UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 128931 

DP Barcode: 426789 

Date: March 24, 2016 

MEMORANDUM 

SUBJECT: 

TO: 

FROM: 

THRU: 

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 

Grant Rowland, Risk Manager Reviewer 

Kathryn Montague, Product Manager 23 

Daniel Kenny, Branch Chief 

Herbicide Branch 

Registration Division (7505P) 

~ A I,,( A -"/J'. ~ 'SI';) I.-( !1 (;, Michael Wagman, Biologist '/YI_(/ - 7! 1.fJ ~ Amy Blankinship, Senior Science Advisor i.y-Jffl.fJ9 d [w \3 c) J -L VJ-J b 

William P. Eckel, PhD., Senior Science AdvisorV1fUAJL/' 

Environmental Risk Branch 6 

Environmental Fate and Effects Division (7507P) 

Mark Corbin, Branch Chief r_ / 0. 1l) --\I 

Monica Wait, RAPL l)J\ J · t,, I D 

Environmental Risk Branch 6 '""/l _____ \;'1 ~ °3>1-Z..L\- / J lo 

Environmental Fate and Effects Division (7507P) Y .. vv vv-...-- - · 

This is an addendum to the Environmental Fate and Effects Division's (EFED) ecological risk 

assessment for dicamba DGA salt (Clarity® formulation or M1691 , EPA Reg No. 524-582) and 

its degradate, 3,6-dichlorosalicyclic acid (DCSA), for the proposed new use on dicamba-tolerant 

soybean. It includes analysis of information that was not previously included in the original 

soybean new use risk assessment (USEPA, 2011, DP 378444). Since the original risk 

assessment was conducted, the registrant, Monsanto, has submitted: 

1) field trial data that impacts EFED's previous analysis of spray drift, 

2) data for incidents and inquiries from the use of dicamba DGA salt, 
2

3) laboratory volatility data for dicamba DGA and DMA salt formulations, and 

4) terrestrial plant reproductive effects data. 

Additionally, this addendum includes analysis conducted by EFED regarding:

5) the implication of new mammalian chronic effects endpoints for parent dicamba and the 

metabolite DCSA from the Health Effects Division (HED; USEPA 2016, D378366+), 

6) a revised T-REX run using refined estimates of foliar dissipation half-lives and variable 

application rates, 

7) the potential for effects to beneficial terrestrial invertebrates,

8) effects posed by runoff, and

9) potential synergistic interactions with glyphosate.

1. Spray Drift and Buffers (Field Trial Data)

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 the 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). 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 
3

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. 

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 
4

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,

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 the maximum

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.e./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. 
5

Table 1. 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 only 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. 
6

Weight of Evidence Conclusions

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 this 

field trial data is not suitable for the purpose of establishing an appropriate 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 (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 extracoarse and ultra-coarse droplet spectra for an estimated average distance of 107 feet 

(rounded up to 110 feet) for a 0.5 lbs a.e./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 ultra-coarse 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.e./A application rate and with the described nozzles 

restricting the droplet spectra extra-coarse and ultra-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.

2. Incidents

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 
7

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.

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 postemergence 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. The air temperature and humidity at the time of dicamba 

application were reported to be 820

F and 55%, respectively. Dicamba residues were found in 

one foliage sample taken from the affected field at 42 μg/kg. In the other two samples, dicamba 

residues were not detected (limit of detection not reported, but a limit of quantification of 3.8 
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 whether 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 from the treated field, is justified. The current proposed labels only 

apply a unidirectional spray drift buffer in the direction wind is blowing. Further discussion of 

volatility is provided in Section 3 below.

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 (discussed previously in this section). With the current information available, 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, then 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.
9

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). 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. The remaining incidents were pre-emergent applications of 

dicamba or at this time remain uncertain as to whether any application of dicamba was made. 

Conclusions Regarding Incident Information 2012—2015

For the purposes of the registration of dicamba on dicamba-tolerant soybean, 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 were not followed included 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 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

since 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 likely impacted by the climatic conditions (temperature and humidity) in the days 

following the application which fall outside of the range of submitted laboratory data conditions. 

Additional discussion and characterization of volatility is provided in the next section.

3. Volatility

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. 
10

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, because these factors seem to be important in field conditions. 

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

a.e./A application rate and 220 feet for the 1.0 lb a.e./A 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 unidirectional buffer according to wind direction.

The incident described by MDA in the previous section (MO Case File #81513M00701, EIIS 

Incident report 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 postapplication 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)

4. Potential Effects on Terrestrial Plant Reproduction

EFED is aware of published literature associating dicamba applications with effects to soybean 

progeny. These studies indicate potential effects to the quantity and reproductive quality of 

future soybean generations following dicamba applications that would not be observed in the 

guideline vegetative vigor and seedling emergence studies EFED typically uses to assess risk to 

terrestrial plants. Therefore, these data raise a potential concern that has not been directly 

addressed in OPP assessments, should these effects occur at lower exposures than the effects 

observed in the guideline terrestrial plant studies. In meetings and email correspondence in 

January/February, 2015, OPP asked whether Monsanto was aware of this issue. Monsanto 

requested the references that OPP was aware of, so that they could independently review them.
11

Monsanto reviewed the open literature references provided by OPP and stated that none of the

studies described effects on progeny at application rates lower than OPP’s lowest available 

regulatory endpoint from the available vegetative vigor plant study (0.000261 lb dicamba a.e./A 

equivalent to 0.29 g a.e./ha, based on plant height), therefore any mitigations (i.e. spray drift 

buffers) based on the plant height endpoint would be protective for effects to progeny. 

The open literature studies examined seeds/pod, seed weight, seed quality, delayed maturation, 

pod malformation, reduced germination or progeny emergence, and malformed progeny. The 

vast majority of the studies did not investigate effects at rates as low as the NOAEC from the 

available vegetative vigor study. Monsanto’s review of the available information indicated that 

the lowest effects endpoint reported from these studies was for delayed maturation of soybeans at 

rates as low as 0.56 g a.e./A from Kelley et al. (2005), which would still be almost 2 times less 

sensitive than the regulatory endpoint based on plant height that EFED has used in its risk 

assessments. Monsanto concluded that the open literature studies did not contain information 

that indicated that the in-field buffer based on plant height that is on the draft label would not 

also be protective of these reproductive effects. 

EFED acknowledges Monsanto’s submission of their analysis of the open literature data for 

effects to progeny, but to date has not independently reviewed each of these studies. However, 

for the following reason, EFED does not believe the information would change its risk 

assessments. The most sensitive endpoint reported in the open literature was a LOAEC of 0.56 g 

a.e./A for delayed maturation of soybeans (Kelley et al., 2005; no NOAEC reported). As EFED’s 

determination for risk to listed plant species is based on the most sensitive apical endpoint (i.e.,

the NOAEC for soybean plant height from the available vegetative vigor study with dicamba 

DGA, 0.000261 lb dicamba a.e./A equivalent to 0.29 g a.e./ha, based on plant height), less 

sensitive endpoints reported in the literature for effects to progeny would not impact EPA’s risk 

assessments. EFED’s policy regarding open literature is that typically if endpoints from the open 

literature are not more sensitive than guideline endpoints, than further analysis is not required 

(USEPA, 2011b)

5. Revised Terrestrial Vertebrate Endpoints

Parent Dicamba

The 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. HED

recently reanalyzed the data from this study (USEPA, 2016, 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 revised T-REX run presented in Section 

6 of this addendum reflects the adjusted chronic endpoint for parent dicamba.

Metablolite DCSA
12

Following preliminary review of a rat 2-generation study with DCSA (MRID 47899517), the risk 

assessment for the proposed new use on soybeans (USEPA, 2011 D378444) used 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, 2016) was completed 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, 2016) 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 malepremating 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 T-REX 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 

terrestrial-phase 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 throughout lactation while chicks in the avian reproduction test

would not be exposed to DCSA residues while still in the egg or post hatch.

6. Revised T-REX Analysis for Parent Dicamba and Quantitative 

Assessment of DCSA Exposure and Risk

Dicamba-specific Half-Life

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. However, EFED has refined this analysis 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); a chemical-specific foliar dissipation half-life value for parent dicamba
13

(described below); and the new chronic mammalian endpoint for parent dicamba (described 

previously in Section 5). 

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 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 2. 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 2. 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

Parent Dicamba T-REX Analysis

Modeled maximum residue values (EECs) determined using this refined approach were slightly 

higher (~15%) than those determined in the original dicamba-tolerant soybean Section 3 

assessment, but would not have impacted the screening-level risk conclusions for any assessed 

taxa. The previous risk assessment (2011) concluded that there was potential for direct adverse 

effects to mammals from chronic exposures of dicamba (max chronic RQ was 2.31 for small 

mammals consuming short grass). Following the refinements presented in this section (3 

applications of dicamba to include the two post-emergent applications at the 0.5 lb a.e./A rate, 

foliar dissipation half-life decreased from 35 days to 8.4 days, and an increase in the mammalian 

chronic endpoint from 45 mg/kg-bw to 136 mg/kg-bw), there are no longer any exceedances for 

any size class of mammal consuming any dietary item (max RQ = 0.84, see Appendix 1 for full 

T-REX input and output)

DCSA Metabolite Exposure Analysis

Since the chronic toxicity endpoints are more sensitive for DCSA than dicamba and DCSA 

residues were higher than dicamba residues within dicamba-tolerant soybean plant tissues (see 

below), EFED separately assessed the chronic exposure to DCSA residues for birds and 

mammals. 
14

The available data indicate that DCSA has similar acute toxicity as parent dicamba, but is 

substantially more toxic on a chronic basis to mammals. In conventional soybean plants, DCSA 

residues following dicamba applications prior to planting were less than 2% of total dicamba 

residues in forage, hay and seed (MRIDs 43814101 and 44089307; max of 0.130 ppm DCSA, 

see Appendix 2) and would not be above toxicity thresholds for any taxa. However, in dicambatolerant soybean plants, dicamba is converted to DCSA and its glycosidic conjugates following 

demethylation of the aromatic methoxy moiety of dicamba (USEPA, 2013. HED residue 

chemistry summary) and in comparison to dicamba use on conventional soybeans, the maximum 

residues of DCSA in dicamba-tolerant soybean field trials following one 1-lb/A pre-emergent 

application and two 0.5-lb/A post-emergent applications were a substantially higher proportion 

of dicamba-related residues in forage, hay and seed (Appendix 2 and MRID 47899524; 76%--

88% of total dicamba-related residues). The empirical data from MRID 47899524 found means

and maximums, respectively, of DCSA concentrations of 17.0 and 51.3 ppm, in forage 7-10 days 

following the last application, 32.2 and 61.1 ppm in hay 13-15 days following the last application 

and 0.059 and 0.440 ppm in seeds 73-98 days after the last application. EFED used the 

maximum measured values from the empirical data on forage, hay and seeds to assess risk to 

terrestrial vertebrates. There is some uncertainty in this approach as the maximum DCSA 

residues appear to be slightly increasing (16%) between forage at 7-10 days and hay at 13-15 

days, however this could be due to the difference between fresher forage and drier hay, where 

DCSA has become more concentrated compared to the overall plant biomass, rather than due to

additional conversion of dicamba residues to DCSA. Additionally, the amount of additional 

dicamba available to potentially convert to DCSA appears limited after this point as the 

maximum residues of dicamba were only 2.62 and 1.16 ppm in forage and hay, respectively. 

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 DTsoybean 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. This is because the 

conversion of dicamba to DCSA in plants is only expected to occur in crops modified to be 

tolerant to dicamba.

DCSA Effects Assessment

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, resulting in a 

chronic avian endpoint of 40.9 mg/kg-bw. However, since the DCSA chronic endpoint for 

mammals is based on effects to pups who were continually exposed in utero 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 would not be exposed to DCSA residues during their 

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

Using the empirical dataset for DCSA residues in DT-soybean crops (as described above), the 

maximum residues in soybean forage and hay tissue were 61.1 ppm and in seeds were 0.440 

ppm. Residues in arthropods (as a dietary item for birds and mammals consuming insects that 
15

have consumed soybean tissues with DCSA residues) were assumed to follow the Kenaga 

nomogram relationship between broadleaf plants and arthropods for spray applications and 

therefore were considered to contain 42.5 ppm. This is likely conservative, given that the 

residues from the nomogram are for external residues in food items following a spray application 

while the actual exposures would be internal residue concentrations in the plant. A screening 

assessment using this empirical data for the exposure values results in exceedances of the chronic 

LOC for all size classes of mammals consuming soybean forage and hay tissue or consuming 

insects that had consumed soybean tissues with DCSA residues (RQs range from 1.1—3.3, 

Table 3), for small birds consuming forage and hay tissue or insects that had fed on DT-soybean 

tissues, (RQs range from 1.2—1.7, Table 4) and medium birds feeding on forage/hay tissue 

(marginal exceedance of 1.0) but no exceedances occurred for any size mammalian or avian 

granivore consuming soybean grain (max granivore RQ of < 0.01).

Table 3. Dose-based exposure, body-weight adjusted chronic endpoints and risk quotients for 

mammals consuming DT-soybean tissues containing DCSA residues (maximum 61.1 mg/kg in 

forage/hay, 0.44 mg/kg in seeds) or consuming arthropods that had fed on DT-soybean tissues 

(assumed to contain 42.5 mg/kg DCSA). Bold RQ values exceed the LOC.

Size Class 

(g)

Dietary Item Food Intake 

(k-diet/d)

Dose-based EEC 

(mg/kg-bw)

Adjusted NOAEL 

(mg/kg-bw)

RQ

Small (15g) Forage/Hay 0.0143 58.25 6.2 3.3

Seed 0.00318 0.09 6.2 <0.01

Arthropod 0.0143 40.52 6.2 2.3

Medium 

(35g)

Forage/Hay 0.0231 40.33 14.2 2.8

Seed 0.00513 0.06 14.2 <0.01

Arthropod 0.0231 28.05 14.2 2.0

Large 

(1000g)

Forage/Hay 0.153 9.35 17.6 1.5

Seed 0.0340 0.01 17.6 <0.01

Arthropod 0.153 6.50 17.6 1.1

Table 4. Dose-based exposure and risk quotients for birds consuming DT-soybean tissues 

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

dicamba and DCSA for birds).

Size Class (g) Dietary Item Food Intake 

(k-diet/d)

Dose-based 

EEC (mg/kgbw)

NOAEC 

(mg/kg-bw)

RQ

Small (20g) Forage/Hay 0.0228 69.65 40.9 1.7

Seed 0.0051 0.11 40.9 <0.01

Arthropod 0.0228 48.45 40.9 1.2

Medium 

(100g)

Forage/Hay 0.0649 39.65 40.9 1.0

Seed 0.0144 0.06 40.9 <0.01

Arthropod 0.0649 27.58 40.9 0.7

Large (1000g) Forage/Hay 0.291 17.78 40.9 0.4

Seed 0.065 0.03 40.9 <0.01

Arthropod 0.291 12.37 40.9 0.3

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-soybean tissues (forage from days 7-10, hay from days 13-15 and seeds from 
16

days 73-98) 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-soybean 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.

As noted above, EFED calculated these RQs based on the female lactation dose NOAEL 

endpoint of 8 mg/kg/d from the DCSA 2-generation study where reductions of up to 9% pup 

body weight were observed 2-3 weeks post birth at the next highest dose (78 mg/kg/d). If the 

BMDL5 (the lower 95% confidence level on the estimated benchmark dose to result in a 5% 

body weight change in pups from background levels) of 34.9 mg/kg/d calculated by HED (EPA, 

2016) for DCSA was used in place of the NOAEL, then the maximum residues from the 

empirical data in soybean hay would be below the threshold dose for all size classes of mammals 

feeding on soybean plant tissue or soybean-consuming arthropods (RQs would range from 

0.35—0.76 for mammals feeding on tolerant soybean tissues and 0.24—0.53 for mammals 

feeding on arthropods having consumed soybean tissues).

7. Terrestrial Invertebrates Risk Characterization

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 memorandum1 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 addendum document 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 

 1 https://www.whitehouse.gov/the-press-office/2014/06/20/presidential-memorandum-creating-federal-strategypromote-health-honey-b
17

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 an RQ were calculated for terrestrial invertebrates,

2 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 

 2 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.
18

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. 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 soybean 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./L3 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 Database4

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). 

 3 Test material was dicamba BAPMA salt

4 EU Pesticide Properties Database (PPDB), http://sitem.herts.ac.uk/aeru/ppdb/en/Reports/213.htm
19

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 contact-based bee chronic NOAEC of 38 μg a.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 
20

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 

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 soybean 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 in summer,

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-soybean 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.
21

8. 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). According to the original

ecological risk assessment (USEPA, 2011a), dicamba 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 3 for calculations). Using 

these assumptions in 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 3). 

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.

9. 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 
22

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 application (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 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.

References:

Egan, JF and DA Mortensen, 2012. Quantifying vapor drift of dicamba herbicides applied to 

soybean. Environ. Toxicol. Chem. 31(5) 1023-1031, 2012.

Flint, JL and Barrett, M. , 1989a. Effects of Glyphosate Combinations with 2,4-D or Dicamba 

on field Bindweed (Convolvulus arvensis). Weed Science, 1989, 37: 12 -18.

Flint, JL and Barrett, M. 1898b. Antagonism of Glyphosate Toxicity to Johnsongrass (Sorghum 
23

halepense) by 2,4-D and Dicamba. Weed Science, 1989, 37: 700 – 705.

Kelley, KB, LM Wax, AG Hager and DE Riechers, 2005. Soybean response to plant growth 

regulator herbicides is affected by other postemergence herbicides. Weed Science, 2005, 53: 101 

-112.

Koch, H. and p. Weisser. 1997. Exposure of honey bees during pesticide application under field 

conditions. Apidologie, 28:439—447.

Olszyk, D, T, Pfleeger, EH Lee and M Plocher, 2015. Glyphosate and dicamba herbicide tank 

mixture effects on native plant and non-genetically engineered soybean seedlings. 

Ecotoxicolgy, 2015, 24:1014-1027.

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.

Slade, D. 2015 (Missouri Department of Agriculture, Jefferson City, MO). Letter to: Grant 

Rowland (Environmental Protection Agency, Washington D.C.), 2015 Dec 15.

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. Office of Chemical 

Safety and Pollution Prevention, Washington, D.C. DP 378444. March 8, 2011.

USEPA, 2011b. Evaluation Guidelines for Ecological Toxicity Data in the Open Literature. 

Environmental Fate and Effects Division. Office of Pesticide Programs. Office of Chemical 

Safety and Pollution Prevention, Washington D.C. May 16, 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, 2016. 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.

Winston, M.L. 1987. The Biology of the Honey Bee. Harvard University Press, Cambridge, 

MA. ISBN 0-674-07409-2.
24

Appendix 1: T-REX Inputs and Outputs for Dicamba Use on DicambaTolerant Soybeans

Application No. Rate Day of Application

Chemical Name: Application Rate (lb ai/acre 1 1 0 The value in G6 must be zero

Seed Treatment? (Check if yes) FALSE Seeding Rate (lbs/acre) 2 0.5 7

 Use: 166.7 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 Toxicity Value Reference (MRID)

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 1

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

yes 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

8

Reset Model
25

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 Arthropods Granivore

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

20 135.44 299.47 2.21 137.26 1.01 168.45 1.24 18.72 0.14 117.29 0.87 4.16 0.03

100 172.42 170.77 0.99 78.27 0.45 96.06 0.56 10.67 0.06 66.88 0.39 2.37 0.01

1000 243.55 76.46 0.31 35.04 0.14 43.01 0.18 4.78 0.02 29.94 0.12 1.06 0.00

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
26

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 

Size 

Class

(grams)

Adjusted

LD50

EECs and RQs

Short Grass Tall Grass Broadleaf Plants Fruits/Pods/Seeds Arthropods Granivore

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

15 6022.06 250.70 0.04 114.90 0.02 141.02 0.02 15.67 0.00 98.1893 0.0163 3.4819 0.0006

35 4872.49 173.26 0.04 79.41 0.02 97.46 0.02 10.83 0.00 67.8619 0.0139 2.4065 0.0005

1000 2107.50 40.17 0.02 18.41 0.01 22.60 0.01 2.51 0.00 15.734 0.0075 0.5579 0.0003

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
27

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 Arthropods Granivore

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ

15 298.90 250.70 0.84 114.90 0.38 141.02 0.47 15.67 0.05 98.19 0.33 3.48 0.01

35 241.85 173.26 0.72 79.41 0.33 97.46 0.40 10.83 0.04 67.86 0.28 2.41 0.01

1000 104.61 40.17 0.38 18.41 0.18 22.60 0.22 2.51 0.02 15.73 0.15 0.56 0.01
28

Appendix 2. Dicamba Crop Field Trial Residue Data Which Include the 

Determination of the DCSA Metabolite.

Table 1. Summary of Residues from Conventional Asparagus Crop Field Trials with DCSA as a Dicamba

Residue of Concern.

1

Formulation2 Total 

Application Rate

(lb ae/A)

PHI 

(days)

N3 Residue of 

Concern

Combined Residues (ppm)

Min. Max. LAFT5 HAFT5 Median5 Mean5 SD5

4 lb ae/gal 

DGA SL,

4 lb ae/gal 

DGA SL, and 

2 lb ae/gal Na

SL

Single postemergence 

broadcast 

application of 

0.5 lb ae/A

1 24 Parent 0.266 3.274 0.304 3.144 0.604 0.967 0.852

DCSA4 <0.01 0.071 <0.01 <0.040 0.011 0.014 0.0069

Total 0.271 3.192 0.314 3.166 0.622 0.981 0.854

1 Asparagus data are taken directly from MRID Nos. 43245206 and 43425803 (D204488, D204809, and D209229, L. Cheng, 

07/14/1997) used for tolerance re-assessment in the 2005 RED. 2 Test applications included the dimethylamine (DMA), diglycolamine (DGA), and sodium (Na+)salt formulations. 3 number of samples. 4 DCSA is the 3,6-dichloro-2-hydroxybenzoic acid metabolite. 5 Values based on per-trial averages. LAFT = lowest average field trial, HAFT = highest average field trial, SD = standard 

deviation. For computation of the LAFT, HAFT, median, mean, and standard deviation, values < LOQ are assumed to be at the 

LOQ (0.01 ppm).

Table 2. Summary of Residues from Conventional Soybean Crop Field Trials (Seed) with DCSA as a 

Dicamba Residue of Concern.

1, 2

Formulation3 Total 

Application Rate

(lb ae/A)

PHI 

(days)

N4 Residue of 

Concern

Combined Residues (ppm)

Min. Max. LAFT6 HAFT6 Median6 Mean6 SD6

4 lb ae/gal 

DMA SL

Single 0.5 lb ae 

pre-plant 

treatment 

followed by a 

single postemergence 

application of 

2.0 lb ae/A

7 24 Parent 0.027 8.10 0.038 7.40 0.72 1.022 1.703

DCSA5 <0.01 0.130 <0.01 <0.048 .014 0.02 0.015

5-OH 

dicamba

<0.01 0.360 <0.01 0.26 0.01 0.043 0.071

Total 0.047 8.14 0.084 7.44 0.768 1.085 1.713

1 Soybean grain data are for the 1X rate which used a 0.5 lb ae/A treatment made at 14-days pre-planting followed by a 2.0 lb 

ae/A treatment made at 7-days prior to harvest taken directly from MRID Nos. 43814101 (D223283, S. Knizner, 07/29/1996) and 

44089307 (D228703, S. Chun, 07/16/1998) used for tolerance reassessment in the 2005 RED. 2 The registrant was not supporting tolerances for soybean forage and hay at this time in lieu of a feeding restriction placed on the 

label. However, data were included for these commodities in the study submissions acquired using a single 0.5 lb ae/A treatment 

made at 14-days pre-planting (0.25x the maximum rate). Total residues of dicamba (parent, DCSA, and 5-OH dicamba) were 

<0.03 - <0.097 ppm in soybean forage and <0.03 - <0.04 ppm in soybean hay. 3 Test applications included the dimethylamine (DMA) salt formulation. 4 number of samples. 5 DCSA is the 3,6-dichloro-5-hydroxybenzoic acid metabolite. 6 Values based on per-trial averages. LAFT = lowest average field trial, HAFT = highest average field trial, SD = standard 

deviation. For computation of the LAFT, HAFT, median, mean, and standard deviation, values < LOQ are assumed to be at the 

LOQ (0.01 ppm).
29

Table 3. 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 Median 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

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 1 Except for sample min/max, values reflect per trial averages; n = no. of field trials. For calculation of median, mean, and 

standard deviation, the LOQ (0.02 ppm each analyte in undelinted cotton seed and 0.04 ppm for each analyte in cotton gin 

byproducts) was used for any results reported as <LOQ in Table C.3. Combined residues of dicamba, 5-OH dicamba, DCGA, 

and DCSA are expressed in parent equivalents. Individual analyte results are reported as per se. N/A = Not applicable. 2 LAFT = lowest-average-field-trial; HAFT = highest-average-field-trial.
30

Table 4. Summary of Residues from Dicamba-Tolerant Soybean Crop Field Trials with DCSA as a 

Dicamba Residue of Concern.

Commodity Total Applic. 

Rate

lb a.e./A

(kg a.e./ha)

PHI 

(days)

Residue Levelsa, b

(ppm)

N Min. Max. HAFT Median

(STMdR)

Mean

(STMR)

Std. Dev.

DCGAc

Forage 1.96-2.04

(2.19-2.28)

7-10 44 0.356 5.90 5.27 1.93 2.02 1.02

Hay 13-15 44 0.167 7.26 7.19 2.00 2.66 1.91

Seed 73-98 44 <0.011 0.135 0.131 0.017 0.032 0.029

DCSA

Forage 1.96-2.04

(2.19-2.28)

7-10 44 8.92 51.3 50.4 15.0 17.0 8.00

Hay 13-15 44 12.2 61.1 60.7 31.9 32.2 11.2

Seed 73-98 44 0.010 0.440 0.439 0.033 0.059 0.089

Dicamba

Forage 1.96-2.04

(2.19-2.28)

7-10 44 <LOQ 2.62 2.47 0.068 0.374 0.603

Hay 13-15 44 <LOQ 1.16 1.01 0.051 0.130 0.216

Seed 73-98 44 <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ

5-OH Dicamba

Forage 1.96-2.04

(2.19-2.28)

7-10 44 <LOQ 0.009 0.009 0.005 0.006 <LOQ

Hay 13-15 44 <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ

Seed 73-98 44 <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ

a

Concentrations of the individual analytes are reported as dicamba equivalents

b

Values < LOQ are assumed to be at the LOQ. C DCGA residues were quantitated by a non-validated method
31

Appendix 3: 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

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) @ 40 105
32

°Lat

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
33

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.

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