U. S. ENVIRONMENTAL PROTECTION AGENCY 

Washington, D.C. 20460 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

Date: January 15, 2013 

Chemical: Ethanaminium (2,4-D choline salt) 

PC Code: 051505 

DP Barcodes: 400223, 400230, 400234, 400237, 405028, 405812 

D~Cl SION~ 441'151) 44~:2.44} 4'5 775 5.} 4'5'='460 

MEMORANDUM 

SUBJECT: 

TO: 

FROM: 

Approved 

By: 

EFED Environmental Risk Assessment of Proposed Label for Enlist 

(2,4-D Choline Salt), New Uses on Soybean with DAS 68416-4 (2,4-D 

Tolerant) and Enlist (2,4-D + Glyphosate Tolerant) Corn and Field 

Corn 

Michael Walsh, Risk Manager Reviewer 

Kathryn Montague, Risk Manager 

Dan Kenny, Branch Chief 

Registration Division (7505P) 

Meghan 

Faruque 

Radtke, 

Khan, Ph.D., 

Ph.D., 

Senior 

Biologist 

Scientist ~.:.__~ 

~ ~ :;;;> 

Environmental Risk Branch I 

Environmental Fate and Effects Division (7507P) /1_/ /71/ ( _/ 

EdwardOdenkirchen,Ph.D.ActingBran~Chi~~\ .;\0----I( 15/ /) Sujatha Sankula, Ph.D., Lead Biologist ~"'~ '\ 1 ~~ 

Environmental Risk Branch I · 

Environmental Fate and Effects Division (7507P) 

The Environmental Fate and Effects Division (EFED) has assessed the ecological risks to 

listed and non-listed species associated with the proposed new uses of2,4-D choline salt 

on herbicide-tolerant com and soybean. 

No direct risks from the proposed applications of2,4-D choline salt to herbicide-tolerant 

com and soybeans were identified for the following: 

• Birds (chronic) 

• Aquatic plants 

• Freshwater fish (acute and chronic) 

1 
2

• Estuarine/marine fish (acute and chronic)

• Freshwater invertebrates (acute and chronic)

• Estuarine/marine invertebrates (acute and chronic)

• Aquatic plants

• Terrestrial insects

There was insufficient information to determine if the following groups are at direct risk 

from the proposed new uses of 2,4-D choline salt:

• Mammals (acute and chronic)

• Birds, reptiles, terrestrial-phase amphibians (acute)

• Terrestrial plants

In addition, there was insufficient information for all taxa to determine if indirect effects 

are expected because of potential dependencies (e.g., food, shelter, habitat) on species 

that are directly affected. Information, such as biological distribution, species biology, 

spray drift properties specific to the 2,4-D choline formulations, and mitigation efforts in 

regions where the pesticide is used, could be used to reduce the uncertainty regarding 

potential direct and indirect effects.

A spray drift analysis using the AIXR 11004 nozzle and the GF2726 formulation 

indicated that buffers of 202 feet would reduce risk quotients for birds (acute), mammals 

(acute and chronic), and terrestrial plants below the Agency’s levels of concern. This 

buffer is only achieved through the combination of the AIXR 11004 nozzle and GF2726 

formulation. The locations of the buffers would be dependent on species distribution, 

species biology, and any mitigation efforts proposed by the registrant.

The following major data gaps and uncertainties were identified in this assessment:

• Acute oral toxicity test for passerines (850.2100) is not available; data from 

bobwhite quail and mallard duck were used as surrogates.

• Estuarine/marine invertebrate chronic toxicity test (850.1350) is not available; an 

acute-to-chronic ratio based on freshwater invertebrates was applied. 

• Terrestrial plant seedling emergence and vegetative vigor tests (850.4100, 

850.4150) were not available for the 2,4-D choline salt/glyphosate formulation; 

data from other 2,4-D forms were used as surrogates. There is uncertainty as to 

whether they accurately represent the toxicity of a dual herbicide product to 

terrestrial plants.

• Vapor-phase effects data on terrestrial plants were limited in scientific soundness; 

conservative assumptions were made to characterize the risk to terrestrial plants 

from vapor-phase drift.

Key Uncertainties and Information Gaps

A number of ecological and fate uncertainties were identified. Table 1 lists several 

studies that could be used to address these uncertainties.

Table 1. Fate and Ecological Toxicity Data Gaps for 2,4-D Choline Salt
3

Guideline # Data Gap Justification

835.6100

Terrestrial Field 

Dissipation

To confirm the bridging strategy for the environmental fate 

behavior of 2,4-D choline salt to 2,4-D acid, a terrestrial field 

dissipation study would be needed.

850-6100 Analytical methods

for soil and water

An environmental chemistry method (ECM) in soil and water 

would be used to support the terrestrial field dissipation study. An 

independently validated environmental chemistry method as well 

as ECM in soil and water is necessary to evaluate the results of the 

terrestrial field dissipation study.

850.2100 Avian Oral Toxicity 

Test for Passerines

The CFR requires testing for one passerine species when a 

chemical is intended for outdoor use. The current method of 

calculating a weight-adjusted LD50 using bobwhite quail or 

mallard duck data may over- or under-estimate risks to passerines 

because these birds may metabolize the chemical differently. This 

study has already been requested through the Problem Formulation 

of Registraton Review. A protocol for the study should be 

submitted to EPA for approval prior to study initiation.

850.4150

Terrestrial Plant 

Vegetative Vigor 

Test

The CFR requires typical end-use data for terrestrial plants. In the 

case of the new 2,4-D choline salt registrations, no information is 

available for the two 2,4-D choline salt formulations, nor the 

EnlistTM formulation, which is a mixture with glyphosate. For the 

2,4-D choline salt-glyphosate formulation, it is anticipated that 

there could be additional toxicological effects (synergistic or 

additive) because of the presence of two herbicides. This could 

change the outcome of the assessment by yielding more sensitive 

toxicity values for terrestrial plants, thus modifying minimum 

buffer distances.

Non-guideline

Terrestrial Plant 

Vegetative Vigor 

Test with Vapor 

Phase Exposure

2,4-D is known to volatilize from the field and drift off site under 

certain environmental conditions. EFED evaluated a vapor-phase 

study on grapes, cotton, soybean, and tomato; however, the 

methodology did not include a control, did not measure 

growth/weight parameters, and was not well-aligned with the 

850.4150 protocol. Open literature studies and field data from the 

vapor-flux study suggest 20% plant damage corresponds to 

significant reductions in yield. A vapor-phase study with 

vegetative vigor endpoints would confirm these conclusions. At a 

minimum, grape and cotton should be tested as these were the 

most sensitive species in the submitted vapor-phase study.

Labeling Recommendations

According to the Label Review Manual, the following label statements are recommended 

for 2,4-D choline formulations for ground boom application 

(http://www.epa.gov/oppfead1/labeling/lrm/index.htm):

Application Instructions

A combination of AIXR 11004 spray nozzle, GF-2726 formulation, and 

appropriate pressure are required to be selected to provide ASABE Standard 

S571.1 droplet size category of coarse/very coarse (DV0.5 of ≥ 439μm) or coarser. 

Directions from the equipment manufacturer or vendor, pesticide registrant or a 

test facility using a laser-based measurement instrument must be used to adjust 
4

equipment to produce acceptable droplet size spectra. Application equipment 

must be tested at least annually to confirm that pressure at the nozzle and nozzle 

flow rate(s) are properly calibrated”.

To add additional nozzles to the label, spray drift curves for the new nozzle and/or 

droplet spectrum information must document that the additional nozzle(s) is 

expected to perform similarily to the nozzles already specified on the label with 

the GF-2726 formulation.

Environmental Hazards

“This pesticide is toxic to birds, mammals, fish, and aquatic invertebrates. Do not 

apply directly to water, or to areas where surface water is present or to intertidal 

areas below the mean high water mark. Do not contaminate water when 

disposing of equipment washwater or rinsate.”

Groundwater Advisory

"2,4-D, an anion of 2,4-D choline is known to leach through soil into groundwater 

under certain conditions as a result of label use. This chemical may leach into 

groundwater if used in areas where soils are permeable, particularly where the 

water table is shallow.”

Surface Water Label Advisories

“2,4-D from 2,4-D choline application may impact surface water quality through 

spray and runoff of rain water. This product has a high potential for runoff for 

several months or more after application. Poorly draining soils and soils with 

shallow water tables are more prone to produce runoff that contains this product. 

A level, well maintained vegetative buffer strip between areas to which this 

product is applied and surface water features such as ponds, streams, and springs 

will reduce the potential for contamination of water from rainfall-runoff. Runoff 

of this product will be reduced by avoiding applications when rainfall is 

forecasted to occur within 48 hours.”
5

Ecological Risk Assessment for the Section 3 New Use Registration of 

2,4-D Choline Salt on Soybean with DAS 68416-4 (2,4-D Tolerant) 

and 2,4-D + Glyphosate Tolerant Corn and Field Corn

Team Members

Meghan Radtke, Ph.D., Biologist

Faruque Khan, Ph.D., Senior Scientist

Peer Reviewers

Anita Ullagaddi, M.S., Biologist

Stephen Wente, Ph.D., Biologist

Acting Branch Chief 

Edward Odenkirchen, Ph.D.

Date of Approval

January 15, 2013
6

TABLE OF CONTENTS

1. Executive Summary ................................................................................................................. 9

1.1 Nature of the Chemical Stressor........................................................................................... 9

1.2 Potential Risk to Non-Target Organisms........................................................................... 11

1.3 Listed Species Assessment ................................................................................................. 11

1.4 Data Gaps and Uncertainties............................................................................................... 12

2. Problem Formulation............................................................................................................. 14

2.1 Nature of Regulatory Action............................................................................................... 14

2.2 Stressor Source and Distribution ........................................................................................ 14

2.2.1 Nature of the Chemical Stressor................................................................................. 14

 2.2.2 Environmental Fate Bridging Strategy....................................................................... 14

 2.2.3 Environmental Fate and Transport ............................................................................. 15

 2.2.4 Degradates.................................................................................................................... 19

2.2.5 2,4-D Choline Salt Use Characterization................................................................... 20

2.3 Receptors.............................................................................................................................. 21

2.3.1 Aquatic and Terrestrial Receptors and Effects.......................................................... 21

2.3.2 Ecosystems Potentially at Risk ................................................................................... 21

2.4 Assessment Endpoints......................................................................................................... 21

2.5 Conceptual Model ............................................................................................................... 22

2.5.1 Risk Hypothesis........................................................................................................... 22

2.5.2 Conceptual Diagrams.................................................................................................. 22

2.6 Analysis Plan ....................................................................................................................... 24

2.6.1 Measures of Effect and Exposure............................................................................... 25

3. Exposure Assessment............................................................................................................. 27

3.1 Proposed 2,4-D Choline Salt Application Information..................................................... 27

3.2 Aquatic Exposure Modeling for 2,4-D Choline Salt......................................................... 29

3.2.1 PRZM-EXAMS Model ............................................................................................... 29

3.2.2 Model Inputs................................................................................................................ 30

3.2.3 Model Outputs: Aquatic Exposure Estimates............................................................ 34

3.3 Monitoring Data .................................................................................................................. 35

3.4 Terrestrial Exposure Estimates for 2,4-D Choline Salt..................................................... 36

3.4.1 Exposure Estimates for Birds and Mammals............................................................. 37

3.4.2 Exposure Estimates for Plants.................................................................................... 38

4. Ecological Effects Assessment .............................................................................................. 40

4.1 Aquatic Effects Summary ................................................................................................... 41

4.1.1 Toxicity Effects on Fish and Amphibians.................................................................. 41
7

4.1.2 Toxicity Effects on Invertebrates ............................................................................... 42

4.1.3 Toxicity Effects on Plants........................................................................................... 43

4.2 Terrestrial Effects Summary............................................................................................... 48

4.2.1 Toxicity Effects on Birds............................................................................................ 44

4.2.2 Toxicity Effects on Mammals..................................................................................... 45

4.2.3 Toxicity Effects on Non-Target Insects..................................................................... 46

4.2.4 Toxicity Effects on Plants........................................................................................... 47

4.3 Incident Database Review................................................................................................... 48

5. Risk Characterization............................................................................................................ 51

5.1 Risk Estimation.................................................................................................................... 51

5.1.1 Summary of Aquatic Risk Quotients.......................................................................... 51

5.1.2 Summary of Terrestrial Risk Quotients ..................................................................... 55

5.2 Risk Description .................................................................................................................. 57

 5.2.1 Probit Slope Response Analysis of LOC Values and Acute RQ Values................ 58

 5.2.2 AgDrift Analysis for Terrestrial Plants, Birds, and Mammals................................ 58

5.2.3 Risk to Aquatic Vertebrates and Invertebrates.......................................................... 59

5.2.4 Risk to Terrestrial Animals......................................................................................... 60

5.2.5 Risks to Terrestrial Plants........................................................................................... 62

5.2.6 Risks to Aquatic Plants ............................................................................................... 65

5.3 Listed Species Assessment ................................................................................................. 66

5.3.1 Data Gaps and Uncertainties....................................................................................... 67

6. References.............................................................................................................................. 699

Appendix A. Registrations and Uses for 2,4-D Choline Salt .................................................... 71

Appendix B. Major and Minor Degradates Identified in Environmental Fate 

Studies ................................................................................................................................................ 73

Appendix C. SIP and STIR Results for 2,4-D Choline Salt...................................................... 74

Appendix D. Examples of PRZM/EXAMS Outputs for 2,4-D and 2,4-DCP.........................77

Appendix E. Avian and Mammalian Risk Quotient Calculations for 2,4-D Choline 

Salt Based on T-REX ....................................................................................................................... 82

Appendix F. Terrestrial Plant Risk Quotient Calculations for 2,4-D Choline Salt 

Based on TerrPlant .......................................................................................................................... 84

Appendix G. Risk Quotient Method and Levels of Concern.................................................... 86

Appendix H. Preliminary Volatility Flux Data........................................................................... 88

Appendix I. Example of PERFUM Input and Output .............................................................. 94

Appendix J. Spray Drift Deposition Curves .............................................................................103
8

Acknowledgements

The 2,4-D chemical team (Dr. Meghan Radtke, Dr. Faruque Khan, Dr. Sujatha Sankula, 

and Dr. Edward Odenkirchen) would like to acknowledge the contributions and 

invaluable assistance and technical support from the Plant Tech Team, Mr. Charles Peck, 

Mr. Gabe Rothman, and Dr. James Hetrick in analyzing and modeling the spray drift and 

volatilization of 2,4-D. This assessment has been peer reviewed in draft form by 

individuals chosen for their diverse perspectives and technical expertise. We wish to 

thank the following individuals for their peer review of this document: Mr. Nelson 

Thurman, Dr. Katrina White, Ms. Elizabeth Riley, Mr. Reuben Baris, Dr. Stephen Wente,

and Ms. Anita Ullagaddi.
9

1. Executive Summary

2,4-D choline salt is an herbicide in the phenoxy or phenoxyacetic acid family that is 

used preplant, preemergence and postemergence for selective control of broadleaf weeds. 

It is currently registered on a number of crops including: sugarcane, rice, pome fruits, 

stone fruits, conventional corn and soybeans, fallow land, turf, and tree and brush control. 

Dow Agrosciences LLC, the manufacturer and registrant of 2,4-D choline salt, submitted 

an application to the U.S. Environmental Protection Agency (EPA) to add the following 

uses to the 2,4-D choline salt label: 1) Enlist corn and field corn (containing DAS 68416-

4 trait), and 2) Enlist soybean (containing DAS 68416-4). All of these crops contain 

genetic traits that make them tolerant to the herbicide, 2,4-D; unless specifically stated, 

all mentions of corn and soybean in this assessment will refer to the 2,4-D herbicidetolerant varieties that are being assessed. Two of the proposed registrations contain only 

2,4-D choline salt as the active ingredient whereas the other two labels are for a 2,4-Dcholine salt/glyphosate mixture. The latter would allow applications to herbicide-tolerant 

corn and soybeans with resistance to both 2,4-D choline salt and glyphosate.

2,4-D choline salt has already been registered for use on conventional corn and soybean,

but an ecological risk assessment was not performed on these crops. The proposed 

application rates and seasonal/annual maximums are similar to those that have been 

previously assessed for other crops; however, the timing of the applications is different 

because of the herbicide tolerant trait in corn and soybeans. The trait allows for 

applications to herbicide-tolerant corn and soybeans that are later in the growing season

(later growth stages) than conventional varieties of these crops. Ground boom 

application is the only recommended method for Enlist corn, field corn containing DAS 

68416-4 trait, and herbicide-tolerant soybean. 

1.1 Nature of the Chemical Stressor 

2,4-D choline salt is a quaternary ammonium salt that rapidly dissociates into a 2,4-D 

anion and a choline cation. 2,4-D is a plant growth regulator (synthetic auxin herbicide) 

in the phenoxy or phenoxyacetic acid family that is used as a preplant, pre-emergence, 

and post-emergence herbicide for the selective control of broadleaf weeds.

Based on the registrant-submitted studies, the following conclusions were drawn 

regarding the environmental fate and ecological effects of 2,4-D choline salt:

Fate

• The environmental fate strategy for 2,4-D is based on bridging the degradation 

of 2,4-D esters and 2,4-D salts to 2,4-D acid.

• 2,4-D is an anionic (X-COO- H+) acid under most environmental conditions; it is 

expected to be mobile to moderately mobile. The vapor pressure (1.4 × 10-7 mm 

Hg) and Henry's Law Constant (8.56 x 10-6 atm-m3

/mol) indicate that 2,4-D acid 

has a low volatility.
10

• The degradation of 2,4-D appears to be dependent on oxidative microbiallymediated mineralization in the terrestrial environment and photodegradation in 

water. 

• Preliminary results from a field volatility study performed with 2,4-D choline salt, 

2,4-D ethylhexyl ester (EHE), and 2,4-D dimethylamine salt (DMA salt) suggest 

that the estimated volatility flux rate of 2,4-D choline salt is lower than the amine 

and EHE formulations.

• Three major degradates were identified in the submitted environmental fate 

studies for 2,4-D: 1,2,4-benezenetriol (maximum formed = 37%); 2,4-Ddichlorophenol (2,4-DCP) (maximum formed = 32.6%); and chlorohydroquinone 

(CHQ) (maximum formed = 16%).

• 2,4-D is unlikely to bioaccumulate in fish given the low value of the log noctanol/water partition coefficient (log Kow = 0.18 at neutral pH).

Ecological

• Ecotoxicity studies for algae, freshwater fish, and honeybee support bridging 2,4-

D choline salt data to 2,4-D acid data.

• On an acute basis, 2,4-D acid is practically non-toxic to freshwater fish, 

estuarine/marine fish, and amphibians; and slightly toxic to freshwater and 

estuarine/marine invertebrates.

• NOAECs are available for freshwater fish (based on length), estuarine/marine fish 

(based on survival), and freshwater invertebrates (based on survival and 

reproduction).

• 2,4-D acid is more toxic to vascular aquatic plants than non-vascular aquatic 

plants.

• 2,4-D acid is more toxic to dicots than monocots. Seedling emergence toxicity is 

based on shoot length whereas vegetative vigor toxicity is based on weight. 

• 2,4-D acid is slightly (based on dietary study) to moderately toxic (based on oral 

dose) to birds on an acute basis. There were no observed chronic effects in the 

reproductive study.

• On an acute basis, 2,4-D acid is slightly toxic to mammals. Chronic effects 

include: decreased female body weight gain, male renal tubule alteration, 

increased gestation length, and decreased pup body weight.

• 2,4-D acid is practically non-toxic to adult honeybees. 
11

1.2 Potential Risk to Non-Target Organisms 

Ecological risks associated with the new uses of 2,4-D choline salt are summarized 

below: 

• Proposed 2,4-D choline salt uses are not expected to directly adversely affect 

freshwater or estuarine/marine fish, and freshwater or estuarine/marine 

invertebrates. 

• Proposed 2,4-D choline salt uses are not expected to directly adversely affect 

aquatic vascular and non-vascular plants. 

• Birds may be directly affected by the proposed uses of 2,4-D choline salt on an

acute basis. Chronic effects are not expected. 

• Mammals may be directly affected by the proposed uses of 2,4-D choline salt on 

an acute and chronic basis. 

• Terrestrial plants (monocots and dicots) may be directly affected by the proposed 

uses of 2,4-D choline salt. 

• The acute contact toxicity test on honeybees demonstrated that 2,4-D choline salt

is practically non-toxic; however, the lack of direct effects on terrestrial 

invertebrates does not preclude concerns for indirect effects.

1.3 Listed Species Assessment

The screening-level analysis for 2,4-D choline salt indicated that there was insufficient 

information to determine if there were direct effects to mammals (acute and chronic);

birds, reptiles, and land-phase amphibians (acute); and terrestrial plants. Indirect effects 

are determined by assessing the potential for reduction of biologically mediated 

resources, or habitat modification of listed taxa; however, there was insufficient 

information to determine if there were indirect effects to any taxa (Table 2). 

Table 2. Listed Species Risks Associated with the Proposed New Uses for 2,4-D 

Choline Salt

Listed Taxa Direct Effects Indirect Effects1

Terrestrial and semiaquatic plants: 

Monocots

Dicots

Insufficient information

Insufficient information

Insufficient information

Insufficient information

Birds Insufficient information – Acute 

No – Chronic Insufficient information
12

Listed Taxa Direct Effects Indirect Effects1

Terrestrial-phase 

amphibians2

Insufficient information – Acute 

No – Chronic Insufficient information

Reptiles2 Insufficient information – Acute 

No – Chronic Insufficient information

Mammals Insufficient information – Acute 

Insufficient information – Chronic Insufficient information

Aquatic plants:

Vascular

Non-vascular

No

No

Insufficient information

Insufficient information

Freshwater fish No – Acute

No – Chronic Insufficient information

Aquatic-phase 

amphibians3

No – Acute

No – Chronic Insufficient information

Freshwater 

invertebrates

No – Acute

No – Chronic Insufficient information

Mollusks No – Acute

No – Chronic Insufficient information

Marine/estuarine fish No – Acute

No – Chronic Insufficient information

Marine/estuarine 

invertebrates

No – Acute

No – Chronic Insufficient information

Terrestrial invertebrates No Insufficient information

1

Indirect effects to a listed species occur when its resource base is reduced or habitat is 

modified; indirect effects are possible for all taxa based on potential effects to terrestrial 

plants and obligate relationships with birds and/or mammals.

2

Birds are surrogates for reptiles and terrestrial-phase amphibians.

3

Freshwater fish are surrogates for aquatic-phase amphibians.

1.4 Data Gaps and Uncertainties

Avian Acute Oral Toxicity Test for Passerines (850.2100): The CFR requires data for 

one passerine species when a chemical is intended for outdoor use. The current method 

of calculating a weight-adjusted LD50 using bobwhite quail or mallard duck data may 

over- or under-estimate risks to passerines because these birds may metabolize the 

chemical differently. This study has already been requested through Problem 

Formulation. A protocol for the study of passerine species should be submitted to EPA 

for approval prior to study initiation.

Estuarine/Marine Invertebrate Chronic Toxicity Test (850.1350): No acceptable data 

are available for the chronic toxicity of 2,4-D choline salt to marine/estuarine 
13

invertebrates. Currently, the toxicological effects for estuarine/marine invertebrates on a

chronic basis remain uncertain for 2,4-D choline salt, although this assessment estimated 

a chronic value based on an acute-to-chronic ratio using freshwater invertebrate data. 

Additionally, chronic risk quotients for freshwater invertebrates did not come close to 

exceeding the level of concern; thus increasing EFED’s confidence that the available 

aquatic invertebrate information is sufficient to evaluate risk.

Terrestrial Plant Seedling Emergence and Vegetative Vigor Tests (850.4100, 

850.4150): Typical end-use data are required for terrestrial plants. In the case of the new 

2,4-D choline salt registrations, no information is available for the two 2,4-D choline salt 

formulations, nor the EnlistTM formulation, which is a mixture with glyphosate. For the 

2,4-D choline salt-glyphosate formulation, it is anticipated that there could be additional 

toxicological effects (synergistic or additive) because of the presence of two herbicides. 

In lieu of these data, toxicity information from other 2,4-D formulations were used as 

surrogates on an acid-adjusted basis.

Terrestrial Plant Vegetative Vigor Test with Vapor Phase Exposure (non-guideline): 

2,4-D is known to volatilize from the field and drift off site under certain environmental 

conditions. EFED evaluated a vapor-phase study on grapes, cotton, soybean, and tomato; 

however, the methodology did not include a control, did not measure growth/weight 

parameters, and was not well-aligned with the 850.4150 protocol. A vapor-phase study 

with vegetative vigor endpoints is being recommended to further characterize the risk to 

plants from this exposure route. Grape and cotton should be tested as these were the two 

most sensitive species in the vapor-phase study that was submitted.

Additionally, the following uncertainties were identified for this risk assessment:

• For terrestrial organisms, only dietary exposure to 2,4-D choline salt was 

assessed.

 

• For freshwater fish, birds, and terrestrial insects, definitive acute toxicity data 

were not available. Acute data were presented as greater than values, making it 

possible to conservatively compare the toxicity value directly to the EECs to 

estimate potential risk. 

• For terrestrial plants, vegetative vigor toxicity values for monocots were nondefinitive. The EC25 (a greater than value) was compared directly to the EECs to 

assess the likelihood of risk. The monocot vegetative vigor NOAEC was a less 

than value; however, given that seedling emergence data were more sensitive for 

dicots than the vegetative vigor data, the same pattern was assumed to hold true 

for monocots. Consequently, the monocot seedling emergence NOAEC was 

assumed to be the most sensitive monocot NOAEC and used to calculate risk 

quotients. If this assumption is false, the risk to listed monocots may have been 

under-estimated.
14

• In the absence of chronic data for marine/estuarine invertebrates, an acute-tochronic ratio was calculated to estimate the NOAEC for Eastern oyster.

• This risk assessment only considered the most sensitive of the species evaluated 

in the registrant-submitted studies. The position of the tested species relative to 

the distribution of all species’ sensitivities to 2,4-D choline salt is unknown. 

Extrapolating the risk conclusions from the most sensitive tested species to nontested species may either underestimate or overestimate the potential risks to 

those species. 

• 2,4-D is currently undergoing a Tier I Endocrine Disruptor Screening as required 

by FFDCA section 408(p). The results of the screening analysis are not yet 

available.

2. Problem Formulation

The problem formulation provides the foundation for the ecological risk assessment 

being conducted for the proposed new uses of the herbicide 2,4-D choline salt. As such, 

it articulates the purpose and objectives of the risk assessment, evaluates the nature of the 

problem, and provides a plan for analyzing the data and characterizing the risk. 

2.1 Nature of Regulatory Action

Under the authority of the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), 

Dow Agrosciences LLC is seeking registration for the new uses of 2,4-D choline salt as a 

foliar spray for the control of broadleaf weeds in herbicide-tolerant soybeans containing 

DAS-68416-4, and for the control of annual and perennial weeds in herbicide-tolerant 

EnlistTM corn and herbicide-tolerant field corn (Table 6). See Appendix A for a list of 

current uses and application information for 2,4-D choline salt.

2.2 Stressor Source and Distribution

2.2.1 Nature of the Chemical Stressor 

2,4-D is a plant growth regulator (synthetic auxin herbicide) in the phenoxy or 

phenoxyacetic acid family. It is used as a preplant, pre-emergence, and post-emergence 

herbicide for the selective control of broadleaf weeds. 2,4-D causes disruption of 

multiple growth processes in susceptible plants by affecting proteins in the plasma 

membrane, interfering with RNA production, and changing the properties and integrity 

of the plasma membrane. Disruption of reproductive processes may occur resulting in 

sterile or multiple florets and nonviable seed production. Symptoms may appear on 

young growth almost immediately after application, but death may not occur for several 

weeks.

2.2.2. Environmental Fate Bridging Strategy 

The salt and ester forms of 2,4-D are derivatives of 2,4-D acid. The environmental fate 
15

strategy for 2,4-D is based on bridging the data on the degradation of 2,4-D esters and 

2,4-D salts to 2,4-D acid [Registration Standard for 2,4 Dichlorophenoxyacetic acid (2,4-

D), 1988, 540/RS-88-115]. The bridging data provide information on the time of 

dissociation of 2,4-D amine salts and the rate of hydrolysis of 2,4-D esters. The choline 

salt of 2,4-D is also considered equivalent to 2,4-D acid. The 2,4-D choline salt has been 

shown to dissociate rapidly (within 6 seconds) in water (MRID 48260701). However, a 

field terrestrial study conducted with 2,4-D choline is required to confirm the bridging 

strategy. Table 3 provides selected physico-chemical properties of 2,4-D choline salt. A 

detailed environmental fate data bridging strategy can be found in the 2005 Registration 

Eligibility Document (RED) for 2,4-D (USEPA 2005).

Table 3. Selected Physical and Chemical Properties of 2,4-D Choline Salt

Parameter Value Source

Chemical structure MRID 48260701

Molecular formula and 

weight (g/mol)

C13H19Cl2NO4

324.2 MRID 48260701

Water solubility (mg/L) Not applicable for end use 

product MRID 48208302

Density (g/cm3) 1.21 MRID 48208302

LogKOW Not applicable for end use 

product Vapor pressure (mm MRID 48208302

Hg@~25oC)

2.2.3. Environmental Fate and Transport

The physicochemical properties in Table 4 suggest that 2,4-D acid is soluble in water 

(569 mg/L). The vapor pressure (1.4 × 10-7 mm Hg) and Henry's Law Constant (8.56 x 

10-6 atm-m3

/mol) indicate that 2,4-D acid has low volatility. The Agency received a field 

volatility study (MRID 48862902) performed with 2,4-D choline salt, 2,4-D ethylhexyl 

ester (EHE), and 2,4-D dimethylamine salt (DMA salt). This study is in review at this 

time; however, preliminary results suggest that the volatility rate of 2,4-D from the 

choline salt is lower than the DMA and EHE formulations (Figure 1). Additional data 

and figures are provided in Appendix H. 2,4-D acid is unlikely to bioaccumulate in fish 

given the low value of the log n-octanol/water partition coefficient (log Kow 0.18 at 

neutral pH). 

Table 4 also provides environmental fate properties of 2,4-D acid, along with the major 

and minor degradates detected in the submitted environmental fate and transport studies. 

2,4-D acid is stable abiotic hydrolysis in buffered aqueous solutions at pH 4, 5, 7, and 9. 

The degradation of 2,4-D acid appears to be dependent on oxidative microbiallymediated mineralization in the terrestrial environment and photodegradation in water. 

Results from laboratory studies indicate rapid to moderately rapid degradation under 

aerobic soil conditions with half-lives ranging from 1.4 days to 12.4 days and a median 
16

half-life of 2.9 days. Only minor degradates, 2,4-dichlorophenol (2,4-DCP) and 2,4-

dichloroanisol (2,4-DCA), were identified in soils. The photodegradation half-life of 2,4-

D acid was 12.9 days in a pH 5.0 buffer solution and a major degradate, 1,2,4-

benezenetriol (37% of applied) was identified. 2,4-D acid was stable to photodegradation 

in soil.

2,4-D acid was not stable in aerobic aquatic environments (t1/2=15.0 days) but was 

moderately persistent to persistent (t1/2=28.5 to 333 days) in anaerobic aquatic laboratory 

studies. The major degradates were chlorohydroquinone (CHQ) (maximum of 16.0 % of 

applied) in aerobic aquatic conditions and 2,4-DCP (maximum of 32.6 % of applied) 

under anaerobic aquatic environment. 

The registrant conducted a total of 30 terrestrial field dissipation studies in CA, CO, NC, 

ND, NE, OH, and TX on bare ground plots as well as plots cropped with corn, pasture, 

turf, and wheat. The 2,4-D acid half-lives ranged from 1.1 to 42.5 days with a median 

half-life of 6.1 days. These half-lives reflect dissipation from the surface soil layer (0 to 

6 inches). The data indicate a rapid to moderately rapid dissipation rate for 2,4-D acid. 

The results of this study are also consistent with half-lives from laboratory studies and 

confirm the conceptual model for 2,4-D dissipation. The half-lives of 2,4-D degradation 

products (2,4-DCP and 2,4-DCA) were not estimated because of their sporadic 

occurrence patterns in surface soils. To address the behavior of 2,4-D in aquatic water 

systems, a series of aquatic field dissipation studies were conducted. Three studies were 

conducted using 2,4-D DMA while a fourth study was conducted using 2,4-D BEE. In 

addition, two field dissipation studies using 2,4-D DMA were also submitted to address 

forest field dissipation. 

Table 4. Physical Chemical and Environmental Fate Properties of 2,4-D Acid

Parameter Value Source

Selected Physical/Chemical Parameters

Chemical Structure

TOXNET

IUAPC Name (2,4-dichlorophenoxy)acetic acid U.S. EPA, 2005b

CAS Name (2,4-dichlorophenoxy)acetic acid U.S. EPA, 2005b

CAS No. 94-75-7 U.S. EPA, 2005b

Molecular Weight (molecular formula) 221.04 g/mol (C8H6Cl2O3) U.S. EPA, 2005b

Smiles Code O=C(O)COc(c(cc(c1)Cl)Cl)c1 EPISUITE4.1

Vapor pressure (25°C) 1.4 x 10-7 mm Hg U.S. EPA, 2005b
17

Parameter Value Source

Aqueous solubility (20°C) 569 mg/L U.S. EPA, 2005b

Dissociation constants (pKa ) in water 

(25°C)

2.60 MRID 471122-02

Henry’s Law Constant (25°C) 8.56 x10-6 atm-m3/mol Rice et al, 1997

Log octanol-to-water partition coefficient

(log KOW)

2.14 @ pH 5.0

0.18 @ pH 7.0

0.10 @ pH 9.0

U.S. EPA, 2005b

Persistence

Hydrolysis half-life Stable MRID 41007301

Aqueous photolysis half-life 12.98 days

Degradates1

1,2,4-benzenetriol (37% of applied) 

MRID 41125306

Soil photolysis half-life Stable 

CO2 (5% of applied)

MRID 41125305

Aerobic soil metabolism half-life (days) Catlin Silty clay loam – 1.7 day

Commerce Loam – 4.62 days

Catlin Silty clay loam – 1.4 days

Fargo Clay – 12.4 days

Keith Clay loam – 4.4 days

Walla Walla silt loam – 2.0 days

Cecil Sandy loam -2.9 days

Degradates 

2,4-DCP (3.5%) 

2,4-DCA (2.8%)

MRID 43167501, 

MRID 00116625

Anaerobic aquatic metabolism half-life 

(total system)

333 days

28.5 days

41.0 days 

Degradates

2,4 DCP (Maximum 32.6% of applied)

4-chlorophenoxy acetic acid(4 - CPA) 

<2.0% of applied), 

4-chlorophenol (4 - CPP) <2.0% of 

applied),

2,4- DCA (<2% of applied)

MRID 43356001

MRID 42979201

MRID 41557901
18

Parameter Value Source

Aerobic aquatic metabolism half-life (total 

system)

15 days

Inconclusive2

Degradates

Chlorohydroquinone (CHQ) (maximum 

16% of Applied)

2,4-DCP (4.9 of applied) 

MRIDs 42045301 

44188601

Mobility

Adsorption/desorption

Kd-ads / Kd-des (mL/g)

Kfoc-ads / (mL/g)

Adsorption

Freundlich adsorption Kf-ads values 

Sand 0.36

Sandy loam 0.17

Loam 0.28

Silty clay loam 0.52

Clay 1.27

Desorption

Freundlich adsorption Kf-des values 

Sand 1.16

Sandy loam 0.87

Loam 1.58

Silty clay loam 1.99

Clay 1.64

Adsorption

Freundlich adsorption Kfoc values 

Sand 76

Sandy loam 70

Loam 117

Silty clay loam 59

Clay 58.1

MRID 44117901

MRID 42045302 

MRID 44117901

MRID 42045302 

MRID 44117901

MRID 42045302 

Leaching Thin Layer Chromatography (TLC) 

Retention Value (Rf, unitless)

(Un-aged sample)

Sand 1.0

Sandy loam 0.77

Silt loam 0.60

Loam 0.41

Column Study

(Aged sample)

Immobile

MRID 00057313

MRID 00080124
19

Parameter Value Source

Field Dissipation

Terrestrial field dissipation half-life The first order half-lives ranged from 1.1 

to 42.5 days with a median half-life of 

6.1 days

MRID 43914701

MRID 43762401 

MRID 43762402

MRID 43514601

MRID 43533401

MRID 43864001

MRID 43592801

MRID 43762403

MRID 43762404

MRID 43640601

MRID 43831702

MRID 43872703

MRID 43849102

MRID 43831701

MRID 43705202

Aquatic field dissipation half-life Estimated dissipation half-lives of 20.7 

and 2.7 days from the North Carolina 

pond after the first and second 

applications, 14 days and 6.1 days in 

water from a North Dakota pond after the 

first and second applications, and 1.0 day 

in water from the Louisiana rice paddy 

after a single application

MRID 43908302

MRID 43491601

Forest Field Dissipation half-life The estimated half lives for 2,4-D were 59 

days in exposed soil, 68 days in protected 

soil, 42 days on foliage, and 72 days on 

leaf litter.

MRID 43954702

Field Volatility3

Maximum Flux Rate (g/m2-s)

5.50E-09 (Farmland, IN)

1.53E-08 (Fowler, IN)

1.88E-08 (Little Rock, AR)

1.48E-09 (Little Rock, AR)4

1.48E-09 (Ty Ty, GA)

2.44E-09 (Ty Ty, GA)4

MRID 488629025

1 Structures of major and minor degradates of 2,4-D are provided in Appendix B 2

Half-life cannot be calculated because study duration was insufficient 3

Agency estimated flux rate based on 1.0 lb a.e./A of 2,4-D choline salt 4

Flux study performed with 2,4-D choline plus glyphosate 5

Study in review 

2.2.4 Degradates

There were three major degradates identified in the submitted environmental fate studies 

for 2,4-D; 1,2,4-benzenetriol (37% formed), 2,4-dichlorophenol (2,4-DCP) (32.6% 

formed), and chlorohydroquinone (CHQ) (16% formed). Minor degradates included 4-

chlorophenol, 4-CPA and 2,4-DCA (Table 4). The major degradate, 1,2,4-benzenetriol, 
20

is a photodegradate that was observed under abiotic conditions and is less likely to occur 

under natural conditions where microbe-mediated degradation occurs. The 1,2,4-

benzenetriol degradate was eliminated from concern because it is formed only via 

aqueous photolysis. It may be less likely to occur in many environments because

degradation of 2,4-D appears to be dependent primarily on oxidative microbial-mediated 

mineralization. The exposure of CHQ in the environment is likely to be low since it 

formed in aerobic aquatic environments to a significant extent only on day 27, and, from 

that point, dissipation was rapid (half-life of 5 days). Although 2,4-DCP is a minor 

degradate in the terrestrial environment, it is a major degradate (<32%) under anaerobic 

aquatic conditions. There are some toxicity data for 2-DCP available in the ECOTOX 

database1 and the European Footprint database2 that suggest it is more toxic than 2,4-D 

for selected aquatic organisms. Therefore, 2,4-D as well as its degradate, 2,4-DCP, will 

be considered as independent stressors of concern in ecological risk assessment.

Figure 1. Volatility Flux Profile of 2,4-D Formulations

2.2.5 2,4-D Choline Salt Use Characterization

2,4-D choline salt is a non-selective herbicide for the control of broadleaf weeds. The 

proposed new uses for 2,4-D choline salt include soybeans containing DAS-68416-4, 

EnlistTM corn, and field corn containing DAS-40278-9. The current action refers to GF2654 TS (soybean), GF-2654 TC (field corn), GF-2726 (EnlistTM corn), and GF-2727 

(EnlistTM corn). GF-2726 and GF-2727 are products that contain both 2,4-D choline salt 

and glyphosate. Table 6 shows the application rates for each use. All products are 

applied as a ground foliar spray. The herbicide-tolerant trait allows for a later application 

in both soybean (R2 stage) and corn (V8 stage). 

 1 http://cfpub.epa.gov/ecotox/

2 http://sitem.herts.ac.uk/aeru/footprint/en/index.htm
21

2.3 Receptors

2.3.1 Aquatic and Terrestrial Receptors and Effects

The receptor is a biological entity that is exposed to a stressor (USEPA 1998). Consistent 

with the process described in the Overview Document, this risk assessment uses a 

surrogate species approach in its evaluation of 2,4-D choline salt (USEPA 2004). 

Toxicological data generated from surrogate test species, which are intended to be 

representative of broad taxonomic groups, are used to extrapolate the potential effects on 

a variety of species (receptors) included under these taxonomic groupings. 

Acute and chronic toxicity data from studies submitted by the pesticide registrant along 

with available open literature studies are used to evaluate potential direct effects of 

pesticides to the aquatic and terrestrial receptors. Open literature studies are identified 

through EPA’s ECOTOX database (http://cfpub.epa.gov/ecotox/), which employs a 

literature search engine for locating chemical toxicity data for aquatic and terrestrial flora 

and fauna. The evaluation of both sources of data provides insight into the direct and 

indirect effects of pesticides on biotic communities from loss of species that are sensitive 

to the chemical and changes in structural and functional characteristics of the affected 

communities. 

A search of the ECOTOX database on October 24, 2012 yielded no studies for 2,4-D 

choline salt. Therefore, only studies submitted by the registrant were evaluated to 

determine the effects of 2,4-D choline salt on non-target organisms. 

2.3.2 Ecosystems Potentially at Risk 

The ecosystems at risk from exposure to a stressor are often extensive in scope; it may 

not be possible to identify specific ecosystems at the screening level. In general terms, 

terrestrial ecosystems potentially at risk could include the treated field and areas 

immediately adjacent to the treated field that may receive drift or runoff. Areas adjacent 

to the treated field could include other cultivated fields, fencerows and hedgerows, 

meadows, fallow fields or grasslands, woodlands, riparian habitats, and other 

uncultivated areas. 

Aquatic ecosystems potentially at risk from a stressor include water bodies adjacent to or 

downstream from the treated field; impounded bodies such as ponds, lakes, and 

reservoirs; and flowing waterways such as streams or rivers. For uses in coastal areas, 

aquatic habitat also includes marine ecosystems and estuaries. 

2.4 Assessment Endpoints

Assessment endpoints represent the actual environmental value that is to be protected, 

defined by an ecological entity (species, community, or other entity), and its attribute or 

characteristics (USEPA 1998, 2005a). For 2,4-D choline salt, the ecological entities 

include the following: birds, amphibians, reptiles, mammals, insects, freshwater fish and 
22

invertebrates, estuarine/marine fish and invertebrates, and aquatic plants and algae. The 

attributes for each of these entities include survival, growth, and reproduction. 

2.5 Conceptual Model

For a pesticide to pose an ecological risk, it must reach ecological receptors in 

biologically significant concentrations. An exposure pathway is the means by which a 

pesticide moves in the environment from a source to an ecological receptor. For an 

ecological pathway to be complete, it must have a source, a release mechanism, an 

environmental transport medium, a point of exposure for ecological receptors, and a 

feasible route of exposure.

A conceptual model is intended to provide a written description and visual representation 

of the predicted relationships between the stressor, potential routes of exposure, and the 

predicted effects for the assessment endpoint. A conceptual model consists of two major 

components: the risk hypothesis and the conceptual diagram (USEPA 1998, 2005a).

2.5.1 Risk Hypothesis

Based on the environmental fate characteristics and mode of action, it is presumed in this 

screening-level assessment that 2,4-D choline salt, when used in accordance with the 

label, has the potential to adversely affect survival, growth, and/or reproduction of both 

terrestrial and aquatic organisms. 

Birds, mammals, amphibians, reptiles, and non-target insects may be exposed to 2,4-D 

choline salt through direct ingestion of the pesticide on the dietary items, incidental 

ingestion of contaminated soil, dermal contact with treated plant surfaces and soil during 

activities in the treated areas, direct impingement of sprayed material on the body at the 

time of application, preening activities, and ingestion of drinking water contaminated by 

the pesticide. 2,4-D exposures to aquatic species may occur through spray drift, runoff, 

volatilization, wet/dry deposition, and leaching to groundwater. The seedling emergence 

and vegetative vigor of non-target terrestrial plants and growth and biomass accumulation 

of aquatic plants adjacent to the site of 2,4-D choline salt application could be affected by 

runoff, drift, and volatilization from treated fields. 

2.5.2 Conceptual Diagrams

The potential exposure pathways and effects of 2,4-D and its degradates, 2,4-

dichlorophenol (2,4-DCP), from applications of 2,4-D choline salt on terrestrial and 

aquatic environments are depicted in Figure 2. Solid arrows represent the most likely 

routes of exposure and effects while dashed lines represent potential routes of exposure 

that are considered less likely for 2,4-D choline salt and its degradates. 

Figure 2. Conceptual Model for Exposure Routes for 2,4-D and its Degradates
23

Stressors

2,4-D choline salt products applied to use site 

Transport

Pathways

Spray 

Drift

Exposure

Media

Exposure 

Route

Ingestion/

Inhalation

Direct contact/

Root Uptake

Direct contact/

Root Uptake

Gill/ Integument 

Uptake

Receptors

Attribute

Changes

Birds/ Mammals/ Insects Terrestrial

Plants

Acute Effects

•Mortality

Aquatic Invertebrates

Aquatic Vertebrates

Amphibians 

Wetland/ Riparian

Plants/Aquatic plant

Individual Plants

• Seedling emergence

• Vegetative vigor

Terrestrial

Aquatic

Volatilization and 

Atmospheric transport

and deposition

Leaching to 

groundwater

Terrestrial Environment Aquatic Environment

Chronic Effects

•Reproduction

•Growth

•Survival

Acute Effects

•Mortality

Chronic Effects

•Reproduction

•Growth

•Survival

Chronic Effects

•Reproduction

•Growth

•Survival

Source/

In-situ deposition

(Soil and Foliage)

Runoff

Airplant 

transfer

Exposure Routes for 2,4-D
24

The conceptual model used to depict the potential ecological risk associated with 2,4-D 

choline salt (and the degradates of concern) assumes that as an herbicide, 2,4-D choline 

salt can affect terrestrial and aquatic organisms if environmental exposure is expected as 

a result of the proposed label uses. The use scenarios for 2,4-D choline salt involve 

ground spray applications of the herbicide to herbicide-tolerant corn and soybean foliage. 

While 2,4-D choline salt is already registered on conventional varieties of these crops, the 

herbicide-tolerant varieties allow for the application of 2,4-D choline salt at later dates in 

the season. Altering the timing of 2,4-D choline salt applications may change the 

window of exposure for non-target vegetation. A variety of food types (e.g., short grass, 

long grass, broadleaf plants) will be assessed regardless of the type represented by the 

target crop, as a variety of food types exist on and off the treated field.

As there are multiple spray applications to foliage, degradation of the chemical on the 

foliage between applications is considered in the terrestrial assessments. A default foliar 

dissipation rate of 35 days will be used in the terrestrial risk assessments since no foliar 

dissipation studies are available; this may be an over-estimate. For aquatic assessments, 

the microbial degradation on foliage is assumed stable, but wash-off of the foliage will be 

considered using the default wash-off coefficient assumption of 0.5 cm-1

. Spray drift will 

be considered in the aquatic assessments as a route of loading to the pond, with higher 

levels of spray drift for aerial applications than ground spray applications.

2,4-D choline salt will dissociate into its salt and 2,4-D acid upon reaching a water body. 

It is then partitioned between the water column, suspended sediment, and bed sediment at 

a ratio based on the pesticide’s physical/chemical properties. Degradation by abiotic 

hydrolysis, photolysis, and microbial mediated metabolism is taken into account. 

2,4-D choline salt is unlikely to bio-accumulate in fish; the study was waived because of 

the low Log Kow. 

For birds and mammals, only the dietary route of exposure is considered. Solubility 

information for 2,4-D choline salt was not available, however; because it rapidly 

dissociates into 2,4-D acid and the choline salt, it is appropriate to use 2,4-D acid’s 

solubility. Screening for risks from drinking water was performed with the Screening 

Imbibition Program (SIP). The model identified that exposure through drinking water 

may present acute and chronic concerns for birds and mammals (Appendix C). The 

Screening Tool for Inhalation Risk (STIR) did not identify concerns for birds or 

mammals (Appendix C). Other uncertainties in this risk assessment include the lack of 

information on exposure from soil ingestion and dermal routes.

2.6 Analysis Plan 

The first-tier screening-level risk assessment is used as the approach to determine risk to 

non-target organisms from the proposed new uses for 2,4-D choline salt. Measures of 

exposure and effect are used to evaluate the potential risk for a specific assessment 

endpoint. A risk quotient is obtained by dividing the measures of exposure for a 

particular assessment endpoint by the measures of effect for that endpoint. The risk 

quotient is a deterministic point-estimate-based approach and does not provide a 
25

quantitative estimate of likelihood and/or magnitude of an adverse effect. Risk quotients 

are compared to specified levels of concern; if the risk quotient exceeds the level of 

concern, then the potential for risk exists.

Polychloro dibenzo-p-dioxin (PCDD) and polychloro dibenzo-p-furans (PCDF) may be 

formed during the manufacture of 2,4-D and can remain in the products as impurities. 

According to 2,4-D registrants, since the 1990’s, the manufacturing processes for 2,4-D 

and its chemical intermediate, dichlorophenol, have been modified to reduce 

concentrations of PCDD and PCDF in the technical 2,4-D products. The Agency 

reviewed recently submitted product chemistry data for the end-use (product “GF-2654 

TC” to evaluate toxic impurities in 2,4-D choline formulation (U.S. EPA 2012, D405897, 

Confidential Memo). No PCDD and PCDF were detected above 30% of the limit of 

quantitation (LOQ) in the registered sources of the active ingredients for 2,4-D choline.

Assuming 30% of the LOQ, the toxicity equivalence (TEQ) of PCDD and PCDF was 

estimated using toxicity equivalence factors derived from the World Health Organization 

(WHO 20053

). The estimated TEQ of the technical product used in 2,4-D choline is 

approximately 4.8 times lower than the the estimated TEQ calculated for the formulated 

products of 2,4-dichlorophenoxyacetic acid (2,4-D) and ethylhexyl 

dichlorophenoxyacetate (2,4-D EHE) in a previous assessment (US EPA 2005, D317729, 

Confidential Memo). The previous assessment concluded that the environmental loading 

of PCDD and PCDF from terrestrial and aquatic uses of 2,4-D is not expected to pose a 

risk for reproductive effects to piscivorous birds and mammals. Therefore, 2,4-D choline 

salt use in corn and soybean is not expected to pose risk to terrestrial organisms since the 

TEQ for 2,4-D choline is 4.8 times lower than the formulated products of 2,4-D acid and 

EHE. No ecological risk assessment is warranted for PCDD and PCDF for 2,4-D choline 

salt formulations at this time. 

The stressors of ecological concern for terrestrial and aquatic organisms are 2,4-D acid 

and its degradates. However, as outlined in the degradate section above, all but 2,4-DCP 

can be eliminated as likely degradates of concern. 2,4-DCP is primarily formed in 

aquatic environments; it is only a minor degradate in terrestrial environments. Therefore, 

2,4-D and degradate 2,4-DCP will be considered as stressors of concern in the aquatic 

analyses and only 2,4-D will be considered for terrestrial environments.

2.6.1 Measures of Effect and Exposure

The assessment will examine the effects of 2,4-D choline salt (and 2,4-DCP, when 

relevant) on aquatic and terrestrial environments primarily through the routes of spray 

drif and runoff. Model details are described later in this section. Several other models 

are used to characterize effects from vapor exposures, buffer distances, and individual 

effects probabilities; these are included in the risk description section.

Table 5 lists the measures of environmental exposure and ecological effects used to 

assess the potential risks of 2,4-D choline salt to non-target organisms. The methods 

 3 van den Berg et al., 2006
26

used to assess risks are consistent with those outlined in the Overview Document

(USEPA 2004).

Table 5. Measures of Effect and Exposure for 2,4-D Choline Salt

Assessment Endpoint Surrogate Species and 

Measures of Ecological 

Effect1

Measures of 

Exposure

Birds2

Survival

Bobwhite quail oral LD50

Bobwhite quail and mallard 

duck dietary LC50

Maximum residues on 

food items and soil

Reproduction and 

growth

Bobwhite quail reproduction 

NOAEC

Mammals

Survival Laboratory rat oral LD50

Reproduction and 

growth

Laboratory rat two-generation 

NOAEC and NOAEL

Freshwater 

amphibians Survival Leopard frog LC50 Peak EEC

Freshwater 

fish3

Survival Rainbow trout LC50 Peak EEC

Reproduction and 

growth Fathead minnow NOAEC 60-day average EEC

Estuarine/ 

marine fish

Survival Tidewater silverside LC50 Peak EEC

Reproduction and 

growth Sheepshead minnow NOAEC 60-day average EEC

Freshwater 

invertebrates

Survival

Water flea EC50

Peak EEC

Reproduction and 

growth Water flea NOAEC 21-day average EEC

Estuarine/ 

marine 

invertebrates

Survival Eastern oyster LC50 Peak EEC

Reproduction and 

growth Eastern oyster NOAEC4 21-day average EEC

Terrestrial 

plants

Seedling emergence

and vegetative 

vigor

Onion (monocot) 

Lettuce (dicot) 

EC25 and NOAEC

Maximum residues on 

foliage and soil
27

Assessment Endpoint Surrogate Species and 

Measures of Ecological 

Effect1

Measures of 

Exposure

Aquatic plants Survival and growth

Vascular – Duckweed 

Non-vascular – Freshwater 

diatom 

EC50 and NOAEC

Peak EEC

Terrestrial 

insects Survival Honeybee acute LD50 N/A

1

Risk assessment guidance indicates that the most sensitive species tested within a taxonomic group 

are to be used for screening level risk assessments.

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

3 Freshwater fish are surrogates for aquatic-phase amphibians.

4

Based on acute-to-chronic ratio calculation.

3. Exposure Assessment

3.1 Proposed 2,4-D Choline Salt Application Information 

Table 6 shows the maximum application rates for each new use of 2,4-D choline salt, 

based on the proposed labels. Uses are being expanded to include Enlist corn and 

soybean containing DAS-68416-4. Additional information regarding the proposed 2,4-D 

choline salt uses can be found in Section 2.2.2.
28

Table 6. Proposed 2,4-D Choline Salt Uses

Proposed Label Use Active Ingredients Method Maximum Application Rate

(Interval)

GF-2726, GF-2727

EnlistTM corn

2,4-D choline salt

Glyphosate

Ground 

broadcast spray

Preplant (Burndown)

Single application at 1 lb ae/A

Preemergence

Single application at 1 lb ae/A

Postemergence (up to V8 stage - 48 

inches)

Maximum single application at 1 lb 

ae/A

2 applications, maximum (12 days)

Seasonal maximum of 3 lb ae/A

GF-2654 TC

Field corn 

containing DAS40278-92,4-D choline salt Ground 

broadcast spray

Preplant

Single application at 1 lb ae/A

Preemergence

Single application at 1 lb ae/A

Postemergence (up to V8 stage - 48 

inches)

Maximum single application at 1 lb 

ae/A

2 applications, maximum (12 days)

Seasonal maximum of 3 lb ae/A

GF-2654 TS
29

Proposed Label Use Active Ingredients Method Maximum Application Rate

(Interval)

Soybeans containing 

DAS-68416-4 2,4-D choline salt Ground broadcast spray

Preplant

Single application at 1 lb ae/A

Preemergence

Single application at 1 lb ae/A

Postemergence (up to R2 stage)

Maximum single application at 1 lb 

ae/A

2 applications, maximum (12 days)

Seasonal maximum of 3 lb ae/A

3.2 Aquatic Exposure Modeling for 2,4-D Choline Salt

A Tier II screening-level surface water exposure for aquatic risk assessment was 

conducted for the Section 3 proposed new use registration. Modeled application rates 

represent the maximum use patterns of the proposed labels for use on herbicide-tolerant 

corn and soybean. Since the 2,4-D choline salt dissociates rapidly in less than 6 seconds 

in the environment (MRID 48260701), the aquatic exposure was based on the 2,4-D acid 

equivalent. Aquatic exposure was also modeled for its major degradate 2,4-DCP, which 

is more toxic than 2,4-D for selected aquatic organisms (http://cfpub.epa.gov/ecotox/)

using the highest exposure scenario for 2,4-D (i.e., MS Corn STD). An additional 

PRZM/EXAMS model run was performed to estimate the no drift exposure using MS 

Corn STD scenario for 2,4-D to determine the contribution of spray drift in surface water.

3.2.1 PRZM-EXAMS Model 

The Pesticide Root Zone Model, (PRZM, Carsel et al., 1997) and the Exposure Analysis 

Modeling System (EXAMS, Burns, 1997) were used in tandem to generate aquatic 

estimated environmental concentrations (EECs). PRZM (3.12.2 dated May 12, 2005) 

simulates fate and transport on the agricultural field whereas EXAMS (2.98.04.06, dated 

April 25, 2005) simulates the fate and resulting daily concentrations in the water body. 

Simulations are carried out with the linkage program shell, PE5V01.pl (dated November 

15, 2006), which incorporates the standard agricultural and non-agricultural scenarios 

developed by EFED. Simulations are run for multiple (usually 30) years, and the EECs 

represent peak values that are expected once every ten years based on the thirty years of 

daily values generated during the simulation. Additional information on these models 

can be found at: http://www.epa.gov/oppefed1/models/water /index.htm.

The aquatic exposure is estimated for the maximum application pattern to a 10-ha field 

bordering a 1-ha pond, 2-m deep (20,000 m3

) with no outlet. Exposure estimates 

generated using this standard pond are intended to represent a wide variety of vulnerable 
30

water bodies that occur at the top of watersheds including prairie pot holes, playa lakes, 

wetlands, vernal pools, man-made and natural ponds, and intermittent and first-order 

streams. As a group, there are factors that make these water bodies more or less 

vulnerable than the standard surrogate pond. Static water bodies that have larger ratios of 

pesticide-treated drainage area to water body volume would be expected to have higher 

peak EECs than the standard pond. These water bodies will be either smaller in size or 

have large drainage areas. Smaller water bodies have limited storage capacity and thus 

may overflow and carry pesticide in the discharge, whereas the standard pond has no 

discharge. As watershed size increases beyond 10-ha, it becomes increasingly unlikely 

that the entire watershed is planted with a non-major single crop that is all treated 

simultaneously with the pesticide. Headwater streams can also have peak concentrations 

higher than the standard pond, but they likely persist for only short periods of time and 

are then carried and dissipated downstream.

Standard PRZM crop scenarios, which consist of location-specific soils, weather, and 

cropping practices, were used in the simulations to represent proposed labeled uses of 

2,4-D choline salt. Standard corn scenarios are available for Illinois, Indiana, Iowa,

Kansas, Minnesota, Mississippi, North Carolina, Nebraska, Ohio, and Pennsylvania. A 

single standard scenario, Mississippi, is available for soybean. The proposed label 

recommends applying 2,4-D choline salt on corn through the V8 stage (48 inches tall). 

Applications to soybeans can occur up to the R2 stage (full bloom). 2,4-D choline salt 

application dates were chosen based on the geographical location of the crop and 

explored the extended application time period. 

2,4-D choline salt labels have a recommendation to use the 2,4-D choline product with an

ASABE S-572 droplet size classification of coarse/coarser (Dv0.5 of ≈ 439 μm) spray 

quality. Given the limited option for droplet size in the current AgDRIFT ground 

module, the droplet size of Dv0.5 of 341μm option was used as the nearest label 

recommended coarse/coarser droplet size. Maximum spray drift fractions (0.017) using a 

droplet size of Dv0.5 of 341μm as well as no spray drift fraction were evaluated to 

account for the spray drift contribution in the EECs. Table 9 presents all simulated 

surface water concentrations of 2,4-D for corn and soybean. 

3.2.2 Model Inputs 

Input parameters for the PRZM/EXAMS models are listed in Table 7 and Table 8 for 2,4-

D and 2,4-DCP respectively. Explanations of the various model input parameters are 

discussed below. Since 2,4-DCP is a major degradate (32.6%) formed in the aquatic 

environment but only 3.5% formed in the terrestrial environment, two application rates 

were used in PRZM/EXAMS modeling to account contribution from terrestrial via 

runoff/erosion of parent as well as 2,4-DCP and direct deposition from spray drift. 2,4-

DCP exposures were modeled for the highest EECs from the parent (MS corn scenario). 

Using these contributions in tandem requires post-processing of the modeled output to 

derive a weighted EEC that represents the contribution of runoff/erosion from the 

terrestrial environment and direct deposition of spray drift to an aquatic environment. A 

Microsoft Excel spreadsheet was used that allows for the weighting and aggregation of 

exposure from both scenarios. The daily time series from each model run was copied 
31

from the times series file (*_TS.out) generated from the PRZM into the EXCEL 

spreadsheet. Rolling averages for the relevant durations of exposure (4, 14, 21, and 60 

day averages) were estimated using Table 20 of PRZM/EXAMS. Example

PRZM/EXAMS model outputs of 2,4-D and post-processed 2,4-DCP EECs were 

provided in Appendix D.

Table 7. PRZM/EXAMS Input Parameter Values for 2,4-D

Parameter Value Source Comments

Maximum application 

Rate lb a.e./A (kg 

a.e./ha)

Corn and Soybean: 1.0 

(1.12)

GF-2654 TC

2,4-D (56.3%)

Application rate for GF-2654 for 

corn and soybean are much higher 

than GF-2726 (24.4% of 2,4-D,). 

Therefore no exposure assessment 

was performed for GF-2726.

Number of Applications 3 GF-2654 TC

2,4-D (56.3%)

1 application for pre-emergence

1 application on emergence date 

1 application for post emergence

Minimum Application 

Intervals (days)

12 (Pre emergence)

12, 21 and 28 (Post 

emergence)

GF-2654 TC

2,4-D (56.3%)

Minimum application interval 

before emergence

Application intervals for postemergence

Scenario and 

Application Date2

IA Corn-13-051

IL Corn 19-042

IN Corn 03-051

KS Corn 28-042

MN Corn 03-051

MS Corn 29-032

MS Soybean 04-042

NC Corn 03-031

NE Corn13-052

OH Corn 19-042

PA Corn 04-041

Based on 

PRZM/EXAMS 

Scenarios

12 days before crop emergence

12-28 days from the date of crop 

emergence in the scenario

Depth of Incorporation 

(inches) 0

GF-2654 TC

2,4-D (56.3%)

For foliar application according to 

Input parameter guidance (USEPA 

2009)

Method of Application Ground spray GF-2654 TC

2,4-D (56.3%) ---

Application Efficiency

Ground: 0.99 Input parameter 

guidance (USEPA, 

2009)

Default values for ground spray

Spray Drift Fraction 0.0173 Estimated Based on nearest droplet size 

specification in the submitted labels

Molecular Mass (g/mol) 221.04 USEPA 2005 Product chemistry data

Vapor Pressure (Torr) 1.4 x 10-7 USEPA 2005 Product chemistry data

Henry’s Law Constant 

(atm m3/mol) 8.56 x 10-6 Rice et al., 1997 Measured value
32

Parameter Value Source Comments

Solubility in Water 

(mg/L) 569 USEPA 2005 ---

Hydrolysis t1/2 at pH 7.0 

(days)

0 MRID 41007301 Stable to hydrolysis @ pH 7.0

Aquatic Photolysis t1/2

(days)

12.98 MRID 41125306 ---

Aerobic Soil 

Metabolism t1/2 (d) 6.92 MRID 43167501

MRID 00116625

90th percentile upper confidence 

bound on the mean half-life of 2,4-

D

Aerobic Aquatic 

Metabolism t1/2 (d) 15 x 34 MRID 420445301 Input parameter guidance (USEPA

2009)

Anaerobic Aquatic 

Metabolism t1/2 (d) 321

MRID 43356001

MRID 42979201

MRID 41557901

90th percentile upper confidence 

bound on the mean half-life of 2,4-

D

Soil Partitioning 

Coefficient (Koc; ml/goc) 76.02 MRID 44117901

MRID 42045302

Represent average Kfoc from 5 soils 

1 = PRZM/EXAMS simulations were performed with a single post-emergence crop scenario (12 days after crop 

emergence) 

2 = PRZM/EXAMS simulations were performed with varied post-emergence crop scenarios (12, 21 and 28 days 

after crop emergence) 

3 = Spray drift fraction was estimated using AgDRIFT 2.1.1 model – Tier I Ground scenario (High boom, DV0.5 

341 μm and 90th percentile data) 

4= Due to reported half-life for a single aerobic aquatic metabolism study, the input half-life was multiplied by 3 

according to guidance for selecting input parameters in modeling for environmental fate and transport of 

pesticides. Version 2.1 October 22, 2009.

Table 8. PRZM/EXAMS Input Parameter Values for 2,4-DCP

Parameter Value Source Comments

Scenario and 

Application Date

MS Corn1 Based on 

PRZM/EXAMS 

Scenario. 

12 days before crop emergence. 

12days from the date of crop 

emergence in the scenario

Spray Drift Fraction

1.0 Assumed To evaluate 2,4-DCP contribution 

from 2,4-D spray drift 

2,4-DCP mass/acre 

from runoff 

contribution of parent 

(lb a.e./A)

0.024

Estimated2 To evaluate 2,4-DCP contribution 

from 2,4-D runoff (10%) based on 

PRZM manual3

Molecular Mass (g/mol) 162.9 USEPA 2005 Product chemistry data
33

Parameter Value Source Comments

Maximum application 

Rate lb a.e./A (kg 

a.e./ha)

Corn 

0.026 (0.029)

4

0.028 (0.031)

5

Estimated See calculations in footnotes 3and 4

Number of Applications 3 

GF-2654 TC

2,4-D (56.3%)

1 application for pre-emergence

1 application on emergence date 

1 application for post emergence

Method of Application

Ground Assumed

2,4-DCP from post-application 

parent degradation in terrestrial 

environment

Direct Application Assumed

2,4-DCP contributions from spray 

drift and 10% of parent from runoff 

were simulated as a direct 

application via spray drift into 

surface water body 

Depth of Incorporation 

(inches) 0 Assumed

For ground application according to 

Input parameter guidance (USEPA 

2009)

Application Efficiency

1.0 Assumed To evaluate 2,4-DCP contribution 

from runoff and erosion

0

Assumed 2,4-DCP ontributions from spray 

drift and 10% of parent from runoff 

were simulated via spray drift as a 

direct application into surface water 

body

Solubility in Water 

(mg/L) 4500 TOXNET6 Open literature value

Hydrolysis t1/2 at pH 7.0 

(days)

0 Assumed No Data

Aquatic Photolysis t1/2

(days)

0 Assumed No Data

Aerobic Soil 

Metabolism t1/2 (d) 75 x 37 EPISUITE8 Single value multiplied by 3 

according to the Input parameter 

guidance (USEPA 2009)

Aerobic Aquatic 

Metabolism t1/2 (d) 35.5 x 37 EPISUITE8 Single value multiplied by 3 

according to the Input parameter 

guidance (USEPA 2009)

Anaerobic Aquatic 

Metabolism t1/2 (d) 337.5 x 37 EPISUITE8 Single value multiplied by 3 

according to the Input parameter 

guidance (USEPA 2009)

Soil Partitioning 

Coefficient (Koc; ml/goc) 609 EPISUITE8 ---
34

Parameter Value Source Comments

1

 PRZM/EXAMS simulation was performed only for MS Corn based on highest EECs observed in 2,4-D 

exposure

2

 2,4-D application rate (1.0 lb a.e/A) x [(0.1, runoff contribution of 2,4-D) x (0.326, the maximum conversion 

rate from the anaerobic aquatic degradation of 2,4-D to 2,4-DCP in laboratory studies) x (0.74, the molecular 

weight ratio of 2,4-D to 2,4-DCP)] 3 http://www.epa.gov/ceampubl/gwater/przm3/przm3123.html

4 2,4-D application rate (1.0 lb a.e/A) x [(0.035, the maximum conversion rate for the terrestrial degradation of 

2,4-D in the terrestrial environment to 2,4-DCP in laboratory studies) x (0.74, the molecular weight ratio of 2,4-D 

to 2,4-DCP)] 5 2,4-D application rate (1.0 lb a.e/A) x [(0.017, spray drift contribution of 2,4-D) x (0.326, the maximum 

conversion rate from the anaerobic aquatic degradation of 2,4-D to 2,4-DCP in laboratory studies) x (0.74, the 

molecular weight ratio of 2,4-D to 2,4-DCP) + 0.024 lbof runoff contribution] 6 http://toxnet.nlm.nih.gov/cgi-bin/sis/search/f?./temp/~nU3WAu:1 7 Due to reported half-life for a single value, the input half-life was multiplied by 3 according to guidance for 

selecting input parameters in modeling for environmental fate and transport of pesticides. Version 2.1 October 22, 

2009

8 http://www.epa.gov/opptintr/exposure/pubs/episuite.htm

3.2.3 Model Outputs: Aquatic Exposure Estimates

The Mississippi scenario that modeled three applications of 2,4-D choline salt at 1 lb 

ae/A produced the highest EECs for herbicide-tolerant corn. This same scenario was also 

modeled without spray drift. In general, results suggest that the EECs for the droplet size 

of Dv0.5 of 341μm are slightly higher (≤ 4%) as compared to the no spray drift fraction

(Table 9), which indicates that most of the aquatic exposure is driven by runoff. EECs 

for the 2,4-DCP degradate were also modeled for Mississippi and were much lower than 

those for 2,4-D acid (e.g. 6.4 versus 58 μg ae/L for peak EEC; Table 9). 

Table 9. Range of Estimated Environmental Concentrations (EECs) of 2,4-D and 

2,4-DCP for Surface Water 

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Peak EEC 

(μg ae/L)

21-Day

EEC (μg ae/L)

60-Day 

EEC

(μg ae/L)

Surface Water 

(PRZM/EXAMS)

Aquatic Exposure for 2,4-D (Parent)

Corn:

IA Corn STD

IL Corn STD

IN Corn STD

KS Corn STD

MN Corn STD

MS Corn STD

NC Corn STD

NE Corn STD

OH Corn STD

PA Corn STD

(3app. X 1.0 lbs 

a.e./acre)

26.5-21.9

30.1-24.4

26.5-24.5

28.3-23.5

16.0-14.1

58.0-57.8

28.0-21.0

40.8-36.6

26.8-16.7

11.8-10.9 

23.8-20.2

26.6-21.9

25.3-22.1

20.9-18.7

15.1-12.8

41.6-41.0

26.5-19.7

36.4-33.1

19.3-11.7

9.6-8.9

19.1-16.7

22.7-20.0

20.9-18.9

13.7-12.5

13.6-11.7

29.2-26.4

23.2-17.5

28.9-27.9

11.4-8.8

6.6-6.6
35

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Peak EEC 

(μg ae/L)

21-Day

EEC (μg ae/L)

60-Day 

EEC

(μg ae/L)

Soybean:

MS Soybean STD

(3app. X 1.0 lbs 

a.e./acre)

42.8-42.8 32.5-32.5 21.4-21.3

Corn:

MS Corn STD1

(2app. @ 1.0 lbs and 

2 App @0.50 lb 

a.e./acre)

57.8 41.3 27.3

Corn:

MS Corn STD2

(3app. X 1.0 lbs 

a.e./acre)

56.5 40.1 25.3

Aquatic Exposure for 2,4-DCP (Degradate)

Corn:

MS Corn STD3

(3app. x 0.026 lbs 

a.e./acre)

4

and 

3app. x 0.028 lbs 

a.e./acre)

5

4.666 3.846 3.62

1

Post Emergence 2 applications at 0.50 lb a.e./A 2 PRZM/EXAMS simulated EECs without spray drift fraction to evaluate spray drift 

contribution

3 PRZM/EXAMS simulation was performed only for MS Corn based on highest EECs 

observed in 2,4-D exposure 4 = 2,4-D application rate (1.0 lb a.e/A) x [(0.035, the maximum conversion rate for the 

terrestrial degradation of 2,4-D in the terrestrial environment to 2,4-DCP in laboratory 

studies) x (0.74, the molecular weight ratio of 2,4-D to 2,4-DCP)] 5

= Sum of 0.024 lb from runoff contribution and 0.004 lb from spray drift] 6

= Reported values are based on combining exposures from terrestrial runoff/erosion and 

spray drift contribution using post processing

3.3 Monitoring Data

Monitoring data considered in the previous assessment (USEPA 2004) were the United 

States Geological Survey’s (USGS) National Water Quality Assessment Program 

(NAWQA) groundwater and surface water database, USGS/EPA reservoir monitoring 

database, National Drinking Water Contaminant Occurrence Database (NCOD), and US 

EPA’s Storage and Retrieval environmental data system (STORET). Review of these 

databases was conducted to provide peak and median concentrations. Additionally, the 

quality of data was evaluated for targeting pesticide use areas, detection limits, and 

analytical recoveries. The monitoring data indicate that 2,4-D is detected in groundwater 

and surface water. The highest time-weighted annual mean (TWAM) concentration was 

1.45 μg/L from the NAWQA database containing non-targeted data reflecting pesticide 

concentrations in flowing water as opposed to more stationary bodies of water such as 

ponds, lakes, and reservoirs. Also, 2,4-D is detected in treated (finished) drinking water. 
36

Maximum concentrations of 2,4-D in surface source water and ambient groundwater are 

58 μg/L and 14.8 ug/L, respectively.

Monitoring data from the USGS NAWQA program were accessed on September 21, 

2012 to evaluate the current trend of 2,4-D concentrations in surface water and 

groundwater. All data of filtered surface water and groundwater were downloaded since 

the drinking water memorandum was issued in 2004. For surface water, a total of 931 

water samples were analyzed for 2,4-D. 2,4-D was detected in 47.0% (i.e. 434 samples 

from a total national dataset of 931 samples). The maximum concentrations of 2,4-D 

ranged from 0.008 μg/l to 8.700 μg/L. Even though the maximum concentration of 8.7 

μg/L is lower than the previously reported value of 15 μg/L, 2,4-D was detected at a

higher percentage in surface water sampled after 2003 as compared to the 23.5% (i.e. 

1030 samples from a total national dataset of 4377) reported in the previous drinking 

water memorandum (USEPA 2004). For groundwater, a total of 1184 water samples 

were analyzed for 2,4-D. 2,4-D was detected in 1.0% of the samples (i.e. 12 samples 

from a total national dataset of 1184 samples). The maximum concentrations of 2,4-D 

ranged from 0.008 μg/L to 1.4 μg/L. Percent of detection and the reported concentration 

are lower than in the previously reported drinking water memorandum (USEPA 2004). 

Several mitigation measures may have contributed to the lower concentrations of 2,4-D 

in NAWQA surface water and groundwater monitoring in the years since the RED was 

issued (2005). The following mitigation measures were placed in the RED to reduce 2,4-

D loading in the environment (USEPA 2005):

• The application rate was reduced from 2.0 to 1.5 lb ae/A per year for turf uses.

• Master label rates were lower than the existing labels rates for various uses. All 

registrants must conform use rates to those set forth in the 2,4-D master label and 

reflected in the 2,4-D RED label table.

• Measures to control spray drift described in the “Spray Drift Management” 

section in the RED to reduce the risk of 2,4-D to non-target plants.

3.4 Terrestrial Exposure Estimates for 2,4-D Choline Salt

The Terrestrial Exposure (T-REX) model (Version 1.5.1), an EFED computer model that 

uses a first-order dissipation relationship to account for residue dissipation between 

applications, was used to estimate exposure concentrations of 2,4-D to terrestrial wildlife. 

The T-REX simulation model incorporates the Kenaga nomogram (Fletcher et al. 1994; 

Hoerger and Kenaga 1972; Pfleeger et al. 1996) (relationship between the amount of 

pesticide applied and the amount of pesticide residue present on a given food item). The 

model also calculates the peak concentration of pesticide residues on each food item on a 

daily interval for one year using a first order decay function based on the concentrations 

present from both the initial and additional applications. In addition to exposure 

concentrations (dose and diet-based), the T-REX model calculates risk quotients based on 

food items for mammals and birds, including herbivores, insectivores, and granivores. 

For dose-based exposures, three weight classes of mammals (15, 35, and 1000 g) and 

birds (20, 100, and 1000 g) are considered. 
37

A default foliar dissipation half-life of 35 days was used in this assessment. Example 

input parameters, such as application rate, interval, and number of applications, used in 

the T-REX model are presented with corresponding EECs in Tables 10 and 11 (also 

Appendix E).

3.4.1 Exposure Estimates for Birds and Mammals

The estimated exposure concentrations of 2,4-D choline salt for birds and mammals are 

presented in Tables 10 and 11, respectively. These estimates are based on the upperbound Kenaga dose and the assumption that the species in question eat one type of food 

item and forage only in the treated and/or overspray areas. 

EECs ranged from 26 to 578 mg ae/kg-diet for dietary exposures (birds and mammals). 

Avian dose-based EECs ranged from 2.34 to 659 mg ae/kg-bw while mammalian dosebased EECs ranged from 1.23 to 552 mg ae/kg-bw (Tables 10 and 11). 

Table 10. Avian Exposure Concentration Estimates for Foliar Applications

Feeding Category

Dietarybased EECs 

(mg/kg-food 

item)

Dose-based EECs (mg/kg-bw)

Small

(20 g)

Medium

(100 g)

Large

(1000 g)

2,4-D choline salt on herbicide-tolerant corn and soybean – 3 applications at 1 lb ae/A (12 

day interval)

Short grass 578.44 658.79 375.67 168.19

Tall grass 265.12 301.94 172.18 77.09

Broadleaf plants 325.37 370.57 211.31 94.61

Fruits/pods 36.15 41.17 23.48 10.51

Arthropods 226.56 258.03 147.14 65.88

Seeds 36.15 9.15 5.22 2.34

Table 11. Mammalian Exposure Concentration Estimates for Foliar Applications

Feeding Category

Dietarybased EECs 

(mg/kg-food 

item)

Dose-based EECs (mg/kg-bw)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

2,4-D choline salt on herbicide-tolerant corn and soybean – 3 applications at 1 lb ae/A (12 

day interval)

Short grass 578.44 551.50 381.16 88.37

Tall grass 265.12 252.77 174.70 40.50

Broadleaf plants 325.37 310.22 214.40 49.71

Fruits/pods 36.15 34.47 23.82 5.52

Arthropods 226.56 216.00 149.29 34.61

Seeds 36.15 7.66 5.29 1.23
38

3.4.2 Exposure Estimates for Plants

TerrPlant 1.2.2 (10/29/09) was used as a Tier 1 model for screening level assessments of 

pesticides. The model provides estimates of exposure to terrestrial plants from single 

pesticide applications; however, the model does not consider exposures to plants from 

multiple pesticide applications. TerrPlant derives pesticide EECs in runoff and in spray 

drift, and develops risk quotients for non-listed and listed species of monocots and dicots 

inhabiting dry and semi-aquatic areas (Appendix F). 

The estimated exposure concentrations of 2,4-D choline salt for terrestrial plants are 

presented below (Table 12). The most protective application scenario for each use was 

selected, based on information from the label. EECs ranged from 0.01 (spray drift) to 

0.51 (total for semi-aquatic areas). 

Table 12. Terrestrial Plant Exposure Concentration Estimates

Description Equation EEC (lb ae/A)

2,4-D Choline Salt and EnlistTM on Corn and Soybeans – single ground application at 1 

lb ae/A

Runoff to dry areas (A/I)*R 0.05

Runoff to semi-aquatic areas (A/I)*R*10 0.5

Spray drift A*D 0.01

Total for dry areas ((A/I)*R)+(A*D) 0.06

Total for semi-aquatic areas ((A/I)*R*10)+(A*D) 0.51

Atmospheric Concentration Analysis 

Henry's Law constant of 8.56 x10-6 atm-m3

/mol indicates that 2,4-D is expected to 

volatilize from moist soil and water surfaces. The Probablistic Exposure and Risk Model 

for Fumigants (PERFUM, v. 2.5.1, 7/2/2008) is used in estimating the vapor phase 

(secondary drift) of 2,4-D in the atmosphere. The PERFUM model is a processor of the 

ISCST3 model (Reiss and Griffin 20084

), which has been developed from EPA’s Office 

of Air Quality Planning and Standards5

. PERFUM can calculate upper-bound EECs in 

air near the edge of a field or at a downwind distance of a treated field. PERFUM

incorporates actual weather data and flux distribution estimates, and then accounts for 

changes and altering conditions. Analyses based on a variety of model outputs can be 

used to compare the potential risks at a range of distances. The PERFUM model and 

users manual are public and can be downloaded at 

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

The Agency estimated the volatility flux from a submitted field volatility study 

performed with 2,4-D choline as well as 2,4-D dimethylamine salt (DMA) and 2,4-D 

ethylhexyl ester (EHE) at four different sites (MRID 48862902). This study is in review 

 4 http://www.exponent.com/

5 The ISCST3 model documentation can be accessed on-line at: http://www.epa.gov/scram001/

dispersion_alt.htm
39

at this time and preliminary flux profiles are provided in Appendix H. The Agencyestimated highest volatility flux of 1.88 x 10-8 g/m2-

s from Little Rock, AR was used in 

the PERFUM modeling. Atmospheric concentrations at the 90th percentile of 2,4-D for 

corn are depicted at various distances in Figure 3. An example of the input for the 

PERFUM model is provided in Appendix I. 

Figure 3. Estimated 90th Percentile of 2,4-D Concentrations in the Atmosphere 

Since the PERFUM model is limited to vapor drift estimation, the AERSCREEN model 

is used to calculate wet and dry deposition in the environment. AERSCREEN is a 

screening model, based on the USEPA AERMOD model, designed to produce estimates 

of "worst-case" 1-hour concentrations for a single source without the need for hourly 

meteorological data (USEPA 2011). Details of the AERSCREEN model can be obtained 

from the cited URL6 below. The highest volatility flux of 1.88 x 10-8 g/m2-

s from Little 

Rock, AR was also used in the AERSCREEN modeling. Table 13 provides maximum 

wet and dry depositions of 2,4-D. The estimated wet and dry depositions were negligible 

(≤9.60 x 10-4 lb a.e/A). Examples of input and output of the AERSCREEN model are

provided in Appendix I. 

Table 13. Daily AERSCREEN 2,4-D Maximum Total 24-Hour Deposition Results

Time 

Period

(Hours)

Total 

Field 

Emissions

(lb a.i./A)

Total 

Field 

Emissions

(% of 

applied)

Total Dry 

Deposition

(lb a.i./A)

Total Wet 

Deposition

(lb a.i./A)

Total 

Deposition

(lb a.i./A)

Total 

Deposition

(% of 

applied)

Total 

Deposition

(% of 

emitted)

Choline Formulation, Corn and Soybeans (EPA Field Emissions)1

0 - 5 1.20 x 10-2 1.204% 1.05 x 10-3 5.00 x 10-5 1.10 x 10-3 - -

5 – 10 8.43 x 10-5 0.008% 5.00 x 10-5 0 5.00 x 10-5 - -

 6 http://www.epa.gov/ttn/scram/models/screen/aerscreen_userguide.pdf)
40

Time 

Period

(Hours)

Total 

Field 

Emissions

(lb a.i./A)

Total 

Field 

Emissions

(% of 

applied)

Total Dry 

Deposition

(lb a.i./A)

Total Wet 

Deposition

(lb a.i./A)

Total 

Deposition

(lb a.i./A)

Total 

Deposition

(% of 

applied)

Total 

Deposition

(% of 

emitted)

10 - 16 1.59 x 10-5 0.002% 0 0 0 - -

16 - 23 7.30 x 10-6 0.001% 0 0 0 - -

23 - 24 4.90 x 10-5 0.005% 2.00 x 10-5 0 2.00 x 10-5 - -

Total 1.22 x 10-2 1.220% 9.60 x 10-4 5.00 x 10-5 1.17 x 10-3 0.117% 9.48% 1 Deposition values for corn and soybeans the same despite 50 cm and 15 cm release heights modeled, 

respectively

Atmospheric Monitoring Data

Data collected in the 1960s and 1970s, and summarized in Majewski and Capel (1995), 

indicate that 2,4-D has been detected in rainwater samples at concentrations between 50 

nanograms per liter (ng/L) and 204,000 ng/L, while 2,4-D was detected in air samples at 

concentrations between 1.15 nanograms per gram (ng/g) and 1410 ng/g. Majewski and 

Capel noted that the higher concentrations were infrequently detected, and the authors 

also noted that the high detections were located near areas where pesticides were applied 

and may have resulted from unusual conditions. More recent data reported by Anderson 

et al. (2002) on water and rainfall samples in a wetland environment in Alberta, Canada 

indicate that 2,4-D was one of the most frequently detected pesticides in rainfall samples 

with a frequency of detection of 65%; however, concentrations did not exceed 1 μg/L. In 

a study conducted in southern Manitoba by Rawn et al. (1999), 2,4-D was detected in 

rainfall at concentrations less than 1 μg/L and was detected in air as both vapor and 

particle phase at a maximum concentration of 3500 picograms per cubic meter (pg/m3

). 

Both rainfall and air detections were closely associated with local use; however, the 

authors noted that the relative contribution of these compartments to surface water was 

low compared to runoff. 

4. Ecological Effects Assessment

Ecotoxicity data are available for 2,4-D choline salt for a limited number of taxa (acute 

freshwater fish, acute honeybee, acute freshwater invertebrate, and green algae). These 

studies support the bridging strategy that has been used for other forms of 2,4-D; 2,4-D 

toxicity is usually reported in “acid equivalents” so that toxicity data can be compared 

among forms. For this assessment, 2,4-D choline salt specific data, when available, will 

be discussed as well as the most sensitive 2,4-D acid equivalent data (the 2005 RED can 

be consulted for a complete listing of 2,4-D data – USEPA 2005b). Only the most 

sensitive 2,4-D toxicity value from the broader 2,4-D dataset will be used in risk quotient 

calculations. Given that there is a difference between the toxicity of esters and 

amines/salts/acid in aquatic systems, only toxicity data from the latter will be considered 

for aquatic exposures, unless it is unavailable. Ester data will be used in the absence of 

salt/amine/acid data. For terrestrial scenarios, the most sensitive 2,4-D toxicity value will 

be used, regardless of the chemical form.
41

A search of the ECOTOX database on October 24, 2012 did not yield any 2,4-D choline 

toxicity studies. Therefore, only studies submitted by the registrant were evaluated to 

determine the effects of 2,4-D choline salt on non-target organisms. The Ecological 

Incident Information System (EIIS) was also reviewed to provide a refined 

characterization of the ecological effects for 2,4-D. The ECOTOX database and 

European Footprint database yielded toxicological information for aquatic organisms 

indicating that 2,4-DCP, a major degradate of 2,4-D, is more toxic than 2,4-D. 

References from the Footprint database were not available for review. ECOTOX sources 

were not formally reviewed; however the implication of 2,4-DCP’s higher toxicity is

incorporated into the risk characterization section of the document.

4.1 Aquatic Effects Summary

2,4-D choline salt’s effects on aquatic fauna were determined by evaluating freshwater

and estuarine/marine fish and invertebrates, and freshwater amphibians. Acute and 

chronic studies were submitted for freshwater fish, freshwater invertebrates, and 

estuarine/marine fish, whereas only acute studies were submitted for estuarine/marine 

invertebrates and freshwater amphibians. In general, 2,4-D choline salt is slightly toxic 

to fish and invertebrates, and practically non-toxic to aquatic-phase amphibians, on an 

acute basis. 

4.1.1 Toxicity Effects on Fish and Amphibians

An acute freshwater fish limit test for 2,4-D choline salt was available. Neither 

mortalities nor sub-lethal effects were observed. This study yielded the lowest registrantsubmitted endpoint to date for 2,4-D moieties and classifies 2,4-D choline salt as 

“slightly toxic” to freshwater fish. Acute and chronic studies for other fish and 

amphibians were not available for 2,4-D choline salt; however, data were available for 

other 2,4-D forms. The lowest chronic endpoint for freshwater fish was in an early life 

stage study with the fathead minnow. The study found a decrease in length (most 

sensitive endpoint) as well as mortality at the highest two concentrations (98.1 and 62.9 

mg ae/L) and weight loss at 37.6 mg ae/L. An acute estuarine/marine fish study with the 

tidewater silverside classified 2,4-D as “slightly toxic” to estuarine/marine fish. No 

mortalities or sub-lethal effects were observed. Chronic estuarine/marine fish endpoints 

were only available for ester forms of 2,4-D. The lowest endpoint was based on survival 

of sheepshead minnow in an early life study. Other effects included a reduction of 

hatchability at the two highest concentrations (287 and 194 μg ae/L), and reduced length 

and weight (133 μg ae/L). An acute study with the leopard frog suggests that 2,4-D is 

“practically non-toxic” to freshwater amphibians. No sub-lethal effects were observed. 

The study was classified as “supplemental” because it was a non-guideline study; 

however, it is considered scientifically sound and appropriate for use in risk assessments 

(Table 14).
42

Table 14. Toxic Effects in Fish and Amphibians

Test Species 2,4-D Form Tested Toxicity 

Value

(mg ae/L)

Study 

Classification

MRID #

Acute 

Freshwater 

Fish

Rainbow 

trout 

(Oncorhynch

us mykiss)

2,4-D choline salt

96-h LC50 > 

48 Acceptable 48892401

Chronic 

Freshwater 

Fish (early life 

cycle)

Fathead 

minnow 

(Pimphales 

promelas)

Dimethylamine salt 

of 2,4-D

NOAEC = 

14.2*

Based on 

length

Acceptable 41767701

Acute 

Estuarine/ 

Marine Fish

Tidewater 

silverside 

(Menidia 

beryllina)

Diethanolamine salt 

of 2,4-D

96-h LC50 > 

80.24 Acceptable 42018301

Chronic 

Estuarine/ 

Marine Fish

Sheepshead 

minnow 

(Cyprinodon 

variegatus)

Butoxyethyl ester 

of 2,4-D

NOAEC = 

0.05554*

Based on 

survival

Acceptable 41345701

Acute 

Freshwater 

Amphibians

Leopard frog 

tadpoles 

(Rana 

pipiens)

Dimethylamine salt 

of 2,4-D

96-h LC50 = 

278* Supplemental 44517306

*Denotes value used for calculating risk quotients

4.1.2 Toxicity Effects on Invertebrates

An acute freshwater invertebrate limit test for 2,4-D choline salt was available using the 

water flea. One mortality was reported; however, it was considered incidental and not 

related to the treatment. No sub-lethal effects were observed. The most sensitive 

endpoint for acute exposures to freshwater invertebrates occurred in a water flea study 

conducted with 2,4-D acid (MRID 41158301). Sub-lethal effects were not reported for 

this study; the study classifies 2,4-D as “slightly toxic” to freshwater invertebrates on an 

acute basis. A freshwater invertebrate chronic study with the water flea yielded 

endpoints based on survival and reproduction (number of neonates produced, number of 

broods produced per daphnid, and brood size). No other effects were observed. An acute 

study with the eastern oyster classified 2,4-D as “slightly” toxic to estuarine/marine 

invertebrates. One mortality occurred at the highest treatment level (155 mg ae/L) and 

reduced feeding was also observed at this concentration. Shell growth reduction was 

noted in the other concentrations and the EC50 was based on this parameter (Table 15). 

Estuarine/marine invertebrate chronic toxicity data were not available. A toxicity value 

can be estimated based on the assumption that the acute-to-chronic ratio (ACR) for 

freshwater invertebrates applies to estuarine/marine invertebrates also. Thus, the 
43

following equation was used to estimate a NOAEC for eastern oyster, the most sensitive 

estuarine/marine species on an acute basis (Table 15).

EC50(oyster) = EC50 (waterflea) = 49.6 = 25 = 31.8 = NOAEC(oyster)

NOAEC(oyster) NOAEC(waterflea) X 16.05

Table 15. Toxic Effects in Invertebrates

Test Species 2,4-D Form Tested Toxicity Value 

(mg ae/L)

Study 

Classification MRID #

Acute 

Freshwater 

Invertebrates

Water flea 

(Daphnia 

magna)

2,4-D choline salt 48-h EC50 > 

40.7 Acceptable 48892402

Acute 

Freshwater 

Invertebrates

Water flea 

(Daphnia 

magna)

2,4-D acid 48-h LC50 = 25* Acceptable 41158301

Chronic 

Freshwater 

Invertebrates

Water flea 

(Daphnia 

magna)

Diethanolamine salt 

of 2,4-D

NOAEC = 

16.05*

Based on 

survival and 

reproduction

Acceptable 42018303

Acute 

Estuarine/ 

Marine 

Invertebrates

Eastern 

oyster 

(Crassostr

ea 

virginica)

Isopropylamine salt 

of 2,4-D

96-h EC50 = 

49.6* Acceptable 41429003

Chronic 

Estuarine/ 

Marine 

Invertebrates

Eastern 

oyster 

(Crassostr

ea 

virginica)

ACR

NOAEC = 

31.8*

Based on ACR

ACR ACR

*Denotes value used for calculating risk quotients

4.1.3 Toxicity Effects on Plants

A non-vascular aquatic plant study for 2,4-D choline salt was available using green algae. 

Yield and growth rates were affected at the 23.3 mg ae/L treatment. The most sensitive 

endpoint for non-vascular plants was derived from a freshwater diatom study and was 

based on growth inhibition. An aquatic vascular plant study with duckweed was 

available. The most sensitive endpoints were a reduction in frond number and plant 

number. At the two highest treatment levels (2.11 and 1.03 mg ae/L), colony break up 

and root destruction were observed; there was a statistically significant increase in frond

chlorosis as well (Table 16). 
44

Table 16. Toxic Effects in Aquatic Plants

Test

Species 2,4-D Form

Tested

Toxicity 

Value 

(mg ae/L)

Study 

Classification MRID #

Non-Vascular 

Aquatic Plant

Green algae 

(Pseudokirc

hneriella 

subcapitata)

2,4-D 

choline salt

72-h IC50 > 

45.85

NOAEC = 

23.3

Acceptable 48892405

Non-Vascular 

Aquatic Plant

Freshwater 

diatom 

(Navicula 

pelliculosa)

Dimethylam

ine salt of 

2,4-D EC50 = 3.88* Acceptable 41505903

Vascular 

Aquatic Plant

Duckweed 

(Lemna 

gibba)

Diethanola

mine salt of 

2,4-D

EC50 = 

0.2992* Acceptable 42712204

*Denotes value used in risk quotient calculations

4.2 Terrestrial Effects Summary

2,4-D choline salt’s effects on terrestrial organisms were determined by evaluating 

toxicity data for birds, mammals, insects, and terrestrial plants. Acute and chronic 

studies were submitted for birds and mammals; acute contact data were available for 

honeybees. Vegetative vigor and seedling emergence data were available for plants. In 

general, 2,4-D choline salt is practically non-toxic to terrestrial insects, slightly toxic to 

mammals, and moderately toxic to birds, on an acute basis. Terrestrial dicots appear to 

be more sensitive than monocots. 

4.2.1 Toxicity Effects on Birds

2,4,-D choline salt’s toxicity to birds was evaluated through acute oral exposure, acute 

dietary exposure, and chronic dietary exposure. Birds are considered a surrogate for 

terrestrial-phase amphibians and reptiles, in the absence of taxa-specific data.

The acute oral toxicity study was conducted with the northern bobwhite quail and 

resulted in a classification of “moderately toxic” to birds on an acute oral basis. Toxic 

symptoms prior to death were lethargy, reduced reaction to external stimuli, depression, 

lower limb weakness, wing droop, prostrate posture, loss of righting reflex, and a ruffled 

appearance. Sub-lethal effects included a drop in body weight at two of the treatment 

levels (218.7 and 135 mg ae/kg-bw). There was also a decrease in food consumption at 

the 218.7 mg ae/kg-bw treatment level during the first 3 days after dosing, but this was 

compensated for by a 2-3 times higher food consumption rate from days 4 through 14 

(Table 17).

Two acute dietary studies were available, classifying 2,4-D choline salt as “practically 

non-toxic” on an acute dietary basis to birds. No mortalities occurred in either study. 
45

The northern bobwhite quail study exhibited a slight decrease in body weight gain at the 

3035 and 1706 mg ae/kg-diet treatment levels. The mallard duck study exhibited a 

decrease in body weight gain and feed consumption, but only at the highest treatment 

level (3035 mg ae/kg-diet) (Table 17).

The chronic reproduction study was performed with the northern bobwhite quail. Two 

mortalities occurred, but they were considered incidental and not related to the treatment. 

No sub-lethal effects were observed.

Table 17. Toxic Effects in Birds

Test Species 2,4-D Form

Tested

Toxicity 

Value

Study 

Classification MRID #

Acute Oral 

Toxicity

Northern 

bobwhite quail 

(Colinius 

virginianus)

Triisopropanol

amine salt of 

2,4-D

LD50 = 218.7 

mg ae/kg-bw*

NOAEL = 67.5 

mg ae/kg-bw

Acceptable 41644401

Acute 

Dietary 

Toxicity

Northern 

bobwhite quail 

(Colinius 

virginianus)

Triisopropanol

amine salt of 

2,4-D 

LC50 > 3035

mg ae/kg-diet 

NOAEL = 961 

mg ae/kg-diet

Acceptable 41644402

Acute 

Dietary 

Toxicity

Mallard Duck

(Anas 

platyrhynchos)

Triisopropanol

amine salt of 

2,4-D

LC50 > 3035 

mg ae/kg-diet

NOAEL = 

1706 mg 

ae/kg-diet

Acceptable 41644403

Chronic 

Reproduction

Northern 

bobwhite quail 

(Colinius 

virginianus)

2,4-D acid

LOAEC > 962 

mg ae/kg-diet

NOAEC = 962 

mg ae/kg-diet*

No effects

Acceptable 45336401

*Denotes value used for risk quotient calculation

4.2.2 Toxicity Effects on Mammals

The acute toxicity of 2,4-D choline salt to mammals was assessed using the oral gavage 

study conducted on laboratory rat. Based on the LD50, 2,4-D choline salt is moderately 

toxic to mammals on an acute basis. The two-generation chronic study with the 
46

laboratory rat indicated several endpoints in parents and offspring growth (see Table 18) 

to be the most sensitive. Some reproductive effects were also identified (Table 18).

Table 18. Toxic Effects in Mammals from Acute Oral Exposure to 2,4-D Choline 

Salt

Test Species 2,4-D Form

Tested Toxicity Value Study Classification MRID #

Acute Oral Rat (Rattus 

norvegicus)

Triisopropanol

amine salt of 

2,4-D

LD50 = 441 mg 

ae/kg-bw* Acceptable 41413501

Chronic 

TwoGeneration 

Reproduction

Rat (Rattus 

norvegicus) 2,4-D acid

Parental

NOAEL = 5 mg 

ae/kg-bw

Based on 

decreased female 

weight gain (F1) 

and male renal 

tubule alteration 

(F0 and F1)

Reproductive

NOAEL = 20 mg 

ae/kg-bw

Based on increase 

in gestation 

length

Offspring

NOAEL = 5 mg 

ae/kg-bw

Based on 

decreased pup 

body weight

Acceptable 00150557

00163996

*Denotes value used for calculating risk quotients

4.2.3 Toxicity Effects on Non-Target Insects

There are contact toxicity data for 2,4-D choline salt. The study had several incidental 

deaths in the control and four of the five treatment groups; none were considered 

statistically significant. Three bees were observed to be either apathetic (slow response 

to stimuli) or affected (reduced coordination), but the effects were not dose responsive. 

The most sensitive 2,4-D toxicity value is derived from a honeybee study with 2-

ethylhexyl ester of 2,4-D. It classifies 2,4-D as “practically non-toxic” to honeybees on 

an acute contact basis. Sub-lethal signs of toxicity included lethargy, immobility, loss of 

equilibrium and hyper excitability (Table 19). 
47

Table 19. Toxic Effects in Honeybee (Apis mellifera) 

Test 2,4-D Form

Tested 

Toxicity Value

(μg ae/bee)

Toxicity 

Classification

Study 

Classification MRID #

Contact 

Toxicity

2,4-D choline 

salt

LD50 > 89.4

NOAEL = 89.4

Practically 

non-toxic Acceptable 48892404

Contact 

Toxicity

2-ethylhexyl 

ester of 2,4-D

LD50 > 66

NOAEL = 33

Practically 

non-toxic Acceptable 43982101

4.2.4 Toxicity Effects on Plants

Typical end-use product data are not available for the 2,4-D choline salt. Given that a 

dual herbicide product is being registered – 2,4-D choline salt/glyphosate – data for this 

herbicide combination are necessary for a thorough toxicological assessment. The stress 

of simultaneous exposure to two herbicides may cause additive or synergistic effects in 

terrestrial plants and lead to increased toxicity. Conversely, the herbicides could interfere 

with each other and cause a decrease in the toxicity of the product. Finally, the 

herbicides may operate independently and the observed toxicity may be equivalent to the 

more toxic of the two chemicals.

For this risk assessment, data from other forms of 2,4-D were used as surrogates for the 

2,4-D choline salt and 2,4-D choline salt/glyphosate products. For seedling emergence, 

onion was the most sensitive monocot and lettuce was the most sensitive dicot. Shoot 

length was the most sensitive parameter for both species, as well as for four of the other 

species that were tested (tomato, cucumber, soybean, turnip). Seedling emergence was 

the most sensitive parameter for one species (cabbage) and other toxicological 

observations included chlorosis and leaf curl. Onion and lettuce were also the most 

sensitive species for the vegetative vigor test. The most sensitive parameter for onion 

was fresh weight. Leaf distortion and necrosis were also observed. The most sensitive 

parameter for lettuce was dry weight. Chlorosis, necrosis, leaf curl, stem curl, wilt, and 

adventitious growth were also reported as effects. Overall, the vegetative vigor and 

seedling emergence studies indicate that 2,4-D choline salt is slightly more toxic to dicots

than monocots (Table 20).

Table 20. Toxic Effects in Terrestrial Vascular Plants 

Test Species 2,4-D Form

Tested

Toxicity Value 

(lb ae/A)

Study 

Classification MRID #

Seedling 

Emergence Monocot = 

Onion (Cepa 

allium)

Dicot = Lettuce 

(Lactuca sativa)

Isopropylamine 

salt of 2,4-D

Monocot 

EC25 = 0.010*

NOAEC = 

0.005628*

Based on shoot 

length

Dicot 

EC25 = 0.00081*

NOAEC = 0.00047*

Acceptable 43982101
48

Test Species 2,4-D Form

Tested

Toxicity Value 

(lb ae/A)

Study 

Classification MRID #

Based on shoot 

length

Vegetative 

Vigor Onion (Cepa 

allium) 2,4-D acid

EC25 > 0.0075

NOAEC < 0.0075

Based on fresh 

weight

Acceptable 42416801

Vegetative 

Vigor Lettuce 

(Lactuca sativa)

2-ethylhexyl 

ester of 2,4-D

EC25 = 0.0021*

NOAEC = 0.00167*

Based on dry weight

Acceptable 47106004

*Denotes value used for calculating risk quotients

4.3 Incident Database Review

The Environmental Protection Agency maintains an incident database system called the 

Ecological Incident Information System (EIIS) to track and evaluate accidental kills 

associated with pesticide use. Based on the information contained in the incident report, 

the likelihood of a particular pesticide causing the incident is classified as highly 

probable, probable, possible, or unlikely. If there are incidents, this information will be 

reviewed and considered in conjunction with the degree to which the levels of concern 

were exceeded, information on sales, local use practices, and monitored levels in the 

environment to determine whether the predicted effect based on the labeled use of the 

product is likely to occur or not.

A search of the EIIS, Avian Incident Monitoring System, and the Incident Database 

System on October 31, 2012 did not identify any ecological incidents attributed to 2,4-D 

choline salt. Although incident reporting is required under FIFRA Section 6(a)(2), the 

absence of additional reports in EIIS does not indicate that the chemical has no effects on 

wildlife; rather, it is possible that incidents have gone unreported. Registrations for 2,4-D 

choline salt are relatively recent (first registration occurred March 2011), thus, the time 

frame for collecting incidents is short. Given that 2,4-D choline salt is similar in its 

toxicity and mode of action to other 2,4-D forms, the full suite of 2,4-D incidents was 

considered in this risk assessment. The 2,4-D Problem Formulation contains a 

comprehensive list of all incidents reported in the EIIS as of August 6, 2012 (USEPA 

2012). The database contained approximately 460 plant incidents, 22 fish incidents, 2 

non-specified aquatic incidents, 4 mammal incidents, 4 bird incidents, and 1 honeybee

incident. 

Given that 2,4-D is an herbicide, it is not surprising that the vast number of reported 

incidents are related to plants. Many of these were lawn/turf grass incidents where 

browning or mortality occurred as a result of the application (some applications were 

considered “misuse,” but many were registered uses). In agricultural settings, direct 

treatment and spray drift were commonly cited as the cause of damage. Overall, the 

diversity and number of reported plant incidents supports the premise that 2,4-D has the 

potential to affect non-target plants. 
49

Many of the other incidents (fish, aquatics, bird, mammals, honeybee) involved other 

chemicals in addition to 2,4-D. The presence of more than one pesticide, especially if 

2,4-D is not explicitly tested for and detected, increases the uncertainty of the cause of 

the incident. Below is a list of the incidents that were most likely caused by 2,4-D (Table

21). These incidents demonstrate that registered uses of 2,4-D may have adverse effects 

on non-target fish and birds. 
50

Table 21. Selected Ecological Incidents Associated with 2,4-D from the EIIS Database

Incident 

Number 

(Source)

Taxa Magnitude Year State Use Legality of 

Use

Certainty 

Category

2,4-D 

Residues

Other Chemicals 

Involved Comments

2,4-D I000636-

017 Catfish Several 1987 MO Home/ lawn Registered High probability N/R No

Mortality caused 

by runoff into a 

pond

2,4-D I000799-

003

Turkey

Cardinal

Blackbird

Duck

Bream

Bass

Duck, 

Bream, 

Bass –

hundreds

All others –

Unknown

1991 NC Home/lawn Not 

determined Probable

2,4-D tested 

for in duck 

and blackbird 

tissue; 2,4-D 

detected in 

water – 1 ppb

Dicamba

Mecoprop

Carbaryl

Diazinon

Pentachlorophenol

Oxamyl

Mortality caused 

by ingestion 

(birds) and runoff 

into stream (fish)

2,4-D B0000-

300-37

Drum

Bream

Croaker

100 fish 

each 1984 SC N/R Not determined Probable N/R No Mortality caused by ingestion

2,4-D I000925-

001 Unknown fish 23000 1993 WV Right-of-way, rail Registered High probability

Confirmed in 

water, 

concentration 

not reported

Triclopyr

Mortality caused 

by drift into 

stream
51

5. Risk Characterization

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

ecological risk associated with the proposed new uses for 2,4-D choline salt. The risk 

characterization provides estimates and descriptions of the risk; articulates risk 

assessment assumptions, limitations, and uncertainties; synthesizes an overall conclusion; 

and provides risk managers with information to make regulatory decisions. 

5.1 Risk Estimation

The risk quotient method was used to determine if 2,4-D choline salt has the potential to 

cause adverse effects to non-target organisms. In the risk quotient method, the estimated 

environmental concentrations (EECs) are divided by acute and chronic toxicity values. 

The resulting unit-less risk quotients are compared to the Agency’s levels of concern to 

determine the need for regulatory action (Appendix G). 

5.1.1 Summary of Aquatic Risk Quotients

5.1.1.1 Risk Quotients for Fish

Risk quotients were calculated for acute effects on freshwater amphibians and chronic 

effects on freshwater and estuarine/marine fish (Table 22). Acute risk quotients for 

freshwater amphibians and fish ranged from <0.001 to 0.001 and did not exceed the acute 

listed or non-listed species LOCs (0.05 and 0.1, respectively). Likewise, the chronic 

LOC of 1 was not exceeded for freshwater fish (range of < 0.001 to 0.002) or 

estuarine/marine fish (range of 0.09 to 0.53). 
52

Table 22. Acute Risk Quotients for Freshwater Amphibians and Chronic Risk 

Quotients for Freshwater Fish and Estuarine/Marine

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Chronic risk 

to freshwater 

fish

60-day 

EEC/NOAEC

(NOAEC = 

14200 μg 

ae/L)

Acute risk to 

freshwater 

amphibians

Peak EEC/LC50

(LC50 = 278000 

μg ae/L)

Chronic risk 

to estuarine/ 

marine fish

60-day 

EEC/NOAEC

(NOAEC = 

55.4 μg ae/L)

Surface Water 

(PRZM/EXAMS)

Aquatic Exposure for 2,4-D (Parent)

Corn:

IA Corn STD

IL Corn STD

IN Corn STD

KS Corn STD

MN Corn STD

MS Corn STD

NC Corn STD

NE Corn STD

OH Corn STD

PA Corn STD

(3app. X 1.0 lbs 

a.e./acre)

0.001-0.001

0.001-0.002

0.001-0.002

0.001-0.001

0.001-0.001

0.002-0.002

0.001-0.002

0.002-0.002

0.001-0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

<0.001-<0.001

0.34-0.30

0.41-0.36

0.38-0.43

0.25-0.23

0.25-0.21

0.53-0.48

0.42-0.32

0.52-0.50

0.21-0.16

0.12-0.12

Soybean:

MS Soybean STD

(3app. X 1.0 lbs 

a.e./acre)

0.002-0.002 <0.001-<0.001 0.39-0.38

Corn:

MS Corn STD1

(2app. @ 1.0 lbs and 

2 App @0.50 lb 

a.e./acre)

0.002 <0.001 0.49

Corn:

MS Corn STD2

(3app. X 1.0 lbs 

a.e./acre)

0.002 <0.001 0.46

1

Post Emergence 2 applications at 0.50 lb a.e./A 2 PRZM/EXAMS simulated EECs without spray drift fraction to evaluate spray drift 

contribution

3 PRZM/EXAMS simulation was performed only for MS Corn based on highest EECs 

observed in 2,4-D exposure 4 = 2,4-D application rate (1.0 lb a.e/A) x [(0.035, the maximum conversion rate for the 

terrestrial degradation of 2,4-D in the terrestrial environment to 2,4-DCP in laboratory 

studies) x (0.74, the molecular weight ratio of 2,4-D to 2,4-DCP)] 5

= 2,4-D application rate (1.0 lb a.e/A) x [(0.017, spray drift contribution of 2,4-D) x 

(0.326, the maximum conversion rate from the anaerobic aquatic degradation of 2,4-D to 

2,4-DCP from laboratory studies) x (0.74, the molecular weight ratio of 2,4-D to 2,4-

DCP)]
53

5.1.1.2 Risk Quotients for Aquatic Invertebrates

A summary of risk quotients for acute and chronic exposures of 2,4-D choline salt to 

freshwater and estuarine/marine invertebrates is presented in Table 23, below. Risk 

quotients ranged from <0.001 to 0.002 (acute) and <0.001 to 0.003 (chronic) for 

freshwater invertebrates. Risk quotients ranged from <0.001 to 0.001 (acute and chronic) 

for estuarine/marine invertebrates. None of the LOCs were exceeded for any scenario 

(acute listed = 0.05; acute non-listed = 0.1; chronic = 1). 

Table 23. Acute and Chronic Risk Quotients for Aquatic Invertebrates Exposed to 

2,4-D Choline Salt in Surface Water

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Acute risk to 

freshwater 

invertebrates

Peak 

EEC/LC50

(LC50 = 25000 

μg ae/L)

Chronic risk to 

freshwater

invertebrates

21-day 

EEC/NOAEC 

(NOAEC = 

16050 μg ae/L)

Acute risk to 

estuarine/ 

marine 

invertebrates

Peak 

EEC/EC50

(EC50 = 49600 

μg ae/L)

Chronic risk 

to estuarine/ 

marine 

invertebrates

21-day 

EEC/NOAEC 

(NOAEC = 

31800 μg 

ae/L)

Surface Water 

(PRZM/EXAMS)

Aquatic Exposure for 2,4-D (Parent)

Corn:

IA Corn STD

IL Corn STD

IN Corn STD

KS Corn STD

MN Corn STD

MS Corn STD

NC Corn STD

NE Corn STD

OH Corn STD

PA Corn STD

(3app. X 1.0 lbs 

a.e./acre)

0.001-0.001

0.001-0.001

0.001-0.001

0.001-0.001

0.001-0.001

0.002-0.002

0.001-0.001

0.002-0.001

0.001-0.001

<0.001-<0.001

0.001-0.001

0.002-0.001

0.002-0.001

0.001-0.001

<0.001-<0.001

0.003-0.003

0.002-0.001

0.002-0.002

0.001-0.001

0.001-0.001

0.001-<0.001

0.001-<0.001

0.001-<0.001

0.001-<0.001

<0.001-<0.001

0.001-0.001

0.001-<0.001

0.001-0.001

0.001-<0.001

<0.001-<0.001

0.001-0.001

0.001-0.001

0.001-0.001

0.001-0.001

<0.001-<0.001

0.001-0.001

0.001-0.001

0.001-0.001

0.001-<0.001

<0.001-<0.001

Soybean:

MS Soybean STD

(3app. X 1.0 lbs 

a.e./acre)

0.002-0.002 0.002-0.002 0.001-0.001 0.001-0.001

Corn:

MS Corn STD1

(2app. @ 1.0 lbs and 

2 App @0.50 lb 

a.e./acre)

0.002 0.003 0.001 0.001

Corn:

MS Corn STD2

(3app. X 1.0 lbs 

a.e./acre)

0.002 0.002 0.001 0.001
54

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Acute risk to 

freshwater 

invertebrates

Peak 

EEC/LC50

(LC50 = 25000 

μg ae/L)

Chronic risk to 

freshwater

invertebrates

21-day 

EEC/NOAEC 

(NOAEC = 

16050 μg ae/L)

Acute risk to 

estuarine/ 

marine 

invertebrates

Peak 

EEC/EC50

(EC50 = 49600 

μg ae/L)

Chronic risk 

to estuarine/ 

marine 

invertebrates

21-day 

EEC/NOAEC 

(NOAEC = 

31800 μg 

ae/L)

1

Post Emergence 2 applications at 0.50 lb a.e./A 2 PRZM/EXAMS simulated EECs without spray drift fraction to evaluate spray drift contribution

3 PRZM/EXAMS simulation was performed only for MS Corn based on highest EECs observed in 2,4-D 

exposure

4 = 2,4-D application rate (1.0 lb a.e/A) x [(0.035, the maximum conversion rate for the terrestrial 

degradation of 2,4-D in the terrestrial environment to 2,4-DCP in laboratory studies) x (0.74, the molecular 

weight ratio of 2,4-D to 2,4-DCP)] 5

= 2,4-D application rate (1.0 lb a.e/A) x [(0.017, spray drift contribution of 2,4-D) x (0.326, the maximum 

conversion rate from the anaerobic aquatic degradation of 2,4-D to 2,4-DCP from laboratory studies) x 

(0.74, the molecular weight ratio of 2,4-D to 2,4-DCP)]

5.1.1.3 Risk Quotients for Aquatic Plants

A summary of risk quotients for acute and chronic exposures of 2,4-D choline salt to 

freshwater and estuarine/marine invertebrates is presented in Table 24, below. Risk 

quotients ranged from 0.002 to 0.015 for non-vascular aquatic plants and 0.021 to 0.194 

for vascular aquatic plants. None of the risk quotients exceeded the LOC of 1 for aquatic 

plants. 

Table 24. Risk Quotients for Aquatic Plants Exposed to 2,4-D Choline Salt in 

Surface Water

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Risk to non-vascular 

plants

Peak EEC/EC50

(EC50 = 3880 μg ae/L)

Risk to aquatic 

vascular plants

Peak EEC/EC50

(EC50 = 299.2 μg ae/L)

Surface Water 

(PRZM/EXAMS)

Aquatic Exposure for 2,4-D (Parent)

Corn:

IA Corn STD

IL Corn STD

IN Corn STD

KS Corn STD

MN Corn STD

MS Corn STD

NC Corn STD

NE Corn STD

OH Corn STD

PA Corn STD

(3app. X 1.0 lbs 

a.e./acre)

0.007-0.006

0.008-0.006

0.007-0.006

0.007-0.006

0.004-0.004

0.015-0.015

0.007-0.005

0.011-0.009

0.007-0.004

0.003-0.003

0.089-0.073

0.101-0.082

0.089-0.082

0.095-0.079

0.053-0.047

0.194-0.193

0.094-0.070

0.136-0.122

0.090-0.056

0.039-0.036
55

Drinking Water 

Source (model)

Use

Scenario

(modeled rate)

Risk to non-vascular 

plants

Peak EEC/EC50

(EC50 = 3880 μg ae/L)

Risk to aquatic 

vascular plants

Peak EEC/EC50

(EC50 = 299.2 μg ae/L)

Soybean:

MS Soybean STD

(3app. X 1.0 lbs 

a.e./acre)

0.011-0.011 0.143-0.143

Corn:

MS Corn STD1

(2app. @ 1.0 lbs 

and 

2 App @0.50 lb 

a.e./acre)

0.015 0.193

Corn:

MS Corn STD2

(3app. X 1.0 lbs 

a.e./acre)

0.015 0.189

1

Post Emergence 2 applications at 0.50 lb a.e./A 2 PRZM/EXAMS simulated EECs without spray drift fraction to evaluate spray drift 

contribution

3 PRZM/EXAMS simulation was performed only for MS Corn based on highest EECs 

observed in 2,4-D exposure 4 = 2,4-D application rate (1.0 lb a.e/A) x [(0.035, the maximum conversion rate for the 

terrestrial degradation of 2,4-D in the terrestrial environment to 2,4-DCP in laboratory 

studies) x (0.74, the molecular weight ratio of 2,4-D to 2,4-DCP)] 5

= 2,4-D application rate (1.0 lb a.e/A) x [(0.017, spray drift contribution of 2,4-D) x (0.326, 

the maximum conversion rate from the anaerobic aquatic degradation of 2,4-D to 2,4-DCP 

from laboratory studies) x (0.74, the molecular weight ratio of 2,4-D to 2,4-DCP)]

5.1.2 Summary of Terrestrial Risk Quotients

5.1.2.1 Avian and Mammalian Risk Quotients

Acute toxicity risk quotients were based on the oral toxicity study for the northern 

bobwhite quail. Risk quotients ranged from 0.01 to 4.18. “Seeds” was the only dietary 

item that did not yield a risk quotient that exceeded the listed species LOC (0.1). The 

non-listed species LOC (0.5) was exceeded for small and medium birds consuming short 

grass, tall grass, broadleaf plants, and arthropods; and for large birds consuming short 

grass. The listed species LOC was exceeded for all of these scenarios as well as small 

and medium birds consuming fruits/pods; and large birds consuming tall grass, broadleaf 

plants, and arthropods. The chronic risk quotients ranged from 0.04 to 0.60 and did not 

exceed the chronic LOC of 1 (Table 25).
56

Table 25. Summary of Avian Acute and Chronic Risk Quotients (RQs) for 2,4-D 

Choline Salt for Foliar Applications

Feeding Category

Chronic 

(dietarybased) RQs

Acute (dose-based) RQs

Small

(20 g)

Medium

(100 g)

Large

(1000 g)

2,4-D choline salt on herbicide-tolerant corn and soybean – 3 applications at 1 lb ae/A (12 

day interval)

Short grass 0.60 4.18** 1.87** 0.59**

Tall grass 0.28 1.92** 0.86** 0.27*

Broadleaf plants 0.34 2.35** 1.05** 0.33*

Fruits/pods 0.04 0.26* 0.12* 0.04

Arthropods 0.24 1.64** 0.73** 0.23*

Seeds 0.04 0.06 0.03 0.01

*Exceeds listed species LOC of 0.1

**Exceeds non-listed species LOC of 0.5

The dose-based acute mammalian risk quotients ranged from <0.01 to 0.57. The listed 

species LOC (0.1) was exceeded for small, medium, and large mammals consuming short 

grass, tall grass, broadleaf plants, and arthropods. In addition, small mammals 

consuming short grass also exceeded the non-listed species LOC (0.5). Chronic dosebased risk quotients ranged from 0.32 to 50.2. The chronic LOC of 1 was exceeded for 

all size classes of mammals consuming all food items except for seeds. Chronic dietarybased risk quotients ranged from 0.36 to 5.78. The chronic LOC of 1 was exceeded for 

diets of short grass, tall grass, broadleaf plants, and arthropods (Table 26). 

Table 26. Summary of Mammalian Acute and Chronic Risk Quotients (RQs) for 

2,4-D Choline Salt Foliar Applications

Risk Quotients 

Based

on Kenaga

Upper Bound EEC

Dose-Based RQs Chronic 

DietaryBased RQs

15 g 35 g 1000 g

Acute Chronic Acute Chronic Acute Chronic

2,4-D choline salt on herbicide-tolerant corn and soybean – 3 applications at 1 lb ae/A (12 day 

interval)

Short grass 0.57** 50.2*** 0.49* 42.9*** 0.26* 23.0*** 5.78***

Tall grass 0.26* 23.0*** 0.22* 19.7*** 0.12* 10.5*** 2.65***

Broadleaf plants 0.32* 28.2*** 0.27* 24.1*** 0.15* 12.9*** 3.25***

Fruits/pods 0.04 3.14*** 0.03 2.68*** 0.02 1.44*** 0.36

Arthropods 0.22* 19.7*** 0.19* 16.8*** 0.10* 9.00*** 2.27***

Seeds 0.01 0.70 0.01 0.60 <0.01 0.32 0.36

*Exceeds the acute listed species LOC of 0.1

**Exceeds the acute non-listed species LOC of 0.5

***Exceeds chronic LOC of 1.0
57

5.1.2.2 Plant Risk Quotients

Risk quotients exceeded the LOC (1) for all exposure scenarios. Risk quotients ranged 

from 1 to 90.62 for monocots and 12.35 to 1085.11 for dicots. Risk was attributed to 

both spray drift and runoff from treated fields (Table 27).

Table 27. Summary of Risk Quotients for Terrestrial Plants Exposed to 2,4-D 

Choline Salt through Runoff and/or Spray Drift

Plant Type Listed Status Dry Semi-Aquatic Spray Drift

2,4-D Choline Salt and EnlistTM on Corn and Soybeans – single ground application at 

1 lb ae/A

Monocot non-listed 6.00* 51.00* 1.00*

Monocot listed 10.66* 90.62* 1.78*

Dicot non-listed 74.07* 629.63* 12.35*

Dicot listed 127.66* 1085.11* 21.28*

*Exceeds LOC of 1.

5.1.2.3 Non-Target Insect Risk Quotients

Currently, EFED does not routinely quantify potential risks of pesticides to terrestrial 

non-target adult insects. Based on the acute contact data submitted for honeybees, 2,4-D 

choline salt is practically non-toxic (LD50 > 66 μg ae/bee). Bees or other beneficial 

insects could be exposed to 2,4-D choline salt through residues left on foliage or direct 

contact during the foliar spray application. Current data indicate that it is unlikely that 

such exposures would be lethal, although some sub-lethal effects were noted in the study 

(lethargy, immobility, loss of equilibrium, and hyper excitability). While the honeybee is 

used as a surrogate for all terrestrial insects, there is uncertainty in this assumption in that 

some other species of insects may be more sensitive to 2,4-D choline salt.

5.2 Risk Description

The results of this screening-level risk assessment indicate that direct effects are not 

expected for aquatic organisms; birds, reptiles, and land-phase amphibians (chronic), and 

terrestrial insects. There is insufficient information to determine how the proposed new 

uses of 2,4-D choline salt will directly affect mammals (acute and chronic), birds (acute), 

and terrestrial plants, and indirectly affect all taxonomic groups. The registrant submitted 

a stewardship plan (MRID 48862903) that is undergoing evaluation by the Biological 

Economics and Analysis Division. In addition, two risk assessment documents were 

supplied by the registrant and considered by EFED (MRIDs 48897101, 48897102). The 

risk assessments focused on listed species in the six “launch” states; however, the 

methodology employed was not in line with methods used by the Agency (i.e., fitted 

spray drift curve, no air analysis, species sensitivity distributions). The following 

sections discuss the results of the risk quotient analysis, implications of incident data, and 

explore several additional analyses to further characterize the risks and elucidate potential 

mitigation options. 
58

5.2.1 Probit Slope Response Analysis of LOC Values and Acute RQ Values

As part of the risk estimation, the probability of mortality associated with the listed acute 

LOC values is estimated along with the probability of acute mortality occurring if 

exposure at the EEC actually occurs. The probability of mortality calculations are based 

on the probit slope dose-response relationship. The probability of mortality for an 

exposed individual is calculated using an Excel spreadsheet tool IECV1.1 (Individual 

Effect Chance Model Version 1.1) developed by the U.S. EPA, OPP, Environmental Fate 

and Effects Division (June 22, 2004).

The model provides the option of inserting taxa-specific probit slopes and confidence 

intervals. If specific information is not available, the model uses a default value of 4.5 

for the probit slope and 2 and 9 for the upper and lower 95% confidence interval bounds. 

For 2,4-D choline salt, taxa-specific data were only available for birds; default values 

were used for mammals. 

Probabilities of mortality were only calculated for scenarios where the risk quotient 

exceeded the acute listed species LOC. For 2,4-D choline salt, there were two groups 

that met this criterion – birds and mammals (Table 28).

Table 28. Probability of Mortality for an Individual Bird or Mammal for 2,4-D 

Choline Salt

RQ (range) Slope 95% Confidence 

Interval

Odds 

(1 in...)

Birds 4.18 – 0.12 6.4 2.7 and 10.1 1 – 528,000,000

Mammals 0.57 – 0.10 4.5 2 and 9 7.35 – 294,000

5.2.2 AgDrift Analysis for Terrestrial Plants, Birds, and Mammals

Applied 2,4-D choline can drift from the treated area and still be present at concentrations 

that exceed acute levels of concern for birds, mammals, and terrestrial plants. Buffers 

may offer a potential mitigation strategy to reduce the concentration of 2,4-D choline salt 

that reaches non-target organisms. 2,4-D choline labels have a recommendation to use the 

2,4-D choline product with an ASABE S-572 droplet size classification of coarse/coarser 

spray quality. Given the limited option for droplet size in the current AgDRIFT ground 

module, the droplet size distribution of fine to medium/coarse (Dv0.5 of 341μm) option 

with 90th data percentile and high boom was used to estimate spray drift buffer distances.

The results indicate that buffers could be set to mitigate adverse effects from spray drift 

to non-target monocots, mammals, and birds. Buffers for dicots would need to be greater 

than 1000 ft to mitigate the risk from spray drift, which is probably not a practical 

solution (Table 29). 

The Agency explored an alternative approach to determine buffer distances using the 

label-recommended droplet spectrum. The registrant submitted a spray drift field study 

(MRID 48844001, in review) with various drift reduction nozzles and formulations for 

spray drift management. A cursory review of the study showed that the combination of 
59

an AIXR 11004 nozzle and the GF2726 formulation may be a feasible alternative for offsite drift management. The Agency considered this specific combination of nozzle and 

formulation to determine the potential for off-target movement of the 2,4-D choline salt. 

The deposition data show a biphasic distribution (Figure J-1, Appendix J) with a bias 

toward near field deposition. To determine far-field deposition, the Agency truncated the 

deposition data from 0 to10 feet and used 90th percentile data from 25 to 400 feet to 

determine buffer distances (Figure J-2, Appendix J). The deposition curve was 

developed using Exponential Decay, Double, 4 Parameter equation using SigmaPlot 

version 10.0. The Exponential Decay, Double, 4 Parameter (EDDP) and various 

coefficients are listed below:

f= 25(ft) + a*exp(-b*x)+c*exp(-d*x)

Where:

25 ft represents 0 distance for the deposition curve

a = 0.0013, b = 0.0388, c = 0.0006, and d = 0.0012

Table 29 shows that the buffer distances for dicots are shorter (62 to 202 ft) when 

estimated using the above equation as compared with the AgDRIFT model. 

Table 29. Spray Drift Buffer Distances for 2,4-D Choline Salt for Birds, Mammals,

and Terrestrial Plants

Use – herbicide-tolerant soybean 

and corn

Single application 

rate (lb ae/A)

Fraction of 

applied

Buffer distance (ground application) 

(ft)

AgDRIFT1 EDDP2

Listed monocots 1 0.005628 135 NA

Non-listed monocots 1 0.010 66 NA

Listed dicots 1 0.00047 >1000 202

Non-listed dicots 1 0.00081 961 62

Listed mammals 1 0.175 3 NA

Non-listed mammals 1 0.877 3 NA

Listed birds 1 0.024 23 NA

Non-listed birds 1 0.120 7 NA 1 AgDRIFT ground boom scenario with fine to medium/coarse droplet, high boom (50 inches), and 90th data 

percentile

2 Represent deposition data (25 to 400 ft) from AXIR11004 nozzle and GF2726 formulation at boom height of 50 

inches with 7mph (MRID 4884001)

NA- Not applicable due to truncated data.

5.2.3 Risk to Aquatic Vertebrates and Invertebrates

The proposed uses of 2,4-D choline salt did not exceed any levels of concern for chronic 

exposures to freshwater fish and estuarine/marine fish. Likewise, risk quotients for 

freshwater and estuarine/marine invertebrates did not exceed acute or chronic LOCs. 

Risk quotients could not be calculated for acute effects to freshwater and 

estuarine/marine fish because the toxicological values were non-definitive. Thus, the 

non-definitive values were compared directly with the EECs. The highest peak EEC (MS 

Corn Scenario = 58 μg ae/L) is less than the freshwater fish LC50 (LC50 > 48000 μg ae/L) 

and the estuarine/marine fish LC50 (LC50 > 80240 μg ae/L). This suggests that the 
60

concentration of 2,4-D in aquatic environments as a result of the proposed new uses on 

corn and soybean is not high enough to result in acute adverse effects to freshwater and 

estuarine/marine fish. 

Data from the European Footprint database and ECOTOX database indicate that 2,4-

DCP, a major degradate of 2,4-D, is more toxic to fish and invertebrates than 2,4-D. 

Footprint database toxicity values could not be evaluated because the Agency did not 

have access to the actual studies. Likewise, the open literature studies from the 

ECOTOX database were not reviewed in-depth. However, based on the aquatic EECs for 

2,4-DCP that were generated from the new uses on herbicide-tolerant corn and soybean, 

it is unlikely to pose a problem for freshwater fish and invertebrates. In all cases, the 

EECs for 2,4-DCP are much lower than the reported toxicity values (Table 30).

Table 30. 2,4-DCP Toxicity Values and Toxicity Thresholds for Freshwater Fish 

and Invertebrates

Reported 

acute value

Acute EECs Reported 

chronic 

value

Chronic EECs

Freshwater 

fish

1000 μg/L2 4.66 μg/L 290 μg/L2 3.84 μg/L

Freshwater 

invertebrates

1400 μg/L1,2 4.66 μg/L 210 μg/L2 3.62 μg/L

1

European Footprint Database

2

ECOTOX Database

Incident data suggest registered uses of 2,4-D result in mortality and toxic effects in 

freshwater fish. However, the application rate may have been higher than those of the 

proposed new uses. Given that the risk quotients, which were modeled with the highest 

application rates for herbicide-tolerant corn and soybean, were far below the LOCs, it is 

not expected that aquatic organisms will be at direct risk from these proposed new uses.

The proposed 2,4-D choline salt formulations for registration are reported to maintain 

some properties that may reduce spray drift to non-target areas. While EFED has 

received some information regarding these properties, the full suite of data necessary to 

analyze these properties is not available. Given that these data and other potential 

mitigation options are not finalized, EFED has determined that insufficient information is 

available to make an effects call for indirect effects to aquatic invertebrates and fish.

5.2.4 Risk to Terrestrial Animals

Mammals

Risk quotients for mammals exceeded the Agency’s LOCs for mammals for acute dosebased exposure and chronic dose-based and dietary-based exposure. The listed species 

LOC was exceeded for small, medium, and large mammals consuming short grass, tall 

grass, broadleaf plants, and arthropods; small mammals consuming short grass exceeded 

the non-listed species LOC. The chronic dose-based LOC was exceeded for all size 
61

classes of mammals consuming all food items except for seeds. The chronic dietarybased LOC was exceeded for diets of short grass, tall grass, broadleaf plants, and 

arthropods for mammals. Thus, the risk quotient analysis predicts direct risks are 

expected for listed mammals. 

Likewise, indirect effects are predicted for mammals based on the risk quotients that 

exceeded the non-listed mammal LOC, terrestrial plants, and birds. Indirect effects may 

occur when mammals depend on a terrestrial plant, bird, or other mammal that is 

adversely affected by 2,4-D choline salt. Terrestrial plants provide food and habitat for 

mammals. Birds and other mammals may be food sources for listed species as well.

For acute exposures, the probit slope response analysis indicated a 1 in 7.35 chance of an 

individual mammal being affected at the highest RQ and a 1 in 294,000 chance of an 

individual mammal being affected at the lowest RQ that exceeded the LOC.

2,4-D is currently undergoing a Tier I Endocrine Disruptor Screening as required by 

FFDCA section 408(p). Effects noted in the two-generation rat reproduction study 

included decreased female weight gain (F1) and male renal tubule alteration (F0 and F1) in 

the parental generations. An increase in gestation length was documented as a 

reproductive effect and pup body weight was decreased compared with the controls. 

The proposed 2,4-D choline salt formulations for registration are reported to maintain 

some properties that may reduce spray drift to non-target areas. While EFED has 

received some information regarding these properties, the full suite of data necessary to 

analyze these properties is not available. Given that these data and other potential 

mitigation options are not finalized, EFED has determined that insufficient information is 

available to make an effects call for direct and indirect effects to mammals.

Birds, Reptiles, and Terrestrial-Phase Amphibians

Avian risk quotients exceeded the LOC for acute exposures. The non-listed species LOC 

was exceeded for small and medium birds consuming short grass, tall grass, broadleaf 

plants, and arthropods; and for large birds consuming short grass. The listed species 

LOC was exceeded for all of these scenarios as well as small and medium birds 

consuming fruits/pods; and large birds consuming tall grass, broadleaf plants, and 

arthropods. Thus, the risk quotient analysis indicates that direct effects are predicted for 

birds on an acute basis; no chronic risks are expected. Incident data also support this 

conclusion. Several incidents (~4) involving birds have been reported to the Agency. In 

particular, one incident involved four species of birds (blackbird, cardinal, duck, and 

turkey. 2,4-D acid was applied to a lawn and an unknown number of birds were reported 

to have died via ingestion of the pesticide. 2,4-D was detected in surface water at the 

site; however it was not specifically detected in any of the bird tissues that were sampled. 

An additional uncertainty is the application of six other pesticides in a similar time frame 

(dicamba, mecoprop, carbaryl, diazinon, pentachlorophenol, and oxamyl).

Based on the risk quotient analysis, indirect effects from the proposed 2,4-D choline salt 

uses are predicted for birds. Several non-listed bird LOCs were exceeded. Birds that 
62

depend on terrestrial plants, mammals, or other birds for habitat and food may be affected 

as the risk quotient analysis predicts risks to these groups. 

Effects that may be seen in birds prior to death are lethargy, reduced reaction to external 

stimuli, depression, lower limb weakness, wing droop, prostrate posture, loss of righting 

reflex, and a ruffled appearance. Other effects may include a drop in body weight and

food consumption; however, in laboratory studies, the birds began to recover several days 

after the acute exposure had taken place.

The proposed 2,4-D choline salt formulations for registration are reported to maintain 

some properties that may reduce spray drift to non-target areas. While EFED has 

received some information regarding these properties, the full suite of data necessary to 

analyze these properties is not available. Given that these data and other potential 

mitigation options are not finalized, EFED has determined that insufficient information is 

available to make an effects call for direct (acute only) and indirect effects to birds, 

reptiles, and terrestrial-phase amphibians.

Terrestrial insects were not identified to be at risk from the proposed new uses of 2,4-D 

choline salt, based on honeybee toxicity data. 

5.2.5 Risks to Terrestrial Plants

All terrestrial plant risk quotients exceeded the LOC; risk was a result of spray drift and 

runoff. Thus, the risk quotient analysis indicates that direct effects are predicted from the 

proposed new uses of 2,4-D choline salt to terrestrial plants. Incident data support this 

conclusion as the majority of incidents reported to the agency were for plants (~460). 

2,4-D choline salt is an herbicide, thus it is not unexpected that even small amounts of 

spray drift or runoff could cause adverse effects in non-target species. It should be noted 

that none of these incidents were specifically for 2,4-D choline salt as the initial 

registration for this chemical is relatively new; all the incidents were all for other older 

forms of 2,4-D.

A further uncertainty is attached to the 2,4-D choline salt/glyphosate product. The 

product contains two herbicides with different modes of action; however, no terrestrial 

plant data for this formulation are available. It is unknown if such a formulation will 

have additive or synergistic effects (increased toxicity) or interference effects (decreased

toxicity) on terrestrial plants. Taking a conservative approach, it is possible that the 2,4-

D choline salt/glyphosate formulation will be more toxic to plants than the single active 

ingredient products.

The risk quotient analysis predicted indirect effects for terrestrial plants. Plants may 

depend on other plants to modify the environment so that they can grow (e.g., succession 

in a field). Plants may also rely on birds and mammals for pollinator and seed dispersal 

services; the risk quotient analysis suggests that birds and mammals may be at direct risk 

from the proposed 2,4-D choline salt new uses. 
63

Based on the risk quotient analysis, dicots were more sensitive to 2,4-D choline salt than 

monocots. The spray drift analysis, using the AIXR 11004 nozzle and the GF2726 

formulation, indicates that buffers of 62 feet (non-listed) and 202 feet (listed) is sufficient 

to protect dicots from exposures of 2,4-D choline salt that exceed the Agency’s levels of 

concern. 

In addition to death, effects seen in plants may include decreased stem growth, decreased 

weight, necrosis, leaf and stem curl, wilting, and adventitious root growth.

Potential effects from the volatilization of 2,4-D choline salt after application to fields 

were also considered. A plant vapor study was submitted to evaluate the effects that 2,4-

D vapor from a treated field may have on neighboring non-target species (MRID 

48911801). The study did not use controls and presented qualitative observations on the 

percent of visual plant damage (necrosis, cupping of leaves at margins, epinasty, twising 

of foliage, swelling or increased growth on the main stem at petiole attachment) for 

grapes, cotton, tomato, and soybeans. Grape was the most sensitive species followed by 

cotton, tomato, and then soybean. Plant damage endpoints are not normally used 

quantitatively in risk assessments and their sensivity, compared with growth/weight 

endpoints, is unknown.

In addition to the laboratory study, the registrant submitted several field studies that 

related plant damage in grape, cotton, and soybean to growth or yield endpoints (Table 

31). The studies were all considered scientifically sound and appropriate for qualitative 

incorporation into a risk assessment. All were field studies and there was variation in the 

methodology and timing of measurements. Crop yields were measured at the end of the 

growing season or harvest. The height endpoint was measured at 14 and 28 days after 

treatment, but only the 14 day value is presented here as this corresponds to the 

vegetative vigor guideline study. In addition to variation in field sites (location, year, 

month), some fields also received applications of pesticides (not herbicides) to control 

insects and other pests. These variables introduce some uncertainty into the studies; 

however, they do simulate real-life field conditions. Consequently, the results of the 

studies are considered appropriate for risk characterization.

Table 31. Percent of Plant Damage Producing Significant Effects in Growth/Yield 

for Grape, Cotton, and Soybean

Species Percent 

Damage

Endpoint Citation

Grape 20% Growth/Yield Ogg et al. 1991

Cotton 58% Yield Marple et al. 

2008

Cotton 62-66% Yield Everitt and 

Keeling 2009

Soybean 35% Yield Robinson et al. 

unpublished

Soybean 35% Yield Andersen et al. 

2004
64

Soybean 52% Height/Yield Kelley et al. 

2005

 

Information regarding the effects of vapor phase 2,4-D on grapes and cotton was also 

contained within the vapor-flux study (MRID 48862902). In this study, fields were 

treated with a 2,4-D choline salt formulation at an application rate of 5 lb ae/A. Plants 

were placed near a series of air monitoring stations approximately 1 hour after the 

herbicide was applied; this ensured that all of the effects were the result of vapor-phase 

exposures rather than spray drift. Air monitoring stations were set at 5 and 15 m beyond 

the field’s edge and several sets were located along each side of the field to capture air 

concentrations that differed as a result of wind direction. Plants were also placed directly 

on the field; no air monitoring station was associated with these plants. Plants were left 

in the field for 3 days after treatment and then removed to a greenhouse where they were 

observed for 27 days. Phytotoxic effects (qualitative) were scored on a scale of 0 (no 

damage) to 100% (dead plant) and compared with controls. The only plants to show 

outward signs of damage were the grape (0.6% damage) and cotton (40% damage) plants 

located directly on the field. This study suggests that at 5 m from the edge of the field, 

plants do not sustain damage from the revolatilization of 2,4-D choline salt on the treated 

field.

Considering the results from the plant damage studies, the vapor-flux study, and the 

laboratory vapor-phase study, a conservative approach was taken in selecting endpoints 

to characterize risk from vapor-phase transport. At 20% physical damage, the grape had 

the lowest damage rate that resulted in statistically significant reductions in yield/growth. 

Grape was also the most sensitive species tested in the laboratory vapor-phase study. 

Using the percent of visual injury for grape, an endpoint of 1.9 μg ae/m3 per hour was 

visually identified as the concentration that produced 20% physical damage. 

The PERFUM model was used to estimate the off-field distances for various 

concentrations of 2,4-D as predicted from the vapor flux data (see Section 3.2.4 for more 

detail). Plant endpoints were selected based on three possible exposure periods: 1 hour, 

4 hours, and 8 hours. These durations were selected based on the data in the vapor-phase 

study (see above). To achieve a concentration of 1.9 μg ai/m3 per hour in one hour, the 

atmospheric concentration of 2,4-D would need to be 1.9 μg ai/m3 per hour. To achieve 

the same concentration over a 4 hour period, the concentration of 2,4-D would need to be 

0.475 μg ai/m3 per hour (1.9 / 4 hours). Likewise, the air concentration for an 8 hour 

exposure period is 0.238 μg ai/m3 per hour (1.9 / 8 hours). The following graph (Figure 

4) shows the vapor phase curves for the three different exposure durations. Horizontal 

lines mark the endpoint (1 hour = 1.9; 4 hours = 0.475; and 8 hours = 0.238 μg ai/m3 per 

hour). The model predicts no adverse damage to plants off-field for any of the exposure 

scenarios.
65

Figure 4. Vapor Phase Curves for Various Exposure Durations of 2,4-D

The risk quotient analysis indicates there may be risks to terrestrial plants from runoff 

and spray drift; however, the proposed 2,4-D choline salt formulations for registration are 

reported to maintain some properties that may reduce spray drift to non-target areas. 

While EFED has received some information regarding these properties, the full suite of 

data necessary to analyze these properties is not available. Given that these data and 

other potential mitigation options are not finalized, EFED has determined that insufficient 

information is available to make an effects call for direct and indirect effects to terrestrial 

plants.

5.2.6 Risks to Aquatic Plants

Aquatic vascular and non-vascular plants yielded risk quotients below the LOC for 2,4-D. 

Risks from 2,4-DCP were dismissed as a preliminary scan of the open literature indicates 

it is less toxic to vascular and non-vascular plants than 2,4-D. Based on the risk quotient 

analysis, direct risks to aquatic plants are not expected. 

The proposed 2,4-D choline salt formulations for registration are reported to maintain 

some properties that may reduce spray drift to non-target areas. While EFED has

received some information regarding these properties, the full suite of data necessary to 

analyze these properties is not available. Given that these data and other potential 
66

mitigation options are not finalized, EFED has determined that insufficient information is 

available to make an effects call for indirect effects to aquatic plants.

5.3 Listed Species Assessment

The screening-level analysis for 2,4-D choline salt indicated that there was insufficient 

information to determine if there were direct effects to mammals (acute and chronic); 

birds, reptiles, and land-phase amphibians (acute); and terrestrial plants. Indirect effects 

are determined by assessing the potential for reduction of prey base or habitat 

modification of listed taxa; however, there was insufficient information to determine if 

there were indirect effects to any taxa (Table 32). 

Table 32. Listed Species Risks Associated with the Proposed New Uses for 2,4-D 

Choline Salt

Listed Taxa Direct Effects Indirect Effects1

Terrestrial and semiaquatic plants: 

Monocots

Dicots

Insufficient information

Insufficient information

Insufficient information

Insufficient information

Birds Insufficient information – Acute 

No – Chronic Insufficient information

Terrestrial-phase 

amphibians2

Insufficient information – Acute 

No – Chronic Insufficient information

Reptiles2 Insufficient information – Acute 

No – Chronic Insufficient information

Mammals Insufficient information – Acute 

Insufficient information – Chronic Insufficient information

Aquatic plants:

Vascular

Non-vascular

No

No

Insufficient information

Insufficient information

Freshwater fish No – Acute

No – Chronic Insufficient information

Aquatic-phase 

amphibians3

No – Acute

No – Chronic Insufficient information

Freshwater 

invertebrates

No – Acute

No – Chronic Insufficient information

Mollusks No – Acute

No – Chronic Insufficient information

Marine/estuarine fish No – Acute

No – Chronic Insufficient information
67

Listed Taxa Direct Effects Indirect Effects1

Marine/estuarine 

invertebrates

No – Acute

No – Chronic Insufficient information

Terrestrial invertebrates No Insufficient information

1

Indirect effects to a listed species occur when its prey base is reduced or habitat is 

modified; indirect effects are assumed for all taxa based on potential effects to 

terrestrial plants and obligate relationships with birds and/or mammals.

2

Birds are surrogates for reptiles and terrestrial-phase amphibians.

3

Freshwater fish are surrogates for aquatic-phase amphibians.

5.3.1 Data Gaps and Uncertainties

Avian Acute Oral Toxicity Test for Passerines (850.2100): Data are required for one 

passerine species when a chemical is intended for outdoor use. The current method of 

calculating a weight-adjusted LD50 using bobwhite quail or mallard duck data may overor under-estimate risks to passerines because these birds may metabolize the chemical 

differently. A protocol for the study should be submitted to EPA for approval prior to 

study initiation. This study has also been identified as a gap in the 2,4-D Problem 

Formulation (USEPA 2012, D402410).

Estuarine/Marine Invertebrate Chronic Toxicity Test (850.1350): No acceptable data 

are available for the chronic toxicity of 2,4-D choline salt to marine/estuarine

invertebrates. Currently, the toxicological effects for estuarine/marine invertebrates on a 

chronic basis remain unknown for 2,4-D choline salt, although this assessment estimated 

a chronic value based on an acute-to-chronic ratio using freshwater invertebrate data. 

The 2,4-D Problem Formulation identifies this as a gap, but concludes that chronic 

effects are unlikely, given the degradation rate of 2,4-D acid in water (USEPA 2012, 

D402410). The acute-to-chronic ratio method was considered valid and protective in lieu 

of actual data.

Terrestrial Plant Seedling Emergence and Vegetative Vigor Tests (850.4100, 

850.4150): Typical end-use data are required for terrestrial plants. In the case of the new 

2,4-D choline salt registrations, no information is available for the two 2,4-D choline salt 

formulations, nor the EnlistTM formulation, which is a mixture with glyphosate. For the 

2,4-D choline salt-glyphosate formulation, it is anticipated that there could be additive, 

synergistic, or interference between the two herbicides, which could cause increased or 

decreased toxicological effects than are predicted by single ai data. In lieu of these data, 

toxicity information from other 2,4-D formulations was used as a surrogate; however 

these surrogates did not contain glyphosate. This represents a point of uncertainty 

regarding the effects to terrestrial plants in the current analysis.

Terrestrial Plant Vegetative Vigor Test with Vapor Phase Exposure (non-guideline): 

2,4-D is known to volatilize from the field and drift off site under certain environmental 

conditions. EFED evaluated a vapor-phase study on grapes, cotton, soybean, and tomato; 
68

however, the methodology did not include a control, did not measure growth/weight 

parameters, and was not well-aligned with the 850.4150 protocol. As such, the results 

from the current vapor-phase study are of very limited value; however, they represent the 

best information available at the time the risk assessment was prepared. Thus, the data 

were used in the risk description section to characterize potential risk from 2,4-D vapor 

coming off of fields. A vapor-phase study with vegetative vigor endpoints is being 

recommended to further characterize the risk to plants from this exposure route. 

Additionally, the following uncertainties were identified for this risk assessment:

• For terrestrial organisms, only dietary exposure to 2,4-D choline salt was 

assessed.

• The studies with vapor-phase data yielded opposite conclusions regarding the 

potential for effects to terrestrial plants. Neither study measured quantitative 

endpoints and the laboratory study did not use a control. There remains 

uncertainty regarding the effects of vapor-phase exposures on non-target plants.

 

• For freshwater fish, birds, and terrestrial insects, definitive acute toxicity data 

were not available. Acute data were presented as greater than values, making it 

possible to conservatively compare the toxicity value directly to the EECs to 

estimate potential risk. 

• For terrestrial plants, vegetative vigor toxicity values for monocots were nondefinitive. The EC25 (a greater than value) was compared directly to the EECs to 

assess the likelihood of risk. The monocot vegetative vigor NOAEC was a less 

than value; however, given that in dicots, seedling emergence data were more 

sensitive than the vegetative vigor data, the same pattern was assumed to hold true 

for monocots. Consequently, the monocot seedling emergence NOAEC was 

assumed to be the most sensitive monocot NOAEC and used to calculate risk 

quotients. If this assumption was false, then the analysis may have underestimated the risk to listed monocots.

• In the absence of chronic data for marine/estuarine invertebrates, an acute-tochronic ratio was calculated to estimate the NOAEC for Eastern oyster.

• This risk assessment only considered the most sensitive of the species evaluated 

in the registrant-submitted studies. The position of the tested species relative to 

the distribution of all species’ sensitivities to 2,4-D choline salt is unknown. 

Extrapolating the risk conclusions from the most sensitive tested species to nontested species may either underestimate or overestimate the potential risks to 

those species. 

• 2,4-D is currently undergoing a Tier I Endocrine Disruptor Screening as required 

by FFDCA section 408(p). The results of the screening analysis are not yet 

available.
69

6. References 

Andersen, S.M., S.A. Clay, L.J. Wrage, and D. Matthees. 2004. Soybean foliage 

residues of dicamba and 2,4-D and correlation to application rates and yield. 

Agronomy Journal, 96:750-760.

Anderson, A.M, Byrtus, G., Thompson J., Humphries, D., Hill, B., and Bilyk, M., 2002. 

Baseline Pesticide Data for Semi-Permanent Wetlands in the Aspen Parkland of 

Alberta. Albeta Environment, Publication No. T/673.

Everitt, J.D. and J.W. Keeling. 2009. Cotton growth and yield response to simulated 

2,4-D and dicamba drift. Weed Technology, 23:503-506.

Fletcher, J.S., J.E. Nellsen, T.G. Pfleeger. 1994. Literature review and evaluation of the 

EPA food-chain (Kenaga) nomogram, an instrument for estimating pesticide 

residues on plants. Env. Toxicol. Chem. 13:1381-1391.

Hoerger, F. and E. E. Kenaga. 1972. Pesticide residues on plants: correlation of 

representative data as a basis for estimation of their magnitude in the 

environment. in: F. Coulston and F. Korte (editors), Environmental Quality and 

Safety: Chemistry, Toxicology, and Technology. Vol I. Georg Thieme Publishers, 

Stuttgart, West Germany, pp. 9-28.

Kelley, K.B., L.M. Wax, A.G. Hager, and D.E. Riechers. 2005. Soybean response to 

plant growth regulator herbicides is affected by other postemergence herbicides. 

Weed Science, 53:101-112.

Majewski , M.S. and P.D. Capel. 1995. Pesticides in the atmosphere: distribution, trends, 

and governing factors. Ann Arbor Press, Inc. Chelsea, MI.

Marple, M.E., K. Al-Khatib, and D.E. Peterson. 2008. Cotton injury and yield as 

affected by simulated drift of 2,4-D and dicamba. Weed Technology, 22:609-614.

Ogg, A.G. Jr., M.A. Ahmedullah, and G.M. Wright. 1991. Influence of repeated 

applications of 2,4-D on yield and juice quality of concord grapes (Vitis 

labruscana). Weed Science, 39(2):284-295.

Pfleeger, T.G., A. Fong, R. Hayes, H. Ratsch and C. Wickliff. 1996. Field evaluation of 

the EPA (Kenaga) nomogram, a method for estimating wildlife exposure to 

pesticide residues on plants. Env. Toxicol. Chem. 15:535-543, 1996. U.S. 

Environmental Protection Agency. 1998. Guidelines for Ecological Risk 

Assessment. Risk Assessment Forum, Office of Research and Development, 

Washington, D.C. EPA/630/R-95/002F. April 1998. 

Robinson, A.P., V.M. Davis, D.M. Simpson, and W.G. Johnson. In review. Response of 

soybean yield components to 2,4-D. Weed Science.
70

U.S. Environmental Protection Agency. 2003. 2,4-D: Health Effects Division (HED) 

Metabolism Assessment Review Committee (MARC) decision document. DP 

Barcodes D293119 and D293128. Chemical ID Number 030001. Case Number 

0073.

US EPA. 2004. Revised 2,4-D Drinking Water Assessment for the Health Effects 

Division (HED) Reregistration Eligibility Decision Document. (DP 

286666).

U.S. Environmental Protection Agency. 2004. Overview of the Ecological Risk 

Assessment Process in the Office of Pesticide Programs, U.S. Environmental 

Protection Agency. Listed and Threatened Species Effects Determinations. 

Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide 

Programs, Washington, D.C. January 23, 2004.

U.S. Environmental Protection Agency. 2005a. Generic Format and Guidance for the 

Level I Screening Ecological Risk Assessments Conducted in the Environmental 

Fate and Effects Division. Office of Pesticide Programs, Washington, D.C. 

January 24, 2005. 

U.S. Environmental Protection Agency. 2005b. Reregistration eligibility decision for 

2,4-D. Office of Prevention, Pesticides, and Toxic Substances, (7508C) EPA 

738-R-05-002. June 2005.

U.S. Environmental Protection Agency. 2005c. Revised screening level ecological risk

assessment of polychlorinated dibenzodioxins (PCDDs) and polychlorinated 

dibenzofurans (PCDFs) contaminants from technical products of 2,4-

dichlorophenoxyacetic acid (2,4-D). Confidential memo.

U.S. Environmental Protection Agency. 2011. AERSCREEN User’s Guide (EPA-454/B-11-

001). Office of Air Quality Planning and Standards, Air Quality Assessment 

Environmental Fate and Effects Division, Office of Pesticide Programs, Research 

Triangle Park, North Carolina.

U.S. Environmental Protection Agency. 2012. EFED Registration review problem 

formulation for 2,4-D. Environmental Fate and Effects Division, Office of 

Pesticide Programs, Washington, D.C.

van den B. M. Birnbaum, L.S., Denison, M, De vito, M, Farland, W, Feeley, M, Fiedler, 

H, Hakansson, H., Hanberg,A., Laurie Haws, L., h Martin Rose, M Stephen Safe, 

S., Dieter Schrenk, D., Tohyama, C, Angelika Tritscher,A., Tuomisto, J., 

Tysklind, M., Nigel Walker, N., and Peterson, R.E. 2006. Review - The 2005 

World Health Organization Reevaluation of Human and Mammalian Toxic 

Equivalency Factors for Dioxins and Dioxin-Like Compounds. Toxicological Sci. 

93: 223-241.
71

Appendix A. Registrations and Uses for 2,4-D Choline Salt

Use

Max. Single 

App. Rate

(lbs ae/A)

Max. No. of 

App. per Year

Min. Reapp. 

Interval (d)

Max. Annual 

App. Rate

(lbs ae/A)

App. Method

Cereal grains 1.25

1 postemergence

1 preharvest

NS A 1.75

Ground or 

aerial spray

Broadcast

Field corn and 

pop corn

1 (preplant or 

preemergence)

0.5 

(postemergence)

1.5 (preharvest)

(3 total)

1 application 

per crop stage NS 3

Ground or 

aerial spray

Broadcast

Directed band

Sweet corn

1 (preplant or 

preemergence)

0.5 (post 

emergence)

(1.5 total)

1 application 

per crop stage 21 1.5

Ground or 

aerial spray

Broadcast

Directed band

Grain or forage 

sorghum

1 (amines, acid, 

salts)

0.5 (esters)

(1.5 total)

1 NRB

1 (amines, 

acid, salts)

0.5 (esters)

Ground or 

aerial spray

Broadcast

Directed band

Soybeans 1 1 NR 1

Ground or 

aerial spray

Broadcast

Sugarcane

2 

(preemergence)

2 

(postemergence)

(4 total)

1 application 

per crop stage NS 4

Ground or 

aerial spray

Broadcast or 

band

Rice

1 (preplant)

1.5 

(postemergence)

(2.5 total)

1 application 

per crop stage NS 1.5

Ground or 

aerial spray

Broadcast or 

spot treatment

Pome fruits 2 2 75 4

Broadcast 

ground or spot 

treatment

Stone fruits 2 2 75 4

Broadcast 

ground or spot 

treatment

Nut orchards, 

pistachios 2 2 30 4

Broadcast 

ground or spot 

treatment

Established 

grass pastures, 

rangeland, and 

perennial 

grasslands not 

in agricultural 

2 2 30 4

Ground or 

aerial spray

Broadcast or 

spot
72

Appendix A. Registrations and Uses for 2,4-D Choline Salt

Use

Max. Single 

App. Rate

(lbs ae/A)

Max. No. of 

App. per Year

Min. Reapp. 

Interval (d)

Max. Annual 

App. Rate

(lbs ae/A)

App. Method

production

Ornamental 

turf 1.5 2 NS 3

Broadcast 

ground or spot 

treatment

Grass (turf) 

grown for seed 

and sod

2 2 21 4

Broadcast 

ground or spot 

treatment

Fallow land 

and crop 

stubble

2 2 30 4

Ground or 

aerial spray

Broadcast

Forest site 

preparation, 

forest 

roadsides, 

brush control, 

established 

conifer release

4 1 NR 4

Ground or 

aerial spray

Broadcast

Tree and brush 

control

8 lb ai/100 gal 

(basal spray, cut 

surface –

stumps, frill)

2 ml of 4 lb 

ae/injection site 

(injection)

1 NR 4 (broadcast 

only)

Ground or 

aerial spray

Broadcast, 

spot, basal, 

frill, cut stump 

or injection

Non-cropland 4 2 30 4

Ground or 

aerial spray

Broadcast or 

spot

Aquatic weed 

control 

(irrigation 

ditch bank 

application)

2 2 30 (broadcast 

only) 4

Boat or aerial 

spray

Broadcast or 

spot

Aquatic weed 

control 

(surface 

application for 

floating and 

emergent 

weeds)

4 2 21 (broadcast 

only) 8

Boat or aerial 

spray

Broadcast or 

spot

Aquatic weed 

control 

(surface 

application or 

subsurface 

injection for 

submersed 

weeds)

4 ppm ae/A 

(10.8 lb ae/Acre 

foot)

2 21 10.8 lb ae/Acre 

foot

Boat or aerial 

spray

Broadcast or 

spot

A

 NS means “not stated.” B

 NR means “not relevant.”
73

Appendix B. Major and Minor Degradates Identified in Environmental Fate Studies 

Chemical Name

(CAS No.)

Molecular 

Formula

Molecular wt.: 

g/mole

Chemical Structure Maximum 

Formed

2,4-Dichlorophenol 

[2,4-DCP]

(120-83-2)

C6H4Cl2O

163.0

32.6 % of applied in 

Anaerobic aquatic 

study

Chlorohydroquinone 

[CHQ]

(615-67-8)

C6H3(OH)2Cl

144.56

16.0 % of applied in 

aerobic aquatic study

1,2,4-benezenetriol

(533-73-3)

C6H6O3

126.11

37.0% formed of 

applied in aquatic 

photo-degradation 

study

4-chlorophenol 

(106-48-9)

C6H5ClO

128.56

<2.0% formed of 

applied in anaerobic 

aquatic metabolism

study [Intermediate 

degradate]

2,4-dichloroanisol 

[2,4-DCA]

(553-82-2)

C7H6Cl2O

177.03

<2.0% formed of 

applied in aerobic 

soil metabolism 

study

4-

Chlorophenoxyacetic 

acid [4-CPA]

(122-88-3)

C8H7ClO

186.59

<2.0% formed of 

applied in anaerobic 

aquatic metabolism 

study
74

Appendix C. SIP and STIR Results for 2,4-D Choline Salt

SIP

Table 1. Inputs

Parameter Value

Chemical name 2,4-D choline salt

Solubility (in water at 25oC; mg/L) 569

Mammalian LD50 (mg/kg-bw) 441

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian 

species

Mammalian NOAEL (mg/kg-bw) 5

Mammalian test species laboratory rat

Body weight (g) of "other" mammalian 

species

Avian LD50 (mg/kg-bw) 218.7

Avian test species northern bobwhite quail

Body weight (g) of "other" avian species

Mineau scaling factor 1.15

Mallard NOAEC (mg/kg-diet)

Bobwhite quail NOAEC (mg/kg-diet) 962

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) 97.8680 97.8680

Adjusted toxicity value (mg/kg-bw) 339.1998 3.8458

Ratio of exposure to toxicity 0.2885 25.4480

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
75

Parameter Acute Chronic

Upper bound exposure (mg/kg-bw) 460.8900 460.8900

Adjusted toxicity value (mg/kg-bw) 157.5579 102.2590

Ratio of exposure to acute toxicity 2.9252 4.5071

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 nontarget animals.

STIR

Input

Application and Chemical Information

Enter Chemical Name 2,4-D Choline salt

Enter Chemical Use herbicide

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

If Spray What Type (enter ground or air) air

Enter Chemical Molecular Weight (g/mole) 324.7

Enter Chemical Vapor Pressure (mmHg) 1.40E-07

Enter Application Rate (lb a.i./acre) 1

Toxicity Properties

Bird

Enter Lowest Bird Oral LD50 (mg/kg bw) 218.7

Enter Mineau Scaling Factor 1.15

Enter Tested Bird Weight (kg) 0.178

Mammal

Enter Lowest Rat Oral LD50 (mg/kg bw) 441

Enter Lowest Rat Inhalation LC50 (mg/L) 0.78

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) 2.45E-03

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 3.08E-04

Adjusted Inhalation LD50 2.16E+00

Ratio of Vapor Dose to Adjusted Inhalation LD50 1.43E-04

Exposure not 

Likely 

Significant

Maximum Post-treatment Spray Inhalation Dose 9.61E-02
76

(mg/kg)

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 4.45E-02

Exposure not 

Likely 

Significant

Results Mammalian (0.015 kg )

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 2.45E-03

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 3.87E-04

Adjusted Inhalation LD50 4.64E+01

Ratio of Vapor Dose to Adjusted Inhalation LD50 8.32E-06

Exposure not 

Likely 

Significant

Maximum Post-treatment Spray Inhalation Dose 

(mg/kg) 1.21E-01

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 2.60E-03

Exposure not 

Likely 

Significant
77

Appendix D. Examples of PRZM/EXAMS Outputs for 2,4-D and 2,4-

DCP 

2,4-D

stored as MSCornAT1.out

Chemical: 2 4 D choline

PRZM environment: MScornSTD.txt; modified Tueday, 29 May 2007 at 13:57:40

EXAMS environment: pond298.exv;modified Tueday, 26 August 2008 at 06:14:08

Metfile: w03940.dvf;modified Tueday, 26 August 2008 at 06:14:14

Water segment concentrations (ppb)

Year Peak 96 hr 21 Day 60 Day 90 Day Yearly

1961 16.9 15.9 14.0 10.3 8.1 2.3

1962 48.0 46.1 37.3 24.1 18.4 5.0

1963 4.9 4.6 4.1 2.8 2.2 0.6

1964 30.5 28.1 23.6 16.4 12.1 3.2

1965 38.6 37.0 28.9 17.8 13.1 3.4

1966 10.6 9.8 7.4 4.7 3.6 1.0

1967 12.7 11.8 9.4 6.3 4.8 1.3

1968 13.4 12.9 10.9 8.7 6.6 1.8

1969 13.4 12.5 10.6 6.8 5.0 1.4

1970 6.0 5.6 4.3 3.5 2.7 0.7

1971 49.3 46.2 36.9 22.7 17.3 4.8

1972 5.4 5.1 4.0 2.8 2.3 0.6

1973 29.1 27.7 23.5 14.7 11.0 3.0

1974 38.4 35.7 31.8 20.1 14.8 4.0

1975 9.7 9.1 7.3 5.4 4.4 1.2

1976 33.8 31.6 25.1 17.7 13.5 3.7

1977 15.8 14.8 13.4 10.1 7.5 2.0

1978 7.3 6.9 6.2 4.8 3.7 1.0

1979 93.4 88.6 75.4 50.6 38.9 10.8

1980 98.3 94.1 83.9 61.1 48.9 13.6

1981 10.8 10.1 8.8 5.9 4.4 1.2

1982 10.4 9.7 8.6 5.5 4.2 1.2

1983 58.9 54.9 42.1 26.7 20.1 5.4

1984 9.8 9.2 8.3 6.7 5.1 1.4

1985 8.9 8.5 6.8 5.5 4.2 1.1

1986 3.3 3.1 2.7 1.9 1.5 0.4

1987 4.1 3.8 3.2 2.7 2.2 0.6

1988 38.5 36.2 28.4 22.4 17.6 4.8

1989 18.4 17.2 14.5 11.7 9.2 2.5

1990 11.7 11.0 8.5 5.2 4.0 1.1

Sorted results
78

Prob. Peak 96 hr 21 Day 60 Day 90 Day Yearly

0.03 98.27 94.09 83.86 61.11 48.91 13.55

0.06 93.43 88.55 75.44 50.64 38.87 10.83

0.10 58.93 54.93 42.07 26.65 20.08 5.38

0.13 49.28 46.21 37.29 24.05 18.36 4.98

0.16 48.03 46.11 36.94 22.69 17.61 4.85

0.19 38.64 36.99 31.77 22.35 17.25 4.77

0.23 38.50 36.23 28.93 20.14 14.81 3.97

0.26 38.40 35.70 28.36 17.80 13.50 3.66

0.29 33.75 31.55 25.10 17.66 13.05 3.44

0.32 30.52 28.12 23.59 16.39 12.14 3.19

0.35 29.13 27.74 23.51 14.69 10.99 2.98

0.39 18.37 17.22 14.52 11.67 9.16 2.51

0.42 16.85 15.89 14.01 10.32 8.08 2.27

0.45 15.81 14.79 13.43 10.05 7.52 2.00

0.48 13.42 12.86 10.90 8.68 6.57 1.76

0.52 13.36 12.52 10.58 6.79 5.12 1.39

0.55 12.67 11.77 9.35 6.70 5.02 1.35

0.58 11.67 10.95 8.82 6.33 4.83 1.33

0.61 10.75 10.13 8.65 5.92 4.44 1.24

0.65 10.57 9.76 8.50 5.53 4.42 1.22

0.68 10.36 9.66 8.31 5.49 4.25 1.18

0.71 9.83 9.20 7.40 5.41 4.23 1.14

0.74 9.67 9.14 7.28 5.22 4.02 1.13

0.77 8.86 8.46 6.79 4.82 3.67 1.01

0.81 7.34 6.90 6.23 4.73 3.56 0.99

0.84 5.97 5.58 4.34 3.47 2.66 0.74

0.87 5.38 5.05 4.06 2.84 2.26 0.65

0.90 4.95 4.62 4.02 2.83 2.20 0.62

0.94 4.07 3.83 3.19 2.66 2.18 0.60

0.97 3.28 3.08 2.75 1.94 1.53 0.42

0.10 57.97 54.06 41.59 26.39 19.91 5.34

Average 

of yearly 

averages

:

2.84

Inputs generated by pe5.pl - Novemeber 2006

Data used for this run:

Output File: MSCornAT1

Metfile: w03940.dvf

PRZM 

scenario:

MScornSTD.txt
79

EXAMS 

environ

ment 

file:

pond298.exv

Chemica

l Name:

2 4 D choline

Descripti

on

Variable 

Name

Value Units Comments

Molecul

ar 

weight

mwt 221.4 g/mol

Henry's 

Law 

Const.

henry 8.56E-06 atm-m^3/mol

Vapor 

Pressure

vapr 1.40E-07 torr

Solubilit

y

sol 569 mg/L

Kd Kd mg/L

Koc Koc 61.7 mg/L

Photolys

is halflife

kdp 13 days Half-life

Aerobic 

Aquatic 

Metaboli

sm

kbacw 45 days Halfife

Anaerob

ic 

Aquatic 

Metaboli

sm

kbacs 321 days Halfife

Aerobic 

Soil 

Metaboli

sm

asm 6.9 days Halfife

Hydroly

sis:

pH 7 0 days Half-life

Method: CAM 2 integer See PRZM manual

Incorpor

ation 

Depth:

DEPI cm

Applicat

ion Rate:

TAPP 1.12 kg/ha

Applicat

ion 

Efficienc

y:

APPEFF 0.99 fraction

Spray 

Drift

DRFT 0.017 fraction of application rate applied to pond
80

Applicat

ion Date

Date 29-03 dd/mm or dd/mmm or dd-mm or dd-mmm

Interval 

1

interval 12 days Set to 0 or delete line for single app.

app. rate 

1

apprate kg/ha

Interval 

2

interval 12 days Set to 0 or delete line for single app.

app. rate 

2

apprate kg/ha

Record 

17:

FILTRA

IPSCND 1

UPTKF

Record 

18:

PLVKR

T

0.079

PLDKR

T

FEXTR

C

0.5

Flag for 

Index 

Res. Run

IR EPA 

Pond

Flag for 

runoff 

calc.

RUNOF

F

none none, monthly or total(average of entire run)

Pre Process 2,4-DCP PRZM/EXAMS Outputs for Terrestrial and Spray Drift 

date Terrestrial EEC Spray Drift 

EEC

Percent Terr Percent Spray 

Drift

% Treated Adjusted EEC

1/1/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/2/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/3/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/4/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/5/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/6/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/7/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/8/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/9/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/10/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/11/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/12/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/13/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/14/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/15/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

1/16/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00
81

1/17/1961 0.00E+00 0.00E+00 1.00E+00 1.00E+00 1.00E+00 0.00E+00

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

12/27/1990 7.18E-05 2.26E-04 1.00E+00 1.00E+00 1.00E+00 2.98E-04

12/28/1990 7.12E-05 2.24E-04 1.00E+00 1.00E+00 1.00E+00 2.95E-04

12/29/1990 7.06E-05 2.22E-04 1.00E+00 1.00E+00 1.00E+00 2.93E-04

12/30/1990 7.00E-05 2.20E-04 1.00E+00 1.00E+00 1.00E+00 2.90E-04

12/31/1990 6.94E-05 2.19E-04 1.00E+00 1.00E+00 1.00E+00 2.88E-04

1/1/1991 6.89E-05 2.17E-04 1.00E+00 1.00E+00 1.00E+00 2.86E-04

Post Process 2,4-DCP PRZM/EXAMS Outputs for Terrestrial and Spray Drift

Outputs

Year Max Peak 4-day Max 14 day Max 21 day Max 30 day Max 60 day Max 90 day Annual

2.0E+03 3.7E-03 3.5E-03 3.1E-03 3.0E-03 2.8E-03 2.5E-03 2.1E-03 1.1E-03

2.0E+03 4.4E-03 4.3E-03 3.9E-03 3.7E-03 3.6E-03 3.0E-03 2.5E-03 1.1E-03

2.0E+03 3.7E-03 3.6E-03 3.1E-03 2.9E-03 2.8E-03 2.3E-03 2.0E-03 9.3E-04

2.0E+03 4.2E-03 4.0E-03 3.7E-03 3.5E-03 3.3E-03 2.7E-03 2.3E-03 9.3E-04

2.0E+03 3.7E-03 3.6E-03 3.1E-03 2.9E-03 2.8E-03 2.3E-03 2.0E-03 8.4E-04

2.0E+03 3.8E-03 3.6E-03 3.3E-03 3.0E-03 2.8E-03 2.5E-03 2.1E-03 8.7E-04

2.0E+03 3.5E-03 3.4E-03 3.2E-03 3.2E-03 3.0E-03 2.6E-03 2.2E-03 9.0E-04

2.0E+03 3.8E-03 3.6E-03 3.3E-03 3.1E-03 3.0E-03 2.5E-03 2.2E-03 8.8E-04

2.0E+03 4.4E-03 4.2E-03 3.7E-03 3.5E-03 3.3E-03 2.7E-03 2.2E-03 8.9E-04

2.0E+03 3.6E-03 3.4E-03 3.0E-03 2.8E-03 2.7E-03 2.3E-03 1.9E-03 9.4E-04

2.0E+03 4.6E-03 4.4E-03 4.0E-03 3.8E-03 3.6E-03 3.0E-03 2.5E-03 1.0E-03

2.0E+03 3.5E-03 3.4E-03 3.1E-03 2.9E-03 2.7E-03 2.4E-03 2.0E-03 9.0E-04

2.0E+03 4.5E-03 4.4E-03 3.9E-03 3.7E-03 3.5E-03 2.9E-03 2.4E-03 9.5E-04

2.0E+03 4.4E-03 4.2E-03 3.7E-03 3.5E-03 3.3E-03 2.7E-03 2.3E-03 9.1E-04

2.0E+03 3.6E-03 3.5E-03 3.3E-03 3.2E-03 3.0E-03 2.6E-03 2.2E-03 9.6E-04

2.0E+03 3.9E-03 3.8E-03 3.4E-03 3.2E-03 3.1E-03 2.8E-03 2.5E-03 1.1E-03

2.0E+03 4.2E-03 4.0E-03 3.6E-03 3.3E-03 3.2E-03 2.6E-03 2.2E-03 9.8E-04

2.0E+03 3.8E-03 3.6E-03 3.3E-03 3.2E-03 3.0E-03 2.6E-03 2.3E-03 1.2E-03

2.0E+03 5.6E-03 5.5E-03 5.0E-03 4.8E-03 4.7E-03 4.0E-03 3.4E-03 1.5E-03

2.0E+03 5.7E-03 5.6E-03 5.3E-03 5.0E-03 4.9E-03 4.2E-03 3.6E-03 1.5E-03

2.0E+03 3.8E-03 3.6E-03 3.2E-03 3.0E-03 2.9E-03 2.5E-03 2.2E-03 1.2E-03

2.0E+03 4.2E-03 4.0E-03 3.6E-03 3.3E-03 3.1E-03 2.5E-03 2.2E-03 1.1E-03

2.0E+03 5.0E-03 4.7E-03 4.2E-03 4.1E-03 3.9E-03 3.4E-03 2.9E-03 1.2E-03

2.0E+03 3.8E-03 3.6E-03 3.3E-03 3.1E-03 2.9E-03 2.5E-03 2.1E-03 9.5E-04

2.0E+03 3.6E-03 3.5E-03 3.1E-03 2.9E-03 2.8E-03 2.3E-03 1.9E-03 8.4E-04

2.0E+03 3.6E-03 3.5E-03 3.1E-03 2.8E-03 2.7E-03 2.4E-03 2.1E-03 9.0E-04

2.0E+03 3.7E-03 3.5E-03 3.2E-03 3.0E-03 2.9E-03 2.4E-03 2.1E-03 9.5E-04

2.0E+03 4.4E-03 4.2E-03 3.8E-03 3.6E-03 3.3E-03 2.8E-03 2.4E-03 1.0E-03

2.0E+03 3.8E-03 3.6E-03 3.4E-03 3.3E-03 3.2E-03 2.8E-03 2.4E-03 1.0E-03

2.0E+03 3.6E-03 3.5E-03 3.3E-03 3.1E-03 2.9E-03 2.6E-03 2.2E-03 1.1E-03

90th % (ppm) 0.00 0.00 0.00 0.00 0.00 0.00 0.00

90th % (ppb) 4.66 4.05 3.84 3.62 3.01 2.55 1.17
82

Appendix E. Avian and Mammalian Risk Quotient Calculations for 

2,4-D Choline Salt Based on T-REX 

Applications to herbicide-tolerant soybean and corn at 1 lb ae/A, 3 applications that 

are 12 days apart

Upper Bound Kenaga, Acute Avian Dose-Based Risk Quotients

Size 

Class

(grams)

Adjusted

LD50

EECs and RQs

Short Grass Tall Grass Broadleaf 

Plants Fruits/Pods/Seeds Arthropods Granivore

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

20 0.00 658.79 #DIV/0! 301.94 #DIV/0! 370.57 ### 41.17 #DIV/0! 258.03 ### 9.15 ###

100 0.00 375.67 #DIV/0! 172.18 #DIV/0! 211.31 ### 23.48 #DIV/0! 147.14 ### 5.22 ###

1000 0.00 168.19 #DIV/0! 77.09 #DIV/0! 94.61 ### 10.51 #DIV/0! 65.88 ### 2.34 ###

Upper Bound Kenaga, Subacute Avian Dietary Based Risk Quotients

LC5

0

EECs and RQs

Short Grass Tall Grass Broadleaf 

Plants

Fruits/Pods/Seed

s Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

0

578.4

4

#DIV/0

!

265.1

2

#DIV/0

!

325.3

7

#DIV/0

! 36.15 #####

226.5

6

#DIV/0

!

Size class not used for dietary risk quotients 

Upper Bound Kenaga, Chronic Avian Dietary Based Risk Quotients

NOAEC 

(ppm)

EECs and RQs

Short Grass Tall Grass Broadleaf 

Plants

Fruits/Pods/Seed

s Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

962 578.44

0.6

0 265.12

0.2

8 325.37 0.34 36.15 0.04 226.56

0.2

4

Size class not used for dietary risk quotients

Upper Bound Kenaga, Acute Mammalian Dose-Based Risk Quotients 

Size Adjusted EECs and RQs
83

Class

(grams)

LD50

Short Grass Tall Grass Broadleaf 

Plants

Fruits/Pods/

Seeds Arthropods Granivore

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

15 969.24 551.50 0.57 252.77 0.26 310.22 0.32 34.47 0.04 216.004 0.2229 7.6597 0.0079

35 784.22 381.16 0.49 174.70 0.22 214.40 0.27 23.82 0.03 149.288 0.1904 5.2939 0.0068

1000 339.20 88.37 0.26 40.50 0.12 49.71 0.15 5.52 0.02 34.6129 0.102 1.2274 0.0036

Upper Bound Kenaga, Acute Mammalian Dietary Based Risk Quotients

LC50 

(ppm)

EECs and RQs

Short Grass Tall Grass Broadleaf 

Plants

Fruits/Pods/See

ds Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

0

578.4

4

#DIV/0

!

265.1

2

#DIV/0

!

325.3

7

#DIV/0

! 36.15 #####

226.5

6

#DIV/0

!

Size class not used for dietary risk quotients

Upper Bound Kenaga, Chronic Mammalian Dietary Based Risk Quotients

NOAEC 

(ppm)

EECs and RQs

Short Grass Tall Grass Broadleaf 

Plants

Fruits/Pods/Seeds/Larg

e Insects Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

100

578.4

4

5.7

8

265.1

2

2.6

5

325.3

7

3.2

5 36.15 0.36

226.5

6

2.2

7

Size class not used for dietary risk quotients 

Upper Bound Kenaga, Chronic Mammalian Dose-Based Risk Quotients

Size 

Class

(grams)

Adjusted 

NOAEL

EECs and RQs

Short Grass Tall Grass Broadleaf 

Plants

Fruits/Pods/

Seeds Arthropods Granivore

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

15 10.99 551.50 50.19 252.77 23.00 310.22 28.23 34.47 3.14 216.00 19.66 7.66 0.70

35 8.89 381.16 42.87 174.70 19.65 214.40 24.11 23.82 2.68 149.29 16.79 5.29 0.60

1000 3.85 88.37 22.98 40.50 10.53 49.71 12.93 5.52 1.44 34.61 9.00 1.23 0.32
84

Appendix F. Terrestrial Plant Risk Quotient Calculations for 2,4-D 

Choline Salt Based on TerrPlant 

2,4-D choline salt application to herbicide-tolerant corn and soybean at 

1 lb ae/A

Table 1. Chemical Identity.

Parameter User Inputs

Chemical Name 2,4-D Choline Salt

PC code 52505

Use Herbicide-tolerant corn and soybean

Application Method foliar spray

Application Form liquid 

Solubility in Water 

(ppm) 569

Table 2. Input parameters used to derive EECs.

Input Parameter Symbol

Value (user 

inputs) Units

Application Rate A 1

Incorporation I 1 none

Runoff Fraction R 0.05 none

Drift Fraction D 0.01 none

Table 3. EECs for 2,4-D Choline Salt. Units in .

Description Equation EEC

Runoff to dry areas (A/I)*R 0.05

Runoff to semi-aquatic areas (A/I)*R*10 0.5

Spray drift A*D 0.01

Total for dry areas ((A/I)*R)+(A*D) 0.06

Total for semi-aquatic areas ((A/I)*R*10)+(A*D) 0.51

Table 4. Plant survival and growth data used for RQ derivation. Units are in . All values are user 

inputs

Seedling Emergence Vegetative Vigor

Plant type EC25 NOAEC EC25 NOAEC 

Monocot 0.01 0.005628 x x

Dicot 0.00081 0.00047 0.0021 0.00167

Table 5. RQ values for plants in dry and semi-aquatic areas exposed to 2,4-D Choline Salt through 

runoff and/or spray drift.*
85

Plant Type Listed Status Dry Semi-Aquatic Spray Drift

Monocot non-listed 6.00 51.00 1.00

Monocot listed 10.66 90.62 1.78

Dicot non-listed 74.07 629.63 12.35

Dicot listed 127.66 1085.11 21.28

*If RQ > 1.0, the LOC is exceeded, resulting in potential for risk to that plant group.
86

Appendix G. Risk Quotient Method and Levels of Concern

The Risk Quotient Method is the means by which the Environmental Fate and Effects 

Division (EFED) integrates the results of exposure and ecotoxicity data. In this method, 

both acute and chronic risk quotients are calculated by dividing exposure estimates by the 

most sensitive ecotoxicity values derived from the studies. Calculated risk quotients are 

then compared to OPP's levels of concern. The levels of concern are the criteria used by 

OPP to indicate potential risk to non-target organisms and the need to consider regulatory 

action. EFED has defined levels of concern for acute risk, potential restricted use, and 

for listed species. Risk presumptions, along with the corresponding risk quotients and 

levels of concern are summarized in the table below.

Table G-1. Levels of Concern for Assessed Taxa

Risk Presumption Risk Quotient Level of 

Concern

Birds

Acute Risk EEC/LC50 or LD50/sqft or LD50/day 0.5

Acute Restricted Use EEC/LC50 or LD50/sqft or LD50/day (or LD50 < 

50 mg/kg)

0.2

Acute Listed Species EEC/LC50 or LD50/sqft or LD50/day 0.1

Chronic Risk EEC/NOAEC 1

Mammals

Acute Risk EEC/LC50 or LD50/sqft or LD50/day 0.5

Acute Restricted Use EEC/LC50 or LD50/sqft or LD50/day (or LD50 < 

50 mg/kg)

0.2

Acute Listed Species EEC/LC50 or LD50/sqft or LD50/day 0.1

Chronic Risk EEC/NOAEC 1

Aquatic Animals

Acute Risk EEC/LC50 or EC50 0.5

Acute Restricted Use EEC/LC50 or EC50 0.1

Acute Listed Species EEC/LC50 or EC50 0.05

Chronic Risk EEC/NOAEC 1

Terrestrial and Semi-Aquatic Plants 

Acute Risk EEC/EC25 1

Acute Listed Species EEC/EC05 or NOAEC 1
87

Risk Presumption Risk Quotient Level of 

Concern

Aquatic Plants

Acute Risk EEC/EC50 1

Acute Listed Species EEC/EC05 or NOAEC 1
88

Appendix H: Preliminary Volatility Flux Data

Table X-1. 2,4-D volatility flux rate from soybean at Farmland, IN

Period1

Flux Rate (g/m2-s)

Unadjusted Flux Rate2 Adjusted Flux Rate3

EPA Registrant EPA Registrant

Field 1 (2,4 D choline)

1 2.20E-08 1.80E-10 5.50E-09 4.50E-11

2 2.90E-10 1.60E-11 7.25E-11 4.00E-12

3 8.30E-12 1.40E-11 2.08E-12 3.50E-12

4 5.80E-12 5.00E-12 1.45E-12 1.25E-12

5 5.10E-10 3.50E-11 1.28E-10 8.75E-12

6 4.80E-11 1.50E-10 1.20E-11 3.75E-11

7 5.90E-10 1.60E-10 1.48E-10 4.00E-11

8 6.00E-11 1.10E-10 1.50E-11 2.75E-11

Field 2 (2,4-D DMA)

1 6.00E-08 1.30E-08 3.00E-08 6.50E-09

2 1.60E-09 2.50E-11 8.00E-10 1.25E-11

3 2.20E-10 1.20E-10 1.10E-10 6.00E-11

4 8.00E-11 8.40E-11 4.00E-11 4.20E-11

5 1.20E-09 7.00E-10 6.00E-10 3.50E-10

6 1.50E-10 2.60E-10 7.50E-11 1.30E-10

7 3.40E-10 3.50E-10 1.70E-10 1.75E-10

8 1.40E-10 1.50E-10 7.00E-11 7.50E-11

Field 3 (2,4-D EHE)

1 7.80E-08 7.60E-08 7.80E-08 7.60E-08

2 5.10E-08 4.00E-08 5.10E-08 4.00E-08

3 3.40E-09 5.40E-09 3.40E-09 5.40E-09

4 6.90E-09 2.80E-09 6.90E-09 2.80E-09

5 2.80E-08 2.50E-08 2.80E-08 2.50E-08

6 6.60E-09 2.70E-09 6.60E-09 2.70E-09

7 1.50E-08 4.40E-09 1.50E-08 4.40E-09

8 2.50E-09 3.20E-09 2.50E-09 3.20E-09 1 = Air Monitoring Period (varies-2 to 11 hours) 2

Flux rate based on application rate used in study

3 = Flux rate based on 1.0 lb a.e./A

Table X-1. 2,4-D volatility flux rate from Bare field. Fowler, IN

Period1

Flux Rate (g/m2-s)

Unadjusted Flux Rate2 Adjusted Flux Rate3

EPA Registrant EPA Registrant
89

Field 1 (2,4-D Choline)

1 7.70E-08 2.00E-08 1.53E-08 3.97E-09

2 9.30E-10 1.20E-09 1.85E-10 2.38E-10

3 6.20E-10 1.60E-10 1.23E-10 3.17E-11

4 6.90E-09 3.20E-09 1.37E-09 6.35E-10

5 2.60E-10 2.30E-10 5.16E-11 4.56E-11

6 1.60E-12 4.60E-12 3.17E-13 9.13E-13

7 5.20E-12 2.50E-11 1.03E-12 4.96E-12

8 3.30E-12 2.80E-12 6.55E-13 5.56E-13

Field 2 (2,4-D DMA)

1 1.20E-07 5.60E-08 4.56E-08 2.13E-08

2 4.00E-09 5.40E-09 1.52E-09 2.05E-09

3 2.60E-10 3.50E-10 9.89E-11 1.33E-10

4 8.50E-09 4.80E-09 3.23E-09 1.83E-09

5 5.30E-10 3.30E-10 2.02E-10 1.25E-10

6 2.10E-12 9.00E-13 7.98E-13 3.42E-13

7 2.00E-11 6.40E-11 7.60E-12 2.43E-11

8 0.00E+00 1.00E-12 --- 3.80E-13

Field 3 (2,4-D EHE)

1 2.50E-08 3.60E-08 2.50E-08 3.60E-08

2 5.20E-09 5.70E-09 5.20E-09 5.70E-09

3 8.50E-10 1.10E-09 8.50E-10 1.10E-09

4 4.80E-09 5.50E-09 4.80E-09 5.50E-09

5 8.10E-09 3.20E-09 8.10E-09 3.20E-09

6 1.40E-09 6.20E-10 1.40E-09 6.20E-10

7 1.30E-09 2.30E-09 1.30E-09 2.30E-09

8 7.80E-11 2.30E-11 7.80E-11 2.30E-11 1 = Air Monitoring Period (varies-5 to 11 hours) 2 

=Flux rate based on application rate used in study 3 = Flux rate based on 1.0 lb a.e./A

Table 3. 2,4-D volatility flux rate from soybean at Little Rock, AR

Period1

Flux Rate (g/m2-s)

Unadjusted Flux Rate2 Adjusted Flux Rate3

EPA Registrant EPA Registrant

Field 1 (2,4-D Choline)

1 7.50E-08 1.50E-08 1.88E-08 3.75E-09

2 2.10E-09 2.70E-09 5.25E-10 6.75E-10

3 3.30E-10 4.00E-10 8.25E-11 1.00E-10

4 1.30E-10 1.40E-10 3.25E-11 3.50E-11

5 6.10E-09 6.60E-10 1.53E-09 1.65E-10

6 2.10E-10 2.40E-10 5.25E-11 6.00E-11
90

Table 3. 2,4-D volatility flux rate from soybean at Little Rock, AR

Period1

Flux Rate (g/m2-s)

Unadjusted Flux Rate2 Adjusted Flux Rate3

EPA Registrant EPA Registrant

7 2.00E-10 2.50E-10 5.00E-11 6.25E-11

8 1.20E-10 1.40E+10 3.00E-11 3.50E+09

Field 2 (2,4 D choline plus glyphosate DMA)

1 2.00E-08 6.00E-08 2.44E-09 7.31E-09

2 3.40E-09 6.00E-09 4.14E-10 7.31E-10

3 4.40E-10 6.40E-10 5.36E-11 7.80E-11

4 1.80E-10 2.00E-10 2.19E-11 2.44E-11

5 3.90E-09 2.30E-09 4.75E-10 2.80E-10

6 1.20E-10 1.60E-10 1.46E-11 1.95E-11

7 3.00E-10 4.20E-10 3.65E-11 5.12E-11

8 9.80E-11 1.50E-10 1.19E-11 1.83E-11

Field 3 ( 2,4 D DMA)

1 3.70E-08 3.00E-08 9.02E-08 7.32E-08

2 3.00E-09 4.00E-09 7.32E-09 9.76E-09

3 3.40E-10 6.10E-10 8.29E-10 1.49E-09

4 9.10E-11 1.70E-10 2.22E-10 4.15E-10

5 3.70E-09 6.50E-10 9.02E-09 1.59E-09

6 1.60E-10 1.50E-10 3.90E-10 3.66E-10

7 1.30E-10 1.90E-10 3.17E-10 4.63E-10

8 8.30E-11 9.30E-11 2.02E-10 2.27E-10

Field 4 ( 2,4 D EHE)

1 3.50E-08 1.70E-07 8.54E-08 4.15E-07

2 2.60E-08 6.70E-09 6.34E-08 1.63E-08

3 2.60E-09 5.80E-10 6.34E-09 1.41E-09

4 1.20E-09 1.90E-09 2.93E-09 4.63E-09

5 9.60E-09 3.60E-09 2.34E-08 8.78E-09

6 7.20E-09 1.30E-09 1.76E-08 3.17E-09

7 5.80E-10 6.20E-10 1.41E-09 1.51E-09

8 7.30E-10 3.70E-10 1.78E-09 9.02E-10 1 Air Monitoring Period (varies-5 to 11 hours) 2 = Flux rate based on application rate used in study

3 = Flux rate based on 1.0 lb a.e./A

Table 4. 2,4-D volatility flux rate from cotton field at Ty Ty, Ga

Period1 Flux Rate (g/m2-s)
91

Unadjusted Flux Rate2 Adjusted Flux Rate3

EPA Registrant EPA Registrant

Field 1 (2,4 D choline plus glyphosate DMA))

1 1.17E-08 6.00E-10 1.48E-09 7.59E-11

2 2.83E-09 5.00E-09 3.58E-10 6.33E-10

3 1.25E-09 6.00E-10 1.58E-10 7.59E-11

4 3.16E-10 1.00E-10 4.00E-11 1.27E-11

5 4.84E-10 8.00E-10 6.13E-11 1.01E-10

6 1.10E-10 1.00E-10 1.39E-11 1.27E-11

7 2.94E-10 5.00E-10 3.72E-11 6.33E-11

8 9.87E-11 1.00E-10 1.25E-11 1.27E-11

Field 2 (2,4 D choline

1 1.05E-08 3.00E-09 2.63E-09 7.50E-10

2 1.01E-09 7.00E-10 2.53E-10 1.75E-10

3 3.11E-10 3.00E-11 7.78E-11 7.50E-12

4 1.16E-10 7.50E-11 2.90E-11 1.88E-11

5 5.50E-10 2.00E-10 1.38E-10 5.00E-11

6 2.76E-10 3.50E-11 6.90E-11 8.75E-12

7 5.38E-10 2.50E-10 1.35E-10 6.25E-11

8 8.83E-11 5.50E-11 2.21E-11 1.38E-11

Field 3 ( 2,4 D DMA)

1 1.23E-08 1.00E-08 3.00E-08 2.44E-08

2 5.85E-09 1.00E-08 1.43E-08 2.44E-08

3 8.65E-10 3.50E-10 2.11E-09 8.54E-10

4 1.38E-10 1.50E-10 3.37E-10 3.66E-10

5 6.35E-11 3.50E-10 1.55E-10 8.54E-10

6 7.50E-11 7.50E-11 1.83E-10 1.83E-10

7 2.75E-10 4.00E-10 6.71E-10 9.76E-10

8 7.33E-11 8.00E-11 1.79E-10 1.95E-10

Field 4 ( 2,4 D EHE)

1 5.88E-09 3.00E-09 1.43E-08 7.32E-09

2 4.29E-08 3.00E-08 1.05E-07 7.32E-08

3 2.05E-08 5.00E-09 5.00E-08 1.22E-08

4 8.10E-09 3.00E-09 1.98E-08 7.32E-09

5 1.38E-08 7.00E-09 3.37E-08 1.71E-08

6 1.39E-09 3.50E-10 3.39E-09 8.54E-10

7 2.02E-09 1.20E-09 4.93E-09 2.93E-09

8 8.50E-10 4.00E-10 2.07E-09 9.76E-10

1 Air Monitoring Period (varies-3 to 13 hours) 2 = Flux rate based on application rate used in study

3 = Flux rate based on 1.0 lb a.e./A
92

Agency Estimated Volatility Flux Profiles of Various 2,4-D Formulations
93

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1 2 3 4 5 6 7 8

Volatility Flux (μg/m2-s)

Monitoring Period

Ty Ty, GA (Cotton)

Choline

Choline+Glyphosate

Amine

Ester
94

Appendix I

Example of PERFUM Input and Output

******************************************************

** PERFUM Output File 

******************************************************

Version 2.5.1 - compiled on 7/2/2008

Run finished on: 11/14/2012 at 13:03

******************************************************

** Basic information about the model run 

******************************************************

Scenario Type: GRN

Source of flux data: Application Rate 

Source of meteorological data: Peoria, IL 

ISCST3 meterological file: ..\PERFUM2\MET\IL14842.MET 

Field size (acres): 79.969

Length in x-direction (m): 569.00

Length in y-direction (m): 569.00

Grid density: FINE 

******************************************************

** Toxicity Inputs 

******************************************************

Human Equivalent Conc (ug/m3) : 120.0

Uncertainty factor: 1.0

Threshold (ug/m3): 120.0

******************************************************

** Exposure Assumptions

******************************************************

Exposure averaging period (hours): 1

Distribution averaging time (hours): 1

******************************************************

** Time Assumptions 

******************************************************

Starting year: 1984

Ending year: 1988

Application Start Hour: 7

******************************************************

** Additional assumptions for greenhouse scenario
95

******************************************************

Greenhouse source type: Area

Height of greenhouse + stack (m): 0.0

Greenhouse height (m): 0.5

Adjusted greenhouse height (m): 0.5

Source of flux data: Manually entered by user

******************************************************

** Fumigant Flux Profiles 

******************************************************

Flux rates for day number: 1

 HOUR Flux Rate

_______________________

 1 0.032

 2 0.032

 3 0.032

 4 0.032

 5 0.032

 6 0.032

 7 18.800

 8 18.800

 9 18.800

 10 18.800

 11 18.800

 12 18.800

 13 0.525

 14 0.525

 15 0.525

 16 0.525

 17 0.525

 18 0.525

 19 0.082

 20 0.082

 21 0.082

 22 0.082

 23 0.082

 24 0.082

Flux rates for day number: 2

 HOUR Flux Rate

_______________________

 1 0.052

 2 0.052

 3 0.052

 4 0.052

 5 0.052

 6 0.052

 7 1.530

 8 1.530

 9 1.530

 10 1.530
96

 11 1.530

 12 1.530

 13 1.530

 14 1.530

 15 1.530

 16 1.530

 17 1.530

 18 1.530

 19 1.530

 20 0.052

 21 0.052

 22 0.052

 23 0.052

 24 0.052

Flux rates for day number: 3

 HOUR Flux Rate

_______________________

 1 0.030

 2 0.030

 3 0.030

 4 0.030

 5 0.030

 6 0.030

 7 0.050

 8 0.050

 9 0.050

 10 0.050

 11 0.050

 12 0.050

 13 0.050

 14 0.050

 15 0.050

 16 0.050

 17 0.050

 18 0.050

 19 0.050

 20 0.030

 21 0.030

 22 0.030

 23 0.030

 24 0.030

** All flux rates in micrograms per meter squared per second 

----- Number of Periods with Buffer Length Estimates -----

Period Valid Periods Calm Periods

_______________________________________

 1 1694 131

 2 1658 167

 3 1647 178

 4 1639 186
97

 5 1631 194

 6 1622 203

 7 1615 210

 8 1609 216

 9 1604 221

 10 1597 228

 11 1584 241

 12 1565 260

 13 1517 308

 14 1446 379

 15 1393 432

 16 1357 468

 17 1315 510

 18 1281 544

 19 1243 582

 20 1214 611

 21 1194 631

 22 1172 653

 23 1146 679

 24 1122 703

*** Due to some large buffers distances, estimates are valid only at or below the following percentiles

Period Maximum Percentile

___________________________

 1 99.525

 2 99.854

 3 99.978

 4 99.996

 5 99.990

 6 99.989

 7 100.000

 8 100.000

 9 100.000

 10 100.000

 11 100.000

 12 100.000

 13 100.000

 14 100.000

 15 100.000

 16 100.000

 17 100.000

 18 100.000

 19 100.000

 20 100.000

 21 100.000

 22 100.000

 23 100.000

 24 100.000

------- Definition of Flux Averaging Periods ----------

Period 1: Hours 7 to 7

Period 2: Hours 8 to 8

Period 3: Hours 9 to 9
98

Period 4: Hours 10 to 10

Period 5: Hours 11 to 11

Period 6: Hours 12 to 12

Period 7: Hours 13 to 13

Period 8: Hours 14 to 14

Period 9: Hours 15 to 15

Period 10: Hours 16 to 16

Period 11: Hours 17 to 17

Period 12: Hours 18 to 18

Period 13: Hours 19 to 19

Period 14: Hours 20 to 20

Period 15: Hours 21 to 21

Period 16: Hours 22 to 22

Period 17: Hours 23 to 23

Period 18: Hours 24 to 24

Period 19: Hours 1 to 1

Period 20: Hours 2 to 2

Period 21: Hours 3 to 3

Period 22: Hours 4 to 4

Period 23: Hours 5 to 5

Period 24: Hours 6 to 6

--------- PERFUM Model Results -----------

Concentration distribution results for rings the field

Ring No. Distance (meters)

____________________________________

 1 5.

 2 7.

 3 10.

 4 15.

 5 20.

 6 30.

 7 50.

 8 70.

 9 80.

10 90.

11 100.

12 120.

13 150.

14 180.

15 210.

16 240.

17 270.

18 300.

19 360.

20 420.

21 480.

22 540.

23 600.

24 720.

25 840.

26 960.
99

27 1080.

28 1200.

29 1320.

30 1440.

Table I-1. AERSCREEN Inputs for 2,4-D Air Modeling Runs

Input Parameter Option/Value

Source Parameters

Source Type Area

Maximum Field Emission Rate 1 Choline Formulation, Corn and Soybean Crops: 

1.88 x 10-8 g/m2-s (EPA Value)

Field Size and Dimensions 80 acres

(569 meters x 569 meters)

Height of Release Corn (Choline): 50 cm

Soybean (Choline): 15 cm

Land Surface Parameters

Local Seasonal Characteristics 1 – Midsummer with lush vegetation

Land Surface Characteristics 2 – Agricultural Lands

Albedo Corn and Soybean Crops: 0.17

Bowen Ratio Corn and Soybean Crops: 0.4

Aerodynamic Surface Roughness 

Length Corn and Soybean Crops: 0.25 m

Geochemical and Resistance Parameters

Diffusion in Air Coefficient 2 0.06 cm2/s

Diffusion in Water Coefficient 2 0.57 cm2/s

Bulk Canopy Cuticular Resistance 

Parameter 3 2.08 s/cm

Henry’s Law Constant4 0.867 Pa·m3/mol

Atmospheric Half-Life5 69,732 seconds

Meteorological Parameters

Wind Speed Range 0.5 - 18 m/s

Hourly Precipitation Amount 2 mm (0.08 inches)

Temperature Range 288K – 311K

Surface Atmospheric Pressure 1013 hPa

Cloud Cover 5/10

Relative Humidity 75%

Output Parameters

Averaging Period 1-hour

Downwind Distances 6

0 m – 100 m: Every 10 meters

100 m - 3,000 m: Every 100 meters

3,000 m – 10,000 m: Every 500 meters

10,000 m – 20,000 m Every 5,000 meters

Receptor Height 0 m (Ground-level) 1. Maximum flux rates in field volatility studies scaled to labeled maximum application rate from 

MRID No. 48862902: EPA maximum identified from 2,4-D Choline formulation corn crops flux 

rate based on its application to soybeans in Little Rock, AR. 7.50 x 10-8 g/m2

-s flux rate 

determined from study at 4.0 lbs. ae./A scaled to value in table at 1 lb. ae/A. 2. Diffusion in air coefficient and diffusion in water coefficient for 2,4-D retrieved from 

Appendix C of, “AERMOD Deposition Algorithms – Science Document (Revised Draft)”, from 

EPA Office of Air Quality Planning and Standards.

3. Cuticular resistance parameter for 2,4-D retrieved from Appendix D of, “AERMOD 
100

Input Parameter Option/Value

Deposition Algorithms – Science Document (Revised Draft)”, from EPA Office of Air Quality 

Planning and Standards.

4. Table 8 5. Atmospheric half-life of 2,4-D retrieved from hydroxyl radicals rate constant EpiSuite 

AOPWIN v. 1.92.

6. Distances referenced from the center of the treated field. The edge of the field is at a downwind 

distance of 1333.3 feet (402.3 m).

Example output of AERSCREEN

AERSCREEN 11076 / AERMOD 

09292 11/15/12

09:12:09

TITLE: soybean

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

****************************** AREA PARAMETERS ****************************

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

SOURCE EMISSION RATE: 0.609E-02 g/s 0.483E-01 lb/hr

AREA EMISSION RATE: 0.188E-07 g/(s-m2) 0.149E-06 lb/(hr-m2)

AREA HEIGHT: 0.01 meters 0.05 feet

AREA SOURCE LONG SIDE: 569.00 meters 1866.80 feet

AREA SOURCE SHORT SIDE: 569.00 meters 1866.80 feet

INITIAL VERTICAL DIMENSION: 0.00 meters 0.00 feet

RURAL OR URBAN: RURAL

INITIAL PROBE DISTANCE = 5000. meters 16404. feet

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

*********************** BUILDING DOWNWASH PARAMETERS **********************

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

BUILDING DOWNWASH NOT USED FOR NON-POINT SOURCES

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

************************** FLOW SECTOR ANALYSIS ***************************

25 meter receptor spacing: 1. meters - 5000. meters

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

MAXIMUM IMPACT RECEPTOR

Zo SURFACE 1-HR CONC RADIAL DIST TEMPORAL

SECTOR ROUGHNESS (ug/m3) (deg) (m) PERIOD

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

1* 0.250 11.28 45 400.0 ANN

* = worst case diagonal

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

********************** MAKEMET METEOROLOGY PARAMETERS *********************

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

MIN/MAX TEMPERATURE: 297.0 / 299.0 (K)

MINIMUM WIND SPEED: 0.5 m/s

ANEMOMETER HEIGHT: 10.000 meters

SURFACE CHARACTERISTICS INPUT: USER ENTERED

ALBEDO: 0.17

BOWEN RATIO: 0.40

ROUGHNESS LENGTH: 0.250 (meters)

METEOROLOGY CONDITIONS USED TO PREDICT OVERALL MAXIMUM IMPACT

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

YR MO DY JDY HR

-- -- -- --- --

10 01 10 10 01

H0 U* W* DT/DZ ZICNV ZIMCH M-O LEN Z0 BOWEN ALBEDO REF WS

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

-0.63 0.027 -9.000 0.020 -999. 10. 2.9 0.250 0.40 0.17 0.50

HT REF TA HT

- - - - - - - - - - -

10.0 299.0 2.0

METEOROLOGY CONDITIONS USED TO PREDICT AMBIENT BOUNDARY IMPACT

--------------------------------------------------------------
101

YR MO DY JDY HR

-- -- -- --- --

10 01 10 10 01

H0 U* W* DT/DZ ZICNV ZIMCH M-O LEN Z0 BOWEN ALBEDO REF WS

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

-0.63 0.027 -9.000 0.020 -999. 10. 2.9 0.250 0.40 0.17 0.50

HT REF TA HT

- - - - - - - - - - -

10.0 299.0 2.0

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

************************ AERSCREEN AUTOMATED DISTANCES **********************

OVERALL MAXIMUM CONCENTRATIONS BY DISTANCE

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

MAXIMUM MAXIMUM

DIST 1-HR CONC DIST 1-HR CONC

(m) (ug/m3) (m) (ug/m3)

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

1.00 8.800 2524.99 1.565

25.00 8.990 2550.00 1.552

50.01 9.180 2575.00 1.540

75.00 9.363 2600.00 1.527

100.00 9.541 2625.01 1.515

125.00 9.713 2650.00 1.503

150.01 9.880 2675.00 1.491

174.99 10.04 2700.00 1.480

200.00 10.20 2725.01 1.468

225.00 10.35 2749.99 1.457

250.00 10.51 2775.00 1.446

274.99 10.65 2800.00 1.435

300.00 10.80 2825.00 1.424

325.00 10.94 2849.99 1.415

350.00 11.07 2875.00 1.404

375.01 11.16 2900.00 1.394

400.00 11.28 2925.00 1.384

425.00 8.361 2950.01 1.374

450.00 7.399 2975.00 1.364

475.01 6.780 3000.00 1.355

500.00 6.250 3025.00 1.345

525.00 5.909 3050.01 1.336

550.00 5.620 3075.00 1.326

575.01 5.367 3100.00 1.317

599.99 5.145 3125.00 1.308

625.00 4.946 3150.00 1.300

650.00 4.767 3174.99 1.290

675.00 4.604 3200.00 1.281

699.99 4.456 3225.00 1.272

725.00 4.319 3250.00 1.264

750.00 4.193 3274.99 1.255

775.00 4.075 3300.00 1.246

800.01 3.966 3325.00 1.238

825.00 3.864 3350.00 1.230

850.00 3.769 3375.01 1.222

875.00 3.678 3400.00 1.214

900.01 3.593 3425.00 1.206

924.99 3.513 3450.00 1.198

950.00 3.437 3475.01 1.190

975.00 3.365 3499.99 1.182

1000.00 3.296 3525.00 1.175

1024.99 3.231 3550.00 1.168

1050.00 3.169 3575.00 1.160

1075.00 3.109 3599.99 1.153

1100.00 3.052 3625.00 1.146

1125.01 2.997 3650.00 1.139

1150.00 2.945 3675.00 1.132

1175.00 2.895 3700.01 1.125

1200.00 2.846 3725.00 1.118

1225.01 2.800 3750.00 1.111

1250.00 2.755 3775.00 1.105

1275.00 2.712 3800.01 1.098

1300.00 2.671 3825.00 1.091

1325.01 2.630 3850.00 1.085
102

1349.99 2.592 3875.00 1.079

1375.00 2.554 3900.00 1.072

1400.00 2.518 3924.99 1.066

1425.00 2.483 3950.00 1.060

1449.99 2.449 3975.00 1.054

1475.00 2.416 4000.00 1.048

1500.00 2.384 4024.99 1.042

1525.00 2.352 4050.00 1.036

1550.01 2.321 4075.00 1.030

1575.00 2.292 4100.00 1.025

1600.00 2.264 4125.01 1.019

1625.00 2.236 4150.00 1.014

1650.01 2.209 4175.00 1.008

1674.99 2.182 4200.00 1.003

1700.00 2.157 4225.01 0.9972

1725.00 2.132 4249.99 0.9918

1750.00 2.107 4275.00 0.9865

1774.99 2.084 4300.00 0.9813

1800.00 2.060 4325.00 0.9761

1825.00 2.038 4349.99 0.9710

1850.00 2.016 4375.00 0.9659

1875.01 1.994 4400.00 0.9608

1900.00 1.973 4425.00 0.9558

1925.00 1.952 4450.01 0.9509

1950.00 1.932 4475.00 0.9460

1975.01 1.912 4500.00 0.9411

1999.99 1.893 4525.00 0.9363

2025.00 1.874 4550.01 0.9316

2050.00 1.856 4575.00 0.9269

2075.00 1.837 4600.00 0.9222

2099.99 1.820 4625.00 0.9176

2125.00 1.802 4650.00 0.9130

2150.00 1.785 4674.99 0.9085

2175.00 1.769 4700.00 0.9040

2199.99 1.752 4725.00 0.8996

2225.00 1.736 4750.00 0.8951

2250.00 1.720 4774.99 0.8907

2275.00 1.705 4800.00 0.8864

2300.01 1.690 4825.00 0.8821

2325.00 1.675 4850.00 0.8779

2350.00 1.660 4875.01 0.8737

2375.00 1.646 4900.00 0.8695

2400.01 1.632 4925.00 0.8654

2424.99 1.618 4950.00 0.8612

2450.00 1.604 4975.01 0.8572

2475.00 1.591 4999.99 0.8532

2500.00 1.578

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

********************** AERSCREEN MAXIMUM IMPACT SUMMARY *********************

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

3-hour, 8-hour, and 24-hour scaled

concentrations are equal to the 1-hour concentration as referenced in

SCREENING PROCEDURES FOR ESTIMATING THE AIR QUALITY

IMPACT OF STATIONARY SOURCES, REVISED (Section 4.5.4)

Report number EPA-454/R-92-019

http://www.epa.gov/scram001/guidance_permit.htm

under Screening Guidance

MAXIMUM SCALED SCALED SCALED SCALED

1-HOUR 3-HOUR 8-HOUR 24-HOUR ANNUAL

CALCULATION CONC CONC CONC CONC CONC

PROCEDURE (ug/m3) (ug/m3) (ug/m3) (ug/m3) (ug/m3)

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

FLAT TERRAIN 11.29 11.29 11.29 11.29 N/A

DISTANCE FROM SOURCE 402.00 meters

IMPACT AT THE

AMBIENT BOUNDARY 8.800 8.800 8.800 8.800 N/A

DISTANCE FROM SOURCE 1.00 meters
103

Appendix J 

Spray Drift Deposition Curves

Figure J-1

Figure J-2

90th Percentile Depostion Curve of AIXR Nozzle for GF-2726 2,4-D Choline Formulation

Distance (FT)

0 100 200 300 400 500

Fraction of Applied

0.0

0.2

0.4

0.6

0.8

1.0

1.2