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Final Registration of Enlist DuoTM Herbicide

Regulatory Rationale

The Agency has granted a registration for Enlist DuoTM, a herbicide containing the active 

ingredients 2,4-dichlorophenoxyacetic acid (2,4-D) choline salt and glyphosate 

dimethylammonium salt (glyphosate) to Dow AgroSciences (DAS). On November 1, 2011, DAS 

submitted an application for registration to the Environmental Protection Agency (EPA or the 

Agency) for Enlist DuoTM, which is used on corn and soybean crops that are genetically 

engineered (GE) to be resistant to 2,4-D and glyphosate (GE crops). Under the Plant Protection 

Act, the United States Department of Agriculture (USDA) reviewed a separate request from 

DAS to deregulate its EnlistTM GE herbicide tolerant corn and soybean crops and granted the 

request on September 18, 2014.

2,4-D is an active ingredient that is currently registered in a variety of salt, amine, and ester 

formulations and is registered for a variety of food and feed uses, including corn and soybeans. 

This new use will expand the current timing of applications of 2,4-D on corn and soybeans, 

thereby enhancing the flexibility in weed control. 2,4-D was previously registered for over-thetop applications to corn up to 8 inches tall and only pre-plant applications to soybeans. This 

registration allows over-the-top applications of 2,4-D choline salt formulation to GE corn up to 

48 inches tall and over-the-top applications to GE soybeans. 

Although the use on GE crops is a new use pattern for the 2,4 D component of this product, it is 

not a new use for glyphosate containing products. All uses for this product are already registered 

on other glyphosate products and are currently in use on GE corn and soybeans for the same use 

pattern. Since no new use pattern and no new exposures for glyphosate were being considered 

with this registration action, no new assessments were performed for glyphosate. However, as 

described above, the expanded timing of applications is a change in the use pattern for the 

choline salt of 2,4-D. Therefore, this document is intended to discuss the results of the Agency’s 

findings specifically to the assessment of the choline salt of 2,4-D on GE corn and soybeans.

I. Chemical Information

Chemical Name: Ethanaminium, 2-hydroxy-N,N,N-trimethyl-, 2-(2,4-dichlorophenoxy)acetic 

acid hydroxide (1:1:1)

EPA PC Code: 051505

Chemical Abstracts Service (CAS) Number: 1048373-72-3

Mode of Action: 2,4-D is an herbicide in the phenoxyacetic acid family that is used 

postemergence for selective control of broadleaf weeds. 2,4-D, a synthetic auxin herbicide, 

causes disruption of plant hormone responses.

Registrant: Dow AgroSciences, LLC
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Product: GF-2726 (Enlist DuoTM) – EPA Registration Number 62719-649, an end-use product 

containing 24.4% 2,4-D choline salt and 22.1% glyphosate, used on EnlistTM AAD-1 Corn (Trait 

Code: DAS-40278-9) and EnlistTM AAD-12 Soybean (Trait Code: DAS-68416-4).

II. Human Health Risk

A summary of the human health effects and risk of 2,4-D choline salt as assessed in the EPA

document entitled, 2,4-D. Human Health Risk Assessment for a Proposed Use of 2,4-D Choline 

on Herbicide-Tolerant Corn and Soybean, is provided below.

A. Summary of Toxicological Effects

The toxicology database on 2,4-D is complete and sufficient for assessing the toxicity and 

characterizing the hazard of all formulations of 2,4-D, including the choline salt. Data on 

other forms of 2,4-D were also used to assess the choline formulation.

2,4-D has been classified as having low acute toxicity via the oral, dermal, and inhalation 

routes of exposure (Toxicity Category III). It is not a dermal irritant (Toxicity Category IV) 

or dermal sensitizer but it is a severe eye irritant (Toxicity Category I).

The toxicity profile of the active ingredient 2,4-D shows that the principal toxic effects are 

changes in the kidney, thyroid, liver, adrenal, eye, and ovaries/testes in the rat following 

exposure to 2,4-D via the oral route only at dose levels above the threshold of saturation of 

renal clearance. No systemic toxicity was observed in rabbits following repeated exposure via 

the dermal route at dose levels up to the limit dose. Neurotoxicity was observed in the acute 

neurotoxicity study in rats at the high dose. In an extended one-generation reproductive 

toxicity study in rats, reproductive toxicity, developmental neurotoxicity, and immunotoxicity 

were not observed. The thyroid effects observed at dose levels approaching renal saturation 

were considered treatment-related (i.e., resulted from dosing with 2,4-D), although not 

adverse (i.e., the effects are not harmful to the organism). Maternal and developmental 

toxicity were observed at high dose levels exceeding the threshold of saturation of renal 

clearance. There are no residual uncertainties for pre- and/or postnatal toxicity. 

2,4-D is classified as “not classifiable as to human carcinogenicity,” based upon bioassays in 

rats and mice that showed no statistically significant tumor response in either species. The 

Agency determined, based on several reviews of epidemiological studies, in addition to the 

animal studies, that the existing data did not support a conclusion that links human cancer to 

2,4-D exposure.

B. Toxicological End Points and Doses Used in the Human Health Risk Assessment

Once a pesticide’s toxicological profile is determined, EPA identifies toxicological Points of 

Departure (POD) and Levels of Concern (LOC) to use in evaluating the risk posed by human 

exposure to the pesticide. For hazards that have a threshold below which there is no 

appreciable risk, the toxicological POD is used as the basis for derivation of reference values 

for risk assessment. PODs are developed based on a careful analysis of the doses in each 
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toxicological study to determine the dose at which no adverse effects are observed (the 

NOAEL; No Observed Adverse Effect Level) and the lowest dose at which adverse effects of 

concern are identified (the LOAEL; Lowest Observed Adverse Effect Level). 

Uncertainty/safety factors are used in conjunction with the POD to calculate a safe exposure 

level - generally referred to as a Population-adjusted Dose (PAD) or a Reference Dose (RfD) -

and a safe Margin of Exposure (MOE). For non-threshold risks (e.g., cancer), EPA assumes 

that any amount of exposure will lead to some degree of risk. Thus, EPA estimates risk in 

terms of the probability of an occurrence of the adverse effect expected in a lifetime. For more 

information on the general principles EPA uses in risk characterization and a complete 

description of the risk assessment process, see

http://www.epa.gov/pesticides/factsheets/riskassess.htm.

1. Acute Dietary

a. General Population (Including Infants and Children)

An acute dietary endpoint for the general population, including infants and children, 

was selected from the acute neurotoxicity study in rats with a NOAEL of 67 mg/kg. At 

the study LOAEL of 225 mg/kg, an increased incidence of incoordination and slight 

gait abnormalities (forepaw flexing or knuckling) and decreased motor activity were 

observed. A 100X uncertainty factor was applied to account for inter- and intra-species 

variability. As discussed in section C below, the Food Quality and Protection Act 

(FQPA) safety factor was reduced to 1X resulting in an acute Population Adjusted Dose 

(aPAD) of 0.67 mg/kg/day.

b. Females of Child-Bearing Age (13-49 years old)

An acute dietary endpoint for females 13+ was selected from the developmental toxicity 

study in rats with a NOAEL of 25 mg/kg/day. At the study LOAEL of 75 mg/kg/day, 

fetal skeletal malformations (14th rudimentary ribs) were observed. A 100X uncertainty 

factor was applied to account for inter- and intra-species variability and the FQPA 

safety factor was reduced to 1X resulting in an acute Population Adjusted Dose aPAD 

of 0.25 mg/kg/day.

2. Chronic Dietary

The chronic dietary endpoint was selected from the extended one-generation reproduction 

toxicity study in rats with a NOAEL of 21 mg/kg/day. At the study LOAEL of 55.6 

mg/kg/day for males and 46.7 mg/kg/day for females, kidney toxicity, manifested as 

increased kidney weights and increased incidence of degeneration of the proximal 

convoluted tubules, was observed and decreased body weight in pups was observed 

throughout lactation. A 100X uncertainty factor was applied to account for inter- and intraspecies variability and the FQPA safety factor was reduced to 1X resulting in a chronic 

Population Adjusted Dose (cPAD) of 0.21 mg/kg/day.

3. Incidental Oral, Short and Intermediate Term
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Short-term and intermediate-term incidental oral endpoints for risk assessment were 

selected from the extended one-generation reproduction toxicity study in rats with a 

NOAEL of 21 mg/kg/day. At the study LOAEL of 55.6 mg/kg/day for males and 46.7 

mg/kg/day for females, kidney toxicity, manifested as increased kidney weights and 

increased incidence of degeneration of the proximal convoluted tubules, was observed and 

decreased body weight in pups was observed throughout lactation. A 100X uncertainty 

factor was applied to account for inter- and intra-species variability and the FQPA safety 

factor was reduced to 1X resulting in a target MOE of 100 for non-dietary risk assessment.

4. Inhalation, Short and Intermediate Term

Short-term and intermediate-term inhalation endpoints for risk assessment were selected 

from the route-specific 28-day inhalation toxicity study in rats with a LOAEL of 0.05 

mg/L/day. A NOAEL for portal-of-entry effects was not determined. At the study LOAEL 

of 0.05 mg/L/day, squamous metaplasia and epithelial hyperplasia with increased mixed 

inflammatory cells within the larynx, which was not totally resolved following a 4-week 

recovery period, were observed. Human Equivalent Concentrations (HEC)/Human 

Equivalent Doses (HED) for residential and occupational scenarios were calculated. A 3X 

uncertainty factor was applied to account for inter-species variability (to account for the 

pharmacodynamic differences), a 10X uncertainty factor was applied to account for intraspecies variability, and a 10X uncertainty factor was applied to account for the lack of a

NOAEL. The lack of an assessment of the thyroid and other systemic effects in the 

inhalation study is considered inconsequential because the portal of entry endpoint is 

protective of potential systemic effects expected to occur at higher concentrations; i.e., at 

doses that exceed the level of renal clearance. Portal-of-entry effects were observed at all 

dose levels, and an additional 10X uncertainty factor is applied to the LOAEL to obtain an 

extrapolated NOAEL used for the inhalation risk assessments. The use pattern indicates 

that dose levels required to exceed the renal clearance mechanism will not be attained 

following human inhalation exposure.

5. Dermal (All Durations)

No quantification of dermal risk is required. Although the dermal toxicity study did not 

evaluate developmental endpoints, the following were noted:

a. There was no dermal or systemic toxicity observed following repeated dermal 

applications to rabbits at the Limit Dose (1000 mg/kg/day).

b. There was no qualitative susceptibility observed in the developmental or 

reproductive toxicity studies.

c. The use of a 10% human dermal absorption factor (DAF) with the oral 

developmental LOAEL of 90 mg/kg/day established in the rabbit developmental 

toxicity study yields a dermal equivalent dose (DED) of 900 mg/kg/day, which is 
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numerically similar to the high-end dermal NOAEL (1000 mg/kg/day) in the dermal 

rabbit study.

d. No quantitative susceptibility/sensitivity was observed in the developmental and/or 

reproductive toxicity studies between adults and young animals (maternal and 

developmental/offspring NOAELs/LOAELs are the same). Therefore, any dose that 

would be protective of effects in adults will also be protective of effects in the 

developing organism.

e. The use pattern indicates that dose levels required to exceed the renal clearance 

mechanism will not be attained following human dermal exposure.

6. Cancer

The Cancer Peer Review Committee (CPRC; TXR No. 0050017, dated January 29, 1997)

classified 2,4-D as “not classifiable as to human carcinogenicity,” based upon bioassays in 

rats and mice that showed no statistically significant tumor response in either species. EPA

determined, based on several reviews of epidemiological studies, in addition to the animal 

studies, that the existing data did not support a conclusion that links human cancer to 2,4-D 

exposure.

C. FQPA Safety Factor

EPA determined that the 10X FQPA Safety Factor (for the protection of infants and children

mentioned above) could be reduced to 1X for all oral endpoints for the following reasons:

The toxicity database is complete and adequate to assess safety for infants and children. There 

is evidence of increased qualitative susceptibility (i.e., severity of effects) in the rat 

developmental toxicity study and in the rat two-generation reproduction study. However, 

there is no quantitative susceptibility since the maternal and developmental/offspring 

NOAELs/LOAELs identified in these studies are the same. Moreover, the points of departure 

used in the risk assessment are below where these findings occur and are protective. There are 

acute and subchronic neurotoxicity studies, a developmental neurotoxicity cohort in the 

extended one-generation reproductive toxicity study in rats, a detailed evaluation of thyroid 

function across life stages, and a developmental immunotoxicity cohort in the extended onegeneration reproductive toxicity study in rats. Therefore, the Agency has a complete database 

addressing potential hazard to infants and children. The exposure assessment will not 

underestimate children’s exposure to 2,4-D. Further details may be found in the following 

sections:

1. Completeness of the Toxicology Database

The toxicology database for 2,4-D is complete. Acceptable rat and rabbit developmental 

toxicity studies, a rat two-generation reproduction study, an extended one-generation rat 

reproduction toxicity study (F1 offspring evaluated for potential effects on the nervous 

system, immune system, reproductive and endocrine systems, thyroid function, and other 
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systemic toxicity parameters), and acute, subchronic, and developmental neurotoxicity 

studies in rats are available.

2. Evidence of Neurotoxicity

Evidence of neurotoxicity was observed in the acute neurotoxicity study in rats, as 

evidenced by an increase in the incidence of incoordination and slight gait abnormalities 

(forepaw flexing or knuckling) during the Functional Observation Battery (FOB)

assessment at the high dose in both sexes. In the subchronic neurotoxicity study, relative 

forelimb grip strength was significantly increased in rats of both sexes at the high-dose 

level, although there was no treatment-related change in absolute grip strength. Clinical 

signs of neurotoxicity (decreased motor activity, ataxia, loss of righting reflex, extremities 

cold to the touch) were observed in maternal rabbits in the developmental toxicity study. 

Developmental neurotoxicity was not observed in the developmental neurotoxicity cohort 

in the extended one-generation reproductive toxicity study in rats. Neuropathological 

effects were not observed in any study. While neurotoxic effects were observed in some 

studies, these effects all occurred at doses exceeding renal saturation; endpoints were 

selected to assure that the risk assessment is protective for these effects.

3. Evidence of Sensitivity/Susceptibility in the Developing or Young Animal

There is evidence of increased qualitative susceptibility (i.e., severity of effects) following 

in utero exposure to 2,4-D in the rat developmental toxicity study and following in utero 

and/or pre-/post-natal exposure in the rat two-generation reproduction study. However, 

there is no quantitative difference between young and adult animals (i.e., the NOAELs are 

the same in adult and young animals). There is no evidence of increased susceptibility 

following in utero exposure to 2,4-D in the rabbit developmental toxicity study or 

following in utero and/or pre-/post-natal exposure in the rat extended one-generation 

reproduction toxicity study.

2,4-D has been evaluated for potential developmental effects in the rat and rabbit. Maternal

toxicity included decreased body weight gains in the rat study at the same dose level where

developmental effects (occurrence of skeletal malformations) were observed. Maternal 

toxicity in the rabbit included decreased body weight gain, clinical signs of toxicity 

(decreased motor activity, ataxia, loss of righting reflex, extremities cold to the touch), and 

abortions, the latter being indicative of potential developmental toxicity. Decreased 

maternal body weight gains were observed in the rat two-generation reproduction study at 

a dose that exceeded renal saturation and resulted in reduced viability of the F1 pups. 

There are clearly established NOAELs and LOAELs for the population of concern, there 

are no data gaps in the toxicology database, and the PODs are protective of susceptibility.

4. Residual Uncertainty in the Exposure Database

There are no residual uncertainties in the exposure database. The dietary exposure 

estimates are unrefined and reflect primarily tolerance-level residue in food, 100% crop 
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treated, and upper-bound drinking water estimates based on modeling. Additionally, nonoccupational exposure estimates were determined using the Residential Standard Operating 

Procedures which utilize a combination of central tendency and high end inputs designed 

to result in protective exposure estimates which will not underestimate residential 

exposures.

D. Cumulative Effects

2,4-D is an herbicide in the phenoxyacetic acid family of pesticides. This class also includes

MCPA, 2,4-DB, and 2,4-DP. A cumulative risk assessment has not been performed as part of

this human health risk assessment because EPA has not made a determination as to which of 

these compounds, if any, to which humans may be exposed, have a common mechanism of 

toxicity. Unlike other pesticides for which EPA has followed a cumulative risk approach 

based on a common mechanism of toxicity, EPA has not made a common mechanism of 

toxicity finding as to 2,4-D and any other substances. For the purposes of this action, 

therefore, EPA has not assumed that 2,4-D has a common mechanism of toxicity with other 

substances.

For information regarding EPA’s efforts to determine which chemicals have a common

mechanism of toxicity and to evaluate the cumulative effects of such chemicals, see the policy 

statements released by EPA’s Office of Pesticide Programs concerning common mechanism 

determinations and procedures for cumulating effects from substances found to have a 

common mechanism on EPA’s website at http://www.epa.gov/pesticides/cumulative/.

E. Dietary (Food + Drinking Water) Risk

2,4-D is a phenoxyacetic acid herbicide used to control a variety of broadleaf weeds. It is a 

longstanding active ingredient (ai) registered for a variety of food/feed uses. Permanent 

tolerances for 2,4-D are established under 40 CFR 180.142 for a wide variety of crops and 

livestock commodities.

Acute and chronic aggregate (food + dietary drinking water) exposure and risk assessments 

were conducted using the Dietary Exposure Evaluation Model software with the Food 

Commodity Intake Database (DEEM-FCID) Version 3.16. This software uses 2003-2008 

food consumption data from the U.S. Department of Agriculture’s (USDA’s) National Health 

and Nutrition Examination Survey, What We Eat in America, (NHANES/WWEIA).

1. Acute Dietary Risk

For acute exposure assessments, individual one-day food consumption data are used on an

individual-by-individual basis. The reported consumption amounts of each food item can 

be multiplied by a residue point estimate and summed to obtain a total daily pesticide 

exposure for a deterministic exposure assessment, or “matched” in multiple random 

pairings with residue values and then summed in a probabilistic assessment. The resulting 

distribution of exposures is expressed as a percentage of the aPAD on both a user (i.e., 

only those who reported eating relevant commodities/food forms) and a per-capita (i.e., 
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those who reported eating the relevant commodities as well as those who did not) basis. In 

accordance with EPA policy, per capita exposure and risk are reported for analyses. 

The resulting acute food plus drinking water risk estimates are not of concern to EPA

(≤100% aPAD) at the 95th percentile of the exposure distribution for the general population 

and all population subgroups. The resulting acute risk estimate for children 1 to 2 years 

old, the subgroup with the greatest exposure, was 14% of the aPAD at the 95th percentile of 

the exposure. The acute dietary assessment is unrefined; to further refine the 2,4-D dietary

exposure and risk estimates, percent crop treated (%CT), anticipated residues, or 

monitoring data, if available, could be used.

2. Chronic Dietary Risk

For chronic dietary exposure assessment, an estimate of the residue level in each food or 

food-form on the food commodity residue list is multiplied by the average daily 

consumption estimate for that food/food form to produce a residue intake estimate. The 

resulting residue intake estimate for each food/food form is summed with the residue 

intake estimates for all other food/food forms on the commodity residue list to arrive at the 

total average estimated exposure. Exposure is expressed in mg/kg body weight/day and as 

a percent of the cPAD. This procedure is performed for each population subgroup.

The resulting chronic food plus drinking water risk estimates are not of concern to EPA for 

the general population and all population subgroups. The most highly exposed population 

was children 1 to 2 years old utilizing 15% of the cPAD. The chronic dietary assessment is 

unrefined; to further refine the 2,4-D dietary exposure and risk estimates, percent crop 

treated, anticipated residues, or available monitoring data could be used.

F. Residential (Non-Occupational) Exposure/Risk Characterization

There are registered uses of 2,4-D on turf including golf courses and parks as well as aquatic 

uses; therefore, residential handler exposure and post-application exposure to treated turf and 

aquatic sites is possible. There is no hazard via the dermal route for 2,4-D, therefore the 

handler assessment included quantification of risks for only the inhalation route of exposure 

and the post-application assessment included only the inhalation and incidental oral route of 

exposure. The residential handler and post-application risk estimates are not of concern for 

2,4-D for all scenarios and all routes of exposure.

For non-dietary exposures, EPA uses the term Margin of Exposure (MOE) to refer to the risk 

associated with the exposure estimate. The MOE is defined as the ratio of the selected 

toxicological POD, usually the NOAEL, to the estimated human exposure. A target MOE of 

300 means that the estimated maximum safe level of human exposure is 300 times lower than 

the highest dose that produced no adverse effects in the relevant toxicology study; the target 

MOE is calculated by multiplying together the safety / uncertainty factors appropriate for each 

assessment. Risk estimates that are not of concern are indicated by an actual MOE of 300 or 

greater for residential handler exposure and 100 or greater for post-application exposure.
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1. Residential Handler Exposure

Residential handlers may receive short-term dermal and inhalation exposure to 2,4-D when 

mixing, loading, and applying the pesticide to ornamental turf as well as aquatic uses. 

Only inhalation risk estimates were quantitatively assessed because there is no hazard via 

the dermal route for 2,4-D. The handler inhalation exposure scenarios considered were

mixing, loading and applying:

 Liquid/Wettable Powder (WP)/Dry Flowable (DF) to Lawns/Turf with Hose-End Sprayer

 Liquid/WP in Water Soluble Packets (WSP) to Lawns/Turf/Aquatic Sites with manuallypressurized handwand

 Ready-to-Use/WP in WSP to Lawns/Turf with Hose-End Sprayer

 Liquid to Lawns/Turf/Aquatic Sites with Backpack

 Liquid/WP/DF to Lawns/Turf with Manually-pressurized handwand or backpack

 Granule to Lawns/Turf with Push-type spreader, Belly Grinder, Spoon, Shaker Can, Cup 

or Hand dispersal

The MOEs for the six exposure scenarios range from 460 to 3,400,000. Since there is 

potential risk concern only when MOEs are less than 300, residential handler exposures are 

not a concern.

2. Post-Application Exposure

There is the potential for post-application exposure for individuals exposed as a result of 

being in an environment that has been previously treated with 2,4-D. The quantitative 

exposure/risk assessment for residential post-application exposures is based on the 

following scenarios:

 Incidental ingestion (i.e., hand-to-mouth, object-to-mouth, soil ingestion exposure) from

contact with treated turf (children 1 to < 2 years old only)

 Episodic granular ingestion on treated turf (children 1 to < 2 years old only)

 Incidental ingestion of water during recreational swimming (both adults and children).

Assessment of post-application exposure to turf treated with liquid formulations is 

protective of exposure to solid formulations since exposures to liquid formulations are 

generally higher than exposures to solid formulations. The lifestages selected for 

assessment are health protective for the exposures and risk estimates for any other 

potentially exposed lifestages.

a. Residential Post-application Exposure for Turf Use

Incidental oral risk estimates were quantitatively assessed for residential postapplication exposure for turf use. The incidental oral scenarios (i.e., hand-to-mouth and 

object-to-mouth) have been considered inter-related as it is likely that they occur 

interspersed amongst each other over time. Episodic granular ingestion on treated turf 
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was not combined as this exposure would not occur as a result of routine behavior and 

is considered an episodic event related to poisoning. 

The residential post-application risk estimates for turf use have MOEs that range from 

490 to 430,000 for all incidental oral scenarios so are not of concern for 2,4-D.

b. Residential Post-application Exposure for Aquatic Use

2,4-D is used for aquatic weed control of surface and submerged weeds. Many 

treatments are applied to aquatic areas where recreational swimming is not likely to 

occur but some subsurface treatments are made at recreational lakes. Since this can 

result in individuals being exposed to 2,4-D residues in water by entering these areas if 

they have been previously treated, there is a 24-hour swimming restriction. The extent 

of exposure during recreational swimming is assumed to be short-term in duration. Risk 

estimates were calculated for post-application incidental oral ingestion while swimming 

in treated lakes or ponds. Inhalation exposure is expected to be negligible for 

swimmers; therefore, a post-application inhalation assessment was not conducted. 

Furthermore, the inhalation assessment for residential handlers is expected to be 

protective of potential post-application exposure and risk.

The residential post-application risk estimates for aquatic use have MOEs that range 

from 690 to 5,300 for incidental oral ingestion so are not of concern for 2,4-D.

3. Residential Bystander Post-application Inhalation Exposure (Volatilization)

The potential exposure to bystanders from the vapor phase of the 2,4-D choline salt 

formulation residues emitted from treated fields was evaluated using data submitted to the 

Agency that used the choline salt formulation in its study for this use on GE corn and 

soybean. The two main factors that bystander exposure depends on are the rate at which 

these chemicals come off of a treated field which is described as the off-gassing, emission 

or flux, and how those vapors are dispersed in the air over and around the treated field. 

Volatilization can occur during the application process or thereafter. It can result from 

aerosols evaporating during application, while deposited sprays are still drying or after as 

dried deposited residues volatilize. The volatilization assessment used an analysis after 

sprays dried.

Flux data was submitted measuring flux rates of 2,4-D ethylhexyl ester (EHE), 2,4-D 

dimethylamine salt (DMA salt) and 2,4-D choline salt. 2,4-D choline salt was found to 

have a reduced potential for volatility. For this assessment, the data from the 2,4-D choline

salt applications only were used as this action specifically sought registration for 2,4-D 

choline salt product use in conjunction with GE soybean and corn with resistance traits.

Volatilization modeling for a single day was completed using Probabilistic Exposure and 

Risk model for fumigants (PERFUM). There are a variety of factors that potentially affect 

the emission rates of 2,4-D choline salt and subsequent offsite transport and to the extent 

possible, these factors were considered. They include field condition (e.g., bare soil, 
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growing, or mature crop canopy), field parameters (e.g., soil type, moisture, etc.), 

formulation type, meteorological conditions, and application scenario (e.g., rate, method).

Flux estimates from all monitored trials, a number of field sizes, and various 

meteorological data were used with PERFUM to estimate risk based on the 2,4-D choline 

salt field volatility study data.

The field volatility study suggests that volatilization of 2,4-D choline salt from treated 

crops does occur and could result in bystander exposure to vapor phase 2,4-D choline salt. 

However, results of PERFUM modeling indicate that airborne concentrations, even at the 

edge of the treated fields, are not above our levels of concern. 

4. Spray Drift

Spray drift is always a potential source of exposure to residents nearby to spraying 

operations. Off-target movement of pesticides can occur via many types of pathways and it 

is governed by a variety of factors. Sprays that are released and do not deposit in the 

application area end up off-target and can lead to exposures to those it may directly 

contact. They can also deposit on surfaces where contact with residues can eventually lead 

to indirect exposures (e.g., children playing on lawns where residues have deposited next 

to treated fields). The potential risk estimates from these residues can be calculated using 

drift modeling coupled with methods employed for residential risk assessments for turf 

products.

The approach to be used for quantitatively incorporating spray drift into risk assessment is 

based on a premise of compliant applications which, by definition, should not result in 

direct exposures to individuals because of existing label language and other regulatory 

requirements intended to prevent them. Direct exposures would include inhalation of the 

spray plume or being sprayed directly. Rather, the exposures addressed here are thought to 

occur indirectly through contact with impacted areas, such as residential lawns, when 

compliant applications are conducted. Given this premise, exposures for children (1 to 2 

years old) and adults who have contact with turf where residues are assumed to have 

deposited via spray drift thus resulting in an indirect exposure are the focus of this analysis 

analogous to how exposures to turf products are considered in risk assessment.

Several currently-registered 2,4-D products have labels that allow for use on turf, thus it 

was considered whether the risk assessment for that use may be considered protective of 

any type of exposure that would be associated with spray drift. If the maximum application 

rate on crops adjusted by the amount of drift expected is less than or equal to existing turf 

application rates, the existing turf assessment is considered protective of spray drift 

exposure. The maximum single application rate of 2,4-D choline salt on GE corn and 

soybean is 1 lb ae/acre. This is less than the previously registered application rate on turf of 

1.5 lb ae/acre, which was previously assessed and was updated based on the revised 

Standard Operating Procedures (SOPs) for Residential Exposure Assessment. Thus, even if 

100% of the application rate of the choline salt formulation on GE corn and soybean is 

deposited on an adjacent lawn, calculated risk estimates from drift are not be of concern. 
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Again, this is because all existing registered uses on lawns had been previously assessed 

and no risks of concern were identified.

5. Aggregate Risk Assessment

In accordance with the FQPA, EPA must aggregate pesticide exposures and risks from 

three major sources: food, drinking water, and residential exposures. In an aggregate 

assessment, exposures from relevant sources are added together and compared to 

quantitative estimates of hazard (e.g., a NOAEL or PAD), or the risks themselves can be 

aggregated. When aggregating exposures and risks from various sources, EPA considers 

both the route and duration of exposure.

a. Acute Aggregate Risk

The acute aggregate risk assessment includes only food and water exposure. The acute 

food plus drinking water risk estimates are not of concern to EPA (≤100% aPAD) at the 

95th percentile of the exposure distribution for the general population and all population 

subgroups.

b. Short-Term Aggregate Risk

The short-term aggregate risk assessment includes food, water, and residential 

exposure. The resulting short-term aggregate risks are not of concern to EPA (MOEs > 

LOC of 100) for adults and children.

c. Intermediate-Term Aggregate Risk

Intermediate-term residential exposures are not likely because of the intermittent 

application of 2,4-D by homeowners; therefore, the intermediate-term aggregate risk 

assessment includes only food and drinking water exposure, and are less than or 

equivalent to the chronic food plus drinking water exposure. The chronic food plus 

drinking water risk estimates are not of concern to EPA for the general population and 

all population subgroups.

d. Long-Term Aggregate Risk

The chronic (long-term) aggregate risk assessment includes only food and water 

exposure. The chronic food plus drinking water risk estimates are not of concern to 

EPA for the general population and all population subgroups.

6. Occupational Risk Assessment

a. Short- and Intermediate-Term Handler Risk

EPA uses the term occupational handler to describe people who mix, load and / or apply 

pesticides professionally (e.g., farmers, professional pesticide applicators). Based on 
13

the anticipated use patterns and current labeling, types of equipment and techniques that 

can potentially be used (mixing/loading liquid groundboom application, applying spray 

by groundboom equipment), occupational handler exposure is expected from the new

uses. 

Occupational handler risk estimates are not of concern (i.e., MOEs > LOC of 300) for 

all scenarios for use of 2,4-D choline salt on GE corn and soybean. At baseline personal 

protective equipment (PPE) (i.e., no respirator), the occupational handler inhalation 

MOE is 4,900 for mixer/loaders and 3,200 for applicators using groundboom 

equipment.

b. Short- and Intermediate-Term Post-Application Risk

EPA uses the term post-application to describe exposures that occur when individuals 

are present in an environment that has been previously treated with a pesticide (also 

referred to as reentry exposure). Such exposures may occur when workers enter 

previously treated areas to perform job functions, including activities related to crop 

production, such as scouting for pests or harvesting. Post-application exposure levels 

vary over time and depend on such things as the type of activity, the nature of the crop 

or target that was treated, the type of pesticide application, and the chemical’s 

degradation properties. In addition, the timing of pesticide applications, relative to 

harvest activities, can greatly reduce the potential for post-application exposure.

i. Dermal Post-application Risk

There is no potential hazard via the dermal route for 2,4-D choline salt; therefore, a 

quantitative occupational post-application dermal risk assessment was not 

completed.

ii. Inhalation Post-application Risk

Based on the EPA’s current practices, a quantitative occupational post-application 

inhalation exposure assessment was not performed for 2,4-D choline salt at this time 

primarily because of the low acute inhalation toxicity (Toxicity Category III) and 

vapor pressure (1.4 x 10-7 mm Hg at 25°C for 2,4-D acid).

Although a quantitative occupational post-application inhalation exposure 

assessment was not performed, an inhalation exposure assessment was performed for 

occupational/commercial handlers and showed no risks of concern. Handler 

exposure resulting from application of pesticides outdoors is anticipated to result in 

higher exposure than post-application exposure. Therefore, it is expected that these 

handler inhalation exposure estimates will be protective of most occupational postapplication inhalation exposure scenarios. Furthermore, a quantitative volatilization 

inhalation exposure assessment was assessed for bystanders and indicates no risk of 

concern for bystanders.
14

III. Environmental Risk

A summary of the environmental fate and ecological effects and risks of 2,4-D choline salt as 

assessed in the Agency document titled, 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, and its addendums entitled, Addendum to 2,4-D 

Choline Salt Section 3 Risk Assessment: Refined Endangered Species Assessment for Proposed 

New Uses on Herbicide-Tolerant Corn and Soybean and Addendum to 2,4-D Choline Salt 

Section 3 Risk Assessment: Refined Terrestrial Plant Exposure Estimates and Effects 

Determination are provided below.

A. Environmental Fate

1. Degradation

The degradation of 2,4-D occurs via oxidative microbially-mediated mineralization in 

terrestrial environments, and photodegradation in water. Degradation under aerobic soil 

conditions is rapid to moderately rapid with half-lives ranging from 1.4 to 12.4 days. In 

terrestrial field dissipation studies, 2,4-D acid half-lives range from 1.1 days to 42.5 days. 

There are three major degradates (2,4-Dichlorophenol (2,4-DCP), 1,2,4-bezenetriol, and 

chlorohydroquinone (CHQ)) and three minor degradates (4-chlorophenol, 4-CPA and 2,4-

DCA) of 2,4-D. Formation of these degradates varies by environmental component (e.g., 

soil vs. water), and availability of oxygen. Under natural conditions certain degradates 

may be less likely to occur.

2. Mobility

Under most environmental conditions 2,4-D is an anionic acid, hence it is expected to be 

mobile to moderately mobile. Risk of bioaccumulation is low for 2,4-D given the low 

value of the log octanol/water partition coefficient (log Kow = 0.18 at neutral pH). The 

vapor pressure (1.4 × 10-7 mm Hg) and Henry's Law Constant (8.56 x 10-6atm-m3/mol) 

indicate that 2,4-D acid has a low volatility. 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) indicate that the estimated volatility flux rate of 2,4-D 

choline salt is lower than the EHE and DMA salt formulations. 

B. Ecological Risk

Ecological risk characterization for 2,4-D choline salt integrates the results of the exposure 

and ecotoxicity data to evaluate the likelihood of adverse ecological effects. The means of 

integrating the results of exposure and ecotoxicity data is called the risk quotient method. For 

this method, risk quotients (RQs) are calculated by dividing exposure estimates by ecotoxicity 

values, both acute and chronic (RQ = Exposure / Toxicity). RQs are then compared to EPA’s 

levels of concern (LOCs). The LOCs are criteria used by the Agency to indicate potential risk 

to non-target organisms. The criteria indicate whether a pesticide, when used as directed, has 

the potential to cause adverse effects to non-target organisms. 
15

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 based on the new use patterns for 2,4-D choline salt. 

Birds are considered a surrogate for terrestrial-phase amphibians and reptiles, and freshwater 

fish are considered surrogates for aquatic-phase amphibians, in the absence of taxa-specific

data. Submitted ecotoxicity data for 2,4-D choline salt (algae, freshwater fish, and honeybee) 

support bridging 2,4-D choline salt to 2,4-D acid ecotoxicity data. Only the most sensitive 

2,4-D toxicity value from the broader 2,4-D dataset were used in risk quotient calculations, as 

needed. The major degradates of 2,4-D were considered, and all except 2,4-DCP were 

eliminated as likely degradates of concern. 2,4-DCP is a major degradate in certain aquatic 

environments; therefore, 2,4-D and 2,4-DCP were considered stressors of concern in aquatic 

environments, and 2,4-D alone was considered in terrestrial environments. 

The results of this screening-level risk assessment indicate that risks did not result in RQs that 

exceeded the Agency’s LOC for freshwater fish, aquatic-phase amphibians, estuarine/marine 

fish, freshwater invertebrates, estuarine/marine invertebrates, terrestrial insects, or aquatic 

plants for either acute or chronic exposures. Risks for chronic exposures to birds, reptiles, 

and land-phase amphibians do not result in RQs that exceed the Agency’s LOC.

The screening-level analysis indicates that risks for acute exposures to birds, reptiles, and 

land-phase amphibians do result in RQs that exceed the Agency’s LOC for acute exposures. 

Additionally, risks for mammals resulted in RQs that exceed the Agency’s LOC for both 

acute and chronic exposures. Risks for terrestrial plants resulted in RQs that exceed the 

Agency’s LOC for both terrestrial monocots and terrestrial dicots. The following sections 

discuss the results of the risk quotient analyses for these taxonomic groups with potential risk 

above the Agency’s LOC, characterization of those risks, and describe mitigation measures to 

reduce these potential risks of exposure to 2,4-D choline salt. 

1. Risk to Birds:

The risk quotient analysis indicates that potential risks from the new 2,4-D choline salt 

uses result in RQs that exceed the Agency’s LOC for birds only on an acute basis. 

Acute Risk: 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.

Two acute dietary studies were available, classifying 2,4-D choline salt as “practically nontoxic” on an acute dietary basis to birds. No mortalities occurred in either study. 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 
16

body weight gain and feed consumption, but only at the highest treatment level (3035 mg 

ae/kg-diet).

In order to make the most conservative risk estimation, acute toxicity risk quotients were 

based on the oral toxicity study for the northerner bobwhite quail. The risk quotients for 

birds of 0.01 to 4.18 were then compared to the Agency’s screening level of concern for 

non-listed species (RQ>0.5). The Agency risk screening assessment employed residue 

estimates based on reasonable upper bound assumptions and the maximum labeled rate of 

the pesticide to determine the RQ values. At this high end exposure, residues for a variety 

of food items combined with a variety of body sizes triggered the screening concern 

threshold when compared to the most sensitive oral dose toxicity estimate. While concern 

levels are triggered, further consideration of all lines of evidence does suggest that risks 

under more usually encountered circumstances may be lower. For example, high end 

residues compared to toxicity study endpoints using chemicals actually incorporated in the 

animal’s diet do not trigger non-listed species concerns, suggesting that 2,4-D choline 

consumed in the diet may possibly be less available than assumed using dose-based 

exposures. Further, more frequently expected residues levels, such as mean or median 

estimates of exposure would be lower by a factor of two or more, suggesting that residues 

are often not likely to trigger concerns for many food items. In addition, screening 

estimates of exposure and risk are maximal at the actual point of application, right on the 

field. Available information in the Agency risk assessment indicates that the transport of 

pesticide off field by spray drift decreases with distance, suggesting that exposures to 2,4-

D choline salt and attendant risks can be substantially lower for organisms with territories 

established at distance from the field. With this last line of evidence in mind, a mitigation

measure has been incorporated into the pesticide label that requires a 30-foot pesticide 

application setback from areas likely to be habitat for birds in order to further reduce offsite exposure for birds.

2. Risk to Mammals:

The screening level assessment indicates that potential risks from the new 2,4-D choline 

salt uses result in RQs that exceed the Agency’s LOC for mammals in both acute and 

chronic scenarios.

Risk quotients for mammals exceeded the Agency’s LOCs for mammals for acute dosebased exposure and chronic dose-based and dietary-based exposure. The chronic dosebased LOC (1.0) was exceeded for all size classes of mammals consuming all food items 

except for seeds. The chronic dietary-based LOC (1.0) was exceeded for diets of short 

grass, tall grass, broadleaf plants, and arthropods for mammals. 

Acute Risk: 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 dose-based acute mammalian risk 

quotients ranged from <0.01 to 0.57.
17

The acute LOC (0.5) was exceeded only for small mammals consuming short grass. All 

other scenarios resulted in RQs that did not exceed the Agency’s LOC. Because this 

assessment is conducted under screening level assumptions designed to be conservative 

(i.e., in the treated field, maximum use rates, eating only the single food source that is 

expected to result in the greatest exposure, high-end environmental exposures), EPA 

expects that actual risks to these mammals is lower. Any deviation from this worst case 

scenario would result in lower risk estimates and would be expected to result in RQs lower 

than the Agency’s LOC.

Chronic Risk: The two-generation chronic study with the laboratory rat indicated 

endpoints in parental and offspring growth to be the most sensitive. Some reproductive 

effects were also identified. Chronic dose based risk quotients ranged from 0.32 to 50.2. 

The chronic LOC of 1.0 was exceeded for all size classes of mammals consuming all food 

items except for seeds. Chronic dietary based risk quotients ranged from 0.36 to 5.78.

As in the case for birds, risk quotients for mammals span an appreciable range of 

outcomes. The principal focus is on the concern levels for reproduction effects, where RQ 

values range from <1 to 50, and span the Agency’s screening level of concern for nonlisted species (RQ>1). 

Again, the Agency risk screening assessment employed residue estimates based on 

reasonable upper bound assumptions and the maximum labeled rate of the pesticide to 

determine the RQ values. Consideration of more realistic residue estimates and other lines 

of evidence such as food preferences and foraging ranges relative to distance from the site 

of application can lead to markedly reduced concerns for adverse effects in larger 

mammals with more varied diets, with larger home ranges with increased potential to be 

feeding well away from treatment areas. 

Consideration of these lines of evidence also produces reduced risk estimates for small 

herbivorous mammals but does not reduce risk estimates for these organisms to the point 

that concern levels are not exceeded. As in the case for birds, the provision of a 30-foot 

buffer from areas potentially comprising habitat for such mammals is intended to reduce 

the areas where such risks may occur.

3. Risk to Plants:

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 
18

and seedling emergence studies indicate that 2,4-D choline salt is slightly more toxic to 

dicots than monocots.

As is expected with herbicides, terrestrial plants are sensitive to 2,4-D residues. All 

terrestrial plant risk quotients exceeded the LOC (1.0). Risk quotients ranged from 1 to 

90.62 for monocots and 12.35 to 1085.11 for dicots. In the initial assessments for the 

proposed decision (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, and its addendums entitled, Addendum to 2,4-D 

Choline Salt Section 3 Risk Assessment: Refined Endangered Species Assessment for 

Proposed New Uses on Herbicide-Tolerant Corn and Soybean), risk was attributed to both 

spray drift and runoff from treated fields, which indicated that effects could be predicted 

from the new uses of 2,4-D choline salt to terrestrial plants.

Although the risk quotient analysis indicated there may be risks to terrestrial plants from 

runoff and spray drift, data conducted on the Enlist DuoTM formulation demonstrates that 

the formulation has some properties that will reduce spray drift to non-target areas. The 

registrant submitted additional studies for spray drift analysis, using the specific low drift 

nozzles and the specific Enlist DuoTM formulation. The analysis indicates that this 2,4-D 

choline salt formulation applied through specific low drift nozzles is protective of nonlisted dicots from exposures of 2,4-D choline when an adequate buffer is incorporated 

between the application equipment and the downwind edge of the treated field. 

Therefore, to mitigate against potential risks to plants from spray drift, the product labeling 

requires the use of a 30-foot buffer zone and specific nozzle specifications, thus reducing 

the potential spray drift exposure of non-target plants to 2,4-D choline salt residues.Public 

comments on the risk assessment and effects determination pointed out that the Agency did 

not explicitly include a consideration of the risk findings for non-target plants as a result of 

off-field runoff. The Agency considered the spray drift exposure to be the principal risk 

issue associated with the proposed labeled use of 2,4-D choline, owing to a variety of lines 

of evidence, including past experience with other 2,4-D formulations and associated spray 

drift incident reporting. However, in light of the public comments, the Agency 

reconsidered the runoff risks and the effects of the proposed mitigation to limit off-site 

runoff in listed species effects determinations.

Spray drift and runoff were considered as exposure pathways for 2,4-D choline to 

terrestrial plants and aquatic organisms. For aquatic organisms, the consideration of both 

spray drift and runoff loadings to surface waters did not trigger concerns. Risk concerns 

from spray drift to terrestrial plants were mitigated with an in-field 30-foot buffer that 

takes into account wind direction during application, and this mitigation yielded no spray 

drift concerns off field, when incorporated into spray drift modeling.

The in-field spray drift buffer does not mitigate concerns from runoff because 2,4-D 

choline can be applied up to the edge of the field; there is no “buffer strip” between the 

edge of the field and sensitive habitat. The Agency does not currently have a tool to 

evaluate the effectiveness of buffers in reducing pesticide exposure via runoff. The 

Agency has implemented vegetative buffer or filter strips in a few instances to lessen 
19

herbicide loading in runoff waters, however in this case there are no risk concerns for 

aquatic organisms. To assess exposure to terrestrial plants the Agency looked at several 

lines of evidence to determine potential effects as described below.

2,4-D is absorbed by both shoots and roots and is active at the growing points of the shoot 

and root. Translocation to the site of action is primarily via the symplastic pathway (with 

photosynthates in the phloem) and accumulates principally at the growing point of the 

shoot and root. 2,4-D is not translocated as well in the apoplast (carried with the water and 

nutrients in the xylem), which would occur with root uptake. Therefore, growth inhibition 

tends to be more pronounced with foliar uptake than with root uptake (Shaner 2002).

Consequently, 2,4-D in runoff waters would not be readily available for mature plant 

uptake. The Agency is including a statement on the label based on the rainfast period for 

2,4-D that prohibits the application of Enlist Duo if rain or irrigation is expected within 24 

hours. A rainfast period is the time required for the herbicide to be absorbed into the plant 

after application and before a rain/irrigation event so as to provide reasonable weed 

control. The provision of a labeled rainfast period would increase the time available for 

on-field herbicide adsorption, thereby reducing the amount available for runoff. This, in 

combination with 2,4-D’s limited uptake by roots of terrestrial plants, is anticipated to 

reduce the amount of 2,4-D choline salt that could adversely affect plants via runoff.

Further, EPA has evaluated the assumptions regarding runoff of 2,4-D from treated fields 

to adjacent terrestrial habitat. The model TerrPlant assumes, for a chemical with the 

solubility of 2,4-D in the most mobile acid form, that runoff would amount to 5% of the 

field applied mass of the herbicide. This modeling approach does not account for pesticide 

degradation and for pesticide partitioning. These processes that account for loss are 

important in the mechanistic pesticide runoff models used by EPA (Pesticide Root Zone 

Model (PRZM)) and in the field. The Agency has compared the TerrPlant assumption of 

5% runoff to the runoff predictions for PRZM runs used to characterize pesticide runoff for 

aquatic exposure. This comparison revealed that runoff predicted by TerrPlant for 2,4-D is 

grossly overestimated. The total annual runoff is less than a fifth of the amount predicted 

by TerrPlant for a single runoff event.

4. Endangered Species for 2,4-D Choline Salt

A summary of the endangered species assessment for 2,4-D choline salt as assessed in the 

EPA document titled, Addendum to 2,4-D Choline Salt Section 3 Risk Assessment: Refined 

Endangered Species Assessment for Proposed New Uses on Herbicide-Tolerant Corn and 

Soybean, is provided below. See also Addendum to 2,4-D Choline Salt Section 3 Risk 

Assessment: Refined Terrestrial Plant Exposure Estimates and Effects Determination.

In the environmental risk assessment performed for new uses of 2,4-D choline salt on GE

corn and soybean, EPA determined that direct concerns were unlikely for aquatic plants 

(vascular and non-vascular), freshwater fish (acute and chronic), estuarine/marine fish 

(acute and chronic), freshwater invertebrates (acute and chronic), estuarine/marine 

invertebrates (acute and chronic), and terrestrial insects. While direct concerns were found 

to be unlikely for birds, reptiles and terrestrial phase amphibians for chronic risk, they 
20

could not be excluded for acute risk. In addition, potential direct risk concerns could not 

be excluded for mammals (acute and chronic) and terrestrial plants. Indirect effect risk 

concerns are possible for any species that has dependencies on species that are directly 

affected.

Registration of Enlist DuoTM is being registered for use in the states of Illinois, Indiana, 

Iowa, Ohio, South Dakota, and Wisconsin. Based on EPAs LOCATES database and data 

submitted by DAS, 53 listed species were identified as inside the “action area” (area of 

concern where use of pesticide may result in exposure to endangered species) associated 

with the new GE corn and soybean uses within these six states. Additional states may be 

added to the labeling once an assessment is completed and demonstrates that a no effects 

determination is appropriate for any such state. 

The following criteria were used to assess listed species in the spray drift action area:

 For listed individuals inside the action area but not part of an affected taxa nor relying on 

the affected taxa for services involving food, shelter, biological mediated resources 

necessary for survival and reproduction, use of a pesticide would be determined to have 

“no effect.”

 For listed individuals outside the action area, use of a pesticide would be determined to 

have “no effect.”

 Listed individuals inside the action area may either fall into the “no effect” or “may effect” 

categories depending upon their specific biological needs and circumstances of exposure. 

 Those that fall under the “may effect” category are found to be either “likely” or “not 

likely to adversely affect” the listed species. 

 A “likely” or “not likely to adversely affect” determination is made using criteria that 

categorizes the effect as insignificant, highly uncertain, or wholly beneficial. 

Spray drift mitigation language on the label is intended to limit off site transport of 2,4-D 

choline salt in spray drift. Therefore, EPA expects that spray drift will remain confined to 

the 2,4-D choline treated field. Consequently, EPA concluded that spray drift will have no 

effect on 49 of the 53 species originally identified as potentially at-risk because they are 

not expected to occur on corn and soybean fields. 

The 4 remaining listed species that were not ruled out because their range contains areas 

that include treated fields were considered in more depth to refine the assessment:

American burying beetle, Canada lynx, Indiana bat, and whooping crane. Species specific 

biological information and 2,4-D choline salt use patterns were considered. After utilizing 

processes such as refined modeling incorporating species specific information and 

migration habits, EPA made a determination that exposure occurring on the field would 

have no effect on these species.

Additionally, as stated above, the Agency reconsidered the potential effects attributed to 

runoff. That analysis concluded that in light of the refined assessment, combined with 

proposed mitigations such as a mandatory rainfast period, the Agency has determined that 

risks to terrestrial plants from runoff as predicted by TerrPlant modeling are grossly 
21

overestimated in the case of 2,4-D choline and a finding of no effects to listed or non-listed 

species off the treated field is appropriate.

For more details on these findings, refer to the EPA document titled, Addendum to 2,4-D 

Choline Salt Section 3 Risk Assessment: Refined Endangered Species Assessment for 

Proposed New Uses on Herbicide-Tolerant Corn and Soybean.

As noted earlier in this decision, glyphosate is already registered for these uses and did not 

undergo a listed species review as part of the assessment for this pesticide product. 

However, glyphosate currently is in the registration review process and a listed species

analysis will be part of that process.

IV. Resistance Management

The emergence of herbicide resistant weeds is an increasing problem that has become a 

significant economic issue to growers. This has led to a concern that the use of 2,4-D on GE 

crops may result in the development of more resistant weeds. In an effort to address this issue 

going forward, EPA is requiring DAS to develop an Herbicide Resistance Management (HRM) 

plan that will promote herbicide resistance management efforts. The plan mandates that DAS 

must investigate any reports of lack of herbicide efficacy. The initial mechanism users can use 

for communicating directly with DAS is a toll-free number to get advice on how to resolve any 

uncontrolled weeds.

Academia, growers, USDA, and other leaders involved with pest management all acknowledge 

the importance of scouting in herbicide resistance management. Fields should be scouted before 

application of Enlist DuoTM to identify the weed species present as well as their stage of growth. 

Fields should be scouted after each Enlist DuoTM application to identify poor performance or 

likely resistance. In the event that a user encounters a non-performance issue, the toll-free 

number is available to report the issue, which will initiate an intervention against that weed 

population.

When a lack of herbicide efficacy is identified, DAS or its representative will investigate and 

conduct a site visit if needed, to evaluate the lack of herbicide efficacy using decision criteria 

identified by leading weed science experts (Norsworthy, et al. 2012), in order to determine if 

“likely herbicide resistance” (possible resistance) is present. This is distinct from the term “lack 

of herbicide efficacy,” as explained below. For purposes of this decision, a report of lack of 

herbicide efficacy to DAS will be the trigger to start this investigation.

“Lack of herbicide efficacy” refers to inadequate weed control with various possible causes, 

including but not limited to: application rate, stage of growth, environmental conditions,

herbicide resistance, plugged nozzle, boom shut off, tank dilution, post-application weed flush, 

unexpected rainfall event, weed misidentification, etc. EPA recognizes that it can be challenging 

to distinguish emerging weed resistance from other causes at an early stage. Therefore, EPA has 

selected criteria that should be used to evaluate instances of “lack of herbicide efficacy” to 

determine if they do in fact constitute “likely herbicide resistance.” These “likely herbicide 

resistance” criteria are: (1) failure to control a weed species normally controlled by the herbicide 
22

at the dose applied, especially if control is achieved on adjacent weeds; (2) a spreading patch of 

uncontrolled plants of a particular weed species; and (3) surviving plants mixed with controlled 

individuals of the same species (Norsworthy, et al., 2012). The identification of one or more of 

these criteria in the field indicates “likely herbicide resistance” is present.

When DAS or its representative applies the Norsworthy, et al., criteria cited above and likely 

herbicide resistance is identified, then to the extent possible, DAS must proactively engage with

the grower to control and contain likely resistant weeds in the infested area. This may be 

accomplished by re-treating with an herbicide or using mechanical control methods. After 

implementing these measures DAS must follow-up with the growers, to the extent possible, to 

determine if the likely resistant weed(s) has/have been controlled. DAS must also annually 

report to EPA findings of likely herbicide resistance. In addition, prior to implementing control 

measures, DAS will make best efforts to obtain samples of the likely herbicide resistant weeds 

and/or seeds, and as soon as practicable, laboratory or greenhouse testing must be initiated in 

order to confirm whether resistance is the reason for the lack of herbicide efficacy.

Beginning January 15th, 2016, on or before January 15th of each year, DAS will submit annual 

summary reports to EPA. These reports must include a summary of the number of instances of 

likely and confirmed resistance to Enlist DuoTM by weed species, crop, county and state. They 

will also summarize the status of laboratory or greenhouse testing for resistance. The annual 

reports will also address the disposition of incidents of likely or confirmed resistance reported in 

previous years.

DAS must report annually any inability to control likely resistant weeds to relevant stakeholders. 

To accomplish this, EPA expects that DAS will establish a website to facilitate delivery of 

resistance information.

Several management practices that are designed to help users avoid initial occurrences of weed 

resistance appear on the product labeling under the Herbicide Resistance Management heading 

of the label. These practices are discussed in Section VII.B.3 of this document.

Refer to Section VII.C below for EPA’s terms of registration to address the issue of weed 

resistance.

V. Response to Comments

The Agency received 417,301 comments in response to the public participation process (Docket 

ID: EPA-HQ-OPP-2014-0195) regarding EPA’s proposed decision to register the use of 2,4-D 

choline salt on GE 2,4-D and glyphosate tolerant corn and soybeans. Comments received were 

both in favor of and opposed to the decision to register Enlist DuoTM. The EPA welcomes input 

from the public during the decision process when registering pesticides, and is committed to 

thoroughly evaluating and mitigating any potential risks from registered pesticides, consistent 

with applicable statutory standards. Also, EPA strives to document and explain the basis of its 

regulatory decisions through these and other public documents. EPA reviewed and evaluated all 

comments received during the comment period before issuing this final regulatory decision. 

Since many of the comments covered similar concerns, the comments were grouped into major 
23

topic areas. Please see Response to Public Comments Received Regarding New Uses of Enlist 

DuoTM on Corn and Soybeans, dated October 14, 2014, for the Agency response to these 

comments.

VI. Benefits

The need for additional tools to manage resistant weeds has become important as resistance to 

glyphosate and other herbicides has become a significant economic and pest management issue 

to growers. The new uses of 2,4-D choline salt will expand options for weed control in corn and 

soybean and enable control of additional broadleaf weeds, including some resistant biotypes. 

Previously registered uses of non-choline 2,4-D in corn allowed for over-the-top broadcast 

applications only up to 8 inches tall which are increased to up to 48 inches tall with GE 2,4-D 

resistant corn. Similarly, the previously registered use of non-choline 2,4-D in soybeans allowed

pre-plant applications only, however new uses of 2,4-D choline salt expand uses to include overthe-top broadcast applications to GE soybeans. The addition of this new tool to the production 

of soybeans is expected to have a significant impact to broadleaf weed control.

The introduction of a premix formulation combining 2,4-D choline salt and glyphosate to be 

used on EnlistTM corn and soybeans will provide additional benefits. The use of a premix of 2,4-

D choline salt and glyphosate utilizes multiple mechanisms of action which delays the 

development of herbicide resistant weeds. The pairing of two well-established herbicides into a 

systems approach with a GE crop will allow growers and applicators the opportunity to control 

many weeds in a way which fulfills the important principle of using multiple mechanisms of 

action, which the weed science community has been touting for many years. 

The use of 2,4-D choline salt and glyphosate on the EnlistTM corn and soybean seed technology 

will provide efficacious control of broadleaf weeds later in the growing season, resulting in 

reduced spread and persistence of many broadleaf weeds, thus maintaining yields. If widely 

adopted by growers, the herbicide combination in this weed control systems approach will

potentially prolong the use of the glyphosate technology if the two herbicides are controlling 

weeds that are not resistant to either herbicide. In addition, this system will maintain the positive 

effect of reducing the need for tillage, thus preventing unnecessary erosion, in areas where 2,4-D

choline salt will control glyphosate resistant broadleaf weeds.

The use of the 2,4-D choline salt offers environmental benefits over the use of traditional forms 

of 2,4-D as well. Specifically, EPA has determined that the choline salt is less volatile than other 

forms of 2,4-D. The 2,4-D choline salt also demonstrates less potential for off-site movement 

through spray drift than other forms of this herbicide. This will reduce the potential for damage 

to non-target plants, including vulnerable crops, where 2,4-D choline salt is to be used.

VII. Registration Decision

Based on these considerations, consistent with the requirements of FIFRA Sec. 3(c)(7)(B), EPA 

concludes that (i) the Agency has satisfactory data pertaining to the expanded uses of Enlist

DuoTM on corn and soybeans; and (ii) approving this application as set forth below will not 

increase the risk of any unreasonable adverse effects on human health or the environment. 
24

Accordingly, the Agency has granted this registration with certain terms necessary to ensure that 

if weed resistance is likely, EPA can act quickly to address the problem.

A. Data Requirements

There are no outstanding data requirements required to support the registration of this action. 

Although there are data that may be required in connection with registration review activities 

for 2,4-D and glyphosate, those requirements are generic to 2,4-D and/or glyphosate uses and 

products in general and will be handled in accordance with the registration review process.

B. Labeling Requirements

In order to mitigate risks to non-target plants and animals, label language is required that is 

intended to keep the pesticide on the treatment area, thereby reducing the potential for 

exposure of non-target plants and animals. For example, spray drift management language is

required on the occupational/commercial labeling that advises users of applicator 

responsibilities and requires specific techniques to reduce the possibility of spray drift. In 

addition, surface and ground water advisories are required on all labeling, which may further 

reduce residues in drinking water and exposure of non-target organisms.

1. Worker Protection

Do not apply this product in a way that will contact workers or other persons, either 

directly or through drift. Only protected handlers may be in the area during application. 

For any requirements specific to your state or tribe, consult the agency responsible for 

pesticide regulation. 

Do not enter or allow worker entry into treated areas during the restricted entry interval 

(REI) of 48 hours.

PPE required for early entry to treated areas that is permitted under the Worker Protection 

Standard and that involves contact with anything that has been treated, such as plants, soil, 

or water, is: 

Long-sleeved shirt and long pants 

Chemical resistant gloves as specified under category A 

 Shoes plus socks 

 Protective eyewear (goggles, faceshield, or safety glasses)

Some materials that are chemical-resistant to this product are barrier laminate, butyl rubber 

>14 mils, nitrile rubber >14 mils, neoprene rubber >14 mils, natural rubber >14 mils, 

polyethylene, polyvinyl chloride (PVC) >14 mils, or viton >14 mils. If you want more 

options, follow the instructions for category A on an EPA chemical-resistance category 

selection chart.

2. Environmental Hazards
25

This pesticide is toxic to fish and aquatic invertebrates. Do not apply directly to water, to 

areas where surface water is present, or to intertidal areas below the mean high water mark. 

Drift or runoff may adversely affect aquatic invertebrates and non-target plants. Drift and 

runoff may be hazardous to aquatic organisms in water adjacent to treated areas. Do not 

contaminate water when disposing of equipment washwaters or rinsate.

This chemical has properties and characteristics associated with chemicals detected in 

groundwater. The use of this chemical in areas where soils are permeable, particularly 

where the water table is shallow, may result in groundwater contamination. Application 

around a cistern or well may result in contamination of drinking water or groundwater.

3. Resistance Management

To aid in the prevention of developing weeds resistant to this product, the following steps 

should be followed: 

 Scout 

fields before application to ensure herbicides and rates will be appropriate for the weed 

species and weed sizes present.

 Apply 

full rates of Enlist Duo for the most difficult to control weed in the field at the specified 

time (correct weed size) to minimize weed escapes.

 Scout 

fields after application to detect weed escapes or shifts in weed species.

 Report 

any incidence of non-performance of this product against a particular weed species to 

your Dow AgroSciences retailer, representative or call 1-855-ENLIST-1(1-855-365-

4781)

 If 

resistance is suspected, treat weed escapes with an herbicide having a mode of action 

other than Group 4 or 9 and/or use non-chemical methods to remove escapes, as 

practical, with the goal of preventing further seed production.

Additionally, users should follow as many of the following herbicide resistance 

management practices practical:

 Use a 

broad spectrum soil-applied herbicide with other modes of action as a foundation in a 

weed control program.

 Utilize 

sequential applications of herbicides with alternative modes of action.

 Rotate 

the use of this product with non-Group 4 and non-Group 9 herbicides.

 Incorp

orate non-chemical weed control practices, such as mechanical cultivation, crop 
26

rotation, cover crops and weed-free crop seeds, as part of an integrated weed control 

program.

 Thorou

ghly clean plant residues from equipment before leaving fields suspected to contain 

resistant weeds.

 Avoid 

using more than two applications of Enlist Duo and any other Group 4 or Group 9 

herbicide within a single growing season unless in conjunction with another mode of 

action herbicide with overlapping spectrum.

 Manag

e weeds in and around fields, during and after harvest to reduce weed seed production.

4. Spray Drift Management

a. Tank Mix Instructions:

TANK-MIXING INSTRUCTIONS: 

ENLIST Duo may only be tank-mixed with products that have been tested and found 

not to adversely affect the spray drift properties of Enlist Duo. A list of those products 

may be found at EnlistTankmix.com DO NOT TANK-MIX ANY PRODUCT WITH 

Enlist Duo unless: 

 You check the list of tested products found not to adversely affect the spray drift 

properties of Enlist Duo at EnlistTankmix.com no more than 7 days before applying 

Enlist Duo; and 

 The product you tank-mix with Enlist Duo is identified on that list of tested products. 

b. Droplet Size:

Use of Enlist Duo requires the use of specific nozzle and spray pressure combinations. 

A chart is included in the product label that lists the specific nozzle and pressure 

combinations that are allowed. 

c. Groundboom Application: 

Use the minimum boom height based upon the nozzle manufacturer’s directions. Spray 

drift potential increases as boom height increases. Spray drift can be minimized if 

nozzle height is not greater than the maximum height specified by the nozzle 

manufacturer for the nozzle selected.

d. Wind Speed:

Do not apply at wind speeds greater than 15 mph.

e. Temperature and Humidity:
27

When making applications in low relative humidity, set up equipment to produce larger 

droplets to compensate for evaporation. Droplet evaporation is most severe when 

conditions are both hot and dry.

f. Temperature Inversions:

Applications should not occur during a local, low level temperature inversion because 

drift potential is high. Temperature inversions restrict vertical air mixing, which causes 

small suspended droplets to remain in a concentrated cloud. This cloud can move in 

unpredictable directions due to the light variable winds common during inversions. 

Temperature inversions are characterized by increasing temperatures with altitude and 

are common on nights with limited cloud cover and light to no wind. They begin to 

form as the sun sets and often continue into the morning. Their presence can be 

indicated by ground fog; however, if fog is not present, inversions can also be identified 

by the movement of the smoke from a ground source generator. Smoke that layers and 

moves laterally in a concentrated cloud (under low wind conditions) indicates an 

inversion, while smoke that moves upward and rapidly dissipates indicates good vertical 

air mixing.

g. Application Restrictions: 

Do not aerially apply this product.

Do not irrigate treated fields for at least 24 hours after application of Enlist Duo.

Do not make application of Enlist Duo if rain is expected in the next 24 hours.

5. Protection of Sensitive Areas:

a. Buffer

You must maintain a 30 foot downwind buffer (in the direction in which the wind is 

blowing) from any area except:

 Roads, paved or gravel surfaces. 

 Planted agricultural fields (except those crops listed in the “Susceptible Plants” section).

 Agricultural fields that have been prepared for planting.

 Areas covered by the footprint of a building, shade house, green house, silo, feed crib, or 

other man made structure with walls and or roof.

b. Wind Direction

To maintain the required downwind buffer zone:

 Measure wind direction prior to the start of any swath that is within 30 feet of a sensitive 

area.
28

 No application swath can be initiated in, or into an area that is within 30 feet of a 

sensitive area if the wind direction is towards the sensitive area.

6. Susceptible Plants:

Do not apply under circumstances where spray drift may occur to food, forage, or other 

plantings that might be damaged or crops thereof rendered unfit for sale, use or 

consumption. Do not allow contact of herbicide with foliage, green stems, exposed 

non-woody roots of crops, desirable plants; including cotton and trees, because severe 

injury or destruction may result. Small amounts of spray drift that may not be visible 

may injure susceptible broadleaf plants. Before making an application, please refer to 

your state’s sensitive crop registry (if available) to identify any commercial specialty or 

certified organic crops that may be located nearby.

At the time of application, the wind cannot be blowing toward adjacent commercially 

grown tomatoes and other fruiting vegetables (EPA crop group 8), cucurbits (EPA crop 

group 9), grapes and cotton.

C. Registration Terms

EPA has determined that certain registration terms are needed to ensure that likely weed 

resistance as discussed in section IV will be adequately addressed. EPA believes that it is 

important to address likely weed resistance and not wait until confirmation that resistance has 

been found. EPA is basing the registration terms on a list of criteria, presented in the peerreviewed publication, Norsworthy, et al., “Reducing the Risks of Herbicide Resistance: Best 

Management Practices and Recommendations,” Weed Science 2012 Special Issue: 31–62

(Norsworthy criteria).

1. Herbicide Resistance Management (HRM) Plan

EPA is issuing this registration with a term that requires DAS to have an Herbicide 

Resistance Management (HRM) Plan for Enlist DuoTM. The HRM Plan will focus on 

educating growers on the appropriate use of EnlistTM technology. EPA is requiring that 

the HRM Plan must include the following measures that will reduce the potential for the 

development of weed resistance.

a. Investigation

EPA is requiring that DAS or its representative investigate reports of lack of herbicide 

efficacy as reported by users following “scouting.” When investigating any reports of 

lack of herbicide efficacy, DAS or its representative must make an effort to evaluate the 

field for “likely resistance” by applying the “Norsworthy criteria.”

b. Remediation

If “likely resistance” is found, EPA is requiring DAS to engage with the grower to 

control and prevent the spread of likely resistant weeds in the affected area as well as 

requiring DAS to collect material, if possible, for further testing. DAS must provide the 
29

grower with specific information and recommendations to control and contain likely 

resistant weeds, including retreatment and/or other nonchemical controls, as 

appropriate, and if requested by the grower, DAS will assist the grower in implementing 

those additional weed control measures.

c. Annual Reporting of Herbicide Resistance to EPA

EPA is requiring that DAS submit annual summary reports to EPA that include a 

summary of the number of instances of likely and confirmed weed resistance by weed 

species, crop, county and state. The annual reports must include summaries of the

status of laboratory or greenhouse testing for resistance. The annual reports will also 

address the disposition of incidents of likely or confirmed resistance reported in 

previous years. These reports will not replace or supplement adverse effects reporting 

required under FIFRA 6(a)(2). 

d. Reporting of Likely Resistance to other Interested Parties

EPA is requiring that DAS inform growers and other stakeholders of cases of likely 

resistance that are not resolved by the application of additional weed control measures. 

e. Monitoring the use of Enlist DuoTM on EnlistTM Seed

EPA is requiring DAS to monitor whether Enlist DuoTM is being used on the EnlistTM

seed purchased from DAS. EPA is requesting DAS provide EPA with a protocol to 

survey whether Enlist DuoTM is being used on EnlistTM seed purchased from DAS and 

not the non-choline 2,4-D products that are not registered for these application

windows. 

f. Education

EPA is requiring DAS to develop an education program that will provide growers with 

the best available information on herbicide resistance management.

2. EPA’s Continued Control over the Registration 

Because the issue of weed resistance is an extremely important issue to keep under control

and can be very fast moving, this registration will expire unless this term is removed or 

modified by EPA. The date of expiration will be 5 years from the date of the Registration 

Notice if use in the first year is equal to or exceeds 100,000 acres. If use in the first year 

does not exceed 100,000 acres, the date of expiration will be 6 years from the date of 

Registration Notice. This will ensure that EPA retains the ability to easily modify the 

registration or allow the registration to terminate if necessary.

3. Geographic Limitation on Use of Enlist DuoTM

EPA is issuing this registration only to be sold and used in Illinois, Indiana, Iowa, Ohio, 

South Dakota, and Wisconsin. Additional states may be added to the labeling in the future.