UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 128931 

DP Barcode: 404823 

Date: March 24, 2016 

MEMORANDUM 

SUBJECT: Ecological Risk Assessment for Dicamba DGA Salt and its Oegradate, 3,6-

dichlorosalicylic acid (DCSA), for the Proposed Post-Emergence New Use on 

Oicamba-Tolerant Cotton (MON 8770 I) 

TO: 

FROM: 

THRU: 

Grant Rowland, Risk Manager Reviewer 

Kathryn Montague, Product Manager Team 23 

Daniel Kenny, Branch Chief 

Herbicide Branch 

Registration Division (7505P) } / I - ~ 2-l/ 6 

Michael Wagman, Biologist S.J,~ /.;7y/{c; Elizabeth Donovan, Biologist ~~~ J (\ ( - I' ( ,... ~ f ) ) . 2, \-\-\ b 

William P. Eckel, Ph.D., Senior Science Advi or [1\1'\VL /J.A nJY\ ~ Amy Blankinship, Senior Science Advisor (/ v en /, I ~/) 

Environmental Risk Branch 6 ~ ~...-i-- /0. o 

Environmental Fate and Effects Division (7507P) 

Mark Corbin, Branch Chief ~u '? 1 , \I b 

Environmental Risk Branch 6 )- L.-\,.,1 

Environmental Fate and Effects Division (7 

The Environmental Fate and Effects Division (EFED) has completed a review of the new use 

request for the herbicide di cam ha [M 1691 Herbicide, EPA Reg. No. 524-582 (58. l % diglycolamine 

salt of dicamba (DGA); PC code 128931)] for post-emergent (in-crop) use on dicamba-tolerant 

cotton (MON 88701, BOLLGARD II® XTENDFLEXrn cotton). Dicamba is currently registered 

for use on cotton at application rates similar to those proposed for the new use as a pre-emergent 

and post-harvest application, not to exceed 2 lbs a.e./A per year. The proposed new use is included 

on the supplemental label ofM1691 herbicide for pre-emergence and post-emergence (in-crop) use 

on MON 8870 l dicamba-tolerant cotton; this risk assessment is based on the proposed label dated 

December, 20 15. The primary difference between the proposed new use on MON 88701 cotton 

and the current registration on cotton is the timing of applications. The proposed new use allows 

1 
2

post-emergence applications at a maximum single rate of 0.5 lbs a.e./A; the total in-crop rate (2 lbs 

a.e./A) is still enforced. This ecological risk assessment only addresses the differences between 

what is currently registered and the change in the timing of application and incorporates updated 

toxicological endpoints. References for additional details on the fate and transport properties and 

effects data for dicamba are provided in the discussion below. 

Based on the proposed maximum application rates and exceedances of the Agency’s Levels of 

Concern (LOCs), at the screening level there is a potential for direct adverse effects to Federally 

endangered and threatened (referred to hereafter as “listed”) and non-listed birds (acute exposure

only), listed vascular aquatic plants, and listed and non-listed terrestrial dicots from the proposed 

new use. This document does not make effects determinations for those taxa where the level of 

concern for listed species has been exceeded. Further evaluation of the potential for effects to listed 

species and modification of their critical habitat will be provided in another assessment.

Risks due to off-site spray drift exposure and from runoff are detailed in the Risk Estimation and 

Characterization section below. 

While dicamba exists in either acid form or as one of several salts (including dicamba DGA), 

bridging data were submitted indicating that the dicamba salts will be rapidly converted to the free 

acid of dicamba (MRID 43288001). Additionally, submitted effects data indicate similar toxicity 

of the acid and salts (based on acid equivalents). EFED determined that fate studies conducted 

with dicamba acid provide “surrogate data” for the dicamba salts and that toxicity data across the 

acid and salts could generally be combined (USEPA, 2005a).

Although the risks, based on standard risk assessment methods used by EFED, are not expected to 

differ from the previous assessment done for dicamba use on cotton (because the rates are similar to 

those already assessed), there is potential for other ecological concerns that are characterized here, 

but were not evaluated in the previous assessment on conventional cotton. These concerns are 

related to a potential increase in usage of dicamba products, the proposed changes in the timing of 

applications and the fate of dicamba and its metabolite, DCSA, within dicamba-tolerant cotton 

plants. In general, there is also a potential for increased susceptibility of late season plants to direct 

impact from off-site transport from volatilization related to dicamba use.

PROBLEM FORMULATION

Dicamba was first registered in the United States in 1967 and is widely used in agricultural,

industrial, and residential settings. Dicamba is a benzoic acid herbicide similar in structure and 

mode of action to phenoxy herbicides. Dicamba controls annual, biennial and perennial broadleaf 

weeds in crops and grasslands, and it is used to control brush and bracken in pastures. Dicamba is 

formulated primarily as a salt in an aqueous solution. Supported forms are: dicamba acid (PC code 

029801), dicamba dimethylamine salt - DMA (029802), diethanolamine salt (029803), dicamba 

sodium salt (029806), dicamba diglycoamine salt - DGA (128931), dicamba isopropylamine salt 

(128944) and dicamba potassium salt (129043).

This assessment is for the new use request for M1691 Herbicide [EPA Reg. No. 524-582 (56.8% 

diglycolamine salt of dicamba (DGA); PC code 128931)] for use on dicamba-tolerant cotton (MON 

87701). Dicamba is currently registered for use on cotton at application rates similar to those 
3

proposed for the new use. The maximum registered application rate for cotton is 2.0 pounds acid 

equivalent per acre (lbs a.e./A). For the proposed new use on dicamba-tolerant cotton, the 

maximum single application rate is 1.0 lbs a.e. /A for a pre-emergence application and 0.5 lbs

a.e./A for a post-emergence application; the maximum seasonal rate is 2.0 lbs a.e./A.

The primary difference between the proposed new use and the current cotton registration is the 

timing of the applications. The current registration for dicamba on cotton is limited to preemergence and post-harvest applications. The proposed new use on dicamba-tolerant cotton 

includes applications pre-emergence and/or post-emergence. The proposed post-emergence 

applications would occur later in the growing season than the current use pattern (but before postharvest applications), which would increase the potential time period of exposure. 

The major degradate under anaerobic conditions is 3,6-dichlorosalicylic acid (DCSA) which is 

persistent, comprising > 60% of the applied after 365 days of anaerobic incubation in a 

sediment:pond water system (MRID 43245208). DCSA is also formed in aerobic soil under 

laboratory conditions at a maximum of 17.4 % of the applied parent. Toxicity data for DCSA 

effects to mammals has been submitted to the Agency. No other toxicity data for DCSA has been 

submitted to the Agency. Based on available data, DCSA appears to be less toxic or equally toxic 

as the parent (see Table 1) for aquatic organisms on an acute basis, but may be substantially more 

toxic on a chronic basis to terrestrial organisms, specifically mammals. 

Therefore, this assessment will consider the parent and its degradate DCSA together in the aquatic 

assessment (with the assumption that dicamba and DCSA are equally toxic), while the terrestrial 

assessment for mammals will consider parent dicamba and DCSA separately. 

Table 1. Toxicity Data for Dicamba and its Degradate DCSA.

SOURCE DICAMBA DCSA

SUBMITTED DATA (Most Sensitive)

Acute oral Rat (LD50; mg/kg-bw) 2,740 2,641 

(MRID 47899504)

Chronic rat (NOAEC; mg/kg-bw)

136 (based on signs 

of parental 

neurotoxicity and 

delayed sexual 

maturation and

decreased pup weight 

at 450 mg a.e./kg-bw)

8 mg a.e./kg-bw

(based on decreased 

pup weight at 78 mg 

a.e./kg-bw, using 

female lactation 

doses ) 

(MRID 47899517)

Acute oral Avian (LD50; mg/kg-bw) 188 --

Acute FW Fish (LC50; mg/L) 28

Chronic FW Fish (NOAEC; mg/L) -- --

Acute FW Invertebrate (EC50; mg/L) 34.6 --

Chronic FW Invertebrate (NOAEC; mg/L) -- --

NV Aquatic Plant (EC50; mg/L) 0.061 --

V Aquatic Plant (EC50; mg/L) >3.25 --

Acute Honeybees (LD50; μg/bee) >90.65 --

Footprint WEBSITE1

Acute oral Rat (LD50; mg/kg-bw) 1,581 >1,560

Acute oral Avian (LD50; mg/kg-bw) 1,373 --

Acute Fish (LC50; mg/L) >100 >100

Chronic Fish (NOAEC; mg/L) 180 --
4

Acute FW Invertebrate (EC50; mg/L) >110.7 89

Chronic FW Invertebrate (NOAEC; mg/L) 97 --

NV Aquatic Plant (EC50; mg/L) 1.8 138

V Aquatic Plant (EC50; mg/L) >3.25 >73

Acute Honeybees (LD50; μg/bee) >100 --

Acute Earthworms (LC50; mg/kg) >1,000 >1,000

1 Pesticide Properties Database (PPDB) (http://sitem.herts.ac.uk/aeru/footprint/en/index.htm)

FW=freshwater, NV=non-vascular, V=vascular

BACKGROUND 

The most recent regulatory actions for dicamba include the following:

• US EPA/EFED (2014) Addendum to the Environmental Fate and Ecological Risk 

Assessment for the Section 3 New Use of Dicamba on Dicamba-Tolerant Soybean (MON 

87708) Clarifying Plant Risks. May 20, 2014. D404138+.

• US EPA/EFED (2011) Ecological Risk Assessment for Dicamba and its Degradate, 3,6-

dichlorosalicylic acid (DCSA), for the Proposed New Use on Dicamba-Tolerant Soybean 

(MON 87708). March 8, 2011. D378444.

• US EPA/EFED (2010) Reduced Risk Request for Dicamba Herbicide Over-The-Top of 

Dicamba-Tolerant Soybean. May 27, 2010.

• US EPA. (2010) EFED Response to a FIFRA Section 18 Emergency Exemption for 

Dicamba co-formulated with 2,4-D (LatigoTM) Use on Turf grown for grain, seed, and hay 

to control broadleaf weeds. Requested by the Oregon Department of Agriculture. May 24, 

2010. D377095

• US EPA (2006) Reregistration Eligibility Decision for Dicamba and Dicamba Salts. 

June 8, 2006.

• US EPA (2005a) EFED Reregistration Chapter for Dicamba/Dicamba Salts. August 31, 

2005. D317696

• US EPA (2005b) Drinking water assessment for dicamba on sugarcane. May 31, 2005. 

D317705

Consistent with the previous assessments, the environmental fate and effects data used in this 

assessment are bridged across the dicamba acid and all of the supported dicamba salts (MRID 

43288001). EFED established a strategy for bridging the environmental fate and effects data 

requirements for the dicamba sodium and potassium salts, dimethylamine salt (DMA), 

isopropylamine salt (IPA) and diglycoamine salt (DGA) to the dicamba acid. Bridging data were 

submitted indicating that the dicamba salts will be rapidly converted to the free acid of dicamba. 

Additionally, the submitted effects data indicate equal toxicity of the acid and salts (based on acid 
5

equivalents). EFED determined that fate studies conducted with dicamba acid provide “surrogate 

data" for the dicamba salts and that toxicity data across the acid and salts could generally be 

combined. (USEPA, 2005a)

MODE OF ACTION

Dicamba is a benzoic acid herbicide similar in structure and mode of action to phenoxy herbicides. 

Like the phenoxy herbicides, dicamba mimics auxins, a type of plant hormone and causes abnormal 

cell growth by affecting cell division. Dicamba acts systemically in plants after it is absorbed 

through leaves and roots. It is easily transported throughout the plant and accumulates in new 

leaves.

USE CHARACTERIZATION

Monsanto Company submitted a new use request for the herbicide dicamba [M1691 Herbicide, 

EPA Reg. No. 524-582 (56.8% diglycolamine salt of dicamba)] for use on dicamba-tolerant cotton 

(MON 87701). M1691 Herbicide is a water-soluble formulation intended for control and 

suppression of many broadleaf weeds, woody brush and vines. Table 2 presents the proposed 

application rates to the dicamba-tolerant cotton. Rates for dicamba salts are normalized to dicamba 

acid equivalent per acre (a.e./A).

Table 2. Dicamba DGA Proposed Use Pattern for Dicamba-Tolerant Cotton. 

Crop

Maximum Individual

Application Rate3

lbs dicamba a.e./A

Number of

Applications

Application 

instructions 

and intervals

(days)

Max Annual 

Application Rate 

in lbs dicamba 

a.e./A/year

Application 

Method

Dicambatolerant 

cotton

MON 87701

Pre-emergence (preplant, at planting, or 

prior to crop 

emergence) 2

1.0 14

Pre-plant, at 

planting, or 

prior to crop 

emergence. 

1.0

2.0

total

Restricted to 

ground 

sprays only Post-emergence1

(Preharvest) 0.5 44

From 

emergence to 7 

days prior to 

harvest, 

minimum 7 

days between 

applications

2.0

1

- M1691 Herbicide 2

- Registered uses 3

- “Acid equivalent” 4

- Calculated by dividing the max application rate by the max individual application rate.

It is common for products like this to be tank mixed with other products and pesticide active

ingredients, but the label for this use prohibits tank-mixing with other herbicides and only allows 

tank-mixes with products that have been tested and found not to increase the likekihood of 

drift/volatility. EFED recommends that additional guideline laboratory plant testing be required if 

proposed tank mixes include additional active ingredients to account for potential synergistic 

phytotoxic effects. Testing of such products should include the standard suite of tested species 

from the already submitted dicamba and other active ingredient’s vegetative vigor studies as well as 

those that the open literature and any other data that may indicate potential for synergistic effects.
6

According to the proposed label, aerial application of dicamba to dicamba-tolerant cotton is not 

permitted (i.e., it is restricted to ground applications only). 

The proposed dicamba registration is for use on dicamba-tolerant cotton (MON 87701). Figure 1

shows acres of cotton harvested in 2014 in the U.S., per USDA. It is assumed that the new use of 

dicamba on dicamba-tolerant cotton would be within this 17-state area. The figure indicates that 

there were approximately 10 million acres of cotton harvested in 2014. The states shaded in red in 

the diagram below indicate a decrease in harvested cotton acres from the previous year while blue 

shading indicates an increase in harvested acres from the previous year.

FIGURE 1. Acres of Cotton Harvested By State in the United States in 2014 (based on 

information from USDA-NASS) 

http://www.nass.usda.gov/Charts_and_Maps/Field_Crops/cotnacm.asp

ENVIRONMENTAL FATE CHACTERIZATION

Dicamba is very soluble (6,100 ppm) and mobile (Koc = 13.4 L/mg o.c.) in the laboratory, and is 
7

not expected to bioaccumulate in aquatic organisms as it is an anion at environmental pHs (pKa = 

1.9). Dicamba is unstable to aerobic metabolism with half-lives on the order of days, while it is 

generally stable to abiotic processes, and it is generally more persistent under anaerobic conditions. 

Dicamba may reach surface water via run-off, spray drift during application, and by vapor drift 

from volatilization (see analysis below in the volatilization characterization). It is important to note 

that multiple literature studies show that there is a high vapor drift from soybean fields resulting in 

non-target plant injury.

1 Therefore, an analysis of drift from particles volatilized from the treated 

field was completed (see below). Dicamba is less likely to be available to leach to groundwater 

because it is so susceptible to aerobic degradation. However, any dicamba reaching groundwater 

would be somewhat persistent (due to its relatively persistent anaerobic half-life). The major route 

of exposure to non-target organisms is likely spray drift and runoff. A refined characterization of 

potential exposures from runoff, spray drift and volatility is presented in this assessment. Available 

incident reports indicate that visual observations of off-field plant damage have followed dicamba 

applications and subsequent spray drift and/or volatilization of dicamba residues (discussed below 

in the incident characterization section).

The major degradate under anaerobic conditions is 3,6-dichlorosalicylic acid (DCSA), comprising 

> 60% of the applied after 365 days of anaerobic incubation in sediment-pond water system (MRID 

43245208). DCSA is not persistent when formed under aerobic conditions and degrades roughly at 

the same rate as the parent (8.2 days, MRID 43245207). DCSA was also found in the two 

acceptable field studies in soil segments deeper than 10 cm, and is believed to be persistent if it was 

to reach anaerobic ground water. This degradate was formed in aerobic soil under laboratory 

conditions at the maximum of 17.4 % of the applied parent. Other minor dicamba degradates of 

concern are DCGA and 5-OH-dicamba, and both are less toxic than the parent and DCSA. The 

formation of DCGA in the laboratory studies did not exceed 3.64%, and the formation of 5-OH 

dicamba did not exceed 1.9 % in soil/water system during anaerobic aquatic degradation of 

dicamba under laboratory condition. DCSA was also a major metabolite in plant metabolism and 

magnitude of residue studies for dicamba-tolerant soybean and cotton.

Chemical structures of dicamba and dicamba salts are presented in USEPA, 2011. Further details 

regarding fate and transport laboratory and field studies submitted for dicamba can be found in 

(USEPA, 2005 (EFED Reregistration Chapter); USEPA, 2011).

Aquatic Exposure Estimates

Tier II modeling was performed for dicamba acid and its major degradate DCSA using the Surface 

Water Concentration Calculator (SWCC v1.106) coupled with the standard pond scenario. The 

standard Mississippi cotton scenario was selected to assess runoff potential from vulnerable use 

sites. The modeling scenario for DCSA was based on the following: (1) assuming 17.4% 

conversion from parent to DCSA and (2) using molecular weight conversion to adjust from parent 

application rate to DCSA application rate. Tables 3 and 4 list the input parameters used for the 

PRZM/EXAMS modeling of dicamba acid and DCSA degradate.

Table 3. SWCC Input Parameters for Parent Dicamba.

 

1 Al-Khatib and Tamhane, 1999; Auch and Arnold, 1978; Everitt and Keeling, 2009; Kelley et al., 2005; Hamilton and 

Arle, 1979; Lanini, 2000; Marple et al., 2008; Wall, 1994; Weidenhamer et al., 1989; Wax et al., 1969.
8

Model Input Variable Input Value Source and Comments

Application rate 

(kg ai/hectare) 

1.12; 0.56; 0.56 M1691; EPA Reg. No. 524-582

Number of appl./season 3 M1691; EPA Reg. No. 524-582

Interval between applications 7 days M1691; EPA Reg. No. 524-582 

Application Method Ground M1691; EPA Reg. No. 524-582

Scenario modeled (Metfile) -

Initial Application Date

MScottonSTD 

(W03940.dvf) - 16 

April

Dates based on the crop profile, date of planting, 

& precipitation data.

Henry’s Law Constant (atm 

m3/mol)

1.6 x 10-9 Estimated

(VP x MW)/(760 torr/1 atm * solubility)

Molecular Weight (g/mol) 221 SANDOZ Safety Data Sheet (Nov, 1989).

Solubility @ 25°C (mg/L) 6100 SANDOZ Safety Data Sheet (Nov, 1989).

Vapor Pressure (torr) 3.41 x 10-5 SANDOZ Safety Data Sheet (Nov, 1989).

Koc (mL/g o.c.) 13.4 (average) MRID 42774101

Aerobic Soil Metabolic Halflife (days)18 MRID 43245207; (6d x 3)

Aerobic Aquatic Metabolic 

Half-life (days)

72.9 MRID 43758509; 3x a single half-life value of 

24.3 days

Anaerobic Aquatic Metabolic 

Half-life (days)

423 A single half-life value was available (MRID 

43245208); 3x the half-life value (141 x 3 = 423) 

Hydrolysis (pH 7) half-life 

(days)

0 Stable. MRID 40547902 

Aquatic Photolysis Half-life 

(days)

105 MRID 42774102. Adjusted half-life to represent 

sun intensity and 12 hours of sunlight per day. 

38.1 day value represented continuous sun 

exposure at an intensity of 1.38 times natural 

sunlight.

Table 4. SWCC Input Parameters for DCSA.

Model Input Variable Input Value Source and Comments

Application rate (kg 

ai/hectare)

0.18; 0.09; and 0.09 (degradate molecular weight)/(parent molecular 

weight) x max%formation x application rate = 

(207/221)x 0.174

x 1.12

Number of appl./season 3 EPA Reg. No. 524-582

Interval between appl. (d) 7 days EPA Reg. No. 524-582

Scenario modeled (Metfile) -

Initial Application Date

MScottonSTD 

(W03940.dvf) - 16 

April

Dates based on the crop profile, date of planting, 

& precipitation data.

Henry’s Law Constant (atm 

m3/mol)

1.6 x 10-9 Estimated for dicamba and used for DCSA

Molecular Weight (g/mol) 207 Product Chemistry
9

Model Input Variable Input Value Source and Comments

Solubility @ 25°C (mg/L) 2112 MRID 43095301

Vapor Pressure (torr) 3.41 x 10-5 For Dicamba

Koc (mL/g) 1208 (average) MRID 43095301

Aerobic Soil Metabolic Halflife (days)24.6 MRID 43245207; (8.2 d x 3) 

Aerobic Aquatic Metabolic 

Half-life (days)

49.2 No acceptable data were available; 2x the half-life 

corresponding to the PRZM aerobic soil 

metabolism rate input value (2x 24.6d)

Anaerobic Aquatic Metabolic 

Half-life (days)

0 Stable. MRID 43245208. 

Hydrolysis (pH 7) Half-life 

(days)

0 Stable. MRID 43245208

Aquatic Photolysis Half-life 

(days)

105 No data for DCSA; therefore, used value for 

dicamba.

SWCC Modeling Output

Table 5 presents combined SWCC estimated environmental concentrations in surface water for 

dicamba acid and the DCSA degradate for the proposed use on dicamba-tolerant cotton. These 

estimated environmental concentrations (EECs) were used to calculate risk to aquatic animals and 

plants. 

TABLE 5. Combined SWCC Estimated Environmental Concentrations (EECs) for Dicamba 

Acid and DCSA Degradate.

Scenario 1-in-10-year Peak 

EEC

1-in-10-year 

21-day mean EEC

1-in-10-year 

60-day mean EEC 

MS cotton 42.6 ppb 40.4 ppb 35.8 ppb

EECs represents a combined value for the parent and degradate

Terrestrial Exposure Estimates

Dicamba-specific Half-Life and Parent Dicamba Exposure Estimates

In the risk assessment for the proposed new use on soybeans (USEPA, 2011), EFED modeled the 

dicamba residues using a single application (the T-REX model available at the time of the 

assessment was incapable of running applications with variable rates) and used the default foliar 

dissipation half-life value of 35 days. For this cotton new use risk assessment, EFED has refined 

the estimates of exposure by modeling the residues using the maximum potential application rates 

with minimum application intervals (a 1 lb a.e./A application followed by two 0.5 lb a.e./A 

applications with a seven day interval between each application) and a chemical-specific foliar 

dissipation half-life value for parent dicamba.

EFED used residue data by Jimenez (1994; MRID 43370701) to calculate a dicamba specific foliar 

dissipation half-life. According to the available Health Effects Division (HED) review (DP 

Barcode 207649, 3/11/1996), this study was acceptable for use in risk assessment and indicated that 
10

there was no difference in foliar dissipation data between the various tested dicamba salt 

formulations (DMA, DGA and sodium salt formulations). Therefore, data for all dicamba salt 

formulations tested were used to calculate the final foliar half-life value. 

Half-lives were calculated for each set of residue decline data based on the NAFTA Guidance for 

Evaluating and Calculating Degradation Kinetics in Environmental Media and using the PestDF 

package in the R statistical program. Each equation was evaluated for appropriateness before 

inclusion in the final half-life calculation. Individual decline data and estimated foliar half-life 

values are available in Table 6. A summary of this information is provided in the table below. The 

upper 90th percentile, one tailed, confidence interval of 8.4 days was used to calculate refined EECs 

in this assessment.

Table 6. Dicamba Half-Life (days) in Foliage 

Arithmetic 

Mean

Standard 

Deviation

Max Value Min Value Number of 

Values

Upper 90% 

CL on the 

mean

7.3 6.6 43.7 1.11 99 8.4

Exposure estimates for terrestrial animals assumed to be in the target area or in an area exposed to

spray drift are derived using the T-REX (Terrestrial Residue EXposure model) model (version

1.5.2., 6/11/2013). This model incorporates the Kenaga nomograph, as modified by Fletcher et al.

(1994), which is based on a large set of actual field residue data. The upper limit values from the

nomograph represent an approximation of the highest residue value observed in the data set

(Hoerger and Kenaga 1972). Consideration is given to different types of feeding strategies for

mammals and birds; including herbivores, insectivores and granivores. For dose-based exposures,

three weight classes of birds (20, 100, and 1000 g) and mammals (15, 35, and 1000 g) are

considered. The dicamba-specific foliar dissipation half-life of 8.4 days was used for risk 

estimation. The assessment assumes a maximum single application rate of 1.0 lb a.e./A followed 

by two 0.5 lb a.e./A applications with a 7 day application interval to estimate terrestrial exposures 

of dicamba. The dose- and dietary-based EECs (upper bound Kenaga) on a variety of food items 

from the use of dicamba applied at the maximum labeled rates is provided in Appendix 1, along 

with the full T-REX inputs and output.

DCSA Metabolite Exposure Analysis

The available data indicate that in mammals, DCSA has similar acute toxicity as parent dicamba, 

but is substantially (17x) more toxic on a chronic basis. DCSA residues following dicamba 

applications prior to planting conventional cotton are generally considered negligible and would not 

be of concern (USEPA, 1983 and 1984). However, in dicamba-tolerant cotton plants, DCSA can 

form in greater amounts (MRIDs 48728701 & 48728703). Based on the available data, EFED 

evaluated the DCSA metabolite separately from parent dicamba in the chronic terrestrial effects 

assessment.

Appendix 2 shows residues of dicamba and its metabolites in cotton plants following a number of 

different treatment regimes (data from MRIDs 48728701 & 48728703). The highest residues for 

both dicamba and its metabolite DCSA were found in cotton gin byproducts following TRT 4 (4 
11

post-emergent applications of 0.5 lb/A for a total seasonal application rate of 2.0 lb/A) where 

maximum DCSA residues were approximately 21% of the maximum total dicamba-related residues 

(6.29 ppm DCSA compared to 23.6 ppm dicamba) while undelinted cotton seed had substantially 

less residues (0.27 ppm DCSA and 1.54 ppm dicamba). EFED used the maximum values from the 

empirical data on gin byproducts and undelinted cotton seeds to assess risk from DCSA residues 

following post-emergent applications of dicamba on DT-cotton plants to terrestrial vertebrates. Gin 

byproducts for cotton can include a number of different plant parts including fragments of burs, 

stems and leaf material and immature cottonseed. Since gin byproducts can include various plant 

parts including immature seeds and since the mature seeds had very low measured DCSA residues

(thereby, the immature seeds could potentially be influencing the overall residue concentration 

when using all gin byproducts), it is possible that the maximum DCSA residues in cotton plant

tissues may be slightly higher. Additional data on the distribution of DCSA residues in the various 

cotton plant parts (e.g. stem, leaves) over a broader temporal range would decrease this uncertainty. 

However, the best available data indicate that DCSA is a much smaller fraction of dicamba related 

residues in the DT-cotton system and using the maximum empirical residues is considered a 

conservative approach. 

Based on the available plant metabolism data for DCSA on non-DT plants, EFED assumed that any 

exposure for terrestrial vertebrates occurs as a result of feeding solely on DCSA in DT-cotton and 

no exposure to DCSA is expected for terrestrial vertebrates feeding off the field, even if dicamba 

residues should occur following spray drift or volatilization. 

ASSUMPTIONS AND UNCERTAINTIES

DCSA percent formation used for the modeling “application rate” calculation was based on the 

amount of degradate formed in the aerobic soil metabolism conducted on silt loam soil. It is 

possible that DCSA may be formed in different amounts in different soil types, and could result in 

DCSA EECs being under- or overestimated. 

ENVIRONMENTAL EFFECTS DATA

Assessment of risk is based on the most sensitive species tested for terrestrial and aquatic 

organisms. The acute and chronic toxicity values for the most sensitive terrestrial and aquatic 

organisms tested are presented in Table 7. These endpoints are based on those presented in the 

most recent assessment conducted for dicamba (USEPA 2011, D378444), except for the terrestrial 

plant endpoints for dicamba DGA and the chronic mammalian endpoints for dicamba acid and the 

metabolite DCSA. 

The risks to terrestrial plants were evaluated using new toxicity information from seedling 

emergence (MRID 47815101) and vegetative vigor (MRID 47815102) terrestrial plant studies 

conducted with a typical end-use product (TEP) representative of the product being proposed here 

for use on dicamba-tolerant soybean. The new vegetative vigor study was classified as

“supplemental” due to a decrease in plant height in lettuce controls. Quantitative data for the other 

nine species in the study may be used in risk assessment, but the endpoints for lettuce may not be 

used quantitatively in risk assessment. However, the qualitative data in MRID 47815102 indicate 
12

that the endpoints for this species are likely not as sensitive as the quantitative endpoint for the 

most sensitive species, soybean (plant height). The new data indicates that the DGA salt may be 

less toxic to monocots than dicots, but has an EC25 approximately 13 times more toxic to the 

vegetative vigor of dicots than dicamba acid. It is unclear if the enhanced toxicity to dicots is due 

to synergistic effects with surfactants and adjuvants in the formulation used (Clarity Herbicide, 

EPA Reg No. 7969-137, 56.8% DGA salt) or due to the DGA salt itself. 

The screening-level risk assessment for the proposed new use on soybeans (USEPA, 2011. 

D378444) used the chronic endpoint from the rat 2-generation study (MRID 43137101), a NOAEL 

of 45 mg/kg-bw, based on decreased pup weight at 136 mg/kg-bw compared to the concurrent 

controls. The Health Effects Division (HED) recently reanalyzed the data from this study (USEPA, 

2016a; D431873) in comparison to the historical control database range and determined that the 

NOAEL and LOAEL should be raised to 136 and 450 mg/kg-bw, respectively, because pup 

weights in each generation in the 136 mg/kg-bw treatment group were within the historical control 

range and above the historical control mean for the F1, F2A and F2B generations. 

The screening-level risk assessment for the proposed new use on soybeans also used data from a 

preliminary review of a rat 2-generation study with DCSA (MRID 47899517) which identified a 

chronic NOAEL endpoint of 37 mg/kg/d based on decreased parental bodyweight in the 362 

mg/kg/d treatment group. However, since that assessment was completed, the chronic endpoint was 

revised in the final DER review (USEPA, 2012) and a recent memorandum for a benchmark dose 

analysis (USEPA, 2016a) to reflect the observed statistically significant decreases (6-9%) in 

offspring weight on 14 and 21 post-natal days (PND). The DER review from 2012 considered the 

LOAEL from effects to pup weight to be based on the male pre-mating dose of 37 mg/kg/d, with a 

corresponding NOAEL of 4 mg/kg/d. The recent benchmark dose analysis conducted by HED 

(USEPA, 2016a) determined BMD5 (estimated benchmark dose (BMD) to result in 5% body 

weight change in pups from background levels) and BMDL5 (the lower 95% confidence level on 

the BMD5) based on both the male pre-mating dose and the female lactation dose and noted that 

female lactation doses are more reflective (than male-premating doses) of pup exposure during the 

nursing period when the pup body weight decreased. This analysis concluded that the pup weight 

LOAEL and NOAEL threshold values based on the dam lactation doses would be 78 mg/kg/d and 8 

mg/kg/d, respectively. HED also calculated a BMD5 (estimated benchmark dose to result in 5% 

body weight change in pups from background levels) and BMDL5 (the lower 95% confidence level 

on the BMD5) of 38.6 and 34.9 mg/kg/d, respectively, based on the female lactation doses. In the 

revised analysis below, EFED used the NOAEL value of 8 mg/kg/d for risk estimation and further 

characterized the risk using the BMDL5 of 34.9 mg/kg/d for DCSA effects to mammals. 

No chronic data is available for the effects of the DCSA degradate to birds (or reptiles or terrestrialphase amphibians, for which birds are surrogates). As a highly conservative approach, EPA will 

consider the toxicity differential for chronic effects between parent dicamba and the metabolite 

DCSA and apply a similar ratio to estimate chronic effects to avian organisms. Therefore, a factor 

of 17x (based on the chronic endpoints of 136 mg/kg-bw for parent dicamba and 8 mg/kg-bw for 

DCSA) is applied to the dicamba chronic NOAEC of 695 mg/kg-diet for the mallard duck, to result 

in a highly conservative estimate of a chronic NOAEC of 40.9 mg/kg-diet for birds for DCSA. This 

is considered a highly conservative approach as the chronic mammalian endpoint is based on 

effects to pups who would have been continually exposed to DCSA residues in utero and 
13

throughout lactation while chicks in the avian reproduction test would not be exposed to DCSA 

residues while still in the egg or post hatch.

TABLE 7. Toxicity Values Used to Assess Risks from Use of Dicamba and its Metabolite, 

DCSA. Test substance was dicamba DGA unless otherwise noted in the footnotes.

SPECIES ACUTE ENDPOINT Chronic Endpoint MRID

Rainbow trout 

(Oncorhynchus mykiss) LC50 = 28 mg a.e./L4 No data available 400980011

Fathead minnow

(Pimphales promelas) LC50 > 56.4 mg a.e./L5 NOAEC = 9.7 mg a.e/L4 487180102

, 487180082

Sheepshead minnow 

(Cyprinodon variegates) LC50 > 180 mg a.e./L NOAEC = 11 mg a.e./L4 000253901, 487180112

Water flea (Daphnia 

magna) EC50 > 100 mg a.e./L NOAEC = 42 mg a.e./L5 40094602, 487180072

Grass shrimp 

(Palaemonetes purgio) EC50 > 100 mg a.e./L No data available 00034702

Mysid shrimp No data available NOAEC = 11 mg a.e./L4 487180122

Duckweed (Lemna gibba) IC50 > 3.25 mg a.e./L NOAEC = 0.20 mg a.e../L 42774111

Blue-green algae 

(Anabaena flos-aquae) IC50 = 0.061 mg a.e./L NOAEC = 0.005 mg a.e./L 42774109

Green algae

(Pseudokirchneriella 

subcapitata)

IC50 = 7.01 mg a.e./L5 EC053 = 0.39 mg a.e./L5 487180092

Bobwhite quail (Colinus 

virginianus) or Mallard 

duck (Anas platyrhynchos)

LD50 = 188 mg a.e./kg-bw 

(quail)

LC50 > 10,000 mg a.e./kgdiet (quail)

NOAEC = 800 mg a.e./kgdiet (duck) (based on a 

reduction in hatchability at 

1,600 mg a.e./kg-diet)

42918001, 00025391, 

43814003

Zebra finch (Taeniopygia 

guttata) LD50 = 207 mg a.e./kg-bw4 No available data 487180132

Rat (Rattus norvegicus) LD50 = 2,740 mg a.e./kgbw

NOAEL = 136 mg a.e./kgbw (based on 

neurotoxicity and delayed 

maturation in parent 

generation, decreased pup 

weight at 450 mg a.e./kgbw)

00078444, 43137101

Rat (Rattus norvegicus) LD50 = 2,6416

NOAEL = 8 mg a.e./kgbw (based on decreased 

pup weight at 37 mg 

ae/kg-bw6.

47899504, 47899517

Honey bee (Apis mellifera) LD50 > 91 μg a.e./bee No data available 00036935

Dicot (Tomato, 

Lycopersicon esculentum) 

– seedling emergence

EC25 = 0.123 lbs ae/A NOAEC = 0.0673 lbs ae/A 47815101

Monocot (Onion, Allium 

cepa) – Seedling 

Emergence

EC25 = 1.68 lbs ae/A NOAEC = 0.64 lbs ae/A 47815101

Dicot (Soybean, Glycine 

max) – Vegetative Vigor EC25 = 0.000513 lbs ae/A NOAEC= 0.000261 lbs ae/A 47815102

Monocot (Onion, Allium 

cepa) – Vegetative Vigor EC25 = 0.472 lbs ae/A EC05 = 0.137 lbs ae/A 47815102

1 The raw data from this study (Mayer and Ellersieck, 1986; MRID 40098001) were not available for review. 

Therefore, per current EFED policy regarding the results from this study, the study was classified as ‘supplemental’. 2 Currently in review.
14

3 EC05 value was used in lieu of non-definitive NOAEC. 4 Test material was dicamba acid. 

5 Test material was dicamba BAPMA salt.

6 Test material was DCSA

“a.e.” = acid equivalent.

RISK ESTIMATION & CHARACTERIZATION

Aquatic Organisms

The aquatic assessment used a total toxic residues (TTR) approach to evaluate risk from dicamba 

and its metabolite DCSA. An RQ was calculated for aquatic animals based on available data for

freshwater fish [specifically rainbow trout (Oncorhynchus mykiss) (MRID 40098001)]. The acute 

RQ for freshwater fish is <0.01 for both dicamba (42.2 μg a.e./L divided by 28,000 μg a.e./L) and 

DCSA (0.4 μg a.e./L divided by 28,000 μg a.e./L), which is below the acute LOC (0.5). The results 

from the remaining acute aquatic animal studies are from limit tests and are non-definitive (i.e., the 

LC50/EC50’s are ‘greater than’ values); therefore, acute RQs were not calculated using these data. 

In order to gain a better understanding of how the EECs for the maximum proposed dicamba 

application rate for cotton relates to the toxicity data currently available for aquatic animals, we 

compared the EECs to the toxicity endpoints using the conservative assumption that the highest 

concentrations tested in the acute aquatic animal studies represent endpoints (e.g., acute: LC50/EC50

= 100 mg a.e./L). This is conservative as it assumes that at that dose, 50% of the animals would not 

have survived, however in these studies there was either no mortality or substantially less than 50% 

mortality at this dose. In this exercise, none of the acute RQs for estuarine/marine fish or aquatic 

invertebrates (freshwater and estuarine/marine) would exceed an Agency level of concern (LOC) 

for dicamba or DCSA (they are all <0.01). 

Chronic RQs for both the fathead minnow and sheepshead minnow would be <0.01, which is well 

below the level of concern (1.0). However, acute toxicity data indicates rainbow trout are more 

sensitive than fathead and sheepshead minnows (LD50 of 28 mg ae/L for trout compared to >56.4 

for the fathead minnow and >180 mg ae/L for the sheepshead minnow). However, the rainbow 

trout would have to be more than 270 times more sensitive than the fathead minnow on a chronic 

basis to result in an exceedance of the chronic LOC. Given that the acute data indicates that 

dicamba is only slightly toxic to rainbow trout, the likelihood that dicamba is more than 2 orders of 

magnitude more sensitive on a chronic basis to rainbow trout compared to minnows is considered 

low. The chronic RQ for aquatic invertebrates is <0.01 based on the most sensitive aquatic 

invertebrate endpoint of 11 mg ae/L for mysid shrimp.

For aquatic plants the only RQ that exceeds an Agency LOC is for listed non-vascular aquatic 

plants for parent dicamba (RQ = 8.5) (see Table 8). RQs for non-listed non-vascular aquatic plants 

and listed and non-listed vascular aquatic plants would all be below the LOC of 1.0.
15

TABLE 8. RQs for Aquatic Plants and the Use of Dicamba on Cotton.

TAXON LISTED/NONLISTED

ENDPOINT (μg 

a.e./L)

Combined DICAMBA and DCSA Degradate

EEC (μg a.e./L) RQ

Vascular Aquatic 

Plant

Non-listed 

species IC50 > 3,250 42.6 (peak) <0.013

Listed species NOAEC = 200 42.6 (peak) 0.21

Non-Vascular 

Aquatic Plant

Non-listed 

species IC50 = 61 42.6 (peak) 0.7

Listed species NOAEC = 5 42.6 (peak) 8.5

Bolded numbers exceed the Agency LOC of ‘1’.

“a.e.” = acid equivalent.

“N/A” = not applicable

Terrestrial Organisms

Assessment for Exposure to Dicamba Residues

Birds

The maximum single application rate for the proposed new use of dicamba on dicamba-tolerant 

cotton is 1.0 lb a.e./A, with a maximum yearly application rate of 2.0 lbs a.e./A. The maximum 

single application rate of 1.0 lb a.e./A can only be used once; the maximum application rate for 

subsequent applications is limited to 0.5 lbs a.e./A. EFED used T-REX version 1.5.2 to simulate 

applications of 1 lb a.e./A, followed by two 0.5 lbs a.e./A at seven-day intervals, with an 8.4-day 

dicamba-specific foliar dissipation half-life. 

In the previous assessments conducted by EFED (USEPA, 2005, 2011), there were risks to birds 

(acute - listed and non-listed) and mammals (acute – listed; chronic – listed and non-listed) 

identified based on LOC exceedances from RQs calculated in T-REX using the 2.0 lbs a.e./A

application rate. At the 1.0 plus 0.5 plus 0.5 lb a.e./A application rates, the Agency’s acute LOCs 

are exceeded for listed and non-listed birds [acute dose-based RQs range from <0.01 (large

granivore birds) to 2.21 (small birds consuming short grass)] (see Table 9 and APPENDIX 1). No 

chronic RQs exceed the Agency’s LOC for chronic risk (chronic dietary-based RQs range from 

0.02 to 0.38). 
16

TABLE 9. Acute Dose-Based RQs for Birds from T-REX 1.5.2 for Dicamba Use on DicambaTolerant Cotton.

Dose-based RQs (Dose-based EEC/adjusted 

LD50)

Avian Acute RQs

Size Class (grams)

20 100 1000

Short Grass 2.21 0.99 0.31

Tall Grass 1.01 0.45 0.14

Broadleaf plants 1.24 0.56 0.18

Fruits/pods/seeds/ 0.14 0.06 0.02

Arthropods 0.87 0.39 0.12

Seeds (granivore) 0.03 0.01 <0.01

Bolded numbers exceed the Agency’s acute risk LOC for non-listed species (RQ > 0.5) and bold italicized numbers 

exceed the acute risk LOC for listed species (RQ > 0.1).

Mammals

For mammals, none of the acute RQs from exposure to dicamba exceed any of the Agency’s LOCs 

(acute dose-based RQs range from <0.01 to 0.04 for dicamba). Additionally, none of the dietarybased chronic RQs exceed the Agency’s LOCs for chronic risk (chronic dietary-based RQs range 

from 0.01 to 0.10 for dicamba). Chronic dose-based RQs also do not exceed the Agency LOC for 

chronic risk from dicamba (RQs range from 0.01 to 0.84; see APPENDIX 1).

DCSA Chronic Effects Assessment for Terrestrial Organisms

No effects data are available for the chronic effects of DCSA to birds. EFED conservatively 

assumed that the ratio of parent dicamba to DCSA toxicity (17x differential) from the mammalian 

toxicity data could be applied to the chronic effects endpoint for birds. However, since the DCSA 

chronic endpoint for mammals is based on effects to pups who were continually exposed in utero

and throughout lactation in the study, it may be conservative to assume that this toxicity differential 

in mammals for parent dicamba and DCSA would be equivalent for chicks who in an avian 

reproduction test would have been presumed to not be exposed to DCSA residues during their 

gestation in the egg (beyond initial maternal transfer into egg during egg development) or posthatch.

Empirical data for DCSA are available from magnitude of residue studies reviewed by HED

(MRIDs 48728701 and 48728703) for dicamba pre and post-emergent applications (4 applications 

at a total of 2.0 lbs a.e./A, 4 different treatment groups with differing timing of applications). This 

data show dicamba and DCSA residues in undelinted cotton seed and gin byproducts (residual 

cotton plant parts) had maximum residues of 23.6 mg/kg-diet (ppm) dicamba and 6.29 ppm DCSA 

at 6-7 days following the last application. Using the maximum DCSA residues in gin byproducts 

(6.29 ppm) or undelinted cotton seed (0.27 ppm) would not result in an exceedance of the chronic 

LOC for any size class of mammal or bird (RQs would range from <0.01—0.34; Table 10). 

Residues in arthropods (as a dietary item for birds and mammals consuming insects that have 

consumed cotton tissues with DCSA residues) were assumed to follow the Kenaga nomogram 

relationship between broadleaf plants and arthropods and therefore were considered to contain 4.4

ppm which also would not result in any exceedances (RQ’s range from 0.11—0.24).
17

While this assessment used the comparison of the maximum residues detected with the chronic 

mammalian endpoint, there is some uncertainty due to the limited temporal sampling of DCSA 

residues in DT-cotton tissues (only measured at one time point) and therefore understanding 

formation/decline rates is not possible (to better understand potential maximum residues). Plant 

metabolism studies that track DCSA residues over time in all parts of DT-cotton plants following 

post-emergent applications would decrease this uncertainty. In the absence of this, EFED has used 

the best available data and the maximum measured residues to evaluate the chronic exposure. This 

risk estimation uses the NOAEC endpoint of 8 mg/kg/d. If the BMDL5 of 34.9 mg/kg/d for DCSA 

effects to mammals calculated by HED were used instead, than the maximum chronic RQ would be 

0.08. Given that the maximum measured DCSA residues are not close to the NOAEC threshold 

endpoint (max RQ of 0.34) and the BMDL5 indicates that biological effects may not be expected 

even if residues were an order of magnitude higher than indicated by the maximum measured 

residues, the lack of a plant metabolism study tracking DCSA residues throughout the DT-cotton 

plant may not be considered a major uncertainty.

Table 10. Dose-based exposure, body-weight adjusted chronic endpoints and risk quotients 

for mammals consuming DT-cotton tissues containing DCSA residues (max empirical values 

of 6.29 mg/kg in broadleaf plant tissue (gin byproducts), 0.27 mg/kg in seeds)

Size Class 

(g)

Dietary Item Food Intake 

(k-diet/d)

Dose-based EEC 

(mg/kg-bw)

Adjusted NOAEL 

(mg/kg-bw)

RQ

Small (15g) Broadleaf 

plant

0.0143

58.25

17.58 0.34

Seed 0.00318 0.09 17.58 <0.01

Arthropod 0.0143 4.19 17.58 0.24

Medium 

(35g)

Broadleaf 

plant

0.0231

40.33

14.23 0.29

Seed 0.00513 0.06 14.23 <0.01

Arthropod 0.0231 2.90 14.23 0.20

Large 

(1000g)

Broadleaf 

plant

0.153 9.35 6.15 0.16

Seed 0.0340 0.01 6.15 <0.01

Arthropod 0.153 0.67 6.15 0.11

Table 11. Dose-based exposure and risk quotients for birds consuming DT-cotton tissues 

containing DCSA residues (chronic endpoint assumes a 17x differential in toxicity between 

parent dicamba and DCSA to birds).

Size Class 

(g)

Dietary Item Food Intake 

(k-diet/d)

Dose-based 

EEC 

(mg/kg-bw)

NOAEC 

(mg/kg-bw)

RQ

Small (20g) Broadleaf 

plant

0.0228 7.17 40.88 0.18

Seed 0.0051 0.07 40.88 <0.01

Arthropod 0.0228 5.02 40.88 0.12

Medium 

(100g)

Broadleaf 

plant

0.0649 4.08 40.88 0.07

Seed 0.0144 0.04 40.88 <0.01
18

Size Class 

(g)

Dietary Item Food Intake 

(k-diet/d)

Dose-based 

EEC 

(mg/kg-bw)

NOAEC 

(mg/kg-bw)

RQ

Arthropod 0.0649 2.86 40.88 0.14

Large 

(1000g)

Broadleaf 

plant

0.291 1.83 40.88 0.04

Seed 0.065 0.02 40.88 <0.01

Arthropod 0.291 1.28 40.88 0.03

Beneficial Terrestrial Invertebrates

The initial 2011 risk assessment for dicamba use on tolerant plants (soybean use D378444, 

3/8/2011) was conducted prior to EPA developing methods to quantitatively evaluate risks to 

pollinators. Consequently, that assessment included no quantitative analysis of the risk to 

beneficial terrestrial invertebrates posed by dicamba use patterns. Based on the acute contact data 

for the honey bee, the 2011 assessment qualitatively concluded that direct effects to listed terrestrial 

insect species were not expected.

In June 2014, President Obama issued a memorandum2 establishing a Pollinator Health Task Force, 

co-chaired by USDA and EPA, to create a National Pollinator Health Strategy that promotes the 

health of honey bees and other pollinators (including birds, bats, butterflies, and insects). 

Consistent with this Presidential memorandum, the Office of Pesticide Programs (OPP) has been 

evaluating its own risk assessment methods to more quantitatively characterize pesticide risks to 

bees. This evolution has identified additional honeybee life stage testing and longer duration 

effects tests for adults (i.e., larval acute and chronic studies, and adult chronic studies) as 

potentially important to the risk assessment process. 

This assessment explains additional lines of evidence not considered in the 2011 risk assessment to

better characterize the risk posed by dicamba uses to terrestrial invertebrates and ascertain if the 

lack of additional bee studies (including chronic adult honey bee data and acute and chronic larval 

honey bee data) is a significant limitation to making a more definitive characterization. The 

characterization of the chronic risks to adult and larval bees presented here is a novel approach 

using the best available data. However, this approach is not meant to preclude potential requests 

for additional terrestrial invertebrate data or represent a new framework for EPA assessments for 

estimating potential risks to beneficial terrestrial invertebrates. EPA’s established methodology for 

evaluating risks to pollinators can be found in the Guidance for Assessing Pesticide Risks to Bees 

(USEPA, 2014) and relies on a full suite of honey bee laboratory data including acute and chronic 

effects studies to adult and larval honey bees to fully estimate risk to bees.

Acute Contact and Dietary Exposure to Adult Honey Bees

On an acute contact exposure basis, dicamba is classified as ‘practically non-toxic’ to non-target 

insects including honey bees (honey bee acute contact LD50 > 91 μg a.e./bee (MRID 00036935). If 

 

2 https://www.whitehouse.gov/the-press-office/2014/06/20/presidential-memorandum-creating-federal-strategypromote-health-honey-b
19

an RQ were calculated for terrestrial invertebrates,3 using this non-definitive LD50 (conservatively 

assuming that the highest tested dose would result in 50% mortality where in that study, mortality 

at that dose was only 2.5%) and determining acute contact exposure from a direct spray of dicamba 

as the maximum single application rate (1.0 lbs a.e./A) * 2.7 μg a.e./bee (upper bound for contact 

exposures from a direct spray of 1 lb a.e./A, based on work by Kock and Weisser, 1997), the 

resultant RQ would be 0.03 (2.7/91) or 1 order of magnitude less than the terrestrial invertebrate 

LOC (0.4). Given that the actual dose that would result in 50% mortality is likely considerably 

higher than 100 μg a.e./bee, this analysis can be considered highly conservative. Although this is 

for only a single application of dicamba, it may be considered an appropriate estimate of exposure 

from a direct spray as there is a 7-day minimum interval between applications and the lifespan of a 

bee as a forager is 4-5 days (Winston, 1987). Therefore, the risk to adult honey bees posed by 

contact with residues of dicamba in spray drift is considered low. 

Acute dietary data for non-target insects has not been submitted to the Agency. However, a review 

of the EU Footprint Pesticide Properties Database reported an acute oral value for honey bees that 

indicates dicamba is also practically non-toxic to non-target insects on an oral exposure basis 

(honey bee acute oral LC50 > 100 μg a.e./bee). If an RQ were calculated for terrestrial 

invertebrates,

2 using this non-definitive LC50 (and again, conservatively assuming that the highest 

tested dose would result in 50% mortality) and determining acute dietary exposure of dicamba 

residues of 32.12 μg/bee/bee/day (1.0 lb a.e./A * 110 μg a.e./g {upper-bound residue for tall grass 

from T-REX} * 0.292 g/day {pollen consumption rate}, the resultant RQ would be 0.32 

(32.12/100) or slightly below the proposed terrestrial invertebrate LOC (0.4). Given that the actual 

dose that would result in 50% mortality is likely considerably higher than 100 μg a.e./bee, this 

analysis can be considered highly conservative.

Chronic Oral Exposures to Adult and Larval Honey Bees

Approach

The Office of Pesticide Program’s overview of the FIFRA risk assessment process (USEPA 2004) 

states the following:

In some instances, a core study may not be available for a particular data 

requirement listed in 40 CFR 158. In this case, the risk assessment team may 

consider other sources of information to address the data gap.... Professional 

judgment is used by the risk assessment team to determine the utility of the available 

supplemental data for the proposed risk assessment. (page 36 USEPA 2004) 

EFED has employed, for the last decade, an acute to chronic ratio approach (USEPA 1999, Mount 

et al. 2003) to address data uncertainty for a variety of endpoints including survival, growth, 

development, and reproduction effects from prolonged exposure. The process uses mathematical 

relationships observed between acute and chronic exposure effects endpoints conducted with one 

organism to draw inferences on the potential effects endpoints associated with longer duration 

exposures in another organism, where only acute exposure effects endpoints have been measured. 

 

3 The employed methodology for calculating an RQ for pollinators was first proposed in the 2012 Pollinator Risk 

Assessment Framework SAP White Paper, and subsequently validated by the panel. The White Paper is accessible 

online at regulations.gov, under docket EPA-HQ-OPP-2012-0543.
20

Consistent with this approach, an evaluation of the invertebrate data uncertainties for dicamba 

focuses on the acute and chronic toxicity data for other invertebrates to determine a relative 

relationship between the acute toxicity endpoints and the chronic endpoints. This relationship is 

then examined relative to the acute toxicity data available for the dicamba equivalence (a.e.) 

endpoints for bees to estimate a chronic endpoint for terrestrial invertebrates. The estimated 

chronic/developmental endpoint is compared to available exposure information to determine if a 

presumption of risk is or is not supported by these lines of evidence.

Aquatic invertebrates were selected for this approach 1) because they represent a relatively closer 

taxonomy to terrestrial invertebrates than would be achieved for tested vertebrate species and 2) 

because of a lack of a specific taxonomically-based mechanism of action for invertebrate responses 

to dicamba DGA salt. Moreover, the chronic data available for aquatic invertebrates includes 

measures of effects on survival, reproduction, growth, and development milestones over protracted 

exposure windows, which are conservatively applicable to the growth, development and survival 

measurement endpoints expected from acute and chronic bee larval and adult bee chronic studies.

Analysis

The available data from the cotton risk assessment for honeybees and for freshwater invertebrates 

are summarized below:

Acute Daphnia magna EC50 > 100 mg a.e./L MRID 40094602

Chronic Daphnia magna NOAEC = 42 mg a.e./L4 MRID 48718007

Acute Honey bee contact LD50 > 91 μg a.e./bee MRID 00036935 

Acute Honey bee oral LD50 > 100 μg a.e./bee EU Footprint Database5

Although the acute daphnid data are non-definitive, a comparison of the acute and chronic 

endpoints, in the case of D. magna, indicates generally low acute and chronic toxicity to aquatic 

invertebrates with a relationship between the acute median lethal toxicity threshold (EC50) and the 

chronic no observed adverse effect concentration (NOAEC) of > 2.38 (>100/42 = >2.38). 

Applying the minimum that this factor could be to the available honey bee median lethal endpoint 

would yield estimates of chronic endpoints of 38 μg a.e./bee for contact exposure and 42 μg 

a.e./bee for oral exposure. Although these estimates may lack conservatism since the ratio of acute 

to chronic toxicity in the daphnid data set may be greater than 2.38, the estimated endpoints may 

still be conservative since the acute values for bees were also not definitive (i.e., the highest dose 

tested was below lethal levels so an actual LD50/LC50 were not demonstrated), so the actual acute 

median lethal value is likely to be greater and yield higher (less sensitive) estimates of chronic 

effect threshold. 

Following EPA’s framework for assessing pesticide risks to bees (USEPA, 2014), the acute contact 

estimated exposure level from dicamba DGA is 2.7 μg a.e./bee for the maximum 1 lb a.e./A 

application rate. This exposure level is over an order of magnitude lower than the estimated 

 

4 Test material was dicamba BAPMA salt

5 EU Pesticide Properties Database (PPDB), http://sitem.herts.ac.uk/aeru/ppdb/en/Reports/213.htm
21

contact-based bee chronic NOAEC of 38 μa.e./bee. The acute dietary estimated exposure level for 

adult honeybees is 32.12 μg/bee/day for the maximum 1 lb a.e./A application rate, which is also 

lower than the ACR-estimated oral exposure chronic bee endpoint of 42 μg a.e./bee. In both cases, 

the estimated exposure level is below the estimated chronic toxicity endpoint for survival, 

developmental, and reproductive effects for honeybees under prolonged exposures, even without 

considering that the acute endpoints in honeybees are very conservative estimates. 

Adult honeybee effects following chronic exposure effects are adequately addressed by this 

approach as well because the endpoint for such a test is survival under prolonged exposure and the 

underlying chronic exposure endpoints used in the approach were conducted at levels producing no 

adult invertebrate mortality. Also it is reasonable to expect that effects measures under longer term 

studies are a conservative surrogate for short duration exposure effects (e.g., an acute larva study 

measuring survival).

Because honeybee risk assessment results serve as a surrogate for assessing risks to other terrestrial 

invertebrates, the findings above indicate no concern for risks to the taxon overall. 

However, there are uncertainties inherent in the evaluation:

1. D. magna chronic studies incorporate long exposure periods and measure survival, 

growth and development of a juvenile crustacean life stage, which are also evaluated 

endpoints in acute and chronic for larval honeybee studies, but the mechanism of action 

may not be exactly a match for non-crustacean arthropods such as larval insects. 

However, given that very similar aquatic effects extrapolations are made across a 

variety of aquatic invertebrate taxa, including insects, the uncertainty herein is no 

greater than the acceptable level of uncertainty in the aquatic invertebrate extrapolations 

routinely employed for aquatic effects characterizations. 

2. Routes of exposure between aquatic organism toxicity tests and honeybee tests are not 

the same. Exposure media in aquatic invertebrate testing is via water, but for chronic 

exposure studies feeding occurs so some oral exposure via diet is probable. Larval 

honeybee testing involves both dietary and dermal exposure under the laboratory testing 

conditions, and adult chronic honeybee studies involve dietary exposure. However, for 

a chemical such as dicamba DGA salt, once dissociated in test solution to the free acid, 

the exposure differences are likely not biologically significant as the uptake for the 

hydrophilic acid is likely diffusion across the membrane with other water soluble 

compounds and therefore absorption for either the gut or gill would be similar, though 

metabolic pathways may be different following uptake through the gill compared with 

the gut.

The refined investigation in this document considered other lines of evidence and focused on:

1. A long standing EFED approach to consider the relationship between acute and 

chronic endpoints in the complete dataset for one organism to draw inferences on 

the same endpoints for another organism when data are lacking (i.e., inferring 

survival, growth and development effects levels for terrestrial invertebrate 
22

developmental life stages and survival in organisms following prolonged exposure); 

and

2. Comparison of resulting extrapolated invertebrate development, growth and 

reproduction endpoints under chronic exposure conditions to estimated levels of 

exposure. 

Considering all lines of evidence, it is reasonable to conclude that risks are likely to be low to larval 

and adult honey bees under the conditions described in the cotton risk assessment scenarios of use 

as the empirical data demonstrate low acute and chronic toxicity to other similar taxa (i.e. aquatic 

invertebrates), low acute toxicity to adult honey bees and based on the ACR analysis presented 

here, chronic toxicity to adult and larval honey bees is also anticipated to be low. Additionally, 

dicamba will be reassessed in registration review [beginning later in 2016]. EPA intends to use 

Registration Review to facilitate submittal of additional pollinator data and is also revising the 

existing insect pollinator data requirements in CFR Part 158 (revision anticipated in 2017) to 

require additional data necessary to complete risk assessments consistent with the risk assessment

framework. Consistent with current use of bee endpoints as a surrogate for other terrestrial 

invertebrates, there is sufficient information to preclude concern for direct terrestrial invertebrate 

risks and any attendant indirect effects mediated through these organisms.

No data is available for the acute or chronic toxicity of dicamba’s degradate DCSA to honey bees 

or other pollinators (the EU footprint database does however report a 14-D LC50 of > 1000 mg/kg 

for earthworms). However, given the low DCSA residues measured in dicamba-tolerant seeds 

(max measured residue of 0.440 ppm), exposures to honey bees and other pollinators from DCSA 

residues in pollen and nectar of dicamba-tolerant soybean are anticipated to be low. 

As noted above for terrestrial vertebrate species, based on the available plant metabolism data for 

DCSA on non-DT plants, EFED assumed that any exposure for terrestrial invertebrates occurs as a 

result of feeding solely on DCSA in DT-cotton plant tissues and no exposure to DCSA is expected 

for terrestrial invertebrates feeding off the field, even if dicamba residues should occur following 

spray drift or volatilization.

Terrestrial Plants

Dicamba exposure to terrestrial and semi-aquatic plants is estimated using the TerrPlant (version 

1.2.2) model. The model generates EECs for plants residing near a use area that may be exposed 

via runoff and/or spray drift. The EECs are generated from one application at the maximum rate 

for a particular use and compound-specific solubility information. Only a single application is 

considered because it is assumed that for plants, toxic effects are likely to manifest shortly after the 

initial exposure and that subsequent exposures do not contribute to the response. Hence, the model 

estimates EECs based on application rate, the solubility factor, and default assumptions of drift. 

Parameter values for application rate, drift assumptions and incorporation depth are based upon the 

use and related application method and can be found in Appendix 3. 

The EECs and resulting RQs for terrestrial and semi-aquatic plants for a single application of 
23

dicamba DGA at the maximum label rate for the proposed use on dicamba-tolerant cotton are 

presented in Tables 12 and 13. RQs were exceeded for listed and non-listed dicots due to spray 

drift or in semi-aquatic areas due to runoff and spray drift. 

Table 12. EECs for Terrestrial and Semi-Aquatic Plants Near Dicamba Use on DicambaTolerant Cotton

Crop

Single Max. 

Application 

Rate (lbs a.e./A)

EECs (lbs a.e./A) 

Ground Spray

Total Loading to 

Adjacent Dry Areas 

(sheet runoff + drift)

Total Loading to SemiAquatic Areas (Channelized 

runoff + drift)

Drift 

EEC

DicambaTolerant 

Cotton

1.0 0.06 0.51 0.01

Table 13. RQ values for plants in dry and semi-aquatic areas exposed to Dicamba DGA 

through runoff and/or spray drift.*

Plant Type Listed Status Dry Semi-Aquatic Spray Drift

Monocot non-listed <0.1 0.30 <0.1

Monocot Listed <0.1 0.80 <0.1

Dicot non-listed 0.49 4.15 19.49

Dicot listed 0.89 7.58 38.31

*If RQ > 1.0, the LOC is exceeded, resulting in potential for risk to that plant group.

EFED’s current screening tool TerrPlant results in a RQ of 0.89 for listed species and 0.49 for nonlisted species of dicots in dry areas, which is less than the LOC for plants of 1.0. Risk quotients 

would be higher if the maximum seasonal rate (2 lb a.e./acre) were applied all at once. Moreover, 

using default assumptions in AgDrift (Tier 1 ground, low boom, fine to medium-coarse spray) the 

LOC for both listed and non-listed dicots (using the most sensitive species, soybean, data, Table 6) 

is exceeded at the maximum distance that the model returns (997 feet). However, the draft label 

contains language restricting applications to a specific nozzle with coarser droplet spectra and other 

restrictions. The implications of these restrictions on spray drift are discussed in the following 

section.

The aforementioned RQ values are for the DGA salt of dicamba. For dicamba acid, which DGA 

salt may dissociate to and which has more sensitive seedling emergence values, RQ values would 

exceed the LOC of 1.0 for all listed and non-listed monocots and dicots in semi-aquatic areas and 

for listed monocots and listed and non-listed dicots in dry areas. It is unclear what the differences 

in observed toxicities of the seedling emergence and vegetative vigor studies between the DGA salt 

and dicamba acid is due to. However, since the vegetative vigor endpoints for dicamba DGA to 

soybean are an order of magnitude more sensitive than any seedling emergence endpoint for 

dicamba acid, this is considered a minor data gap and the major route of concern for terrestrial 

plants is considered to be spray drift of dicamba residues following dicamba DGA applications.
24

Spray Drift Analysis for Exposure to Terrestrial non-Target Organisms

As the application rates are identical between the post-emergent use of dicamba on tolerantsoybean and tolerant-cotton, the following analysis that was conducted for the concurrently issued 

2nd addendum for use on tolerant-soybean (USEPA, 2016b; D426789) is also applicable for the 

present action on cotton.

In the first addendum to the EFED Section 3 risk assessment for dicamba DGA salt for use on 

dicamba-tolerant soybeans (D404138, 5/20/14), EFED estimated that a realistic distance from the 

application site to where no effects are observed to sensitive plants (based on the NOAEC for the 

most sensitive apical endpoint of plant height for the most sensitive tested species, non-dicamba 

tolerant soybeans) ranged from 100 to 175 feet (for the 0.5 lb a.e./A tolerant-soybean postemergent application rate) or approximately double that for the pre-emergent max application rate 

of 1.0 lb/A. However, based on a weight of evidence approach and refined AgDrift modeling for 

coarser droplet spectra (coarse to ultra-course droplet distribution), EFED refined this distance to 

124 feet (rounded up to 125 feet) or to 107 feet if label language were to restrict the droplet size to 

solely extra-coarse and ultra-coarse droplet sizes). 

EFED further refined this analysis after receiving more information including a spray drift 

deposition study submitted by BASF (MRID 49067704). In light of this information, Monsanto 

proposed that the spray drift buffer distance be reduced to 70 feet for M1691 Herbicide using the 

TTI 11004 nozzle at application spray pressures ≤ 63 psi. EFED’s subsequent analysis for 

submitted field trial data (presented below), however, indicates that a larger buffer may be 

necessary in order to limit potential effects to sensitive plants to the sprayed field. Linking this 

data to our previous modeling efforts and employing a weight of evidence approach, EFED 

proposes that the label should be modified to include language to maintain a 100 to 110 foot 

downwind buffer when applying at the 0.5 lbs a.e./A application rate. The July 2015 

amended labels subsequently submitted by Monsanto included a 110 foot buffer and 220 foot 

buffer for 0.5 and 1.0 lbs a.e./A application rates, respectively.

Field Trial Data Discussion

Subsequent to EPA’s 5/20/2014 addendum, Monsanto presented information from academic field 

research that had not previously been submitted to the Agency for review. EPA requested data 

from these field trials and Monsanto submitted the raw data (MRID 49612701 pg. 51) on 4/13/2015 

along with a response document (MRID 49570501 pg. 1). Monsanto’s response document 

included an analysis that the 70 foot buffer would be protective of the no-effect distance for 

sensitive plants (the “no-effect” distance is based on the most sensitive NOAEC for the apical 

endpoint of plant height for the most sensitive tested species, non-dicamba tolerant soybeans) for 7 

of the 9 submitted trials and a proposed rationale for why it may not have been protective in the 

remaining 2 trials. The response document also included Monsanto’s statement that the field trial 

data are not suitable for use in EPA’s regulatory decision-making process, but overall support the 

then-proposed 70 foot buffer. 
25

While EFED agrees that the field trial data are generally not suitable for regulatory decisionmaking, we believe that they demonstrate additional uncertainty that the previously proposed 70 

foot buffer would be sufficient to prevent potential effects to non-target plants that are off the field. 

In an attempt to conduct a quantitative evaluation of the field trial data, EFED considered that the 

data could reasonably represent a dose-response effect, with higher treatment doses expected to be 

closer to the application site. In this context, the distances farthest from the application site were 

considered to be likely to have little to no dicamba residues and loosely were considered controls. 

EFED then considered that plant heights and yield (similar to our apical endpoints of plant height 

and biomass from the standard vegetative vigor plant ecotoxicity tests) at the closer distances (i.e. 

treatment groups) could be compared to those of plants at the “control” distances using statistical 

hypothesis tests, similar to our standard statistical methodologies for data evaluation of ecotoxicity 

tests. In an effort to streamline the data analysis process, EFED used standard t-tests in Excel to 

conduct the analysis.

This statistical analysis indicated that a majority (5/9) of the field trials provided evidence that the 

proposed 70 foot buffer would not be sufficient to keep any effects to sensitive plants’ apical 

endpoints contained to the field. Three of the nine trial sites had significant inhibitions compared 

to the “control groups” at distances greater than EFED’s refined buffer of 125 feet, though EFED 

notes Monsanto’s rationale for the greater distances in two of those sites (Monmouth, IL and 

Haubstadt, IN) might be due to applications not conforming to the currently proposed label 

restrictions for M-1691 Herbicide. The maximum “no effect” spray drift distance that EFED 

determined for the remaining site (Rower, AR) was 147.5 feet. 

Since these field trials involved no true controls and residue analysis was not conducted to confirm 

the lack of residues in the farthest plants, the magnitude of an effect seen between “treatment” 

groups and true control plants might be higher than what this analysis indicates. These field trials 

were all conducted at the 0.5 lbs a.e./A (maximum single post-emergent application rate) and all 

were conducted using the TTI11004 nozzle in accordance with the label directions. The operating 

pressures varied across the sites from 30 psi to 50 psi (other than for the Haubstadt trial site, for 

which nozzle pressures were not recorded), which is less than the labeled maximum operating 

pressure of 63 psi. Higher operating pressures than used in these field trials (but within the 

proposed labeled directions for use), may result in an increased proportion of finer spray droplets 

and consequently result in effects at distances greater than observed in these field trials. The 

specific process, results and conclusions that EFED used in evaluating Monsanto’s submitted field 

trial data and relating it as an additional line of evidence in determining an appropriate buffer that 

would result in no adverse effects to EPA’s apical endpoints for terrestrial plants (the most 

sensitive taxa to the herbicide dicamba), is discussed immediately below.

Details of EFED’s Process to Determine a “No Effect” Spray Drift Buffer from the 

Available Field Trial (MRID 49612701 pg. 51) Data: 

Transects (at each site or for each swath, where multiple swaths were tested) were combined to 

determine mean soybean (non dicamba-tolerant) plant heights (14 & 28 DAT) or yields at set

distances. The farthest two distances for which plant height or yield data were recorded were 

considered “controls,” though there are considerable uncertainties to this approach. Specifically, 
26

no true controls were used, no residue analysis was conducted to confirm that these plants were not 

exposed to dicamba (or other chemical) residues, data were only recorded when there was at least 

5% visual response (which could have been due to a number of factors including potential dicamba 

residues) and for many of these “controls” the height/yield endpoint may not have been recorded in 

all transects, resulting in a lower sample size (n) for controls and therefore a decreased power in the 

statistical t-test. 

All analyses were conducted in MS Excel. Means for each distance towards the sprayer were 

compared to the “control” means to determine the percent inhibition at each distance. T-tests (1-

tailed, assumed equal variances unless an F-test {p<0.05} showed unequal variances) were 

conducted to compare the endpoints of the treatment distances to the controls. Since these were 

field tests and had considerable uncertainties surrounding the controls, EFED considered 

significance at the (α=) 0.1 level which increases the conservatism of the analysis. The buffer for a 

“no effect” distance at each site was considered the first distance greater than a distance which had 

a significant decrease compared to the control group. For example, at the Brooksville, MS site, the 

furthest distance which exhibited a significant decrease (p<0.10) in height at 28 DAT compared to 

the control group was 86.25 feet. The next highest distance at which soybean heights were 

measured was 96.25 feet (not significant, p=0.19), which therefore was considered the “no effect” 

distance buffer for that site.

Results of the Analysis of the Field Trial (MRID 49612701) Data

After reviewing this field trial data, EFED made the following findings. Of the nine field trials 

discussed above, a majority (five) provide evidence that a 70 foot buffer may not be sufficient, and 

four provide evidence that a 100 foot buffer may not be sufficient (Table 1). With a buffer 

distance of 125 feet for a 0.5 lb a.i./A application rate, 3 sites (33%) would provide evidence that a 

larger buffer might be necessary, with Monsanto stating (and subsequently providing information) 

that two of these (Monmouth and Haubstadt) may not have followed the currently proposed label 

by either using a different formulation or applying when wind speed was lower than required by the 

current proposed draft label. 
27

Table 14. Distance (in feet) from Site of Application to a "No Effect" * 

Site Height 

(ft.)

 14 DAT

Height 

(ft.)

28 DAT

Yield

(ft.)

Comments

Brooksville, 

MS

46.25 96.25 66.25

Rower, AR 7.9 20.6 248.7** 14 DAT “controls” had only n=2. 28 DAT controls 

had n=3. **Note, for yield, after the 12% inhibition 

at 223.4’, no treatment group was significantly 

(p<0.1) inhibited compared to controls (inhibitions 

ranged from 1.03—23.75% after this). The higher 

inhibitions were not significant due to the use of the 

nonequal variance t-test, but would have been had 

we assumed equal variances). Therefore, using best 

professional judgment informed by the data and ttest results , EFED has reduced the no effect 

distance for this endpoint to 147.5’, after which all 

inhibitions at shorter distances were > 10% (other 

than 1.1% inhibition at 7.9 feet).

W. Lafeyette, 

IN

66.25 26.25 No Data 14 DAT “controls” had n=3, 28 DAT “controls” 

had n=2

Scott, MS 26.25 26.25 66.25

Jackson, TN 16.25 16.25 16.25 Yield “controls” had n=4.

Kirkwood, IL 116.25 116.25 16.25

Monmouth, 

IL Swath 1

74.2 137.8 0 14 DAT controls had n=4, 28 DAT controls had 

n=3, Yield controls had n=3

Monmouth, 

IL Swath 2

53 95.4 254.4 14 DAT controls had n=3, 28 DAT controls had 

n=2, Yield controls had n=2

Haubstadt, 

IN Swath 1

30 80 10 Swath 1 only took measurements to a maximum of 

100 feet. 14 DAT controls had n=5, 28 DAT 

controls had n=3

Haubstadt, 

IN Swath 2

40 80 150 14 DAT controls had n=3, 28 DAT controls had 

n=3, Yield controls had n=2

Gilbert, IA 

Swath 1

N/A N/A N/A This swath was not evaluated as no field 

measurements were taken past 30 feet.

Gilbert, IA 

Swath 2

35 15 5 14 & 28 DAT and Yield controls had n=4. For 

yield, no distance had lower mean yield compared 

to controls.

* Distance based on Plant Height after 14 and 28 days after treatment (DAT) and Yield (α = 0.10). 

No effect” indicates no reduction in plant height or biomass relative to controls. In controls, the 

sample size (n) is considered 6 (or 10 for Brooksville, MS and Scott, MS trial sites) unless otherwise 

noted in the comments section where fewer controls may affect the power of the test. 
28

Conclusions for the field trial data and weight of evidence approach for spray drift

After reviewing the field trial data submitted to EPA, EFED finds that there is considerable 

uncertainty around the use of a 70 foot in-field buffer with the intent to keep any adverse effects 

(related to our apical endpoints of plant height and biomass) on the field, as the majority of the sites 

appeared to have effects on plant height at distances past this. Though the quality of the data is not 

suitable for the purpose of establishing an quantitative buffer distance (especially as the lack of true 

controls may mean that the magnitude of effects to true control plants could be greater than 

indicated here), EFED believes this data provides a line of evidence that an in-field buffer greater 

than 70 feet is warranted to ensure protection of listed species, such as that determined in our 

previous risk assessment addendum for soybeans (D404138, 5/20/14) which used a refined 

modeling approach extracting out the coarse, extra-coarse and ultra-coarse droplet spectra to 

determine an average 124 foot buffer (rounded up to 125 feet) or solely the extra-coarse and 

ultra-coarse droplet spectra for an estimated average distance of 107 feet (rounded up to 110 

feet) for a 0.5 lbs a.i./A application. The draft label only supports the use of one nozzle (Tee Jet® 

TTI11004) with a maximum operating pressure of 63 psi which restricts droplet spectra to ultracoarse and extremely coarse.

Using a weight of evidence approach (covering the refined modeling analysis conducted in the 

previous risk assessment addendum, the spray drift deposition study submitted by BASF (MRID 

49067704) and the submitted field trial data discussed here), EFED concluded that the label 

should be modified to include language to maintain a 100 to 110 foot downwind buffer when 

applying at the 0.5 lbs a.i./A application rate with nozzles and operating pressures that at a 

minimum restrict droplet spectra to ultra-coarse and extremely coarse. The July 2015 

amended labels subsequently submitted by Monsanto included a 110 foot buffer and 220 foot 

buffer for 0.5 and 1.0 lbs a.e./A application rates, respectively.

Further data that may help refine this estimate would be field trial data with actual controls (and/or 

residue analysis to indicate a lack of dicamba or other herbicide treatments), larger control sample 

sizes and transect replication, field measurements provided regardless of whether plant visual 

response (damage) was observed or not, a greater number of swaths at each trial site (reflective of 

typical practices in soybean agriculture) and using the maximum labeled nozzle operating pressure.

Vapor Analysis for Drift Exposure to non-Target Organisms

After reviewing data submitted to EPA relating to the volatility of dicamba, EFED had concerns 

regarding the volatility of dicamba, and possible post-application, vapor-phase off-site transport 

that might damage non-target plants. Monsanto responded to these concerns with a submission 

(MRID 49612701 pg. 143) that acknowledged the long-recognized volatility of dicamba and 

described measurements of the volatilization in the different formulations. 

The information submitted to address EFED’s concerns was helpful, but the submission did not 

include enough detail to verify the measurements in the studies. EFED determined that it would be 

useful also to perform volatility experiments under varied conditions of temperature and relative 

humidity, as these factors seem to be important in field conditions. 
29

The registrant has agreed to place directional, in-field spray drift buffers of 110 feet for the 0.5 

pound per acre application rate and 220 feet for the 1.0 pound per acre application rate. One open 

literature study (Egan and Mortensen 2012), directly addresses the potential for volatilization and 

transport of dicamba, and the potential for damage to the most sensitive tested species, soybean 

(non dicamba-tolerant). Based on damage assessments of non dicamba-tolerant soybean plants 

placed near treated fields after spray drift from a 0.5 lb/A DGA salt application had dissipated, the 

authors estimated the exposure at distance by correlation to known dose-damage correlations. They 

estimated that the 95% upper bound vapor exposure would drop below the soybean NOAEC at 

approximately a distance of 25 meters (82 feet). This is well within the 110-foot spray drift buffer 

proposed for the 0.5-lb/A rate. Thus, based on at least one study, this buffer distance should be 

adequate to protect against volatilization exposure for EPA’s apical endpoints of plant height and 

yield. However consideration should be made as to whether this buffer distance should be applied 

on all sides of the field, rather than the currently labeled uni-directional buffer according to wind 

direction.

The incident described by MDA in the following section (EIIS incident number I026579-001) 

provides limited information that the proposed 110 to 220-foot spray drift buffers would not be 

adequate to limit off-site plant damage due to post-application volatilization. However, since the 

incident only qualitatively describes visual damage, while the buffer is intended to be protective of 

apical endpoints of height and yield, this remains an uncertainty, and would benefit from additional 

field trial data under varied conditions of temperature and relative humidity. Based on the best 

available data for dicamba residues from vapor drift compared to effects on apical endpoints, EFED 

believes that a 110 foot buffer for the 0.5 lb ae/A application rate should be adequate to protect 

against effects on non-target plants from volatilization of dicamba residues. This analysis similarly 

suggests that a 220-foot buffer is protective for the 1.0 lb ae/A application rate, though this may be 

overly conservative since the 1.0 lb ae/A rate is for pre-emergent applications that may be applied

under conditions less conducive to vapor drift (e.g. cooler temperatures)

Incident Data

A preliminary review on January 20, 2016, of the Ecological Incident Information System (EIIS, 

version 2.1), which is maintained by the Agency’s Office of Pesticide Programs, and the Avian 

Monitoring Information System (AIMS), which is maintained by the American Bird Conservancy,

indicates a total of 11 reported ecological incidents associated with the use of DGA salt. This total 

excludes incidents classified as ‘unlikely’ or ‘unrelated’ and only includes those incidents with 

certainty categories of ‘possible’, ‘probable’, and ‘highly probable’ (for EIIS) and ‘possible’, 

‘probable’, ‘likely’, ‘highly likely’ and ‘certain’ (for AIMS). Incidents classified as ‘unlikely’ the 

result of or ‘unrelated’ to DGA salt will not be included in this ecological risk assessment. The 

EIIS database identifies numerous additional ecological incidents with the dicamba acid (70 

incidents) and various dicamba salts (104 incidents across all salts). EFED investigated whether 

any of the incidents in the database were associated with dicamba use on dicamba-tolerant crops. 

Four of the 11 incidents in the EIIS database involving the DGA salt involved applications to 

dicamba-tolerant soybean and resulting damage to non-dicamba tolerant crops. No incidents are 

currently reported in the EIIS database for incidents stemming from the use on DT-cotton, however 

recent information submitted by Missouri and Arkansas regulatory agencies (discussed below) 
30

include incidents associated with the use of dicamba on DT-cotton that occurred in 2015. Details 

on the recent incidents stemming from use on DT-soybean and cotton are provided below.

In addition to the review of the available incident databases, EFED is aware of other recent incident 

information where non-dicamba tolerant plants were damaged following applications of dicamba 

DGA salt to DT-soybeans or DT-cotton. This recent incident information, submitted by Monsanto 

and state regulatory agencies in Missouri and Arkansas, is discussed below. 

Recent Incident Information Provided by Monsanto and MO and AR Regulatory Agencies

Incident Reports Submitted by Monsanto (2012-2014)

Monsanto provided information for 73 incidents involving the M1691 formulation from 2012 to 

2014. In their response document (MRID 49612701 pg. 68), Monsanto notes that observations 

were solely qualitative visual estimates and that no measurements of apical endpoints such as plant 

height or yield were taken. Monsanto further noted that the incidents related either to seed 

production activities or to activities performed as part of the product development process relating 

to product stewardship. They stated that current proposed label requirements were not in place in 

2012, that all of these incidents either did not follow all of the current draft label requirements 

(including tank mixtures with additional pesticide active ingredients such as glyphosate, nozzle 

type, wind-speed, wind direction, spray volume, etc.) or they were a result of other factors (e.g. 

burndown application, heavy rainfall, equipment contamination, spillage, etc.) and that the 

percentage of incidents as a function of the number of applications made has decreased in each 

subsequent year since 2012.

EFED has conducted an initial review of these incidents and generally agrees with Monsanto that 

the incidents resulted from applications not in accordance with currently proposed draft label 

language or were attributed to other (non-dicamba) factors. However, four incidents (Inquiries 19, 

20, 24, and 30) from 2014 lacked sufficient information in the report (such as on tank mixture, 

application rates, nozzles, wind direction & speed, equipment speed, buffer distance, spray volume 

& pressure or boom height) to determine whether their occurrence followed applications that were 

in accordance with the current proposed draft label requirements. Although, as Monsanto notes,

much of this data arises from seed production activities or activities related to the product 

development process and were not generated for purposes of risk assessment, EFED does not 

discount that they could be suggestive of potential incidents in the field and they could provide 

useful information to that end.

EFED also acknowledges that the incident observations are qualitative measures of visual injury

(e.g. leaf spotting or curling). Nonetheless, the information presented in these incidents may be 

useful if future labels incorporate changes such as potential tank mixes with additional active 

ingredients or additional nozzle types, since some of these incidents include information on tank 

mixes and nozzle types which would be relevant in the case where those changes are made to the

label.
31

Missouri and Arkansas Case files

The Missouri Department of Agriculture (MDA) has submitted information for incidents occurring 

from 2013 to 2015 and the Arkansas Plant Board (APB) has submitted information for incidents 

occurring in 2015, regarding observations of dicamba-type damage to non-tolerant plants following 

either preemergence or postemegence applications to dicamba-tolerant (DT) soybeans or cotton. 

Similar to the incidents reported by Monsanto for 2012-2014, all of the incidents were qualitative 

visual estimates and no observations or measurements of apical endpoints such as plant height or 

yield were taken.

2013-2014 Incidents

MDA has notified EPA of two incidents following potential dicamba applications that occurred in 

2013 and 2014. In 2013, dicamba-type damage was observed in a non-DT soybean field (MO Case 

File #81513M00701, EIIS Incident report number I026579-001). The only dicamba application in 

the area was reported to be a Clarity herbicide application on DT-soybeans 2,800 feet from the 

damaged field. Pesticides were applied using XR T-Jet 11004 nozzle, which is the same nozzle 

proposed for the new dicamba uses on DT cotton. The applicator was informed of this incident in 

January 2014. Dicamba residues were found in one foliage sample taken from the affected field at 

42 μg/kg. In the other two taken samples, dicamba residues were not detected (limit of detection 

not reported, but a limit of quantification of 3.8 μg/kg). The case file submitted to the agency did 

not originally determine the cause of the dicamba damage. In subsequent communication with the 

Agency (2015 letter from D. Slade, MDA to Grant Rowland, EPA), MDA concluded that the 

application of Clarity herbicide was not transported to the affected site by spray drift, but by later 

volatilization. 

In their response document (MRID 49612701 pg. 1, submitted prior to MDA’s December, 2015 

letter), Monsanto noted that it has reviewed the complete incident report from the Missouri 

Department of Agriculture (MRID 49612701 pg. 75). Monsanto stated that the report indicated 

that 1) there was potential the crop visual injury response was observed prior to the dicamba 

application, 2) MO Department of Agriculture did not come to a definitive conclusion on the 

primary cause of the incident and 3) other plausible explanations were not investigated, such as 

temperature inversion, alternative sources of dicamba, such as leaking equipment or damage from 

other herbicides. Therefore, Monsanto concluded that the incident did not provide evidence that 

the observed plant response was a result of exposure to vapor drift of dicamba residues. Monsanto 

also included this incident in their description of the 73 incidents from 2012-2014 discussed 

previously in this section and noted that this incident would not comply with the current proposed 

label requirements as M1691 was tank mixed with glyphosate and other adjuvants. 

EPA notes that MDA has now completed their investigation of this incident, measured residues 

indicating the presence of dicamba residues on the affected site, concluded that dicamba 

volatilization rather than drift was the likely cause of the damage and initiated enforcement action 

against the applicator for allowing the product to move from the target field. The climatic 

conditions at the time of application were slightly outside of the range of conditions from the 

available laboratory studies on dicamba DGA salt’s volatility. Given that effects to EPA’s apical 

endpoints of plant height and biomass were not measured, there is uncertainty regarding whether 
32

this incident indicates that volatilization following dicamba applications may result in impacts to 

apical endpoints beyond the proposed spray drift buffer of 110 feet for a 0.5 lb/A application. 

However, based on the available data, a volatilization buffer equal to the spray drift buffers, and 

extending in all directions form the treated field, is justified. The current proposed labels only 

apply a unidirectional spray drift buffer in the direction wind is blowing. 

MDA also notified EPA of an incident in 2014 (MO Case File #072214MO0701) where “dicamba 

type” damage was observed on a non-DT cotton field where the only nearby dicamba application 

would have been a Clarity herbicide application on DT-soybeans, 2.2 miles from the affected site. 

As with the other incidents, the provided information only indicated observations of visual injury 

and not effects to apical endpoints such as plant height and yield. Residue samples taken from the 

affected site failed to detect dicamba residues. It is unclear whether this incident was also included 

in Monsanto’s submitted information on the 73 incidents from 2012-2014. With the current 

information and due to the lack of identified dicamba residues, it is uncertain whether the damage 

observed in the incident was a result of dicamba applications or due to some other unidentified 

cause. If the observed damage was caused by dicamba, than given the large distance between the 

affected site and the nearest known dicamba application, it would likely have been a result of 

volatilization, rather than spray drift.

2015 Incidents

Missouri and Arkansas recently submitted to EPA a total of 15 incidents in 2015 that might be

attributed to dicamba use (12 in Arkansas and 3 in Missouri) on cotton. The information indicates 

that these incidents resulted from 6 separate instances of applications of dicamba, with 8 of the 

incidents (7 from Arkansas and 1 from Missouri) being a result of a single instance of a postemergent dicamba application to DT-cotton of Strut herbicide (active ingredient Dicamba DGA), 

tank-mixed with glyphosate and applied at two times the labeled rate for the proposed 

Clarity/M1691 post-emergent use. Visual observations of plant damage extended to 1320 feet (1/4 

mile) from the application site, but the provided incident information did not provide sufficient 

details to ascertain damage to EPA’s apical endpoints of plant height or yield. The remaining 

incidents were pre-emergent applications of dicamba or at this time remain uncertain as to whether 

any application of dicamba was made. Results of residue analyses for these incidents are currently 

not available.

Conclusions Regarding Incident Information 2012—2015

Conclusions Regarding Incident Information 2012—2015

For the purposes of the registration of dicamba on dicamba-tolerant soybean and cotton crops, the 

incident information available at this time indicates that the vast majority of incidents occurred 

following applications that were not made according to the current draft label requirements. Label 

requirements that differ from the draft label being evaluated here include the restriction against tank 

mixes with other active ingredients and adjuvants, higher application rates, and applications with 

different nozzle types and climatic conditions than permitted according to the draft label. 

Quantitative measurements of yield loss or decreased plant height were not made in any of the 

incident descriptions. Currently, EPA has no methodology for relating qualitative estimates of 
33

visual damage to quantitative effects to apical endpoints. 

Most of these incidents were likely caused by spray drift off the field following the application. 

The only incident where volatility of dicamba residues has been concluded to be the cause of the 

incident by a regulatory agency (MDA for MO Case File #81513M00701, EIIS Incident report 

number I026579-001) was an incident where the application was also made as a tank mix of 

glyphosate, additional adjuvants and dicamba. However, EFED believes that this difference from 

the draft label is unlikely to have impacted the ability of dicamba residues to volatilize as the 

different active ingredients and adjuvants are generally presumed to have disassociated from each 

other by the time any volatilization would occur. Rather, the volatilization may have been more 

impacted by the climatic conditions (temperature and humidity) in the days following the 

application which fall outside of the range of submitted laboratory conditions for which EPA has 

data. 

Runoff

The Agency considers spray drift exposure to be the principal risk issue associated with the 

proposed label use of dicamba DGA salt, owing to a variety of lines of evidence, including past 

experience with other dicamba formulations and associated spray drift incident reporting. 

However, in the following paragraphs the Agency provides an explicit consideration of the runoff 

risks and effects of proposed mitigation to limit off-site runoff in listed species effects 

determination. 

An important component of the model used for terrestrial risk assessment (TerrPlant) is the 

assumption that the mass of pesticide running off the treated field is directly related to the 

pesticide’s solubility in water. In the case of dicamba DGA salt, the dissociated salt yields highly 

soluble dicamba acid. The TerrPlant model assumes that the high solubility of the acid results in a 

runoff mass of 5 percent of the field-applied mass. The model also includes an assumption that the 

non-target plant receptors in the runoff zone are exposed to a single pulse of the total mass. The 

TerrPlant model does not account for any pesticide degradation or partitioning, nor does it account 

for the temporal aspects of runoff associated with meteorological events (i.e., runoff follows 

precipitation events that exceed field capacity of the soil).The fate characterstics of dicamba 

indicate that it is not environmentally persistent. Therefore, TerrPlant is likely providing a highly 

conservative estimate of runoff loading at any given time because the processes of degradation and 

partitioning combined with the stochastic nature of runoff are important limiting factors for 

dicamba. 

A refined characterization of potential pesticide runoff is routinely considered in aquatic system

exposure modeling through the use of the Surface Water Concentration Calculator (SWCC). EFED 

has turned to this model to ascertain the extent to which explicit consideration of degradation, 

partitioning, and stochastic runoff events would lead to a more mechanistically appropriate runoff 

exposure estimate than is possible using TerrPlant. Using the SWCC Missouri cotton scenario run 

for the cotton risk assessment (D404823) and assuming one application of dicamba DGA at 1 

pound per acre, followed by two half-pound applications at seven-day intervals, EFED has 

determined that the total yearly mass of dicamba in runoff from a 10 hectare (24.7 acre) treated 

field is ~0.055% of the applied mass (see Appendix 4 for calculations). Using these assumptions in 
34

TerrPlant (total 2 lb ae/A application and a 0.055% runoff fraction), and the most sensitive 

endpoint of 0.000261 for the NOAEC for soybeans, the maximum RQ is less than the LOC of 1.0 

by a factor of at least 2 (RQs range from <0.1 to 0.48, see Appendix 4).

Combining the predictions of SWCC modeling and the expectation that much of the off-site plant 

community will not experience foliar contact with dicamba DGA in runoff sheet flow, EFED 

concludes that all available lines of evidence support a “no effects” determination for runoff 

exposure for off-field listed plants for the proposed labeled use of dicamba DGA.

Herbicide Interactions (Synergism)

Mixtures of herbicides, such as those applied in tank mixtures, may cause synergistic, antagonistic, 

or additive effect in non-target plants, compared to the effects of the individual herbicides. The

patent and literature studies cited below indicate that combinations of dicamba and glyphosate may 

cause effects that range from synergistic to antagonistic, depending on the plants tested and the 

doses used.

In the case of dicamba and glyphosate, a patent (Satchivi and Wright 2011) filed by 

DowAgrosciences LLC claimed synergistic effects in Chenopodium album (common 

lambsquarters) and Commelina benghalensis (dayflower), but no synergism in Conyza canadensis

(Horseweed/marestail).

Flint and Barrett (1989a) studied the combined effects of dicamba and glyphosate on field 

bindweed (Convolvulus arvensis) and found synergistic inhibitory effects on the root system. Flint 

and Barrett (1989b) also studied interactions between dicamba and glyphosate in Johnsongrass 

(Sorghum halepense). They found antagonistic effects of dicamba on shoot and root fresh weights, 

compared to lower rates of glyphosate alone.

Kelley et al. (2005) studied the interactions of dicamba with four herbicides, including glyphosate, 

in non-GMO soybeans. Dicamba was not found to be synergistic with glyphosate considering plant 

injury and yield loss as endpoints.

Olszyk et al. (2015) tested the interaction of dicamba and glyphosate (at three different rates) in 

eight plant species, including big bluestem (Andropogon gerardii), Milkweed (Ascelepiassyriaca

spp.), purple node joe pye weed (Eutrochium purpureum), soybean (Glycine max), evening 

primrose (Oenothera biennis), slender knodding smartweed (Polygonum lapathifolium), Canada 

goldenrod (Solidago canadensis), and purple top tridens (Tridens flavus). Generally, antagonistic 

effects were found in plant growth endpoints and the study authors concluded that no combination 

of dicamba with differing glyphosate rates produced a synergistic response for any species.

The current draft label for dicamba use on tolerant soybean and cotton plants specifies that tank 

mixes may only be used for products that have been tested and found not to have unreasonable 

adverse effects on the spray drift properties of M1691 Herbicide. EFED believes that guideline 

laboratory studies of effects to terrestrial plants should be required for any product or tank mixture 

combining dicamba and other active ingredients to assess risks associated with any tank mixture for 

use on dicamba-tolerant soybeans or cotton. Testing of such products should include the standard 
35

suite of tested species from the already submitted dicamba and tank mixed active ingredient

vegetative vigor studies as well as those that the open literature and patent data indicate potential 

for synergistic effects.

FEDERALLY-LISTED SPECIES

This assessment does not include effects determinations for any identified federally-listed 

endangered and threatened species (listed species). A more in-depth listed species-specific effects 

determination will be conducted using the information from this analysis. The identified potential 

risks to listed species from this screening-level risk assessment are summarized in Table 14.

TABLE 14. Listed Species Risks Associated with Potential Screening Level Direct or Indirect 

Effects Due to the Proposed Applications of Dicamba on Dicamba-Tolerant Cotton. 

LISTED TAXON DIRECT EFFECTS INDIRECT EFFECTS

Terrestrial and semi-aquatic 

plants – monocots No1 Yes2

Terrestrial and semi-aquatic 

plants – dicots Yes Yes2

Insects No Yes2

Birds Yes (Acute) Yes2

Terrestrial phase amphibians Yes (Acute) Yes2

Reptiles Yes (Acute) Yes2

Mammals Yes (Chronic) Yes2

Aquatic plants Yes (Non-vascular) Yes2

Freshwater fish No Yes2

Aquatic phase amphibians No Yes2

Freshwater crustaceans No Yes2

Mollusks No Yes2

Marine/estuarine fish No Yes2

Marine/estuarine crustaceans No Yes2

1

Listed species of monocots RQ values did not indicate risk from DGA salt, but risk was indicated for dicamba acid. 
36

DGA salt rapidly disassociates into dicamba acid. 2

The listed chronic LOC was exceeded for mammals, birds and dicot species of plants. Therefore, the potential for 

adverse effects to those species that rely on a specific animal species, multiple animal species, or dicot plant species

cannot be precluded in the screening-level assessment. Indirect effects may include general habitat modification, loss 

of pollinators/seed dispersers, and food supply disruption.

At this time, no federally-listed taxa can be excluded from the potential for direct and/or indirect 

effects from the proposed new uses of dicamba, since there is a potential for indirect effects to taxa 

that might rely on plants, birds, and/or mammals for some stage of their life-cycle. Further 

refinement for the endangered species assessment is provided in a separate document that includes 

species-specific effects determinations. 

UNCERTAINTIES

Based on the usage of other herbicides associated with genetically modified crops that are tolerant 

to a specific herbicide (e.g., glyphosate-tolerant soybean), the use of dicamba on cotton as a result 

of this expanded new use [lbs acid equivalent (a.e.) applied per year] could potentially increase 

when compared to historical dicamba usage data. This is due to a variety of factors including the 

fact that once a tolerant crop is grown in a particular area, the use of the tolerant crop is often 

adopted by neighbors (to minimize the potential risk from spray drift). Additionally, dicamba use 

on tolerant cotton is predicted to increase given the recent resistance issues identified in glyphosatetolerant crops (J. Tooker, D. Mortensen, and F. Egan, pers. comm., Nov. 2010; Mortensen 2010). 

Although EFED does not typically address specific concerns related to the increased usage of a 

chemical, the potential for ecological risks likely increases with increased usage. BEAD should be 

consulted on the potential for increased usage. 

Additionally, applications during a warmer time (i.e., post-emergence as would be included in the 

proposed new use) may increase off-site transport (via volatility) during a time when many plants 

have leafed out (J. Tooker, D. Mortensen, and F. Egan, pers. comm., Nov. 2010; Mortensen 2010). 

Therefore, a post-emergence application may increase the likelihood of effects to non-target plants

through habitat loss. This could indirectly affect those organisms which rely on those plants, 

including pollinators, though this is uncertain and requires additional evaluation. Provided the infield buffer restricts spray drift/volatility of dicamba residues to the field, effects to non-target 

plants should be limited. For endangered species determinations (assessed in separate documents

published concurrently with this risk assessment), effects to listed species critical habitat from the 

use of dicamba on dicamba-tolerant cotton will be considered. 

It is also possible that the proposed new use of dicamba on dicamba-tolerant cotton may increase 

the occurrence of weeds that are resistant to dicamba. The occurrence of weed resistance to 

glyphosate has increased significantly since the adoption of transgenic glyphosate-resistant crops 

(Powles, 2008). Prior to development of glyphosate-resistant crops, there were no known cases of 

evolved glyphosate-resistant weeds (Dyer, 1994). There exists potential that a similar pattern of 

rapidly evolving weed resistance to dicamba could occur where transgenic dicamba-resistant crops 

are used. Weed resistance issues are being considered by the Biological and Economic Analysis 

Division (BEAD) through product labeling and stewardship efforts. 
37

The formation and persistence of dicamba’s metabolite, DCSA, is an uncertainty that impacts the

terrestrial animal risk assessment. Plant metabolism studies that evaluate DCSA residues in DTcotton plant tissues over time would decrease this uncertainty. However, the best available data 

indicate that residues of DCSA in DT-cotton plants are unlikely to result in risk to terrestrial 

animals.

References:

Al-Khatib, K. and A. Tamhane. 1999. Pea (Pisum sativum) response to low rates of selected 

foliar- and soil-applied sulfonyl-urea herbicides. Weed Technology Vol. 13: 753-758.

Auch, D.E., and W.E. Arnold. 1978. Dicamba use and injury on soybeans (Glycine max) in South 

Dakota. Weed Science Vol. 25: 471-475.

Dyer, W.E. 1994. Resistance to glyphosate. In Herbicide Resistance in Plants: Biology and 

Biochemistry. Ed. By Powles, S.B. and J.A. Holturn. CRC Lewis Publishers, New York, NY. 

229—241. 

Everitt, J.D. and J.W. Keeling (2009) Cotton Growth and Yield Response to Simulated 2,4-D and 

Dicamba Drift. Weed Technology: Vol. 23, No. 4, pp. 503-506.

Hamilton, K.C. and H.F. Arle. 1979. Response of Cotton (Gossypium hirsutum) to Dicamba. 

Weed Science. Vol. 27(6): 604-607. 

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: Vol. 53(1): 

101-112. 

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. Vol. 22: 609-614.

Mortensen, D.A. 2010. Statement to the House, Domestic Policy Subcommittee of the Oversight 

and Government Reform Committee. Are 'Superweeds' an Outgrowth of USDA Biotech Policy? 

Hearing, July 28, 2010. Available at:

http://oversight.house.gov/index.php?option=com_content&view=article&id=921%3A07-28-2010; 

Accessed: 03/02/2011.

Powles, S.B. 2008. Evolved glyphosate-resistant weeds around the world: lessons to be learnt. 

Pest Manag Sci. 64: 360-365. 

Satchivi, N and Wright, T, 2011. Synergistic herbicidal composition containing a dicamba 

derivative and a glyphosate derivative. World Intellectual Property Organization publication no. 

WO 2011/140020 A2., 10 November 2011.
38

Teske, M.E., Bird, S.L., Esterly, D.M., Ray, S.L., Perry, S.G. 2001. A User’s Guide for AgDRIFT®

2.01: A Tiered Approach for the Assessment of Spray Drift of Pesticides (Regulatory Version). 

Spray Drift Task Force, Macon, MS. CDI Report No. 01-02.

USEPA, 1983. Dicamba Registration Standard Memorandum. Health Effects Division. Office of 

Pesticide Programs. Washington, D.C. August 12, 1983. 

USEPA, 1984. PP#4G3061 Dicamba on Various Crops (Accession Numbers: 071410, 072411, 

072412, 072413, 072414, 072415, 072416). Evaluation of Analytical Methods and Residue Data.

Health Effects Division, Office of Pesticide Programs. Washington, D.C. August 30, 1984. 

U.S. Environmental Protection Agency (USEPA). 2005a. EFED Reregistration Chapter for 

Dicamba/Dicamba Salts. Environmental Fate and Effects Division. Office of Pesticide Programs. 

August 31, 2005 (DP Barcode 317696).

USEPA 2005b. Human Health Risk Assessment Protocol for Hazardous Waste Combustion 

Facilities. EPA530-R-05-006. United Stated Environmental Protection Agency, Office of Solid 

Waste and Emergency Response, Washington, DC. 

U.S. Environmental Protection Agency (USEPA). 2010. Reduced Risk Request for Dicamba 

Herbicide Over-The-Top of Dicamba-Tolerant Soybean. May 27, 2010.

U.S. Environmental Protection Agency (USEPA). 2010 Environmental Fate and Effects Division 

Response to a FIFRA Section 18 Emergency Exemption for Dicamba co-formulated with 2,4-D 

(LatigoTM) Use on Teff grown for grain, seed, and hay to control broadleaf weeds. Requested by 

the Oregon Department of Agriculture. May 24, 2010. 

USEPA, 2011. Ecological Risk Assessment for Dicamba and its Degradate, 3,6-dichlorosalicylic 

acid (DCSA), for the Proposed New Use on Dicamba-Tolerant Soybean (MON 87708). 

Environmental Fate and Effects Division. Office of Pesticide Programs. Washington, D.C. March 

8, 2011.

USEPA, 2012. Data Evaluation Record for MRID 47899517. Health Effects Division. Office of

Pesticide Programs, Washington D.C. December 7, 2012. 

USEPA, 2014. Guidance for Assessing Pesticide Risks to Bees. Environmental Fate and Effects

Division. Office of Pesticide Programs. Washington, D.C. June 23, 2014. 

USEPA, 2016a. 3,6-Diclorosalicyclic acid (DCSA): Benchmark Dose Analysis of a Reproduction 

Study. Health Effects Division. Office of Pesticide Programs, Washington D.C. March, 2, 2016. 

D431873.

USEPA, 2016b. Dicamba DGA: Second Addendum to the Environmental Fate and Ecological

Risk Assessment for Dicamba DGA salt and its Degradate, 3,6-dichlorosalicylic acid (DCSA) for

the Section 3 New Use on Dicamba-Tolerant Soybean. D426789. Environmental Fate and Effects

Division. Office of Pesticide Programs. Washington, D.C. March 24, 2016.
39

Wall, D.A., 1994. Potato (Solanum tuberosum) Response to Simulated Drift of Dicamba, 

Clopyralid, and Tribenuron. Weed Science Vol. 42(1): 110-114.

Wax, L.M., L.A. Knuth, and F.W. Slife. 1969. Response of soybean to 2,4-D, dicamba, and 

picloram. Weed Science Vol. 17: 388-393.

Woodrow, J., Seiber, J., Baker, L. 1997. Correlation Techniques for Estimating Pesticide 

Volatilization Flux and Downwind Concentrations. Environ. Sci. Technol., 31:523-529.

Woodrow, J., Seiber, J., Dary, C. 2001. Predicting Pesticide Emissions and Downwind 

concentrations Using Correlations with Estimated Vapor Pressures. J. Agric. Food Chem. 

49:3841-3846.

Weidenhamer, J.D., G.B. Triplett, and F.E. Sobotka. 1989. Dicamba injury to soybean. 

Agronomy Journal. Vol. 81: 637-643. 
40

Appendix 1: T-REX Inputs and Outputs for Dicamba Use on Dicamba-Tolerant Cotton
41

TREX MODEL INPUTS You must enable macros for this spreadsheet to work correctly These values will be used in the calculation of exposure estimates for foliar, granular, liquid and/or 

seed applications of pesticides.

Application No. Rate Day of Application

Chemical Name: Application Rate (lb ai/acre 1 1 0

Seed Treatment? (Check if yes) FALSE Seeding Rate (lbs/acre) 2 0.5 7

 Use: 18.9 3 0.5 14

Product name and form: 4

% A.I. (leading zero must be entered for 

formulations <1% a.i.): 5

Application Rate (lb ai/acre) 1 6

Half-life (days): 8.4 7

Application Interval (days): 8

Number of Applications: 9

Are you assessing applications with 

variable rates or intervals? yes 10

You MUST specify application day 11

12

What body weight range is assessed 

(grams)? Birds Mammals 13

Small 20 15 14

Medium 100 35 15

Large 1000 1000 16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

Note: Sources of wildlife diet are assumed to be available for less than one year for this model.

Endpoints

Avian

Endpoint Toxicity value

Indicate test species 

below

Optional Test 

Organism Body 

weight (g) Optional Test Species Name

LD50 (mg/kg-bw) 188.00 1

LC50 (mg/kg-diet) 10000.00 1

NOAEL (mg/kg-bw) 1

NOAEC (mg/kg-diet) 695.00 2

1.15

Mammalian

Acute Study Chronic Study

350 350

Endpoint Toxicity value Reference (MRID)

LD50 (mg/kg-bw) 2740.00

LC50 (mg/kg-diet)

Reported Chronic 

Endpoint 136.00 1

Is dietary concentration 

(mg/kg-diet) reported from 

the available chronic 

mammal study? (yes or no)

no

Enter dietary concentration (mg/kg-diet)

Estimated Chronic Diet 

Concentration Equivalent to 

Reported Chronic Daily Dose

2720 mg/kg-diet based on 

standard FDA lab rat 

conversion

100.00%

Enter the Mineau et al. Scaling Factor

Assessed Species Inputs (optional, use defaults for RQs for national level 

assessments)

Size (g) of mammal used in toxicity study

Default rat body weight is 350 grams

You MUST specify application day in Columns F and G - you may enter up to 30 applications

Chemical Identity and Application Information

Dicamba

5

Reset Model
42

Summary of Risk Quotient Calculations Based on Upper Bound Kenaga EECs

Table X. 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 Arthropo

EEC RQ EEC RQ EEC RQ EEC RQ EEC

20 135.44 299.47 2.21 137.26 1.01 168.45 1.24 18.72 0.14 117.29 0

100 172.42 170.77 0.99 78.27 0.45 96.06 0.56 10.67 0.06 66.88 0

1000 243.55 76.46 0.31 35.04 0.14 43.01 0.18 4.78 0.02 29.94 0

Table X. Upper Bound Kenaga, Subacute Avian Dietary Based Risk Quotients

LC50

EECs and RQs

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

10000 262.94 0.03 120.52 0.01 147.91 0.01 16.43 0.00 102.99 0.01

Size class not used for dietary risk quotients 

Table X. Upper Bound Kenaga, Chronic Avian Dietary Based Risk Quotients

NOAEC 

(ppm)

EECs and RQs

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

695 262.94 0.38 120.52 0.17 147.91 0.21 16.43 0.02 102.99 0.15

Size class not used for dietary risk quotients

Table X. Upper Bound Kenaga, Acute Mammalian Dose-Based Risk Quotients 

EECs and RQs
43

Size 

Class

(grams)

Adjusted

LD50

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds Arthropo

EEC RQ EEC RQ EEC RQ EEC RQ EEC

15 6022.06 250.70 0.04 114.90 0.02 141.02 0.02 15.67 0.00 98.1893 0.

35 4872.49 173.26 0.04 79.41 0.02 97.46 0.02 10.83 0.00 67.8619 0.

1000 2107.50 40.17 0.02 18.41 0.01 22.60 0.01 2.51 0.00 15.734 0.

Table X. Upper Bound Kenaga, Acute Mammalian Dietary Based Risk Quotients

LC50 

(ppm)

EECs and RQs

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

0 262.94 #DIV/0! 120.52 #DIV/0! 147.91 #DIV/0! 16.43 ##### 102.99 #DIV/0!

Size class not used for dietary risk quotients

Table X. Upper Bound Kenaga, Chronic Mammalian Dietary Based Risk Quotients

NOAEC 

(ppm)

EECs and RQs

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds/Large 

Insects Arthropods

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

2720 262.94 0.10 120.52 0.04 147.91 0.05 16.43 0.01 102.99 0.04

Size class not used for dietary risk quotients 

Table X. 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 Arthropo

EEC RQ EEC RQ EEC RQ EEC RQ EEC

15 298.90 250.70 0.84 114.90 0.38 141.02 0.47 15.67 0.05 98.19 0

35 241.85 173.26 0.72 79.41 0.33 97.46 0.40 10.83 0.04 67.86 0

1000 104.61 40.17 0.38 18.41 0.18 22.60 0.22 2.51 0.02 15.73 0
44

Appendix 2. Dicamba and DCSA Residues from Dicamba-Tolerant Cotton Crop Field 

Trials

Summary of Residues from Dicamba-Tolerant Cotton Crop Field Trials with DCSA as 

a Dicamba Residue of Concern.

Commodity Analyte Total 

App.

Rate

lb ae/A

(kg 

ae/ha)

PHI 

(days

)

Residue Levels (ppm)1

n Sample 

Min.

Sample 

Max.

LAFT2 HAFT2 Media

n

Mean Std. 

Dev.

TRT 2 (Applications at Preemergence, 6-leaf stage, and first white flower + 15 days; EP: 

Clarity)

Undelinted 

Cotton seed

Dicamba 2.0

(2.2)

49-

105

13 <0.02 <0.02 <0.02 <0.02 0.02 0.02 N/A

5-OH 

Dicamba

13 <0.02 <0.02 <0.02 <0.02 0.02 0.02 N/A

DCSA 13 <0.02 0.23 <0.02 0.23 0.02 0.04 0.06

Combined 

Residues

13 <0.06 <0.28 <0.06 <0.28 0.06 0.09 0.06

Gin 

byproducts

Dicamba 2.0

(2.2)

82-84 3 <0.04 <0.04 <0.04 <0.04 0.04 0.04 N/A

5-OH 

Dicamba

3 <0.04 <0.04 <0.04 <0.04 0.04 0.04 N/A

DCSA 3 0.39 1.73 0.43 1.58 0.67 0.89 0.61

Combined 

Residues

3 <0.47 <1.82 <0.53 <1.66 0.75 0.97 0.61

TRT 3 (Applications at Preemergence, first open boll stage, and 7 days prior to harvest; 

EP: Clarity)

Undelinted 

Cotton seed

Dicamba 2.0

(2.2)

6-8 13 0.06 1.97 0.06 1.38 0.65 0.64 0.43

5-OH 

Dicamba

13 <0.02 0.02 <0.02 <0.02 0.02 0.02 N/A

DCSA 13 <0.02 0.25 <0.02 0.16 0.03 0.05 0.05

Combined 

Residues

13 <0.12 <2.24 <0.10 <1.56 0.71 0.71 0.48

TRT 4 (Applications at 6-leaf, first white flower + 15 days, first open boll, and 7 days 

prior to harvest; EP: Clarity)

Undelinted 

Cotton seed

Dicamba 2.0-2.1

(2.2-

2.4)

6-8 13 0.09 1.54 0.12 1.42 0.47 0.61 0.41

5-OH 

Dicamba

13 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 N/A

DCSA 13 0.02 0.27 0.02 0.27 0.06 0.08 0.07

Combined 

Residues

13 <0.13 <1.83 <0.16 <1.72 0.56 0.71 0.48

TRT 4 (Applications at 6-leaf, first white flower + 15 days, first open boll, and 7 days 

prior to harvest; EP: Clarity)

Gin 

byproducts

Dicamba 2.0

(2.2)

6-7 3 3.09 23.6 3.13 23.0 14.9 13.7 10.0

5-OH 

Dicamba

3 <0.04 0.04 <0.04 <0.04 0.04 0.04 N/A

DCSA 3 1.70 6.29 1.78 6.17 4.50 4.15 2.22

Combined 

Residues

3 <4.83 29.9 <5.06 <29.6 19.7 18.1 12.2

TRT 5 (Applications at 6-leaf, first white flower + 15 days, first open boll, and 7 days 

prior to harvest; EP: MON 11968)

Undelinted 

Cotton seed

Dicamba 2.0

(2.2)

7-8 4 0.17 0.72 0.20 0.62 0.41 0.41 0.23

5-OH 

Dicamba

4 <0.02 <0.02 <0.02 <0.02 0.02 0.02 N/A
45

Summary of Residues from Dicamba-Tolerant Cotton Crop Field Trials with DCSA as 

a Dicamba Residue of Concern.

Commodity Analyte Total 

App.

Rate

lb ae/A

(kg 

ae/ha)

PHI 

(days

)

Residue Levels (ppm)1

n Sample 

Min.

Sample 

Max.

LAFT2 HAFT2 Media

n

Mean Std. 

Dev.

DCSA 4 0.02 0.17 0.02 0.12 0.04 0.06 0.04

Combined 

Residues

4 <0.21 <0.91 <0.24 <0.76 0.47 0.49 0.27
46

Appendix 3. TerrPlant Inputs and Outputs for Dicamba Use on Dicamba-Tolerant 

Cotton

Green values signify user inputs (Tables 1, 2 and 4). 

Input and output guidance is in popups indicated by red arrows.

Table 1. Chemical Identity.

Chemical Name Dicamba-DGA salt

PC code 128931

Use Cotton

Application Method Ground

Application Form Spray 

Solubility in Water 

(ppm) 6100

Table 2. Input parameters used to derive EECs.

Input Parameter Symbol Value Units

Application Rate A 1 y

Incorporation I 1 none

Runoff Fraction R 0.05 none

Drift Fraction D 0.01 none

Table 3. EECs for Dicamba-DGA salt. Units in y.

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 y.

Seedling Emergence Vegetative Vigor

Plant type EC25 NOAEC EC25 NOAEC 

Monocot 1.68 0.64 0.472 0.137

Dicot 0.123 0.0673 0.000513 0.000261

Table 5. RQ values for plants in dry and semi-aquatic areas exposed to Dicamba-DGA salt through 

runoff and/or spray drift.*

Plant Type Listed Status Dry Semi-Aquatic Spray Drift

Monocot non-listed <0.1 0.30 <0.1

Monocot listed <0.1 0.80 <0.1

Dicot non-listed 0.49 4.15 19.49

Dicot listed 0.89 7.58 38.31

*If RQ > 1.0, the LOC is exceeded, resulting in potential for risk to that plant group.
47

Appendix 4: Runoff Calculations from SWCC and Resulting RQ values 

in TerrPlant

Summary of Water Modeling of dicamba and the USEPA Standard Pond

Estimated Environmental Concentrations for dicamba are presented in Table 1 for the USEPA 

standard pond with the MScottonSTD field scenario. A graphical presentation of the year-to-year 

peaks is presented in Figure 1. These values were generated with the Surface Water Concentration 

Calculator (SWCC Version 1.106). Critical input values for the model are summarized in Tables 2 

and 3.

This model estimates that about 22% of dicamba applied to the field eventually reaches the water 

body. The main mechanism of transport from the field to the water body is by runoff (76.7% of the 

total transport), followed by spray drift (23.1%) and erosion (0.18%).

In the water body, pesticide dissipates with an effective water column half-life of 106.0 days. (This 

value does not include dissipation by transport to the benthic region; it includes only processes that 

result in removal of pesticide from the complete system.) The main source of dissipation in the 

water column is metabolism (effective average half-life = 107.2 days) followed by photolysis (9502 

days) and volatilization (141341.2 days). In the benthic region, pesticide dissipates very slowly 

(622.2 days). The main source of dissipation in the benthic region is metabolism (effective average 

half-life = 622.2 days). The pesticide is about evenly distributed in the benthic region between the 

pore water and sorbed to sediment.

Table 1. Estimated Environmental Concentrations (ppb) for dicamba.

Peak (1-in-10 yr) 42.2

4-day Avg (1-in-10 yr) 41.7

21-day Avg (1-in-10 yr) 40.1

60-day Avg (1-in-10 yr) 35.6

365-day Avg (1-in-10 yr) 13.8

Entire Simulation Mean 8.49
48

Table 2. Summary of Model Inputs for dicamba.

Scenario MScottonSTD

Cropped Area Fraction 1

Koc (ml/g) 13.4

Water Half-Life (days) @ 25 °C 72.9

Benthic Half-Life (days) @ 25 °C 423

Photolysis Half-Life (days) @ 40 

°Lat

105

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 25 °C 18

Foliar Half-Life (days) 35

Molecular Wt 221

Vapor Pressure (torr) 3.41E-5

Solubility (mg/l) 6100

Table 3. Application Schedule for dicamba.

Date (Mon/Day) Type Amount (kg/ha) Eff. Drift

04/16 Ground 1.12 0.99 0.05

04/23 Ground 0.56 0.99 0.05

04/30 Ground 0.56 0.99 0.05

Figure 1. Yearly Peak Concentrations
49

Table 4. Alternative dicamba runoff analysis based on MS cotton SWCC run at 2 lb/A 

(maximum annual application rate for dicamba based on a 1 lb a.e./A application and two 0.5 

lb a.e./A applications.

Sources Contribution 

factor

Total mass 

(kg) based 

on 30 years

Total mass 

applied over 

30 year 

period

Percentage 

of dicamba 

over 30 year 

period

Average 

annual loss 

(%)

Runoff 0.7669 11.14

672

1.66 0.055

Erosion 0.0018 0.02634 0.0039 0.00013

total 0.7687 11.17 1.7 0.056

Comparison of the most sensitive terrestrial and aquatic endpoints for use in the runoff 

calculations.

Comparing most sensitive IC25 /NOAEC for non-vascular aquatic plants and terrestrial plants. 

Most sensitive species are the blue green algae (IC50/NOAEC of 0.061/0.005 mg ae/L) and soybean 

IC25/NOAEC of 0.000513/0.000261 lbs ae/A.
50

To compare, assume exposure is equivalent to a 1-acre field covered with 1 inch of water. 

Therefore the EEC in lb/A that compares to an IC50 of 0.061 mg ae/L for blue-green algae can be 

considered following Equation 1 below:

Equation 1.

EEC lb a.e./Acre = (Z mg/L) * (102,790 L water/ Acre-in) * (1 inch) * (1 lb/ 453,592 mg) which 

reduces to:

Equation 2. 

EEC lb a.e./Acre = (Z mg/L) * (0.226613)

EEC lb ai/A = 0.061 mg/L * 0.226613 = 0.0138 

and the EEC in lb/a that compares to a NOAEC of 0.005 mg ae/L for blue-green algae would be

EEC lb ae/A = 0.005 mg/L * 0.226613 = 0.0011. These endpoints are approximately 1-2 orders of 

magnitude less sensitive than the soybean endpoints. Therefore, the soybean endpoints should be 

used in the revised TerrPlant runoff claculations.

Table 5. RQ values calculated in TerrPlant for plants in dry 

and semi-aquatic areas exposed to Dicamba DGA salt through 

runoff. Conservative assumptions of 2 lb a.e/A application rate 

and 0.06% loss through runoff and erosion. 

Plant Type Listed Status Dry Semi-Aquatic

Monocot non-listed <0.1 <0.1

Monocot listed <0.1 <0.1

Dicot non-listed 0.17 0.26

Dicot listed 0.32 0.48