalidate the study; 

however, the results 

indicate low sorption and 

are consistent with MRID 

45109015 as well as 

known mobility of other 

sulfonylurea herbicides. 

For this reason, the study 

was classified as 

supplemental; however, 

the results should only be 

used for characterization 

and not for quantitative 

risk assessment. 

Loamy sand, 

Germany; pH 5.7, 

2.5 % OC 

0.54 L/kg; 

22 L/kgOC

Sandy loam, 

Germany; pH 6.0, 

1.1 % OC 

0.13 L/kg; 

12 L/kgOC

Silt loam, 

Mississippi, pH 

5.4, 0.69 % OC 

1.1 L/kg; 

152 L/kgOC

Loamy sand, 

Arizona; pH 8.0, 

0.16 % OC 

0.01 L/kg; 

ND 

Sandy Loam, 

Illinois; pH 7.5, 

2.5 % OC 

2.0 L/kg; 

82 L/kgOC

Clay loam, 

Minnesota; pH 

7.2, 2.8% OC 

2.5 L/kg; 

90 L/kgOC

Loamy sand, 

United Kingdom; 

pH 7.9, 1.3% OC 

0.02 L/kg; 

0.8 L/kgOC
12

Parameter Conditions Value Source/Classification

Field Dissipation 

Terrestrial field 

dissipation halflife; 

Silt loam, France; 

pH 7.4 

8.8 d 0.2 lb a.i./A bareground 

7.1 d 0.9 lb a.i./A bareground 

6.4 d 0.9 lb a.i./A grassed 

MRID 45109010/ 

supplemental 

Only one transformation 

product was analyzed and 

the study was conducted in 

Europe. 

Silt, France; pH 

5.8 

7.1 d 0.2 lb a.i./A bareground 

9.4 d 10.9 lb a.i./A bareground 

9.1 d 0.9 lb a.i./A grassed 

Silt loam, Duerrn, 

Germany; pH 6.9 

8.9 d 0.2 lb a.i./A bareground 

14.8 d 0.9 lb a.i./A bareground 

11.6 d 0.9 lb a.i./A grassed 

Silt loam, Warpe, 

Germany; pH 6.4 

6.5 d 0.2 lb a.i./A bareground 

2.0 d 0.9 lb a.i./A bareground 

7.6 d 0.9 lb a.i./A grassed 

Silt loam, 

United Kingdom; 

pH 6.2 

<3 d 0.2 lb a.i./A bareground 

11.2 d 0.9 lb a.i./A bareground 

<1d 0.9 lb a.i./A grassed 

Silt loam, Spain, 

pH 7.8 

11.3 d 0.2 lb a.i./A bareground 

9.3 d 0.9 lb a.i./A bareground 

12.5 d 0.9 lb a.i./A grassed 

Fish Bioconcentration 

Fish 

bioconcentration 

factor (steadystate); depuration 

rate 

-- No data available 

MWRC= maximum water retention capacity 

1. Reported half-life values were calculated using the draft NAFTA degradation kinetics (Guidance for 

Evaluating and Calculating Degradation Kinetics in Environmental Media, Health Canada, U.S. 

Environmental Protection Agency, December 2012.) document: Guidance for Evaluating and 

Calculating Degradation Kinetics in Environmental Media (draft document June 29, 2011); Single 

First Order (SFO), Nth-Order Rate Model or Indeterminate Order Rate Equation Model (IORE), and 

Double First-Order in Parallel (DFOP); Representative half-life to be used to calculate model input 

value for IORE (tR IORE) 

2. First-order rate constant and hydrolysis half-lives at 25 °C were estimated using the Arrhenius 

equation. Half-lives were extrapolated beyond the experimental data. 

3. This study was originally classified as acceptable (noting the deficiency of not having data on the 

triazine moiety); however, it is being reclassified as supplemental because the study only provides 

information on the dissipation of the phenyl moiety of iodosulfuron and formation and decline of the 

observed transformation products is not clear. 

4. Several study deficiencies were noted. The studies were conducted outside the United States. 

5. This study has multiple deficiencies including inadequate sampling times 

pH: One divided by the log of the hydrogen ion concentration; Organic Carbon (OC); days (d) 

Transformation Products 

Several major degradates of iodosulfuron were reported in the submitted environmental fate 

studies. These major transformation products as well as a few minor degradates are listed in 

Table 9 with the maximum amounts of each chemical formed in the various studies. 
13

One of the major environmental transformation products of iodosulfuron is metsulfuron-methyl, 

a registered sulfonylurea herbicide. Metsulfuron forms in high levels (up to 70% applied) under 

aerobic conditions. Two mobility studies have been submitted for metsulfuron-methyl to support 

the registration of iodosulfuron. Metsulfuron does not bind to soils/sediments and is expected to 

be mobile in soils. In addition, since metsulfuron-methyl is registered as an active ingredient 

fate data for metsulfuron has been submitted to EPA to support the metsulfuron registration.3

 As 

part of the registration review process for metsulfuron methyl several studies have been 

requested including: batch equilibrium (adsorption/desorption), soil photolysis, aerobic soil 

metabolism, anaerobic soil, aerobic aquatic, and anaerobic aquatic. Fate characteristics of 

metsulfuron are summarized in Table 10.

4

 In general, metsulfuron is mobile and is expected to 

be more persistent than iodosulfuron. 

No environmental fate data for the other iodosulfuron transformation products have been 

submitted to EPA. Many of these transformation products retain the sulfonyl urea bridge and; 

therefore, may pose toxicological concerns. 

 

3

 Registered uses of metsulfuron methyl include: agricultural crops (wheat, barley, sorghum, and pastureland), noncrop and industrial areas, pasture and rangeland, and turf (residential lawns, golf courses, and ornamental grasses). 4

 With the exception of the highlighted mobility data the data are taken directly from the metsulfuron-methyl 

registration review problem formulation: Kiernan, B. D.; Baris, R. Registration Review: Preliminary Problem 

Formulation for Environmental Fate and Ecological Risk, Endangered Species, and Drinking Water Assessments for 

Metsulfuron-methyl (Case 7205) PC 122010 DP389258 September 22, 2011. 
14

Table 9. Iodosulfuron Environmental Transformation Products

Code Name/ 

Synonym Chemical Name Chemical Structure Study Type Ref. (MRID) Maxi

%AR 

MAJOR 

AE F075736 

Metsulfuron methyl; methyl 2-[3-(4-methoxy-6-methyl1,3,5- triazin-2-y1)ureidosulfonyl]benzoate C14H15N5O6S 

Mol. Wt.: 381.3638 

CO2CH3

SO2

HN

HN

O N N

N

CH3

OCH3

Aerobic Soil 

45108934 55

1 D

45108935 70

14 D

45108936 47

14 D

45109001 5

11 D

Aerobic Aquatic 45109021 67

43 D

Anaerobic 

Aquatic 45109023 1

5 D

AE F161778 

methyl 2-[3-(4-hydroxy6-methyl-1,3,5-triazin-2-yl)ureidosulfonyl] benzoate C13H13N5O6S 

Mol. Wt.: 367.3372 

CO2CH3

SO2

HN

HN

O N N

N

CH3

OH

Aerobic Soil 

45108934 12

28 D

45108935 5

63 D

45108936 13

42 D

45109001 8

28 D

AE 0014966 2-[3-(4-methoxy-6-

methyl-1,3,5-triazin-2-yl)ureidosulfonyl]benzoic acid C13H13N5O6S 

Mol. Wt.: 367.3372 

CO2H

SO2

HN

HN

O N N

N

CH3

OCH3

Aerobic Aquatic 45109021 15

91 D

Anaerobic 

Aquatic 45109023 33

113 

AE 0014965 Dimethoxy-metsulfuron 

free acid 2-[3-(4-hydroxy-6-methyl-1,3,5-triazin-2-yl)ureidosulfonyl]benzoic acid C12H11N5O6S 

Mol. Wt.: 353.3106 

CO2H

SO2

HN

HN

O N N

N

CH3

OH Anaerobic 

Aquatic 45109023 16

113 
15

AE F145740 4-iodo-2-[3-(4-methoxy6-methyl-1,3,5-

~idosulfonyl benzoic acid C14H14IN5O6S Mol. Wt.: 507.2603 CO2CH3

I S

O2

HN

HN

O N N

N

CH3

OCH3

Aerobic Soil 

45108934 3

11 D

45108935 1

4 D

45108936 1

42 D

45109001 4

64 D

Anaerobic 

Aquatic 

45109023 50

28 D

45108936 1

120 

45109001 33

120 

Aerobic Aquatic 45109021 27

365 

AE0002166 Methyl 4-hydroxy-2-[3-

(4-methoxy-6-methyl1,3,5-triazin-2-yl)ureido]benzoate C14H15N5O6S 

Mol. Wt.: 381.3638 

CO2CH3

HO S

O2

HN

HN

O N N

N

CH3

OCH3

Soil Photolysis 45109003 2

144 

AE0 034855 4-hydroxy-6-methyl1,3,5-triazine-2-yl)urea 

C5H7N5O2

Mol. Wt.: 169.1414 

H2N

HN

O N N

N

CH3

OH

Aerobic Aquatic 45109021 24

182 

AE 0000119 (4-methoxy-6-methyl1,3,5-triazin-2-yl)urea 

C6H9N5O2

Mol. Wt.: 183.1680 

H2N

HN

O N N

N

CH3

OCH3

Aerobic Soil 45108934 4

28 D

Aerobic Aquatic

45108936 5

120 

45109001 4

64 D

45109021 24

120 
16

AEF159737 saccharin 

1,2-benzisothiazol3(2H)-one 1,1-dioxide C7H5NO3S 

Mol. Wt.: 183.1845 

SO2

NH

O

Anaerobic 

Aquatic 45109023 9

182 

Unknowns Soil Photolysis 45109003 9.5 (co

327 

Aerobic Soil 

45108934 6

63 D

45108935 7.2 (co

120 

45109001 9.8 (s

120 

Non-extracted Residues 

Aerobic Soil 

45108934 39

120 

45108935 9

91 D

45108936 33

120 

45109001 35

120 

Aerobic Aquatic 45109021 30

365 

MINOR

AE F143133 

Iodo saccharin 6-iodo-1,2-benzisothiazol-3(2H)-one 1, 1-dioxide C7H4INO3S Mol. Wt.: 309.0810 

SO2

NH

O

I

Anaerobic 

Aquatic 45109023 2

3 D

AE FI45741 

methyl 2-[3-(4-hydroxy6-methyl-1,3,5-triazin-2-yl)16reidosulfonyl4-iodobenzoate C13H12IN5O6S Mol. Wt.: 493.2337 

CO2CH3

I S

O2

HN

HN

O N N

N

CH3

OH

Aerobic Soil 

45108934 3

4, 7 

45108935 3

0 D

45108936 4

14 D

45109001 4

64 D

Bold font indicates the highest percentage observed for a given transformation product. MW: molecular weight AR: Applied radioactivity n.d. not detected <LOD 
17

Table 10. General Chemical Properties and Environmental Fate Properties of Metsulfuron 

Methyl

Parameter Value Reference (MRID #) 

/Comment

Physical/Chemical Parameters

Molecular mass 381.4 g/mol ACC 072767 

Vapor pressure (20°C) 2.5 x 10-12 torr ACC 072767 

Henry’s Law Constant 2.35 x 10-16 EPISuite v4.0 

Water solubility (25°C) 109 mg/L (in water) 

Solubility significantly 

increases with increasing pH.

ACC 072767 

Octanol-water partition coefficient (KOW) 0.018 ACC 072767 

Dissociation Constant (pKa) 3.5 ACC 072767 

Mobility

Soil Partitioning Coefficient (Koc, 1/n) †

214, 134, 226, and 46 mL/goc 

0.84, 0.78, 0.84, and 0.87 (1/n values) 

44143301 

supplemental 

Data should not be used 

quantitatively. 

EPISuite calculated value is 

22 mL/g oc. 

0.106 L/kg; 5.3 L/kgOC (silt loam, 

Hattersheium, Germany, pH 7) 

45109016 

supplemental 

0.145 L/kg; 7.7 L/kgOC (loam sand, 

Speyer, Germany, pH 6.0) 

0.065 L/kg; 15.1 L/kgOC (Tor Sud, Speyer, 

Germany, pH 6.0) 

0.5304 L/kg; 24 L/kgOC (loamy sand, 

Hanhofen, Germany, pH 5.8) 45133410 

Supplemental; study was 

conducted at 10 °C 0.2413 L/kg; 27 L/kgOC (silt loam, 

Honville, France pH 6.7) 

Persistence in Water

Hydrolysis half-life (25°C, pH 5, 7, & 9) † Stable ACC 072767 

Aqueous photolysis half-life (25°C)† Stable ACC 072767 

Aerobic aquatic metabolism half-life No available data -- 

Anaerobic aquatic metabolism half-life 35 – 365 days ACC 260973 

41395501 

Persistence in Soil

Soil photolysis† No available data -- 

Aerobic soil metabolism half-life (25°C) 

54.2 days 

25.9, and 28 days 

44491201 

ACC 071434 

Triazine amine‡: 240 days 40340317 

Anaerobic soil metabolism† No available data -- 
18

Parameter Value Reference (MRID #) 

/Comment

Field Dissipation

Terrestrial field dissipation half-life 

(DT50)

 †

1 – 10.5 months 42016507 

Note: sampling/depth of 

analysis was insufficient to 

define the extent of leaching 

(35 cm). 

Aquatic Bioconcentration 

Bioconcentration Factor (BCF) in fish Low Kow indicates low potential to 

bioaccumulate. 

ACC 072767 

‡

 Major degradate of metsulfuron 

†

 At least one study was under review as of September 11, 2011. 

In summary, iodosulfuron and its environmental transformation products may leach to 

groundwater and runoff or drift during application to surface water. Once in aquatic 

environments, iodosulfuron and its environmental transformation products including metsulfuron 

are expected to remain predominantly in the water phase. 

Safeners 

Isoxadifen-ethyl, the safener included in corn use formulations (Reg. No. 264-686) of 

iodosulfuron, is reported to be at concentrations approximately 15-times higher than 

iodosulfuron; therefore, the field application rate is expected to be 0.03 lb/A. Chemical 

identifiers for isoxadifen-ethyl are included in Table 11. Limited environmental fate data for 

isoxadifen-ethyl are provided in Table 12. Another safener, mefenpyr-diethyl, is included in 

wheat and triticale formulations (Reg. Nos. 264-1103; 264-820). The reported concentration of 

the safener is up to 15-times higher than the concentrations of iodosulfuron in the formulated 

products. The corresponding maximum field application rate is assumed to be 0.045 lb/A. 

Chemical identifiers for mefenpyr-diethyl are included in Table 13. Limited environmental fate 

data for mefenpyr-diethyl are provided in Table 14.

Table 11. Isoxadifen-ethyl Chemical Identifiers

Parameter Identifier 

Chemical Structure 

N

O

O O

Chemical Name ethyl 4,5-dihydro-5,5-diphenyl-1,2-oxazole-3-carboxylate 

ethyl 4,5-dihydro-5,5-diphenyl-3-isoxazolecarboxylate 

SMILES C1(C(OCC)=O)=NOC(C1)(c1ccccc1)c1ccccc1 

CAS Number 163520-33-0 
19

Table 12. Physical/Chemical Parameters of Isoxadifen-ethyl

Parameter Value Source/Classification

Molecular mass (molecular formula) 295.34 g/mol (C18H17O3N) Calculated using EPI Suite

Vapor pressure (25°C) 6.79 x 10-5 Pa Calculated using EPI Suite

Modified Grain Method 

Aqueous solubility (20C) 1.26 mg/L MRID 44739133 

(algal toxicity study) 

Log octanol-to-water partition coefficient 

(log KOW) 3.8 MRID 44739121

Koc 9777 L/kg Calculated using EPI Suite

Kow method 

Hydrolysis Stable Calculated using EPI Suite

Table 13. Mefenpyr-diethyl Chemical Identifiers

Parameter Identifier 

Chemical Structure N

N

O

O

Cl

Cl

O

O

Chemical Name 

1H-Pyrazole-3,5-dicarboxylicacid, 1-(2,4-dichlorophenyl)-4,5-dihydro-5-methyl-, 3,5-

diethyl ester 

1-(2,4-Dichlorophenyl)-4,5-dihydro-5-methyl-1H-pyrazole-3,5-dicarboxylic acid 

diethyl 

SMILES CCOC(=O)C2=NN(c1ccc(Cl)cc1Cl)C(C)(C(=O)OCC)C2 

CAS Number 135590-91-9 

Table 14. Physical/Chemical Parameters of Mefenpyr-diethyl

Parameter Value Source/Classification

Molecular mass (molecular formula) 373.24 g/mol (C16H18Cl2N2O4) Calculated using EPI Suite

Vapor pressure (25°C) 4.42 x 10-4 Pa Calculated using EPI Suite

Modified Grain Method 

Aqueous solubility (25C) 2.4 mg/L Calculated using EPI Suite

WSKOW v1.41 

Log octanol-to-water partition coefficient 

(log KOW) 

3.83

Calculated using EPI Suite 

[Exper. database match 

(Tomlin, C 1997)]

4.82 Calculated using EPI Suite 

(KOWWIN v 1.67) 

Koc 740 L/kg Calculated using EPI Suite

Kow method 

Hydrolysis stable Calculated using EPI Suite
20

4. RECEPTORS 

Table 15 and Table 16 provide a summary of the aquatic and terrestrial taxonomic groups, 

respectively, and the most sensitive surrogate species tested to characterize the potential acute 

and chronic ecological effects of iodosulfuron. The tables provide an overview of the available 

acute toxicity information on iodosulfuron, and provide the acute toxicity classifications. Based 

on the available ecotoxicity information, the most sensitive organisms are aquatic and terrestrial 

plants, however, it is noted that there is some uncertainty with the available terrestrial plant 

studies because they have been conducted on formulations containing a safener. There are 

several products that are not formulated with a safener including two for use on turf, which, 

along with a product for use on wheat (formulated with a safener) have the highest use rates. 

Because the safener may influence the final toxicity estimation, terrestrial plant toxicity studies 

for a product formulated for use on turf would reduce the uncertainty in the upcoming analysis. 

Table 15. Summary of the Most Sensitive Endpoints from Aquatic Toxicity Studies for 

Iodosulfuron

Taxonomic 

Group 

Study 

Type 

Surrogate Species 

and 

Test Material 

Toxicity Value 

(all units in terms 

of measured 

active ingredient) 

MRID and 

Classification 

Acute 

Toxicity 

Classification 

Freshwater fish1

Acute 

Rainbow Trout 

(Oncorhynchus 

mykiss) 

TGAI 

LC50 > 88.1 mg 

a.i./L 

Most sensitive 

endpoint: none 

No sublethal 

effects reported. 

45109035 

Supplemental 

classification 

based on water 

quality deviations 

from 850 

guideline 

recommendations. 

No more than 

slightly toxic 

Chronic 

(Early 

Life 

Stage) 

Fathead Minnow 

(Pimephales 

promelas) 

TGAI 

NOAEC 10.2 mg 

a.i./L 

LOAEC >10.2 mg 

a.i./L 

Most sensitive 

endpoint: none 

No sublethal 

effects reported. 

46431804 

Acceptable -- 

Estuarine/Marine 

fish 

Acute 

Sheepshead Minnow 

(Cyprinodon 

variegatus) 

TGAI 

LC50 > 100 mg 

a.i./L 

Most sensitive 

endpoint: none 

No sublethal 

effects reported. 

45109127 

Acceptable 

Practically 

non-toxic 

Chronic No Data 
21

Taxonomic 

Group 

Study 

Type 

Surrogate Species 

and 

Test Material 

Toxicity Value 

(all units in terms 

of measured 

active ingredient) 

MRID and 

Classification 

Acute 

Toxicity 

Classification 

Freshwater 

invertebrates 

Acute 

Water Flea 

(Daphnia magna) 

TGAI 

EC50 > 86.9 mg 

a.i./L 

NOAEC= 28.1 mg 

a.i./L 

Most sensitive 

endpoint: 

immobilization 

No sublethal 

effects reported. 

45109103 

Supplemental 

classification 

based on water 

quality deviations 

from 850 

guideline 

recommendations. 

No more than 

slightly toxic 

Chronic 

Water Flea 

(Daphnia magna) 

TGAI 

NOAEC = 9.1 mg 

a.i./L 

LOAEC = 15.9 

mg a.i./L 

Most sensitive 

endpoint: 

reduced number of 

young per female 

and dry weight 

45109104 

Acceptable -- 

Estuarine/Marine 

invertebrates 

Acute Mysid shrimp 

(Mysidopsis bahia) 

LC50 = >100 mg 

a.i./L 

NOAEC = 100 mg 

a.i./L 

No mortality or 

sublethal effects 

reported 

45109128 

Acceptable 

Practically 

non-toxic 

Chronic No Data 

Aquatic plants 

and algae 

Nonvascular 

Green algae 

(Pseudokirchneriella 

subcapitata) 

TGAI 

EC50 = 0.041 mg 

a.i./L 

NOAEC = 0.014 

mg a.i./L 

Most sensitive 

endpoint: cell 

density 

45109105 

Acceptable -- 

Vascular 

Duckweed 

 (Lemna gibba) 

TGAI 

EC50 = 0.00070 

mg a.i./L 

NOAEC = 

0.00039 mg a.i./L 

Most sensitive 

endpoint: cell 

density 

45109111 

Acceptable -- 

1. Freshwater fish represent surrogates for aquatic-phase amphibians. 

 
22

Table 16. Summary of the Most Sensitive Endpoints from Terrestrial Toxicity Studies for 

Iodosulfuron 

Taxonomic 

Group 

Study 

Type 

Surrogate 

Species 

and 

Test Material 

Toxicity Value 

(all units in terms of 

measured active 

ingredient) 

MRID and 

Classification 

Acute 

Toxicity 

Classification 

Birds1

Acute 

Bobwhite quail 

(Colinus 

virginianus) 

and Mallard 

 (Anas 

platyrhynchos) 

TGAI 

LD50 > 1744 mg/kg-bw 

Most sensitive 

endpoint: none. 

No sublethal effects 

reported. 

45109029/ 

45109028 

Acceptable 

No more than 

slightly toxic

Sub-acute 

dietary 

Bobwhite quail 

(Colinus 

virginianus) 

TGAI 

LC50 = 

> 4360 mg/kg diet 

Most sensitive 

endpoint: none. 

No sublethal effects 

reported. 

45109030 

Acceptable 

Practically 

non-toxic

Chronic 

Bobwhite quail 

(Colinus 

virginianus) 

TGAI 

NOAEC = 372 ppm 

LOAEC = 1077 ppm 

Most sensitive 

endpoint: Adult male 

weight gain 

46431803 

Acceptable -- 

Mammals 

Acute Oral 

Laboratory rat 

(Rattus 

norvegicus) 

LD50 = 2448 mg a.i./kg 45133404 

Acceptable 

Practically 

nontoxic3 

Acute 

Inhalation 

Laboratory rat 

(Rattus 

norvegicus) 

LC50 = > 2.81 mg/L 45108805 

Acceptable -- 

Chronic 

Laboratory rat 

(Rattus 

norvegicus) 

NOAEL = 500 ppm 

LOAEL = 5000 ppm 

Endpoint: Based on pup 

mortality 

45108818 

Acceptable --

Insects 

Acute 

contact 

Honey bee 

(Apis mellifera L.) 

TGAI 

LD50 > 150 μg a.i./bee 

No sublethal effects 

reported. 

45109115 

Acceptable 

Practically 

nontoxic2 

Acute Oral 

Honey bee 

(Apis mellifera L.) 

TGAI 

LD50 > 81 μg a.i./bee 

No sublethal effects 

reported. 

4510914 

Acceptable 

Practically 

nontoxic2

Terrestrial plants Vegetative 

Vigor 

Dicot – Lettuce 

Formulated Corn 

Product 

EC25 = 0.00000037 lbs 

a.i./acre 

EC05= 0.00000000018 

NOAEC = 0.00000066 

lbs a.i./acre 

Endpoint: Shoot length 

451091314

AcceptableSupplemental 

for lettuce and 

onion 

-- 
23

Taxonomic 

Group 

Study 

Type 

Surrogate 

Species 

and 

Test Material 

Toxicity Value 

(all units in terms of 

measured active 

ingredient) 

MRID and 

Classification 

Acute 

Toxicity 

Classification 

Monocot – Onion 

Formulated Corn 

Product 

EC25 = 0.0000067 lbs 

a.i./acre 

EC05= 0.00000043 

NOAEC = 0.0000026 

lbs a.i./acre 

Endpoint: Shoot dry 

weight 

-- 

Terrestrial plants Seedling 

Emergence 

Dicot – Tomato 

Formulated Corn 

Product 

EC25 = 0.0000035 lbs 

a.i./acre 

EC05= 0.000000035 

lbs a.i./acre 

NOAEC = <0.000084 

lbs a.i./acre 

Endpoint: Shoot length 

45109132

AcceptableSupplemental 

for tomato 

-- 

Monocot – Onion 

Formulated Corn 

Product 

EC25 =0.00028 lbs 

a.i./acre 

EC05= 0.00000057 lbs 

a.i./acre 

NOAEC = 0.000011 

lbs a.i./acre 

Endpoint: Shoot length 

1. Birds represent surrogates for terrestrial-phase amphibians and reptiles. 

2. Based on acute contact LD50 (μg a.i./bee) <2 highly toxic; 2-10.99 moderately toxic; ≥11 practically non-toxic 

3. Acute Oral (avian/mammal): Based on LD50 (mg/kg) <10 very highly toxic; 10-50 highly toxic; 51-500 

moderately toxic; 501-2000 slightly toxic; >2000 practically nontoxic 

4. A vegetative vigor study is available (MRID 47731901 with amendment 47746701) but has not been fully 

reviewed. A preliminary review suggests that the toxicity values are less sensitive than the current endpoints.

Effects on Plant Reproduction 

EFED's Plant Technical Team (PTT) conducted an analysis of available SU plant reproduction 

data from the open literature and the registrant-submitted data for chlorsulfuron, a representative 

SU, and concluded that data from the 850.4150 vegetative vigor guideline study are sufficiently 

protective of potential impacts to plant reproduction. Acetolactate synthase (ALS) inhibitors 

primarily affect metabolically active meristem tissues. When plants are exposed at a young, 

vigorously growing life-stage, effects are expressed primarily as inhibition of vegetative growth 

(i.e., biomass, shoot length). As plants mature these effects are less pronounced. However, as 

plants enter their reproductive life-stage, the metabolically active tissue will include flower and 

seed production. Plants exposed to SU herbicides during the reproductive life stage are likely to 

exhibit adverse effects on reproduction, generally expressed as reduced seed number or reduced 

seed weight. Based on the latest data analysis, these effects are expected to occur at similar rates 

to those effects determined in the guideline vegetative vigor studies (850.4150), so further data 

are not requested at this time. 

 
24

Degradate Toxicity 

Metsulfuron, another registered herbicide (PC 122010), is a biotransformation product of 

iodosulfuron in aerobic soils as well as aerobic aquatic environments. The most sensitive 

terrestrial and aquatic toxicity data are summarized in Tables 17 and 18 and are discussed in 

more detail in previous assessments (DP315491, 03/06; D350049, 03/08) and also the recent 

problem formulation for metsulfuron5

. 

Table 17. Aquatic Wildlife Toxicity Profile for Metsulfuron 

Taxonomic Group Study 

Type 

Species 

(Common Name) 

Endpoint 

(mg ai/L) 

Acute 

Classification 

Reference 

(MRID) 

Freshwater Fish 

Acute 

Lepomis 

macrochirus 

(Bluegill sunfish)

LC50>150 Practically 

nontoxic 00125817 

Chronic 

Oncorhynchus 

mykiss 

(rainbow trout) 

NOAEC=4.5 -- 44122801 

Freshwater 

Invertebrates 

Acute Daphnia magna

(Water flea) LC50>150 Practically 

nontoxic 00125818 

Chronic Daphnia magna 

(Water flea) NOAEC = 100b -- 44704901

Estuarine/marine 

Fish No data 

Estuarine/marine 

Invertebrates No data 

Vascular plants Vascular Lemna minor

(duckweed) 

EC50=0.00036 

NOAEC=0.00016 -- 41773902a

Nonvascular plants NonVascular 

Pseudokirchneriella 

subcapitata 

EC50=0.031 

NOAEC=0.005 -- 40639302a

a appears to be the most sensitive endpoint for Metsulfuron (pending full review of other studies) b

 provisional value pending full review. 

 

 

5

 Kiernan, B. D.; Baris, R. Registration Review: Preliminary Problem Formulation for Environmental Fate and 

Ecological Risk, Endangered Species, and Drinking Water Assessments for Metsulfuron-methyl (Case 7205) PC 

122010 DP389258 September 22, 2011.
25

Table 18. Terrestrial Organism Toxicity Profile for Metsulfuron 

In general, the toxicity profiles for iodosulfuron and metsulfuron are similar for terrestrial and 

aquatic animals. Based on the available data, the toxicity to plants is also similar for 

iodosulfuron and metsulfuron. A total toxic residue approach will be used unless the pending 

data review shows that metsulfuron is more toxic to plants. 

Taxonomic 

Group 

Study 

Type 

Surrogate 

Species 

(Common 

Name) 

Endpoint 

Acute 

Classification/ 

Chronic effect 

Reference 

(MRID) 

Birds 

Acute 

Anas 

platyrhynchos

(Mallard duck)

LD50>2150 mg 

a.i./kg-bwa,b

Practically 

nontoxica,b 00125819 

Colinus 

virginianus

(Bobwhite quail) 

LC50>5620 mg 

ai/kg diet 

Practically 

nontoxic 00125821 

Chronic 

Anas 

platyrhynchos

(Mallard duck) 

NOAEC=209 mg 

ai/kg-diet -- 44115702 

Mammals

Acute Rattus norvegicus

(Laboratory rat) 

LD50>5000 mg 

a.i./kg-bw 

Practically 

nontoxic 47018706 

Chronic Laboratory rat 

LOAEL=5000 mg 

a.i./kg-diet 

NOAEC=500 mg 

ai/kg-diet 

Adult body weight 

gain 00151028 

Terrestrial 

Invertebrates Acute Apis mellifera(Honey bee) LD50>25 μg/bee Practically 

nontoxic 00141829 

Terrestrial Plants 

Seedling 

emergence 

Dicot (tomato) 

EC25 = 0.0000032 

lb a.i./Ac 

EC05 or NOAEC = 

0.0000000012 lb 

a.i./Ac 

-- 44050301 c

Monocot (onion)

EC25 = 0.0001 lb 

a.i./Ac 

EC05 or NOAEC 

0.000013 lb a.i./Ac 

-- 44050301 c

Vegetative 

Vigor -- -- -- 44050301d

a

Data under review (provisional data reported here) 

b

Data are incomplete pending passerine submission. c

Data under review(provisional data reported here). The previous iodosulfuron assessment used the Seedling Emergence 

values reported in MRID 44050301, however, the recent problem formulation for metsulfuron identified that MRID 

44050301 is under review and may have deficiencies. Therefore, the values for this endpoint are pending study review d

Data under review. A cursory review of the Vegetative Vigor study submitted in 4405301 shows the toxicity is similar to the 

seedling emergence toxicity however, at this time, the provisional data cannot be reported without further review of the data 

and the supporting statistical analysis. 
26

In addition to metsulfuron, there are also five major degradates for iodosulfuron that contain an 

intact sulfonyl urea (SU) bridge (-NH-CO-NH-SO2-), including: AE F161778, AE 0014966, AE 

0014965 (only formed in anaerobic aquatic environments), AE F145740, and AE 0002166 (see 

structures in Table 9). The intact sulfonyl urea (SU) bridge has been identified as an important 

functional structure of SU toxicity. For degradates AE F161778 and AE 0014966, aquatic 

toxicity data are available and while the data have not been fully reviewed, the data suggest that 

both degradates are less toxic than the parent to aquatic vascular plants (Lemna gibba) with EC50 

values of 0.418 mg/L (MRID 46319201) and >10 mg/L (45791502), for AE F161778 and AE 

0014966, respectively. Data are not available for AE0014965, however, this transformation 

product only forms in anaerobic aquatic environments; thus this degradate will only be 

considered in groundwater simulations (for irrigation). Data are also not available for the other 

two major degradates that retain an intact sulfonyl urea (SU) bridge (AE F145740 and AE 

0002166). In the absence of additional toxicity data, these transformation products will be 

considered to be as toxic as the parent. In summary, four major degradates (AE F161778, AE 

0014966, AE F145740, and AE 0002166) that retain the SU bridge will be included in the TTR 

along with metsulfuron and the parent. 

Safener Toxicity 

For isoxadifen-ethyl (AEF122006), the safener added to formulations intended for corn use, a 

preliminary review of the available acute toxicity data suggests that isoxadifen-ethyl is highly 

toxic to the bluegill (MRID 44739131; LC50 = 0.22 mg /L, NOAEC = 0.09 mg /L) and the 

rainbow trout (MRID 44739130; LC50 = 0.34 mg /L, NOAEC = 0.12 mg /L). For aquatic 

invertebrates and non-vascular plants the available data reports EC50 values of >0.51 mg/L 

(MRID 44739132) and >100 mg/L (MRID 45360901), respectively. For chronic exposure, data 

are not available, however, a structure activity screen using ECOSAR (V 1.1-based on “aliphatic 

amines” as the chemical class/training set) shows a predicted high toxicity to fish, invertebrates, 

and algae with predicted toxicity values of 0.004, 0.005, and 0.007 mg/L for fish, invertebrates, 

and algae, respectively. For terrestrial organisms, the available data suggest that isoxadifen has 

low toxicity to birds and mammals on an acute exposure basis (MRIDs 44739135, 44739126, 

44739127). On a chronic exposure basis, data are only available for mammals with a reported 

NOAEL of 12.6 mg/kg-bw-day (MRID 44995801). See the Stressors of Concern discussion 

below, which discusses a cursory analysis conducted to assess potential exposure of the safener 

to aquatic animals and plants. 
27

Toxicity data are not available to the EPA for the safener mefenpyr-diethyl (added to 

formulations for wheat and triticale uses). According to a screen in ECOSAR (using “esters” as 

the training set), mefenpyr-diethyl is about 1-2 orders of magnitude more toxic to fish and 

invertebrates than iodosulfuron. On an acute exposure basis, mefenpyr-diethyl is estimated to be 

moderately toxic to fish (LD50=1.28 mg/L) and highly toxic to aquatic invertebrates (LC50=0.354 

mg/L). Chronic toxicity estimates are (0.047mg/L) for fish and (0.084 mg/L) for invertebrates. 

Toxicity estimates for terrestrial taxa are not available in ECOSAR, however, an evaluation of 

mefenpyr-diethyl was conducted by the Food and Agriculture Organization of the United 

Nations6

 and the reported toxicity to birds, mammals and invertebrates can be classified as 

practically non-toxic. See the Stressors of Concern discussion below, which discusses a cursory 

analysis conducted to assess potential exposure of the safener to aquatic animals. 

See APPENDIX D for ECOSAR output. 

Ecological Incidents 

A query of the Ecological Incident Information System (EIIS) database identified six incidents 

that were linked to exposure to iodosulfuron (Table 18). All of the incidents involved alleged 

damage to corn following a direct application of the herbicide to the crop. One incident, which 

was assigned a certainty of “probable,” was attributed to misuse of the pesticide. The other four 

incidents were assigned a certainty of “possible” and were attributed to legal use of the pesticide. 

Incidents of this nature, in which an herbicide allegedly damaged a crop on which it was directly 

applied, are no longer entered into EIIS since they have little relevance to risk to nontarget plants 

growing off the treatment site. No incident in the EIIS database attributes damage to crops or 

other nontarget plants resulting from spray drift or runoff of iodosulfuron from a treated field. 

Also, no incident in the EIIS database link exposure to iodosulfuron to any adverse effects to 

fish, wildlife, or invertebrates. 

The Incident Data System (IDS) database was queried for minor ecological incidents that were 

reported by the pesticide registrants in aggregated incident reports. Nineteen minor ecological 

incidents were linked to iodosulfuron exposure. All of these incidents were of damage to crops 

or other nontarget plants. 

A search of the Avian Incident Monitoring System (AIMS) database did not provide any 

additional incident data. 

 

 

6 FAO Specifications and Evaluations for Agricultural Pesticides-Mefenpyr diethyl. Food and Agriculture 

Organization of the United Nations. 2011. 

http://www.fao.org/fileadmin/templates/agphome/documents/Pests_Pesticides/Specs/Mefenpyr_diethyl_2011.pdf
28

Table 18. Ecological Incidents in EIIS Linked to Exposure to Iodosulfuron

Use Site Species 

Affected Date Exposure Certainty Misuse? ID Number 

Corn 

Corn 6/16/2003 Direct Application Probable Yes I014216-035 

Corn 6/2/2004 Direct Application Possible No I015291-020 

Corn 6/24/2004 Direct Application Possible No I015291-037 

Corn 7/1/2004 Direct Application Possible No I015406-004 

Corn 7/1/2004 Direct Application Possible No I015406-003 

Incidents attributed to exposure to metsulfuron are given in Table 19 to provide information on 

the potential risk of this degradation product. It should be recognized, however, that these 

incidents likely resulted from use of herbicide products containing metsulfuron-methyl as an 

active ingredient rather than from degradation of iodosulfuron. Nevertheless, the numerous plant 

damage incidents linked to exposure this ingredient demonstrate that nontarget plants may be 

harmed if they are exposed to this compound through spray drift, runoff , or carryover of soil 

residues. 

Table 19. Ecological Incidents in EIIS Linked to Exposure to Metsulfuron-Methyl 

(Degradation Product of Iodosulfuron)

Use Site Species 

Affected Date Exposure Certainty Misuse? ID Number 

Agricultural 

area 

(unspecified) 

Woody plants 9/1/1991 Spray drift and 

runoff Probable No I000903-001 

Potatoes and 

tomatoes 8/25/1993 Spray drift and runoff Probable No I000903-004 

Fish 7/1/1996 Runoff Unlikely No I005925-008 

Hay 4/25/2000 Carryover Probable NR I012366-047 

Wheat 2001 NR Possible Yes I020627-006 

Barley Vegetables 5/16/2008 Carryover Highly 

Probable No I020340-001 

Conservation 

Reserve Area Cotton 8/12/2003 Spray drift Possible NR I014415-001 

Fence row Birds, fish, 

and algae 5/26/1992 Spray drift and runoff Possible NR I000022-001 

Residential 

property 

NR NR Spray drift Possible Yes I005192-001 

Turf 6/7/2004 Runoff Possible NR I015266-001 

Right-of way 

Peach 7/15/2004 Spray drift Probable No I015218-001 

Vegetables 9/9/1997 Spray drift Probable No I007371-024 

Cantaloupe, 

watermelon, 

and tomato 

6/4/2004 Spry drift Possible No I015280-001 

Ornamental 

plants 7/3/2003 Spray drift Highly probable Yes I014290-001 

Wheat 

Sweet cherry 4/11/1994 Spray drift Possible Yes I003386-001 

Ornamental 

plants 4/18/2001 Spray drift Possible NR I020627-007 
29

Use Site Species 

Affected Date Exposure Certainty Misuse? ID Number 

Wheat 5/17/2005 Treated directly Possible No I06328-036 

5. CONCEPTUAL MODEL 

The environmental fate properties of iodosulfuron indicate that direct spray, spray drift, leaching 

to groundwater, and runoff represent potential transport mechanisms. These pathways may result 

in a wide range of potential aquatic and terrestrial exposure scenarios to both iodosulfuron as 

well as its environmental transformation products. 

6. ANALYSIS PLAN 

Ecological Risk Assessment 

Stressors of Concern 

The exposure assessments will consider the parent, iodosulfuron, the degradate metsulfuron as 

well as four major degradates that retain an intact SU bridge (AE F161778, AE 0014966, AE 

F145740, and AE 0002166) using a TTR approach. For more information on the selection of the 

transformation products included in the TTR analysis see the section above on degradate 

toxicity. Based on available toxicity data for the two safeners included in some iodosulfuronmethyl formulations, a cursory analysis was conducted to determine if additional information or 

data are needed to address potential risk concerns associated with formulated products containing 

the safeners. 

 

While the toxicity of the safener, isoxadifen-ethyl, to some species including aquatic organisms 

may be greater than the parent, aquatic exposure to the safener at levels of toxicological concern 

are not expected based on the current products registered for use on corn. This was determined 

by calculating a high-end exposure concentration [0.21μg/L (peak)] using the GENEEC (v. 2.0 

August 2, 2001)7

. Since the high-end estimated exposure concentration is orders of magnitude 

less than the most sensitive endpoints (freshwater fish acute LC50 value of 220 μg/L; freshwater 

invertebrate acute toxicity estimate = 41 μg/L; fish chronic toxicity estimate = 4 μg/L), risk is 

not expected. Risk to terrestrial animals is not expected based on a preliminary screen using the 

TREX model (V 1.5.1). 

The same analysis was completed for other safener, mefenpyr-diethyl. The high-end estimated 

exposure concentration is 3.93μg/L (peak)8

. Since this high-end exposure concentration is 

orders of magnitude less than the most sensitive acute endpoints (fish toxicity estimate = 1128 

μg/L; invertebrate toxicity estimate = 354 μg/L), risk is not expected on an acute exposure basis. 

On a chronic exposure basis, risk is also not expected based on the estimated toxicity values (fish 

 

7

 Modeling assumptions: 1 applications at 0.03 lb/a; ground application; high boom; no spray drift buffer; fine spray; 

no incorporation; solubility 1.3 mg/L; Koc = 9777 L/kg; and stable to degradation. 

8

 Modeling assumptions: 2 applications at 0.008 lb/A; 3 d minimum retreatment interval; ground application; high 

boom; no spray drift buffer; fine spray no incorporation; solubility 2.4 mg/L; Koc = 740 L/kg; and stable to 

degradation. 
30

toxicity estimate = 47 μg/L; invertebrate toxicity estimate = 84 μg/L). Additionally, there are 

several aquatic toxicity studies cited in the recent FAO evaluation9 that suggest the ECOSAR 

estimates are overestimating the toxicity. A final consideration for this safener is that with a log 

KOW of 4.82 (highest value reported in EPI Suite), this chemical has the potential to 

bioaccumulate; however, a fish bioaccumulation study also referenced in the FAO review of 

mefenpyr diethyl has a reported BCF of 232 which tentatively suggests (until EPA reviews the 

data) that mefenpyr diethyl is unlikely to bioaccumulate in organisms. If these toxicity/BCF data 

are made available to the EPA it will be useful to improve our understanding of the toxicity 

profile for this safener. 

Although this cursory analysis indicates a low potential for risk to aquatic organisms from 

exposures to the safeners at the levels estimated, alternative assumptions on mobility were 

explored to better understand the potential impact of the estimated Koc value on estimated EECs 

for the safeners. For isoxadifen-ethyl [Koc =977 L/kg (10X less than the EPI estimate) the 

estimated peak high-end exposure concentration was estimated to be 0.74 μg/L]. This is 

considerably lower than the endpoint for the most sensitive species assessed. For mefenpyrdiethyl ([Koc =74 L/kg (10X less than the EPI estimate), the estimated peak high-end 

concentration was estimated to be 7.1 μg/L]. Again, these results suggest that concentrations at 

levels of toxicological concern are not like to occur. The results of the GENEEC analysis are 

provided in APPENDIX E. 

Measures of Exposure 

In order to estimate exposure and potential risks of iodosulfuron exposure to aquatic and 

terrestrial organisms, all exposure modeling and resulting risk conclusions will be made based on 

maximum application rates, application methods, and any mitigation measures specifically 

indicated on the label. The models that will be used to predict the estimated environmental 

concentrations (EECs) of iodosulfuron are discussed on OPP’s model website.10 These models 

include: PRZM (Pesticide Root Zone Model) and EXAMS (EXposure Analysis Modeling 

System), PRZM-groundwater (GW), AgDrift, AgDisp, T-REX, TerrPlant, SIP (Screening 

Imbibition Program), and STIR (Screening Tool for Inhalation Risk). 

The results of running the SIP and STIR models are presented in APPENDIX B and 

APPENDIX C. While the SIP model suggests that there are expected concerns for both 

mammals and birds, the model does not perform well with chemicals that have a high aqueous 

solubility and have low toxicity to these organisms. This route of exposure is not likely to be 

significant enough to cause concern. The analysis with STIR indicates no risk to birds or 

mammals through inhalation of iodosulfuron. 

 

9 FAO Specifications and Evaluations for Agricultural Pesticides-Mefenpyr diethyl. Food and Agriculture 

Organization of the United Nations. 2011. 

http://www.fao.org/fileadmin/templates/agphome/documents/Pests_Pesticides/Specs/Mefenpyr_diethyl_2011.pdf

10 http://www.epa.gov/opp00001/science/models_pg.htm 
31

Measure of Effects 

Ecological effects data are used as measures of direct and indirect effects to biological receptors. 

Data are typically obtained from registrant-submitted studies or from literature studies identified 

by ECOTOX. The ECOTOX database provides more ecological effects data in an attempt to 

bridge existing data gaps. ECOTOX is a source for locating single chemical toxicity data and 

potential chemical mixture toxicity data for aquatic life, terrestrial plants, and wildlife. 

ECOTOX was created and is maintained by the USEPA, Office of Research and Development, 

and the National Health and Environmental Effects Research Laboratory's Mid-Continent 

Ecology Division. 

Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects to determine the 

potential ecological risk from the use of pesticides and the likelihood of direct and indirect 

effects to non-target organisms in aquatic and terrestrial habitats. For the assessment of risks, the 

risk quotient (RQ) method is used to compare exposure and measured toxicity values. EECs are 

divided by acute and chronic toxicity values. The resulting RQs are then compared to the 

Agency’s Levels of Concern (LOCs) (USEPA 2004). These criteria will be used to indicate 

when iodosulfuron use, as directed on the label, has the potential to cause adverse direct or 

indirect effects to non-target organisms. In addition, incident data from the EIIS, IDS, and 

AIMS will be considered as part of the risk characterization.

Endangered Species Assessments 

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the 

Agency will evaluate risks to federally listed threatened and/or endangered (listed) species from 

registered uses of pesticides in registration review. 

Endocrine Disruptor Screening Program 

Iodosulfuron is not included in the first group of 67 chemicals issued an order to conduct Tier 1 

EDSP testing. For additional information on the EDSP program, visit http://www.epa.gov/endo/. 

7. PRELIMINARY IDENTIFICATION OF DATA GAPS 

Fate 

Table 20 identifies the environmental fate studies submitted by the registrant, as well as study 

classifications and whether or not additional data are needed in order to support the registration 

review exposure assessments. Rationale for the additional data requested is also presented. 

 
32

Table 20. Submitted Environmental Fate Data for Chemical Iodosulfuron 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Comments 

835.2120 Hydrolysis 45108623 acceptable no 

835.2240 Aqueous 

photolysis 

45108624, 

45108625 supplemental no 

835.2410 Soil photolysis 45109003 supplemental no1

The study only investigated the fate of 

the phenyl ring. The retention of the SU 

bridge is an indication of the potential 

toxicity and the cleavage of this bridge 

can be followed independent of the ring 

structure labeled; therefore additional 

data on the phototransformation of 

triazine labeled iodosulfuron is not 

being requested at this time. 

The formation and decline of the 

transformation products was not clearly 

established during the course of the 

study. The formation of one 

transformation product is increasing at 

study termination and is approaching 

10% of the applied material. 

Identification of this material is needed; 

however, if this information is not 

submitted to EPA it may be considered 

as part of the total toxic residues. 

Iodosulfuron is toxic to plants. 

Knowledge of the persistence of 

iodosulfuron in soil as well as the 

identification of transformation 

products will help the Agency 

determine and estimate exposure of 

iodosulfuron and other residues of 

potential toxicological concern to nontarget organisms. The results of this 

study will be used in the environmental 

fate assessment, and later on, in the 

ecological risk characterization. 
33

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Comments 

835.4100 Aerobic soil 

metabolism 

45108934, 

45109005 supplemental 

no1 

Several of the studies report high levels 

of unextracted residues. The extraction 

methods used in these studies are not 

considered exhaustive. Identification of 

the unextracted materials is needed to 

rule out the potential that these residues 

are of toxicological concern. In 

absence of the data, these residues may 

be considered as part of a TTR 

approach in modeling to account for 

this uncertainty. 

Several of the studies were conducted 

outside the United States additional 

information on soil taxonomy would be 

helpful. 

45108935, 

45109006 supplemental 

45108936, 

45109007 supplemental 

45109001, 

45109008 supplemental 

45108933, 

45109004 supplemental 

835.4200 Anaerobic soil 

metabolism No data no1 

No acceptable study has been 

submitted. 

Iodosulfuron is toxic to plants. 

Knowledge of the persistence of 

iodosulfuron in soil as well as the 

identification of transformation 

products would help the Agency 

determine and estimate exposure of 

iodosulfuron and other residues of 

potential toxicological concern to nontarget organisms. The results of this 

study would be used in the 

environmental fate assessment, 

drinking water assessment and later on, 

in the ecological risk characterization. 

In absence of this data, the aerobic 

aquatic metabolism study will be used 

as a surrogate for identifying residues 

formed under anaerobic conditions that 

may be of toxicological concern. 
34

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications

Are data 

needed to 

conduct risk 

assessment?

Comments 

835.4300 Aerobic aquatic 

metabolism 45109021 supplemental no1

Storage stability data was not provided 

and is needed to confirm the residues 

were not degrading during storage. 

Storage stability studies included as 

part of this study as well as those 

conducted for the TFD study (MRID 

45109010; differences in storage 

conditions is not known) indicate that 

samples are metabolically stable over 

an eight month time period (recoveries 

are within guideline criteria). Although 

samples were stored for up to a year, 

based on these data samples are 

expected to be stable during the 

reported storage period. 

 

Identification of the unextracted 

materials is needed to rule out the 

potential that these residues are of 

toxicological concern. In absence of 

the data, these residues may be 

considered as part of a TTR approach in 

modeling to account for this 

uncertainty. 

Iodosulfuron is toxic to plants. 

Knowledge of the persistence of 

iodosulfuron in soil as well as the 

identification of transformation 

products will help the Agency 

determine and estimate exposure of 

iodosulfuron and other residues of 

potential toxicological concern to nontarget organisms. The results of this 

study will be used in the environmental 

fate assessment, drinking water 

assessment and later on, in the 

ecological risk characterization. 

835.4400 

Anaerobic 

aquatic 

metabolism 

45109002, 

45109009 supplemental no 

45109023 supplemental 

835.1230 

835.1240 

Adsorption/ 

desorption and 

leaching 

45109015 acceptable 

no 45109014 supplemental 

835.1410 Volatility – 

laboratory No data no 

Iodosulfuron is not expected to be 

volatile based on the reported vapor 

pressure. 

 
35

835.6100 Terrestrial field 

dissipation 45109010 supplemental no1 The study only looked for one environmental transformation product. 

In addition, these studies were 

conducted in Europe and no data was 

provided on how the sites were 

representative of locations within the 

US. Storage stability studies conducted 

over an eight month time period 

indicate that iodosulfuron and 

metsulfuron are relatively stable over 

the 8 month time period (recoveries are 

within guideline criteria). Study 

samples were stored for almost one 

year; however, based on the submitted 

storage stability data iodosulfuron and 

metsulfuron are expected to be stable. 

Despite these deficiencies, the 

submitted laboratory studies are 

expected to have identified residues of 

toxicological concern and using a total 

toxic residue approach that includes 

unextracted and unidentified residues 

reported in the submitted laboratory 

studies is expected to be protective. 

No independent lab validations have 

been submitted for the environmental 

chemistry methods for iodosulfuron or 

metsulfuron in soil. 

The reported LOQ is 0.02 μg/kg (ppb) 

for iodosulfuron. Due to the high 

sensitivity of plants to iodosulfuron it is 

important that the detection limit for 

iodosulfuron in soil capture the level of 

concern for the most sensitive species 

including listed plants. 

 

850.6100 Analytical 

method in soil 45108502 incomplete yes 
36

835.6200 Aquatic field 

dissipation No data no No independent lab validations have been submitted for the environmental 

chemistry methods for iodosulfuron in 

water. 

The reported LOQ is 0.1 μg/L (ppb) for 

iodosulfuron. Due to the high 

sensitivity of plants to iodosulfuron it is 

important that the detection limit for 

iodosulfuron in water capture the level 

of concern for the most sensitive 

species including listed plants. 

850.6100 Analytical 

method in water 45108522 incomplete yes 

850.1730 Fish 

bioconcentration No data no - see comments 

Iodosulfuron is not expected to 

bioaccumulate based on the calculated 

Kow. 

The safener mefenpyr diethyl may have 

the potential to bioaccumulate based on 

the calculated Kow. However, a fish 

bioconcentration study is cited in the 

FAO report referenced earlier in this 

document. 

1. Additional data would reduce uncertainty. See comments. 

Effects 

Table 21 and 22 identify the ecological effects studies, as well as study classifications and 

whether or not further data are needed in order to support risk assessment. Rationale for the 

additional data requested is presented below the tables. 
37

Table 21. Submitted Aquatic Ecological Effects Data for Iodosulfuron 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed to 

conduct 

risk 

assessment? 

Comments 

850.1010 Freshwater 

invertebrate 

acute toxicity 

45109103 Supplemental No -- 

850.1025 

850.1035 

850.1045 

850.1055 

Saltwater 

invertebrate 

acute toxicity 

45109128 Acceptable No -- 

850.1075 Freshwater 

fish acute 

toxicity 

45109035 Supplemental No -- 

850.1075 Saltwater fish 

acute toxicity 

45109127 Acceptable No -- 

850.1300 Freshwater 

invertebrate 

life cycle 

45109104 Acceptable No -- 

850.1350 Saltwater 

invertebrates 

life cycle 

No Study -- No Due to the general low toxicity to fish 

and invertebrates, the value of these 

additional data to the ecological risk 

assessment is likely low. 

850.1400 Freshwater 

fish 

early-life stage 

46431804 Supplemental No -- 

850.1400 Saltwater fish 

early-life stage 

No Study -- No Due to the general low toxicity to fish 

and invertebrates, the value of these 

additional data to the ecological risk 

assessment is likely low. 

850.1500 Fish life cycle No Study -- No -- 

850.4400 Aquatic plant 

growth: 

Tier II 

vascular plants 

45109111 Acceptable No 

850.5400 Aquatic Plant 

Growth: 

Tier II algae 

45109105 Acceptable No -- 
38

Table 22. Submitted Terrestrial Ecological Effects Data for Iodosulfuron

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed for 

risk 

assessment? 

Comments 

850.2100 Avian oral 

toxicity 

45109029 Acceptable No Low toxicity to other avian species 

and low use rates suggest that risk to 

passerine species is unlikely to exceed 

levels of concern. 

45109028 Acceptable 

850.2200 Avian dietary 

toxicity 

45109030 Acceptable No -- 

850.2300 Avian 

reproduction 

46431803 Acceptable No -- 

Nonguideline Avian 

inhalation 

45108805 Acceptable No -- 

850.3020 Honey bee 

acute contact 

toxicity 

45109115 Acceptable No -- 

 Honey bee 

acute oral 

toxicity 

45109114 Acceptable No -- 

850.3030 Honey bee 

residue on 

foliage 

No Study -- No -- 

850.3040 Field testing 

for pollinators 

No Study -- No -- 

850.4100 Terrestrial 

plant toxicity: 

seedling 

emergence 

45109132 

45467604 

45467602 

46431802 

45467601 

47731901/ 

47731701 

Acceptable 

Acceptable 

Supplemental 

Supplemental 

Acceptable 

In review 

In review- 

(Amendment) 

Yes A Seedling Emergence study is needed 

for a typical end use product for turf 

uses. Available toxicity data suggest 

that terrestrial plants are highly 

sensitive iodosulfuron, however only 

formulations that contain a chemical 

safener have been tested. The safener 

may influence the final toxicity 

estimation. In addition, performing a 

repeat for the tomato for 45109132 

would reduce the uncertainty in future 

assessments. 

850.4150 Terrestrial 

plant toxicity: 

vegetative 

vigor 

45109131 

45467606 

45467605 

45467603 

Acceptable 

Acceptable 

Acceptable 

Supplemental 

Yes A Vegetative Vigor study is needed 

for a typical end use product for turf 

uses. Available toxicity data suggest 

that terrestrial plants are highly 

sensitive to iodosulfuron, however 

only formulations that contain a 

chemical safener have been tested. The 

safener may influence the final 

toxicity estimation. In addition, 

performing a repeat for lettuce and 

onions for 45109131 would reduce the 

uncertainty in future assessments. 
39

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Study 

Classifications 

Are data 

needed for 

risk 

assessment? 

Comments 

124-1 Terrestrial 

Plant Field 

Testing 

No Study No The November 26, 2002 Notice of 

Registration for EPA Reg. No. 264-

686 identified this study as a 

requirement. However, a waiver was 

submitted and accepted, therefore, data 

are no longer required. 

124-2 Aquatic Plant 

Field Testing 

No study No The November 26, 2002 Notice of 

Registration for EPA Reg. No. 264-

686 identified this study as a 

requirement. However, a waiver was 

submitted and accepted, therefore, data 

are no longer required. 

Additional information that may be useful to the EPA includes: 

1. ecological incidents (non-target plant damage and avian, fish, reptilian, amphibian and 

mammalian mortalities) not already reported to the Agency 

2. major degradate and safener fate and toxicity data 

 
40

APPENDIX A 

Environmental Fate 

Kinetic Analysis Images
41
42
43
44
45
46
47
48
49
50
51
52

 
53

 

APPENDIX B 

SIP Analysis 

Table 1. Inputs 

Parameter Value 

Chemical name Iodosulfuron 

Solubility (in water at 25oC; mg/L) 25000 

 

Mammalian LD50 (mg/kg-bw) 2448 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian 

species 

 

Mammalian NOAEL (mg/kg-bw) 500 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian 

species 

 

Avian LD50 (mg/kg-bw) 1744 

Avian test species mallard duck 

Body weight (g) of "other" avian species 

Mineau scaling factor 1.15 

 

Mallard NOAEC (mg/kg-diet) 

Bobwhite quail NOAEC (mg/kg-diet) 372 

NOAEC (mg/kg-diet) for other bird species 

Body weight (g) of other avian species 

NOAEC (mg/kg-diet) for 2nd other bird 

species 

Body weight (g) of 2nd other avian species 

Table 2. Mammalian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 4300.0000 4300.0000 

Adjusted toxicity value (mg/kg-bw) 1882.9051 384.5803 

Ratio of exposure to toxicity 2.2837 11.1810 

Conclusion* 

Exposure through drinking 

water alone is a potential 

concern for mammals 

Exposure through drinking 

water alone is a potential 

concern for mammals 

 

Table 3. Avian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 20250.0000 20250.0000 

Adjusted toxicity value (mg/kg-bw) 905.5291 39.5430 

Ratio of exposure to acute toxicity 22.3626 512.1011 

Conclusion* 

Exposure through drinking 

water alone is a potential 

concern for birds 

Exposure through drinking 

water alone is a potential 

concern for birds 

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. Therefore, when aggregated 

with other routes (i.e., diet, inhalation, dermal), pesticide exposure through drinking water may contribute to a total exposure 

that has potential for effects to non-target animals. 

 
54

APPENDIX C 

STIR Analysis Results 

Application and Chemical Information 

Enter Chemical Name Iodosulfuron 

Enter Chemical Use Crop 

Is the Application a Spray? (enter y or n) y 

If Spray What Type (enter ground or air) ground 

Enter Chemical Molecular Weight (g/mole) 529.24 

Enter Chemical Vapor Pressure (mmHg) 5.03E-11 

Enter Application Rate (lb a.i./acre) 0.0089 

 

Toxicity Properties 

Bird 

Enter Lowest Bird Oral LD50 (mg/kg bw) 1744 

Enter Mineau Scaling Factor 1.15 

Enter Tested Bird Weight (kg) 0.178 

Mammal 

Enter Lowest Rat Oral LD50 (mg/kg bw) 2448 

Enter Lowest Rat Inhalation LC50 (mg/L) 2.81 

Duration of Rat Inhalation Study (hrs) 4 

Enter Rat Weight (kg) 0.35 

 

Output 

Results Avian (0.020 kg ) 

Maximum Vapor Concentration in Air at Saturation (mg/m3) 1.43E-06 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 1.80E-07 

Adjusted Inhalation LD50 1.12E+01 

Ratio of Vapor Dose to Adjusted Inhalation LD50 1.61E-08 Exposure not Likely Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 9.40E-04 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation LD50 8.43E-05 Exposure not Likely Significant 

 

Results Mammalian (0.015 kg ) 

Maximum Vapor Concentration in Air at Saturation (mg/m3) 1.43E-06 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 2.26E-07 

Adjusted Inhalation LD50 1.67E+02 

Ratio of Vapor Dose to Adjusted Inhalation LD50 1.35E-09 Exposure not Likely Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 1.18E-03 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation LD50 7.07E-06 Exposure not Likely Significant 

APPENDIX D 
55

ECOSAR Results for the Safeners Isoxadifen-ethyl and Mefenpyr-diethyl 

Isoxadifen-ethyl 

ECOSAR Version 1.10 Results Page 

 SMILES : C2(C(=O)(OCC))=NOC(C2)(c3ccccc3)c1ccccc1 

CHEM : 

CAS Num: 

ChemID1: 

MOL FOR: C18 H17 N1 O3 

MOL WT : 295.34 

Log Kow: 5.660 (EPISuite Kowwin v1.68 Estimate) 

Log Kow: (User Entered) 

Log Kow: (PhysProp DB exp value - for comparison only) 

Melt Pt: (User Entered for Wat Sol estimate) 

Melt Pt: (deg C, PhysProp DB exp value for Wat Sol estimate) 

Wat Sol: 0.1917 (mg/L, EPISuite WSKowwin v1.43 Estimate) 

Wat Sol: (User Entered) 

Wat Sol: (PhysProp DB exp value) 

 -------------------------------------- 

Values used to Generate ECOSAR Profile 

-------------------------------------- 

Log Kow: 5.660 (EPISuite Kowwin v1.68 Estimate) 

Wat Sol: 0.1917 (mg/L, EPISuite WSKowwin v1.43 Estimate) 

------------------------------------------------ 

Available Measured Data from ECOSAR Training Set 

------------------------------------------------ 

 Measured 

CAS No Organism Duration End Pt mg/L (ppm) Ecosar Class Reference 

=========== ========== ========== ====== ========== =========================== 

========================= 

163520-33-0 Daphnid 48-hr LC50 NES Aliphatic Amines OPP Pesticide Ecotoxicity DB 

ECOSAR v1.1 Class-specific Estimations 

Aliphatic Amines 

 Predicted 

ECOSAR Class Organism Duration End Pt mg/L (ppm) 

=========================== ================== ======== ====== ========== 

Aliphatic Amines : Fish 96-hr LC50 0.223 * 

Aliphatic Amines : Daphnid 48-hr LC50 0.041 

Aliphatic Amines : Green Algae 96-hr EC50 0.014 

Aliphatic Amines : Fish ChV 0.004 

Aliphatic Amines : Daphnid ChV 0.005 

Aliphatic Amines : Green Algae ChV 0.007 

=========================== ================== ======== ====== ========== 

Neutral Organic SAR : Fish 96-hr LC50 0.125 

(Baseline Toxicity) : Daphnid 48-hr LC50 0.133 

 : Green Algae 96-hr EC50 0.126 

 : Fish ChV 0.018 

 : Daphnid ChV 0.025 

 : Green Algae ChV 0.167 

 Note: * = asterisk designates: Chemical may not be soluble enough to 

 measure this predicted effect. If the effect level exceeds the 

 water solubility by 10X, typically no effects at saturation (NES) 

 are reported. 
56

Mefenpyr-diethyl 

ECOSAR Version 1.10 Results Page 

 SMILES : CCOC(=O)C2=NN(c1ccc(CL)cc1CL)C(C)(C(=O)OCC)C2 

CHEM : 

CAS Num: 

ChemID1: 

MOL FOR: C16 H18 CL2 N2 O4 

MOL WT : 373.24 

Log Kow: 4.818 (EPISuite Kowwin v1.68 Estimate) 

Log Kow: (User Entered) 

Log Kow: 3.83 (PhysProp DB exp value - for comparison only) 

Melt Pt: (User Entered for Wat Sol estimate) 

Melt Pt: 51.00 (deg C, PhysProp DB exp value for Wat Sol est) 

Wat Sol: 15.03 (mg/L, EPISuite WSKowwin v1.43 Estimate) 

Wat Sol: (User Entered) 

Wat Sol: 20 (mg/L, PhysProp DB exp value) 

 

 

Values used to Generate ECOSAR Profile 

-------------------------------------- 

Log Kow: 4.818 (EPISuite Kowwin v1.68 Estimate) 

Wat Sol: 20 (mg/L, PhysProp DB exp value) 

 

Available Measured Data from ECOSAR Training Set 

No Data Available 

ECOSAR v1.1 Class-specific Estimations 

-------------------------------------- 

Esters 

 Predicted 

ECOSAR Class Organism Duration End Pt mg/L (ppm) 

=========================== ================== ======== ====== ========== 

Esters : Fish 96-hr LC50 1.128 

Esters : Daphnid 48-hr LC50 1.734 

Esters : Green Algae 96-hr EC50 0.472 

Esters : Fish ChV 0.047 

Esters : Daphnid ChV 0.527 

Esters : Green Algae ChV 0.302 

Esters : Fish (SW) 96-hr LC50 1.444 

Esters : Mysid 96-hr LC50 0.354 

Esters : Fish (SW) ChV 0.344 

Esters : Mysid (SW) ChV 0.084 

Esters : Earthworm 14-day LC50 600.663 * 

Neutral Organic SAR : Fish 96-hr LC50 0.903 

(Baseline Toxicity) : Daphnid 48-hr LC50 0.815 

 : Green Algae 96-hr EC50 0.769 

 : Fish ChV 0.120 

 : Daphnid ChV 0.134 

 : Green Algae ChV 0.679 

 Note: * = asterisk designates: Chemical may not be soluble enough to 

 measure this predicted effect. If the effect level exceeds the 

 water solubility by 10X, typically no effects at saturation (NES) 

 are reported. 
57

APPENDIX E 

GENEEC Results for the Safeners Isoxadifen-ethyl and Mefenpyr-diethyl 

Isoxadifen-ethyl 

RUN No. 2 FOR isoxadifen-ethyl ON corn * INPUT VALUES * 

 -------------------------------------------------------------------- 

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN) 

 -------------------------------------------------------------------- 

 .030( .030) 1 1 9777.0 1.3 GRHIFI( 6.6) .0 .0 

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 -------------------------------------------------------------------- 

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 -------------------------------------------------------------------- 

 .00 2 N/A .00- .00 .00 .00 

 GENERIC EECs (IN NANOGRAMS/LITER (PPTr)) Version 2.0 Aug 1, 2001 

 -------------------------------------------------------------------- 

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC 

 -------------------------------------------------------------------- 

 207.50 201.57 168.83 118.11 93.90 

RUN No. 4 FOR isoxadifen-ethyl ON any * INPUT VALUES * 

 -------------------------------------------------------------------- 

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN) 

 -------------------------------------------------------------------- 

 .030( .030) 1 1 977.0 1.3 GRHIFI( 6.6) .0 .0 

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 -------------------------------------------------------------------- 

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 -------------------------------------------------------------------- 

 .00 2 N/A .00- .00 .00 .00 

 GENERIC EECs (IN NANOGRAMS/LITER (PPTr)) Version 2.0 Aug 1, 2001 

 -------------------------------------------------------------------- 

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC 

 -------------------------------------------------------------------- 

 742.59 739.67 722.64 686.17 660.49 

 
58

Mefenpyr-diethyl 

RUN No. 1 FOR mefenpyr-diethyl ON any * INPUT VALUES * 

 -------------------------------------------------------------------- 

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN) 

 -------------------------------------------------------------------- 

 .045( .135) 3 3 740.0 2.4 GRHIFI( 6.6) .0 .0 

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 -------------------------------------------------------------------- 

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 -------------------------------------------------------------------- 

 .00 2 N/A .00- .00 .00 .00 

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) Version 2.0 Aug 1, 2001 

 -------------------------------------------------------------------- 

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC 

 -------------------------------------------------------------------- 

 3.93 3.92 3.84 3.69 3.57 

 RUN No. 2 FOR mefenpyr-diethyl ON any * INPUT VALUES * 

 -------------------------------------------------------------------- 

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP 

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) (FT) (IN) 

 -------------------------------------------------------------------- 

 .045( .135) 3 3 74.0 2.4 GRHIFI( 6.6) .0 .0 

 FIELD AND STANDARD POND HALFLIFE VALUES (DAYS) 

 -------------------------------------------------------------------- 

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED 

 (FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

 -------------------------------------------------------------------- 

 .00 2 N/A .00- .00 .00 .00 

 GENERIC EECs (IN MICROGRAMS/LITER (PPB)) Version 2.0 Aug 1, 2001 

 -------------------------------------------------------------------- 

 PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY 

 GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC 

 -------------------------------------------------------------------- 

 7.14 7.14 7.12 7.08 7.04