1 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460

August 26, 2014 

 PC Codes: 117401, 117403, 117404, 117423

MEMORANDUM DP Barcode: 421881

SUBJECT: Response to Public Comments on the EFED Registration Review Problem 

Formulation for Clopyralid 

FROM: Elyssa Arnold, MEM, MPH, Biologist 

James Lin, Environmental Engineer 

 Environmental Risk Branch 2 

Environmental Fate and Effects Division (7507P) 

REVIEWED 

BY: Jean Holmes, DVM, MPH, Risk Assessment Process Leader 

 R. David Jones, Senior Agronomist 

 Environmental Risk Branch 2 

 Environmental Fate and Effects Division (7507P) 

THROUGH: Brian Anderson, Branch Chief 

 Environmental Risk Branch 2 

 Environmental Fate and Effects Division (7507P) 

 

TO: Steven Snyderman, Chemical Review Manager 

 Tracy Perry, Team Leader 

 Patricia Parrott, Branch Chief 

Risk Management and Implementation Branch 3 

Pesticide Re-Evaluation Division (7508P) 

The Environmental Fate and Effects Division (EFED) reviewed comments from the technical 

registrant (Dow AgroSciences) on the “EFED Registration Review Problem Formulation for 

Clopyralid” (Document ID EPA-HQ-OPP-2014-0167-002, referred to as the Problem 

Formulation in this document). EFED also reviewed the registrant’s comments on the Clopyralid 

Preliminary Work Plan for Registration Review (Document ID EPA-HQ-OPP-2014-0167-010) 

that were relevant to the Problem Formulation. The purpose of this memorandum is to respond to 

these comments. EFED appreciates the comments and has considered them carefully; responses 

are provided below each set of paraphrased comments. 
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Comments on Preliminary Work Plan Relevant to the EFED Problem Formulation 

1. Requested Data 

Dow AgroSciences (DAS) Comment: 

DAS plans to generate new data for the following with clopyralid acid technical material:

 Guideline 850.1300 Chronic Freshwater Invertebrate 

 Guideline 850.2100 Avian oral toxicity (passerine species) 

 Guideline 850.4500 Aquatic non-vascular plant growth for 2 additional species: 

 Skeletonema, Navicula 

 Guideline 850.6100 ILV for soil method 

 Guideline 850.6100 ECM and ILV-compost 

DAS plans to submit existing study data for the following:

 Guideline 835.4100 Aerobic Soil Metabolism 

 Guideline 835.4300 Aerobic aquatic Metabolism 

 Guideline 850.1400 Freshwater Fish early-life stage 

 Guideline 850.2300 Avian reproduction (upland game species) 

 Guideline 850.4400 Aquatic Vascular Plant growth 

 Guideline 850.4550 Cyanobacteria toxicity 

 Guideline 850.6100 ECM for soil 

 Guideline 850.6100 ECM and ILV for water 

EFED Response: It appears that DAS has conducted guideline fate and ecotoxicity studies that 

it has not submitted to the Agency. DAS is encouraged to submit any available studies relevant 

to ecological risk assessment, not limited only to the eight studies noted above. EFED will 

review the data when it is received by the Agency. 

2. Terrestrial Plant Data with Clopyralid Acid and Salts 

DAS Comment: Comments addressing the following data requirements: 

 Guideline 850.4100 Terrestrial plant toxicity, seedling emergence 

 Guideline 850.4150 Terrestrial plant toxicity, vegetative vigor 

DAS proposes to continue to rely on existing data MRID 40081401 and supplement it with data 

for formulation EF-243. Where no typical end use products exist for the straight clopyralid salts 

we do not propose to conduct the above studies. 

EFED Response: EFED has classified the seedling emergence and vegetative vigor studies in 

MRID 40081401 as supplemental due to a number of methodological limitations. These 

limitations were listed in DER amendments transmitted by EFED to the Registration Division on 

February 21, 2012 (DP398834). Among other deficiencies, these studies did not report plant 

weight measurements and, therefore, did not measure a potentially sensitive endpoint for 

terrestrial plants exposed to clopyralid. MRID 40081401 does not fulfill the guideline 

requirement for tier 2 vegetative vigor and seedling emergence data for clopyralid. These studies 

are needed to assess risk to non-target terrestrial plants. 
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For the clopyralid acid (PC 117403), EFED notes that, contrary to the statement in Table 1 of the 

comment document, DAS does currently have an active registration for a single active ingredient 

product (i.e., not co-formulated); this registration is for Lontrel Turf and Ornamental, EPA Reg. 

No. 62719-305. For the monoethanolamine (MEA) salt (PC 117401), EFED will evaluate the 

seedling emergence and vegetative vigor studies using EF-243 against OCSPP guidelines when 

they are submitted. For the clopyralid triethylammonium (TEA) salt (PC 117404) and the 

potassium salt (PC 117423), EFED concurs that there are currently no active registrations for 

single active ingredient products (i.e., not co-formulated). 

EFED’s objective in requesting terrestrial plant studies using formulations containing each of the 

four registered clopyralid active ingredients (i.e., PCs 117403, 117401, 117404, and 117423) is 

to identify any differences in toxicity of these chemicals. Where no appropriate formulations 

exist for this purpose, i.e., for PCs 117404 and 117423, studies may be conducted with the 

technical products. If DAS has other information they believe will address this uncertainty, they 

are encouraged to submit it for review. 

3. Terrestrial Plant Data with Clopyralid Co-formulated Products 

DAS Comment: Comments addressing the following data requirements: 

 850.4150 Terrestrial plant toxicity, vegetative vigor co-formulated with other herbicides 

DAS proposes to provide existing data for GF-1362 and conduct studies for Curtail, Sendaro, 

Surestart II, Hornet WDG, and Hat Trick Three Way. Clopyralid-containing products from 

multiple registrants typically have similar compositions of active ingredients and thus DAS 

proposes that the above representative formulations should be adequate for products registered 

by DAS and other registrants. Listed formulations, surrogate or other, containing 3 active 

ingredients are also expected to cover the requirements for a subset of the same actives provided 

they have a similar compositional range. 

EFED Response: EFED will review the studies with the proposed formulations when they are 

submitted to the Agency. 

Comments on the EFED Problem Formulation 

4. Groundwater Incidents and Modeling 

Stated in the Problem Formulation (p. 4): Incidents reported for other picolinic acid herbicides 

demonstrate the potential for groundwater contaminated with these herbicides to damage nontarget terrestrial plants when the groundwater is used for irrigation. 

DAS Comment: We are not aware of any incidents related to damage to non-target plants via 

groundwater contamination with clopyralid and would be interested to evaluate if the incidents 

mentioned can be attributed to picloram only while being used at the historical higher rate. 

The agency intends to use the PRZM-GW model to estimate water concentrations in 

groundwater, for use as inputs into drinking water and non-target plant risk assessments. Recent 
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presentations have shown that proper parameterization of PRZM-GW is critical to yield 

representative estimates of predicted concentrations. The characterization of soil sorption has 

been found to be especially critical and it has been found that including a time-dependent 

formulation of the sorption greatly improves the predictive power of the model (as compared to 

field data). The current implementation of PRZM-GW, however, does not support this 

representation, but various groups have shown that this can be accomplished quite readily. In 

keeping with EPA’s statements that “additional data are recommended to better understand the 

potential for risk due to residues of clopyralid,” Dow AgroSciences proposes parameterizing 

PRZM-GW assessments using best available information and methodology. DAS will undertake 

further field studies or monitoring efforts if refined modeling efforts do not yield sufficient 

refinements of exposure. 

EFED Response: EFED agrees in principle that a tiered study approach can be useful for 

refining risk estimates. EFED completed some sensitivity analysis on incorporation of nonlinear 

sorption and degradation in subsurface soil layers using the PRZM-GW model, to determine the 

usefulness of conducting additional studies in refining risk estimates. EFED explored the 

potential for considering nonlinear sorption and a degradation rate in subsurface layers. 

EFED recommends if any additional analyses are performed, that these analyses focus on better 

understanding/quantifying the potential for degradation in subsurface layers (either via longer 

term hydrolysis studies or degradation in soils from subsurface layers). 

 

It is likely that whether inputs to PRZM-GW can be refined or not, the compound has the 

potential in vulnerable areas to reach groundwater at concentrations that could result in a risk 

concern if the water is used to irrigate sensitive terrestrial plants. In the absence of additional 

data, EFED will use PRZM-GW to estimate the potential exposure to terrestrial plants from 

residues in irrigation water obtained from groundwater sources. 

5. Monitoring Data 

Stated in the Problem Formulation (p. 13): A search of the NAWQA database (11/25/2013) 

returned maximum detection levels for clopyralid of 0.53 ppb in surface water in Oregon and 

0.42 ppb in groundwater in Washington. Data provided by the Montana Department of 

Agriculture include maximum detection levels for clopyralid of 0.80 ppb in surface water and 2.8 

ppb in groundwater. The measured concentrations in groundwater are higher than the previously 

modeled concentration. 

DAS Comment: The maximum level in the NAWQA database was a sample from 1995 and was 

one of only 4 detects in 6803 sampling incidents in the database. When the maximum detected 

level (0.53 ppb) is compared to the lowest human Health Benchmark for pesticides calculated for 

clopyralid (990 ppb) in 2013 reflecting the latest scientific information, concern for exposure to 

clopyralid via ground or surface water is not anticipated. 

EFED Response: Available monitoring data will be considered in the ecological risk assessment 

and risks from exposure to clopyralid via drinking water will be assessed by the Health Effects 

Division. 
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6. Label Ambiguities 

Stated in the Problem Formulation (p. 6): The labels have non-specific use sites. 

DAS Comment: DAS agrees with this Agency comment and has initiated label amendments to 

better define use sites. 

EFED Response: EFED will assess uses consistent with the updated labels at the time of the 

preliminary risk assessment. 

7. Aerobic Soil Metabolism Half-life 

Stated in the Problem Formulation (p. 15): In Table 7, Aerobic soil metabolism DT50 

(25±1°C) 12.8 days (MRID 43891403). 

 

DAS Comment: The DT50 in the cited study should read 7.8 days at 25 °C not 12.8 days. 

EFED Response: The new half-life value of 8.03 days is calculated based on “Standard 

Operating Procedure for Using the NAFTA Guidance to Calculate Representative Half-life 

Values and Characterizing Pesticide Degradation.”1

                                                            

1

 Bohaty, Rochelle, William Eckel, Katrina White, and Dirk Young. 2012. Standard Operating Procedure for Using 

the NAFTA Guidance to Calculate Representative Half-life Values and Characterizing Pesticide Degradation. Office 

of Pesticide Programs, U.S. Environmental Protection Agency. Available online at: 

http://www.epa.gov/oppefed1/ecorisk_ders/degradation_kinetics/ftt_sop_using_nafta_guidance.pdf. 
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8. Avian Reproduction Study with the Mallard Duck 

Stated in the Problem Formulation (p. 18): Results from the avian reproduction study with the 

mallard duck (MRID 00156001) included statistically significant effects on normal hatchlings 

per eggs set and per eggs laid at the lowest treatment concentration (100 ppm) and statistically 

significant effects on 14-day old survivor weight at the lowest and middle treatment 

concentrations (100 and 330 ppm). No effects were seen at the highest treatment concentration 

(1000 ppm), and therefore these statistically significant effects were not considered to be 

treatment related. This study is scientifically sound and classified as acceptable. A low-dose 

effect (hormesis) is another possible explanation for the study results. Reproduction data for an 

additional avian species may help to illuminate the likelihood of this possibility 

DAS Comment: DAS plans to submit the following study: 

 Dabbert, C.B., Mitchell, R.B., and Oberheu, D.T. (1997). Northern Bobwhite Egg 

Hatchability and Chick Immunocompetence following a Field Application of Clopyralid. 

Bulletin of Environmental Contamination and Toxicology 58: 801-806. 

The objective of this study was to determine if field application of clopyralid at the 

recommended label rate would alter viability and hatchability of eggs and subsequently, growth 

and immune function of 28 day-old Northern bobwhite chicks. 

EFED Response: EFED will review the study and determine whether it is sufficient to fulfill the 

850.2300 guideline requirement when it is submitted.

9. Ecological Incidents – Residential Turf Uses of Clopyralid 

Stated in the Problem Formulation (p. 19): Incident I010414-001: There is a warning on the 

label saying that compost containing grass clippings that have been treated with clopyralid 

should not be used in the growing season of application. 

DAS Comment: This incident was reported at a time when clopyralid was registered for use on 

residential turf. The work done by Vandervoort, 1997 indicates that was an appropriate statement 

and shows decline of clopyralid residues from 32,000 ppb to 1,400 ppb in 365 days. This label 

statement became obsolete as residential turf use was voluntarily removed by Dow AgroSciences 

from all clopyralid labels in 2002. From that time clopyralid-containing products could only be 

used by professional applicators on golf courses and other forms of non-residential turf, such as 

commercial complexes, road medians and soccer fields. 

EFED Response: The incident report provides an example of clopyralid persisting at toxic 

levels through a composting process. Only labeled uses will be assessed in the risk assessment.

10. Clopyralid Residues in Compost 

Stated in the Problem Formulation (p. 21): Clopyralid has been found to survive the 

composting process when yard clippings are collected and composted. The chemical becomes 

concentrated in the final compost, which then may be toxic to plants to which it is applied. 
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DAS Comment: The appearance of ‘concentration’ of clopyralid in compost may be an artifact 

of the differential rate of decay of the chemical and the compost feed stocks as they are 

biologically degraded. The effect of the decay of the compost and the residue concentration 

increase needs to be accounted for by bring the compost back to a constant standard (ash basis). 

EFED Response: The results of a degradation study in compost will address the issue as to 

whether clopyralid will be concentrated in the final compost. 

11. Treated Trees in Compost 

Stated in the Problem Formulation (p. 22): Trees treated with clopyralid may be used as 

compost. 

Dow AgroSciences Comment: Clopyralid is labeled for use on woody species pests such as 

mesquite, locust and acacia. The geographical scope of such use implies that these trees are 

unlikely to be close to composting facilities. Trees such as mesquite are unlikely to be 

preferential feedstock for composting facilities. 

EFED Response: EFED will take this information into account in the risk assessment. Potential 

use of trees treated with clopyralid as woodchips or mulch will also be considered. 

12. Dissipation Study in Compost 

Stated in the Problem Formulation (p. 23): Non-guideline Study: Dissipation in Compost. 

A study is requested to demonstrate the rates of degradation and leaching in vegetative and 

manure composts. The study will be used to characterize the risk from clopyralid residues in 

compost and may demonstrate potential mitigations such as compost holding times. The absence 

of information on the persistence of clopyralid residues in composted plant material or animal 

manure prohibits a quantitative assessment of the potential risk to non-target taxa posed by the 

application of composted material containing these residues. A protocol is requested for review 

before the study is conducted. 

DAS Comment: DAS feels that existing data has answered all the questions put forward by the 

Agency and further studies are not required. Labeling restrictions based on said data have 

effectively ensured no large scale repetition of events in 2002. Our comments are outlined in 3 

sections: 

A. Existing data for vegetative composting showing that the questions from the Agency have 

already been answered for appropriate inputs to compost of feedstocks that may contain 

clopyralid, degradation of clopyralid if it does contaminate feedstock and appropriate 

composting conditions to maximize removal of clopyralid from compost. 

B. Current label restrictions for all products. If clopyralid labeling is followed no clopyralid 

should enter waste streams used for compost feedstock. 

C. Lack of clopyralid residues expected in composting using manure feedstocks. DAS 

regulatory animal metabolism studies inform the expected lack of clopyralid in manure. 
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Dow AgroSciences respectfully suggests that sufficient data exists that would render any new 

non-guideline study unnecessary. Clopyralid does indeed decompose in compost, composting 

conditions can be enhanced to increase the speed of clopyralid degradation in green waste 

compost and compost that has been contaminated with clopyralid can be remediated by 

reactivating the degradation process. The place of bioassays in agronomic practices is important 

for protection of succeeding crops and determination of plant back intervals. The same practice 

should be considered BMP for compost feedstock intake. Finally enforcement of current existing 

label language should ensure that clopyralid contamination of compost feedstock is unlikely as 

growers follow and understand the labeling restrictions already in place. 

EFED Response: The fate and transport properties of clopyralid are not well understood in 

compost environments. The compost study will allow EFED to ascertain whether the application 

of compost materials will damage vegetation. Dow is encouraged to submit any currently 

existing data on clopyralid fate transport properties in compost and mulch in addition to the 

requested study.