UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

PC Code: 128850 

DP Barcode: D413093 

MEMORANDUM March 10, 2014 

Subject: Response to Comments on the Preliminary Ecological Risk Assessment for 

Glufosinate-Ammonium 

To: Shirley Keel, Acting Team Leader 

Linda Arrington, Branch Chief 

Risk Management and Implementation Branch V 

Pesticide Re-Evaluation Division (7508P~ /} /A 

From: Chuck Peck, Environmental En ineer ~ /O ~let SA~ w'\ ~~ -f.- Catherine Aubee, Biologist '3 ·11> ·11 

Environmental Risk Branch 4 

Through: Jim Carleton, Ph.D., Senior Scientist A~ c~ :3/Jo/'Jul'f 

Thomas Steeger, Ph.D., Senior Scien~~d -rf_u_~ g/;o/~o/</ Sarah Winfield, Acting Branch Chief · · ti7 tt (/ 

Environmental Risk Branch 4 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

The Environmental Fate and Effects .Division (EFED) has reviewed the comments submitted 

regarding the preliminary ecological risk assessment conducted as part of the registration review 

of the herbicide glufosinate-ammonium (PC Code 128850). The preliminary ecological risk 

assessment was posted to the public docket (EPA-HQ-OPP-2008-0 190) for public comment on 

March 6, 2013. The comments addressed in this memorandum were from the registrant, Bayer 

CropScience (BCS, EPA-HQ-OPP-2008-0 190-0038), and the Center for Food Safety (CFS, 

EPA-HQ-OPP-2008-0190-0031 ). Based on comments received and EFED's responses 

(contained in this memo), EFED will consider the recommendations in revising the ecological 

risk assessment of glufosinate-ammonium. 

In the section below, comments are paraphrased and EFED's response to each of the comment 

follows. 

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BCS Comment 1, page 6:

“BCS requests EPA to re-evaluate all the use patterns using parent and MPP as stated in 

ROCKS [Residues of Concern Knowledgebase Subcommittee] and to re-evaluate rice by 

considering [the] PFAM [Pesticides in Flooded Application Model] with index reservoir, since 

this level of refinement has already been evaluated by EFED and promoted in previous 

presentations at the Exposure Modeling Public Meeting (Rice Modeling: A Case Study, 

10/30/2012, presentation not made during meeting; presentation made 3/19/2013). By 

considering the index reservoir refinement, the acute EDWC [estimated drinking water 

concentration] will be reduced to less than 160 μg/L and less than 40 μg/L for chronic. There is a 

possibility that based on this level of refinement; rice may no longer be the crop that will be 

driving the exposure assessment.”

EFED Response: 

EFED is currently evaluating the use of PFAM coupled with the index reservoir in its drinking 

water assessments for uses on rice. It is anticipated that a decision will be made in the 2014

calendar year regarding the applicability of PFAM. The case study modeling results referenced 

by Bayer and presented at the Exposure Modeling Public Meeting (EMPM), as well as in 

EFED’s refined drinking water assessment of glufosinate-ammonium for rice (DP Barcode 

387412, May 2012) were intended to serve as a means of characterizing qualitatively the

potential reduction of estimated drinking water concentrations (EDWCs) due to dilution. As 

such, the quantitative use of PFAM coupled with the index reservoir is not warranted at this time.

In 2010, EFED conducted a drinking water assessment for agricultural uses of glufosinateammonium using the total toxic residue approach (DP Barocde 372624, June 22, 2010). In that

assessment, the chronic EDWCs for rice use exceeded the human health drinking water level of 

concern (DWLOC), thereby requiring a refined assessment specifically for use on rice (DP 

Barcode 387412, May 2012). The refined assessment for use on rice evaluated only exposures 

from the parent and MPP, as a more recent ROCKS meeting determined that these were the only 

residues of concern. After the use on rice, the next highest chronic EDWC (66 μg/L) was for use 

on grapes, and was below the DWLOC for human health, even when using the TTR approach.

As such, conducting another drinking water assessment for the other use patterns, evaluating 

only parent and MPP, would not change the conclusions.

BCS Comment 2, page 6:

“BCS notes that EPA did not conduct the rice exposure assessment using the aquatic dissipation 

half-life for glufosinate-ammonium. Nor did EPA give any consideration to the very short water 

half-life derived in the study (3.0 days, MRID 45204403). This half-life demonstrates that the 

potential for chronic exposure in the rice paddy is significantly less than the estimates provided 

in the EPA risk assessment.”
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EFED Response:

As outlined in EFED’s response to comments in 2011 (DP Barcode 387412, October 28, 2011), 

use of an aquatic dissipation half-life to represent chemical decay would be inappropriate, 

because dissipation reflects multiple fate processes that operate concurrently. EFED employs

models whose inputs include half-lives reflecting various specific degradation mechanisms 

and/or representative environmental compartments (e.g., aerobic aquatic degradation). The 

purpose of conducting an aquatic field dissipation study is “to determine the extent of dissipation 

and mobility of pesticide residues under actual use conditions” (OCSPP Guideline 835.6200). 

Dissipation, defined as “the overall process leading to the eventual disappearance of substances 

from the site of its application or an environmental compartment” (OCSPP Guideline 835.6100),

is a combination of transport processes, such as volatilization, sorption, leaching, etc., and 

transformation via microbial metabolism, hydrolysis, photodegradation, etc. The PFAM refined 

rice model uses aerobic soil degradation, aerobic aquatic metabolism, and sorption coefficients to 

estimate rice paddy concentrations. Use of a dissipation rate in place of a rate for one of these 

specific mechanisms, would comprise “double counting” a loss process. Additionally, the results 

from an aquatic dissipation study represent a specific site and field conditions, whereas EDWCs 

are intended as conservative representations of exposure applicable in national-scale assessment. 

The aquatic field dissipation study in question used fields flooded at a 12-inch depth; however,

rice paddies are typically flooded to smaller depths (i.e., between 4 and 8 inches). The deeper 

flood depth employed in the aquatic field dissipation studies thus diluted the test chemical to a 

greater degree than would typically occur in a rice paddy, which illustrates how the dissipation 

rate at a specific site may not be conservative or even representative of other sites. Lastly, both 

the Liberty® and Liberty® 280 labels allow for use of glufosinate-ammonium at higher 

application rates (approximately 1.44 lbs a.i./A for rice seed propagation) than those used in the 

aquatic field dissipation study (i.e., 2 applications at 0.44 lbs a.i./A). Because maximum use 

rates were not employed during the study, the resulting concentrations and dissipation rates 

cannot be assumed to be reflective of such use rates. It should also be noted that the submitted 

aquatic field dissipation study has other deficiencies, particularly with respect to the analytical 

method validation and the storage stability data.

The drinking water assessment for glufosinate-ammonium use on rice (DP Barcode 387412, May 

30, 2012), from which the paddy water concentrations were derived, estimated that if the 

maximum concentrations measured in the aquatic field dissipation study were adjusted to reflect

a 4-inch paddy water depth, the maximum glufosinate-ammonium concentrations in the water 

would be between 93 and 288 μg/L. These values were in line with the range of concentrations 

estimated using modeling (i.e., 83 to 390 μg/L), although the latter were based on a higher 

application rate.

BCS Comment 3, pages 6-7:

“BCS notes the following difference in the Efate parameters for glufosinate[-ammonium] and 

requests EPA to review the corresponding MRIDs in order to compare the calculated half-lives 

with those reported by BCS in this Table.”
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Table 1. Summary of Efate properties for glufosinate-ammonium (excerpted from BCS comments).

EFED Response:

In 2010, EFED retrieved the studies and data evaluation records (DERs) for the studies listed in 

Table 1. At the time, EFED planned to estimate drinking water concentrations for glufosinateammonium based on total toxic residues (TTR; glufosinate-ammonium plus its three degradates: 

2-methylphosphinico-acetic acid, 3-methylphosphinico-propionic acid, and 2-acetamido-4-

methylphosphinico-butanoic acid), and needed to calculate half-lives reflecting TTR rather than 

parent chemical. EFED also re-evaluated the half-life estimates for parent glufosinateammonium, since many of the DERs, which were written in the early 1990s, cited the study 

authors’ half-lives without validating them. Using a log-linear regression of the individual 

replicate percentages of radioactivity attributable to glufosinate-ammonium versus time, EFED 

revised the reported half-lives. Table 2 summarizes the half-lives as originally estimated by the 

study authors in the cited studies, and the revised half-lives used by EFED in its assessment. It 

should be noted that EFED’s revised half-lives for MRIDs 43123119, 40345659A, and 

40345660 were also provided in the Problem Formulation for glufosinate-ammonium (DP 

Barcode 345696, January 9, 2008).

Table 2. Summary of half-life estimates by study authors and EFED values for glufosinate-ammonium.

Study Soil designation Study Author

Half-life (days)

EFED

Half-life (days)

43123119

Sandy loam V

Sandy loam 2

Loamy sand 2.2

3.7

8.3

6.4

8.5

8.7

8.8

40345659A1

Sandy loam V

Sandy loam S

Silt loam E

23.3

20.7

21.7

23.0

20.6

21.4

403456602 Sandy loam system

Sand system

46.5 and 51.7

29.1 and 35.2

57

38

45204402/013

Loam system

Sand system (1 ppm)

Sand system (0.1 ppm)

11

42

2

10.9

86.5

1.4 1. It is unclear how the values cited in BCS’ table were derived. Values in table were extracted from study.
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2. For each system depicted, two replicates were conducted and the study authors derived half-lives for each replicate. EFED 

combined the replicates and developed a representative half-life for each system. It is not clear how the values reported in the 

BCS table were derived.

3. The numbers presented in the BCS table reflect those presented in the EFED DER for the study using a non-linear single first 

order method. The values reported in the table above reflect those presented by the study authors.

EFED believes the half-lives used in the assessment for the parent glufosinate-ammonium are 

consistent with those developed in the studies and allow for estimation of protective aquatic 

concentrations.

BCS Comment 4, pages 7-8:

“BCS notes that EPA did not model the same use patterns as was conducted for the aquatic 

exposure assessments and reflected in Table 3.1 [of the preliminary risk assessment], this is 

especially true for the non-agricultural uses. Additionally, BCS notes that these values need to be 

updated to match the use pattern information provided in Appendices 1 and 2 of these comments.

EPA states the default foliar dissipation half-life is 35 days. They also note that the data 

submitted under OCSPP Guideline 860.1500 were not deemed appropriate for determining a 

foliar half-life. BCS in its evaluation of the studies available found 16 trials that can be used to 

derive half-life. While these trials (corn, canola, sugar beet and [p]asture grass) were not

conducted in the United States, they were conducted in areas with similar weather patterns and 

therefore are representative of use patterns for the US. [h]alf-life values ranged from 1.9 to 11.1 

days, with a mean on short/tall grass of 4.4 days and overall mean 5.5 days. BCS will be 

submitting the following report in support of the derivation of the foliar half-life (Sabbagh and 

Demarteau, 2011).”

EFED Response:

EFED will revise the ecological risk assessment using the updated use patterns provided in 

BCS’s Appendices 1 and 2. EFED will review the studies submitted by BCS regarding foliar 

dissipation and, if acceptable, consider these data in the ecological risk assessment.

BCS Comment 5, page 8:

“BCS notes that the fish early life stage test was not included in the Data-Call-In for glufosinate 

[-]ammonium. However we will be conducting a fathead early life stage test (and associated 

acute test) for the European data requirements study during 2013.

With regards to the NOEC [no observed effect concentration] and LOEC [lowest observed effect 

concentration] from the trout 21[-]d test, BCS believes the NOEC and LOEC should be 100 and 

500 μg a.i./L(nominal, which could be converted to mean measured). The only effect observed at 

the 100 μg a.i./L concentration was some slow reactions, but only on Day 2 and for the duration 

of the test.
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BCS notes that the algal species used for the metabolite tests [discussed in Table 3.2.1 of the risk 

assessment] varied, depending on the metabolite, either Desmodesmus or Pseudokirchneriella

were used.”

EFED Response:

The Agency looks forward to the submission of the fish early life stage data. The risk 

assessment and associated DER will be revised to clarify that the no observed adverse effect 

concentration (NOAEC) and lowest observed adverse effect concentration (LOAEC) values for 

the 21-day toxicity test with juvenile rainbow trout (Oncorhynchus mykiss; MRID 48301105) are 

based on sublethal measurement endpoints (behavioral effects) and not survival. Slow reactions 

were observed in all fish from the 100 mg/L (nominal) exposure level on Day 2. The study 

author reported that since this observation occurred only once, it was not considered treatmentrelated. However, slow reactions were not observed in control fish. Moreover, slow reactions 

were observed in up to three surviving fish exposed at 500 mg/L (nominal) on Days 1 through 4,

before subsiding. Therefore, a treatment-related effect, although transient, cannot be excluded; 

therefore, the Agency maintains that the NOAEC for the study is 50 mg/L (nominal) based on 

impaired behavior. We assume that the identification of these treatment levels in terms of μg/L 

in the registrant’s comments is a typographical error.

With respect to the algal toxicity tests with glufosinate-ammonium metabolites, the species used 

in each test is identified in the risk assessment. 

BCS Comment 6, pages 8-9:

“BCS would like to note the following with regards to the DNT [Developmental Neurotoxicity] 

test, and recommends that the morphometric measurements not be used as corroborative 

evidence for evaluating potentially ecologically relevant effects.

• Test levels in the DNT test were: 0, 200, 1,000, or 4,500 ppm (equated to 0, 14, 69, or 

292 mg/kg/d).

• No effects observed in the following: brain weight, brain gross and microscopic 

necropsy, the average onset of preputial separation in males and vaginal opening in 

females, No treatment-related changes in Functional Observational Battery (FOB) during 

any of the testing periods, auditory startle response or learning and memory.

• Total and ambulatory motor activity was increased in males at 4,500 ppm on PNDs

[post-natal days] 13, 17, and 21, in females at 1,000 and 4,500 ppm on PNDs 17, 21, and 

61, and in males at 1,000 ppm on PNDs 21 and 61. Corresponding sub-session interval 

activity was also increased for these groups on each testing day.

• Brain morphometric measurements: [o]nly 1 of 3 measures in the hippocampus showed 

a difference and that endpoint can be problematic. The difference in the mean length of 

the ventral limb of the dentate hilus (Level 3) was observed in both males (9- 15%) and 

females (12-20%) at 200 ppm. No compound[-]related changes in other morphometric 

measurements were observed (PND 21 mean vertical thickness of the cortex on Level 1, 

mean width of the base of the cerebellar lobule 9 on Level 5, PND 72 mean vertical 
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height between the layers of pyramidal neurons in the hippocampal formation in Level 3, 

radial thickness of the cortex in Level 3).

• With regards to the morphometric difference in ventral limb of the dentate hilus, there 

was no evidence of an associated functional deficit (Biel maze) at any dose. Additionally, 

it should be noted that the endpoint in general has high variability associated with how 

measurements are made, and that the difference between left & right side measurements 

in the controls (13%) was greater than the difference between low-dose and control (9-

12%). Measurements at the low dose are within the range of historical controls, while the 

results for controls approach the upper limit of historical control. This measurement is 

also a relatively poor measure of the structure (horizontal measure of structure with radial 

orientation).”

EFED Response:

EFED has used the DNT as a line of evidence in characterizing the potential effects of 

glufosinate-ammonium in mammals; the registrant has not provided a compelling case to discard 

the results of the DNT study. Therefore, EFED will continue to use the DNT results 

qualitatively in the risk characterization. 

BCS Comment 7, pages 9-10:

“BCS notes that the risk quotient exceedances for the estuarine/marine invertebrates have been 

conducted with the exposure assessment within the rice paddy. There will not be any 

estuarine/marine organisms within the rice paddy, the risk assessment should be conducted with 

the exposure estimate for the receiving water body. With regards to the quote noted above on 

tidal flux, only under very specific scenarios (and not necessarily common scenarios) could 

increase exposure duration take place, but an increase in exposure concentration would not 

occur. Of course the incoming tidal flux would increase the dilution of the rice paddy discharge 

water. Also, tidal flux will not introduce estuarine/marine organisms into the rice paddy unless 

there is a significant flood event. This type of an event would significantly impact the production 

of rice and other organisms in rice paddy, as well as any estuarine/marine organisms introduced 

into the rice paddy. Lastly that flood event (if it were to occur) would significantly dilute the 

exposure to glufosinate[-]ammonium.”

EFED Response:

EFED does not agree that exposure to estuarine/marine invertebrates in the rice paddy could not 

occur. The sheepshead minnow (Cyprinodon variegatus), the EPA test species for 

estuarine/marine vertebrates, can exist in shallow freshwater and estuarine/marine environments. 

There is potential for such organisms to be exposed to glufosinate-ammonium once paddy water 

is released. EFED agrees that introduction of estuarine/marine species into a rice paddy due to 

tidal flux would probably only occur in a rare event. However, tidal flux could result in an 

estuarine/marine species being moved upstream to the outlet of a rice paddy prior to release of 

the paddy water, thus resulting in exposure locally to water at concentrations equal to those in 

the paddy.
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BCS Comment 8, page 10:

“BCS acknowledges in EPA’s risk description for aquatic plants (section 4.3.1.3 [of the 

preliminary risk assessment]) it is recognized that the toxicity values across species ranged from 

41μg/L to 100,000 μg/L. However BCS offers the following comments in the potential for 

indirect effects, and would state that the probability of glufosinate[-]ammonium indirectly having 

an effect on organisms through a potential impact on blue-green algae is very low.

• There were no risk quotient exceedances for aquatic macrophytes.

• The other 4 species of algae tested were significantly less sensitive than the blue-green 

algae. If risk quotients were to be calculated for these other species (2 green algae, 

freshwater diatom, saltwater diatom), there would be no risk quotient exceedances (listed 

or non-listed.

• Blue-green algae are a less desirable food item for other aquatic organisms than the 

other species tested.

• If the aquatic field dissipation study water half-life of 3 days (MRID 45204403) were 

used in the exposure estimate for rice, the dissipation of parent glufosinate[-ammonium]

would be significantly greater than the current EPA estimates. Therefore, IF there were a 

potential impact on this less desirable food item, that species would have the potential for 

recovery as the glufosinate[-ammonium] dissipates.

• The exposure estimates are within the rice paddy, not for the surrounding aquatic 

environments. Therefore the risk to algal species outside the rice paddy is low as the 

environmental exposure would be significantly less.”

EFED Response:

Natural algal communities are diverse. Therefore, the Agency consistently requires data for four 

species of aquatic nonvascular plants (freshwater and saltwater algae and diatoms) and uses the 

most sensitive endpoint across species and studies in risk estimation. As stated in EFED’s 

guidance to its risk assessors, “risk estimates should be based on the most sensitive of the four 

unicellular guideline study species independent of presumed ecological compartment” (USEPA 

20081

). As with the use of any surrogate species, they are intended to represent the potential 

sensitivity of a wide range of organisms that may not be limited to their specific taxon. It is for 

this reason that EPA typically requires multiple species of aquatic plants. As such, the

sensitivity of Anabaena flos-aquae is not considered specific to blue-green algae but is 

considered representative of other aquatic non-vascular plants in general.

Because rice paddies may also serve as important habitat for aquatic and semi-aquatic species, 

including nonvascular aquatic plants, the Agency maintains that estimated environmental 

concentrations (EECs) within the treated rice paddy are relevant to risk estimation. Moreover, 

the blue-green alga (cyanobacteria), which was the most sensitive species tested with 

 1 USEPA. 2008. Clarification regarding aquatic plant risk estimation. Memorandum from D. Brady to 

Environmental Fate and Effects Division, dated 30 September 2008. Washington, D.C.
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glufosinate-ammonium, is ecologically important and is critical to the nitrogen cycle (FernándezValiente and Quesada 20042

).

With regards to aquatic dissipation rate of 3 days, please refer to the earlier discussion regarding 

the use of dissipation rates as model inputs in lieu of degradation rates. Lastly, while the EECs 

for rice are taken from estimates of concentrations of water in the rice paddy, these EECs also 

reflect the concentration of glufosinate-ammonium in the release water from a rice paddy. 

Therefore, exposure of algal species directly outside the paddy to these concentrations could 

occur.

BCS Comment 9, page 11:

“BCS provides the following comments for consideration of the risk to birds, reptiles and 

terrestrial[-]phase amphibians.

• EPA did not conduct the assessment (calculate risk quotients) for all the scenarios

provided in Table 3.1 of the EPA Environmental Fate and Ecological Risk Assessment 

for the Registration Review of glufosinate[-]ammonium. This is particularly important 

for the non-agricultural uses of glufosinate ammonium.

• BCS provides updated information on the glufosinate[-]ammonium use patterns for 

further refinement of the assessment.

• Sabbagh and Demarteau, 2011 determined the mean foliar dissipation half-life for 

glufosinate[-]ammonium is 5.5 days.”

EFED Response:

As discussed in the response to Comment 3 above, EFED will revise the ecological risk 

assessment using the updated use patterns provided in Appendices 1 and 2 of BCS’ comment 

document. EFED will review the studies submitted by BCS regarding foliar dissipation and, if 

acceptable, use these data in the ecological risk assessment.

BCS Comment 10, pages 11-13:

“BCS provides the following comments for consideration of the risk to mammals. Collectively 

(and individually) all of this information demonstrates that the potential risk to mammals is 

significantly less than that predicted in the preliminary risk assessment.

• EPA did conduct the assessment with non-agricultural use pattern definitions provided by 

the registrant and in Table 3.1 [of the preliminary risk assessment]. Appendices 1 and 2 

of these comments provides further information with regards to the use patterns for 

glufosinate[-]ammonium.

• BCS provides updated information on the glufosinate[-]ammonium use patterns for 

further refinement of the assessment.

 2 Fernández-Valiente, E., and A. Quesada. 2004. A shallow water ecosystem: rice-fields. The relevance of 

cyanobacteria in the ecosystem. Limnetica 23 (1-2): 95 - 108. 
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• With regards to the acute assessment: The exposure of the indicator species in the field 

(such as herbivorous voles) is better represented by dietary exposure, in which the 

animals can feed on demand in numerous distinct “meals” over the whole day. With the 

gavage exposure in the LD50 studies the animals received the dose in one single bolus. 

This is not representative for the field situation, as wild mammals do not ingest their 

entire daily intake in one meal but rather feed in a diurnal short-term activity rhythm 

(Daan & Slopsema 1978 1, Halle 2000 2). Hansson (1971) also concluded that, e.g. field 

voles would need at least 15 stomach fillings per day. Thus, wild mammals are exposed 

to residues ingested in numerous feeding bouts over the whole day, and these ingested 

residues are mixed with their diet.

• BCS disagrees with the use of a value of 6 mg/kg/day for the NOAEL of the rodent 

multi-generation study. Since there were no relevant effects in male rats, and the effects 

were similar over generations, it would be appropriate to use the female NOAEL of 

approximately 9.6 mg/kg/day for the pre-pairing and gestation periods, the critical 

periods for the effect observed in the study.

• The use patterns with the highest applications rates (orchards and some of the nonagricultural rates) are all ground application that are highly directed banded application or 

directed applications around buildings or other objects within the farmstead. These types 

of applications will have limited off-target movement and in some cases leave untreated 

vegetation in the area (farmstead).

• The glufosinate[-]ammonium-treated vegetation shows very rapid symptoms of 

phytotoxicity, necrosis followed by plant death (see Figure below). Furthermore a study 

by Pallet and Gosch (2004) examined the time course for onset of phytotoxicity to 10 

plant species at 1.3 and 0.89 lb/A application rates (BCS will be submitting this study to 

EPA). Within 4 days, the wilting process is complete in 60% of the species, and after 7 

days all monocotyledonous and dicotyledonous plants are completely wilted. The fast 

wilting process is reducing the food quality of the treated material, because wilted weeds 

are no longer attractive as feed item to herbivorous animals. The animals will then seek 

food items from other areas, such as offsite vegetation where no direct applications of the 

product have been made and potential exposure is significantly reduced due to reduced 

drift with medium droplet size and drift interception by vegetation.

• Barfknecht (2011a) examined the assumption that wild mammals prefer to feed on the 

untreated plants rather than on the wilted weeds. The feeding preference of common 

voles before and after application of [g]lufosinate[-]ammonium was analyzed in 

enclosures with a grass – flower mixture under semi-field conditions. In 2010 and 2011 a 

seed mixture was sown to grow a meadow which was similar to the vegetation which 

occurs in the natural habitats of voles. In each year voles from wild populations were 

equipped with LEDs and released into the enclosure (3 voles in 2010 and 4 voles in 

2011). After one week (in 2011) or two weeks (in 2010) of acclimation, 50 % of the area 

of the semi-field basin (6 m2) was treated with glufosinate-ammonium SL200 (1.5 kg 

a.i./ha). The observation of the voles started at beginning of night and took about 4 hours 

per session. The semi-field basins were continuously observed with the night vision 

device. In 2010, the number of voles observed in the untreated and in the treated part was 

recorded during each minute. The method was slightly changed in 2011. At the end of 

each minute a scan was performed to record the position of each vole. The protocols of 6 

observation sessions were used to analyze the preferences of the voles: day 1, 2 and 5 
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after application in 2010 (day +1, day +2, day+5) and one day before application (day -1) 

and day 1 and 7 after application (day +1, day +7) in 2011. All observations showed that 

after application of [g]lufosinate-ammonium the voles prefer to spend their time in the 

untreated area. Quite different from that finding are the results from the observation 

performed before application. On that day the voles visited both areas similarly with a 

slight preference of the predestined treated area.

• Barfknecht (2001b) further examined glufosinate[-]ammonium treated food acceptance 

with wild mammals by evaluating whether rabbits will reduce their consumption of 

wilting vegetation after an application with glufosinate-ammonium and will prefer fresh 

green vegetation, which is for example readily available beside the treated vegetation 

stripes in vineyards and orchards. Perennial ryegrass (Lolium perenne) was grown in 160 

trays (11 g grass seeds on trays with a size of 46 cm x 28 cm). When height of the grass 

then exceeded a height of about 20 cm, 75 trays were sprayed with glufosinateammonium with an application rate of 1.5 kg a.i./ha. The remaining 85 grass trays were 

used for adaptation of the rabbits and as untreated control food during the test. Two 

months before the exposure, 9 rabbits were separated into two groups and provided a 

time to acclimate to the test cages (size: 2 m x 2 m x 2 m) and eating fresh cut grass. 

About three days before exposure initiation, the rabbits were separated and kept 

individually thereafter until the end of the test. On days 1, 2, 4, and 6 after the spray 

application, each rabbit received an equivalent amount of freshly cut grass from a treated 

and from an untreated tray for 2.5 hours. At all other times the rabbits had free access to 

standard food pellets. Already on day +2 visual differences between treated and untreated 

grass became apparent. The leave tips started to fade. On day +4 and afterwards the 

treated plants were mostly wilted. On day +1, the rabbits consumed similar amounts of 

untreated (50.4 %) and treated (49.4 %) grass. From day +2 on the rabbits preferred the 

untreated grass significantly. On day +6 treated grass was consumed only to 2.2%. The 

feeding behavior was well correlated with the wilting process: The more the plants 

wilted, the less treated grass was consumed by the rabbits, demonstrating that wild 

mammals will seek untreated food.

• With regards to assessing the potential risk to insectivorous mammals, BCS will be 

submitting a study examining the potential residues of glufosinate[-]ammonium to 

ground beetles after an application of 0.67 lb/A under laboratory conditions. The test item 

was applied at a rate of 0.67 lb a.i./A on Poecilus cupreus beetles, the food (Musca 

domestica pupae) and the substrate (natural soil). After spray application of the test item, 

the beetles were maintained in the laboratory on natural soil substrate. After defined time 

intervals, beetles were removed from their exposure units, deep frozen and submitted to 

residue analysis. The residues of glufosinate-ammonium on ground beetles (Poecilus 

cupreus) declined rapidly after a spray application under extended laboratory conditions. 

Time-weighted average concentrations at 7-, 14- and 21-days after the application were 

calculated at 29.0%, 22.1% and 18.7% of the initially measured concentration, 

respectively. This study demonstrates that long-term exposure to glufosinate[-

]ammonium via exposure terrestrial arthropods does not occur.”
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EFED Response:

As noted by the registrant, the Agency considered additional use information provided by the 

registrant in the PRA. Updated use information received in response to the PRA and 

incorporated into labels will be considered, as appropriate, in the revised risk assessment.

The Agency considered both oral (gavage) and dietary toxicity data when assessing risk to 

mammals. Toxicity data obtained through oral exposure are important for most accurately 

determining the dose at which toxicity is observed; these data are then used to assess potential 

toxicity and risk to different size classes of mammals. Dietary toxicity data generally represent a 

more ecologically relevant exposure scenario, but are less useful for calculating dose-based 

estimates of toxicity and risk because habitat use, foraging behavior, and physiology vary across 

mammalian species. 

For estimation of chronic risk to mammals, the Agency will continue to use the NOAEL of 6 

mg/kg/day. The endpoint was selected based on statistically significant (p<0.05) reductions in 

the number of viable rat pups per litter in both offspring generations (F1 and F2) in a rat multigeneration dietary exposure study (MRID 40345612).

The Agency reviewed the data by Barfknecht (2011a, MRID 491358013) and classified the study

as appropriate only for qualitative use in the risk assessment. Barfknecht (2011a) presents very 

limited information for ecological risk characterization regarding glufosinate-treated versus 

untreated habitat utilization by wild-caught specimens of a European vole species. Barfknecht 

(2011a) did not directly evaluate foraging behavior but instead only reported on habitat 

preference based on duration of presence in the treated or untreated portion of the enclosure. 

The report states that foraging behavior was observed both before and after treatment, but this 

refers to only a single day of pre-treatment observation in 2011. The sample size was small and 

the enclosure was not replicated. The study design implied equal availability and proximity of 

treated versus untreated habitat. It is unclear whether observations were performed daily; some 

observations were excluded from the summary. Neither individual level data nor statistics were 

reported. It is not stated whether the results shown represent total time of all animals or an 

average per animal per night. 

For each season, Barfknecht (2011a) indicates that one more vole was introduced into the 

enclosure than was marked. The sex of the marked versus unmarked voles was not reported, and 

it is unclear whether unmarked voles were counted in the observations. Batteries in the LEDs 

had to be changed after four days; however, it is not stated whether the LED had to be removed 

and then re-stitched to the vole during this exchange. The report suggested that the 2010 data 

following battery exchanges were not useful because voles did not emerge from holes after 

release; the author speculated that the availability of an alternate feed source (oat flakes) when 

trapped inhibited subsequent foraging behavior. One of the four marked voles in 2011 lost its 

LED. Overall, Barfknecht (2011a) concluded, “In all observations performed after the 

application, the voles preferred the untreated areas in contrast to the observation before 

 3 MRID 49135801. Barfknecht (2011a), Observation of voles (Microtus arvalis) under semi-field conditions after 

an application of Glufosinate-ammonium SL 200 (1.5 kg a.i./ha). Report No BAR / FS055; Document No. M403847-02-1.
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application”. However, this appears to be a subjective and selective conclusion which is not 

supported by any statistical analysis or by the limited data included in the study report.

In a separate study submitted by the registrant, Barfknecht (2011b4) concluded that rabbits 

presented with treated versus untreated forage preferred the untreated forage. The remaining 

forage was weighed periodically; weights were adjusted for moisture loss based on the weight 

loss of a concurrent control forage to which the rabbits did not have access. The study was 

classified by the Agency as appropriate for qualitative use in the risk characterization. The 

implication of the study author’s conclusion was that plants which have wilted as a result of 

herbicide exposure would be less appealing and possibly less palatable to herbivore and 

omnivores. However, such a preference does not preclude the consumption of treated plant 

material. Moreover, given the national scale of this risk assessment and the relatively small 

territorial ranges of many smaller mammals, it is unreasonable to conclude that alternative 

habitat and untreated forage will be available consistently in glufosinate-ammonium use areas. 

The Agency looks forward to any future submissions addressing potential exposure or toxicity of 

glufosinate-ammonium to insectivorous mammals. Such data will be reviewed and, if 

appropriate, considered in any future ecological risk assessments. With respect to the 

unsubmitted study on residue decline in ground beetles, which was summarized in the 

registrant’s comments on the PRA, the Agency notes that residue decline after a single 

glufosinate-ammonium application does not preclude chronic exposure when multiple 

glufosinate-ammonium applications may occur.

BCS Comment 11, page 14:

“BCS concurs with EPA that both the acute oral and contact toxicity values are >100 μg a.i./bee, 

and therefore the uncertainty associated with the approach EPA has taken with this preliminary 

assessment is very high. Using the EECs from TREX [Terrestrial Exposure model] with multiple 

applications does not consider the life span of the organism or the flowering duration of exposed 

plants, and has resulted in a significantly overly conservative exposure estimates. BCS 

recommends that EPA conduct the assessment using the recently revised bee risk assessment 

methodology released by the EPA. The documentation associated with the EPA methodology 

makes it clear that the multiple applications do not need to be considered given the short life 

span of bees and the short period individual flowers remain viable.”

EFED Response:

The preliminary risk assessment identified uncertainties regarding potential risk to listed species 

of terrestrial invertebrates. Although contact exposure to glufosinate-ammonium in the 

laboratory did not cause toxicity to the young adult honey bee (Apis mellifera), which is used as 

a surrogate for hazard assessment of terrestrial invertebrates, the preliminary risk assessment 

acknowledged that most registered uses of glufosinate-ammonium resulted in screening-level 

 4 MRID 49135802 Barfknecht (2011b), Food acceptance study with rabbits (Oryctolagus cuniculus) and grass 

treated with Glufosinate-ammonium SL200. Report No BAR/FS058; Document No. M-410867-01-1.
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EECs which were too high to preclude risk at the listed species LOC. The risk assessment also 

acknowledged data reviews of the European Food Safety Authority (EFSA), which considered 

potential effects to parasitoid wasp (Aphidius rhopalosiphi), predatory mite (Typhlodromus pyri), 

wolf spider (Pardosa spp.), green lacewing (Chrysoperla carnea), ground beetle (Poecilus 

cupreus), and rove beetle (Aleochara bilineata)

5

. A more recent semi-field study review by 

EFSA6 noted that recovery was not observed for all species (e.g., predatory mite) within one 

season, and concluded that the risk for non-target, non-Apis arthropods associated with the 

representative orchard use was high. The Agency’s preliminary risk assessment considered these 

additional lines of evidence qualitatively in characterizing risk to terrestrial invertebrates. The 

Agency has proposed a new risk assessment process for the honey bee7; future data call-ins and 

risk assessments will consider that process as it evolves.

BCS Comment 12, pages 14-15:

“EPA cites an EFSA document (EFSA 2012) that noted the non[-]target arthropod field study did 

not show recovery.

BCS disagrees with the conclusion drawn by EFSA (2012) that in the non-target arthropod field 

study (Bakker, 2004, M-235671-02-1) recovery of the effected species (especially predatory 

mites) was not demonstrated within one season. Laboratory data demonstrated that predatory 

mites are the most sensitive group concerning the exposure to glufosinate[-]ammonium.

Following two applications of glufosinate[-]ammonium, according to the use pattern tested in 

this study, adverse but transient effects on the non-target mite community were observed in the 

treated strip under the trees, but the tree canopy inhabiting populations were not affected. 

Adversely affected predatory mites (the most sensitive group shown in the laboratory) could 

clearly be demonstrated during the season from the observed initial effects.”

EFED Response:

The study described was not submitted to USEPA for review. The summary of the European 

Food Safety Authority (EFSA 2012) review is referenced for risk characterization in the PRA.

 5 European Food Safety Authority (EFSA). 2005. Conclusion regarding the peer review of the pesticide risk 

assessment of the active substance glufosinate. EFSA Scientific Report 27: 1-81. Available online through the 

EFSA website at http://www.efsa.eu.int. 6 EFSA. 2012. Conclusion on the peer review of the pesticide risk assessment of confirmatory data submitted for 

the active substance glufosinate. EFSA Journal 10 (3): 2609. Available online at 

http://www.efsa.europa.eu/en/efsajournal/doc/2609.pdf. 7 US EPA. 2012. White Paper in Support of the Proposed Risk Assessment Process for Bees. Submitted to the 

FIFRA Scientific Advisory Panel for Review and Comment. September 11 - 14, 2012. Prepared in collaboration 

with Health Canada Pest Management Regulatory Agency (PMRA) and California Department of Pesticide 

Regulation. Available online at http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2012-0543-0004.
15 of 21

BCS Comment 13, page 16:

“BCS recommends that EPA develop a species sensitivity distribution (SSD) for nontarget 

terrestrial plants to better characterize potential effects to this taxonomic group. The SSD and an 

associated defined hazard rate for a specified percent effect on terrestrial plant communities 

could then be used to discuss potential indirect effects on terrestrial organisms for direct effects 

on plant communities.”

EFED Response:

The Agency recognizes the value of species sensitivity distributions (SSDs) and agrees that such 

a tool could be useful in characterizing risk to individual species of non-target plants. However, 

it is unclear how accurately an SSD for the surrogate crop species tested would reflect actual 

species sensitivity differences for non-target, non-crop species. The Agency will likely consider 

the use of SSDs for terrestrial plants as it considers and implements the recommendations of the 

National Academy of Sciences (NAS) for evaluating risk to listed species. However, in the case 

of glufosinate-ammonium, such an analysis is not expected to change the conclusion of potential

risk to non-target plants.

BCS Comment 14, Spray Drift, pages 16-17:

“With regards to the calculation of the possible effect distances using AgDRIFT, it is well 

established the AgDRIFT overestimates drift, particularly with regards to the ground applications 

(Jackson, Ledson and Leggett (2012). Drift estimates should be conducted with models that 

incorporate the data from the Agricultural Agri Food Canada (AAFC)‘2000, ‘2004 and ‘2011 

(e.g., Wolf and Cladwell, 2001). This data has been incorporated into the PMRA [Health 

Canada’s Pest Management Regulatory Agency] drift estimate tool.

BCS would also like to point out the additional information submitted after the problem 

formulation phase of risk assessment regarding spray drift estimates for glufosinate[-]ammonium 

(Chalmers, A.; Sabbagh, G. (2007) MRID No.: 47381901).”

EFED Response: 

With regards to the assertion that AgDRIFT overestimates drift for ground applications, the 

assertion is contradicted by the fact that modeled distances to the effect concentration were less 

than 26 feet, which is well within the distances used by the Spray Drift Task Force to collect 

deposition data that were used to establish the empirical curves in AgDRIFT. With regards to 

using the PMRA drift estimate tool as the comment suggests, the tool employs data from 

Agricultural Agri Food Canada. EFED has not reviewed the underlying data or methodology 

used in the development of the tool to evaluate its suitability for use in EPA risk assessments. 

EFED will not consider use of this or any other model or tool in a regulatory decision making 

context unless and until it has had an opportunity to conduct a thorough review. No such review 

of the PMRA drift tool is planned at this time.
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With regards to the spray drift estimates provided in Chalmers, A.; Sabbagh, G. (2007) MRID 

No. 47381901, EFED has looked at the study and noted the following:

1. The study evaluated spray drift distance using a number of effect concentrations to 25% 

of the plants tested (EC25). These endpoints ranged from 0.063 to 0.994 lbs a.i./A,

whereas EFED evaluated spray drift using lower endpoints of 0.062 and 0.018 lbs a.i./A.

2. Spray deposition distances were based on a single application rate of 0.52 lbs a.i./A, 

while applications for glufosinate-ammonium range from 0.44 to 1.5 lbs a.i./A.

EFED notes though that for an aerial application at a rate of 0.52 lbs a.i./A (corn, cotton, and 

soybean seed propagation) and an endpoint of 0.062 lbs a.i./A, a smaller downwind distance (44 

feet) to the effect concentration was estimated in the EPA risk assessment than that provided in 

the cited study (55 feet).

CFS Comment 1, page 5:

“CFS likewise found no consideration given to adverse impacts on the environment that would 

result from increased prevalence of glufosinate- and multiple herbicide resistant weeds fostered 

by post-emergence glufosinate[-ammonium] use on glufosinate-resistant crops; or from 

increased prevalence of glufosinate- and multiple herbicide-resistant crop volunteers acting as 

weeds.”

EFED Response:

The Agency is aware of one weed species (i.e., Italian ryegrass, Lolium multiflorum) for which 

an introduced trait of glufosinate-resistance was reported in the United States8

. Glufosinateresistance was previously reported in goosegrass (Eleusine indica) in Malaysia9

. Both weed 

species occur in the United States10,11 and have also demonstrated resistance to the herbicide 

glyphosate; in general, these species are associated with orchards, other cultivated crops, and 

landscaped areas.

Consistent with the Overview document12, the Agency’s risk assessment for non-target plants 

currently focuses on the potential for direct impacts of glufosinate-ammonium from off-site 

transport through runoff or spray drift. From a broad perspective, EFED acknowledges that 

 8 Heap, I. 2011. The International Survey of Herbicide Resistant Weeds. Available online at

www.weedscience.org. 9 Lee L J & Ngim J. 2000. A first report of glyphosate-resistant goosegrass (Eleusine indica (L) Gaertn) in 

Malaysia. Pest Management Science, 56, 336-339 10 U.S. Department of Agriculture (USDA). 2013. Plants profile: Lolium perenne L. ssp. multiflorum (Lam.) 

Husnot. Natural Resources Conservation Service (NRCS) National Plant Data Team. Available online at 

http://plants.usda.gov/core/profile?symbol=lopem2. 11 U.S. Department of Agriculture (USDA). 2013. Plants profile: Eleusine indica (L.) Gaertn. Natural Resources 

Conservation Service (NRCS) National Plant Data Team. Available online at 

http://plants.usda.gov/core/profile?symbol=ELIN3. 12 U.S. Environmental Protection Agency (USEPA). 2004. Overview of the Ecological Risk Assessment Process 

in the Office of Pesticide Programs, U.S. Environmental Protection Agency. Endangered and Threatened Species 

Effects Determinations. Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, 

Washington, D.C. Available online at http://www.epa.gov/oppfead1/endanger/consultation/ecorisk-overview.pdf.
17 of 21

indirect ecological effects may occur through changes in the plant community structure (e.g., 

increased numbers of weeds) as a result of disturbance (e.g., herbicide application), and that the 

nature of these changes is partially dependent upon the relative fitness of surviving or colonizing 

plant competitors. However, EFED is unaware of any data which demonstrate specifically that 

glufosinate-resistant weeds are stronger competitors against native plants than non-resistant 

plants within the same weed species or other weed species. More specific to the regulation of 

glufosinate-ammonium use, the best available data do not indicate whether or to what extent 

glufosinate-ammonium (regardless of use on glufosinate-resistant or traditional crops) use may 

affect non-target plant species assemblages in adjacent habitat. Therefore, at this time, the 

quantitative risk assessment will remain limited to an evaluation of (1) the potential for 

phytotoxicity from non-target plant exposure, and (2) indirect effects from direct effects on other 

species with obligate relationships to plants (e.g., pollinators and seed dispersers), based on 

established LOCs. EFED will characterize the general ecological uncertainties related to 

glufosinate-ammonium use on glufosinate-resistant crops qualitatively in the revised risk 

assessment, but these uncertainties will not affect the general risk conclusions because the 

Agency has already identified risks to non-target plants.

CFS Comment 2, pages 9 and 10:

“EPA mentions that glufosinate[-ammonium] inhibits glutamine synthetase in animals, too 

(EFED at 6). EPA fails to include the fact that glufosinate[-ammonium] also inhibits glutamine 

synthetase in fungi and bacteria, sometimes with lethal results (Ahn 2008, Sanyal and Shrestha 

2008, Pampulha et al. 2007, Hsiao et al. 2007). Although pesticide registration reviews do not 

involve assessing risks to fungi and bacteria specifically (EFED at 9), these taxa interact with 

plants and animals in ways that are vital to the survival and reproductive success of plants and 

animals. Therefore, effects of glufosinate[-ammonium] on fungi and bacteria should be taken 

into account by EPA in the registration review, along with impacts to plants and animals. Also, 

plant communities are important aspects of habitat for all taxa, and are sensitive to alteration by 

glufosinate[-ammonium] exposure.”

EFED Response:

The Agency is unaware of a method for quantitatively assessing ecological risks resulting from 

effects on fungi or bacteria. Should appropriate data on glufosinate-ammonium effects on fungi 

and bacteria be submitted for Agency review, EFED may consider such information in 

characterizing potential risk in subsequent risk assessments.

CFS Comment 4, page 11:

“EPA needs to make a more specific and detailed assessment of the extent to which registered 

uses of glufosinate[-ammonium] in the U.S. are likely to harm beneficial insects such as 

predatory mites and spiders, lacewings, and beetles; calling for more species-specific data, as 

necessary. If these species that provide pest reduction services are unable to survive and 
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reproduce, it could result in crop losses and an increase in use of pesticides, with attendant 

impacts to the economy, human health and the environment.”

EFED Response:

The preliminary risk assessment identified uncertainties regarding potential risk to listed species 

of terrestrial invertebrates. Although contact exposure to glufosinate-ammonium in the 

laboratory did not cause toxicity to the young adult honey bee, which is used as a surrogate for 

hazard assessment of terrestrial invertebrates, the preliminary risk assessment acknowledged that 

most registered uses of glufosinate-ammonium resulted in screening-level EECs which were too 

high to preclude risk at the listed species LOC. As noted in a response to a previous comment, 

the risk assessment also acknowledged data reviews by EFSA, which considered potential effects 

to a parasitoid wasp, predatory mite, wolf spider, green lacewing, ground beetle, and rove beetle13. 

A more recent semi-field study review by EFSA14 noted that recovery was not observed for all 

species (e.g., predatory mite) within one season, and concluded that the potential risk for nontarget, non-Apis arthropods associated with the representative orchard use was high. The 

Agency’s preliminary risk assessment considered these additional lines of evidence qualitatively 

in characterizing potential risk to terrestrial invertebrates.

CFS Comment 5, page 12:

“Given the recent honeybee die-offs, and the evidence that pesticides are a stressor involved 

(Mullin et al. 2010), EPA should include more rigorous testing of glufosinate[-ammonium] on 

honeybees in the registration review. In particular, EPA has the flexibility to require studies on 

toxicity of glufosinate[-ammonium] to bee larvae, a life stage that is fed pollen and nectar 

possibly contaminated with glufosinate[-ammonium] residues, and EPA should call for such 

studies. Also, field incidents of non-target injury from herbicides are known to be underreported, as we discuss below. Field studies designed specifically to look at pollinators are thus 

required to determine whether applications of glufosinate[-ammonium] have chronic, sublethal, 

reproductive or behavioral effects, and should be required.

EPA has not considered exposure to glufosinate[-ammonium] residues via pollen and nectar in 

assessing risk to pollinators (EFED at 1, 14, 32 – 33)... In particular, in GE glufosinate-resistant 

crops – the major use of glufosinate[-ammonium] – glufosinate[-ammonium] and metabolites 

move extensively, including to flowers and seeds (Ruhland et al. 2004), and some metabolites 

are stable in the plants, resulting in residues that may be a route of exposure to organisms that 

consume GE glufosinate-resistant plant tissues.”

 13 European Food Safety Authority (EFSA). 2005. Conclusion regarding the peer review of the pesticide risk 

assessment of the active substance glufosinate. EFSA Scientific Report 27: 1-81. Available online through the 

EFSA website at http://www.efsa.eu.int. 14 EFSA. 2012. Conclusion on the peer review of the pesticide risk assessment of confirmatory data submitted for 

the active substance glufosinate. EFSA Journal 10 (3): 2609. Available online at 

http://www.efsa.europa.eu/en/efsajournal/doc/2609.pdf.
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EFED Response:

The Agency notes that pesticides residues reported in Mullin et al. 2010 were not specifically 

linked with honey bee declines; while pesticides have been identified as a factor associated with 

declines in honey bee health, there are no data that have demonstrated that pesticides are a cause 

of the these declines. The preliminary risk assessment considers both acute contact and oral 

toxicity data when describing potential hazard to the honey bee. Specifically, the median lethal 

dose to 50% of the bees tested (LD50)value for technical grade glufosinate-ammonium is >100 

μg a.i./bee; therefore, glufosinate-ammonium technical is classified as practically non-toxic to 

young adult honey bees on both an acute oral and contact exposure basis. Similarly, the 20.4% 

a.i. formulation of glufosinate-ammonium was tested through both contact and oral exposure,

and caused no mortality or sublethal effects at up to the limit dose (LD50 >100 μg a.i./bee). The 

Agency has proposed a new risk assessment process for the honey bee15. Future data call-ins 

and risk assessments will consider that process as it evolves, where additional data will be 

required for assessing the potential acute and chronic effects of pesticides on adult and larval 

bees. However, based on the available data on young adult bees, glufosinate-ammonium is 

classified as practically non-toxic to bees on an acute exposure basis.

CFS Comment 6, pages 14-16:

“Herbicide dosage is important for the effects, and sometimes drift levels can stimulate the 

growth of pathogens, whereas full application rates suppress the same pathogens. Thus nontarget plants may be at higher risk for diseases than the treated crop itself from herbicide 

applications: “It is not unusual for low rates of herbicides to stimulate in vitro pathogen growth 

(e.g., Yu et al., 1988). Hormesis (the stimulatory effect of a subtoxic level of a toxin) is common 

with both fungicide effects on fungi and herbicide effects on plants (Duke et al., 2006). Thus, 

dose rates are likely to be highly important in both direct and indirect effects of herbicides on 

plant disease.” (Duke et al. 2007).”

“The study by Carpenter and Boutin in 2010 is especially relevant to EPA’s registration review 

because it focuses on sublethal effects of glufosinate[-ammonium] at drift levels on a range of 

wild and cultivated plants, and includes reproductive responses.

Drift levels can cause sublethal vegetative effects such as necrotic leaves, chlorosis and stunting, 

and reproductive effects such as flowering delay, deformed flowers, sterility and fewer or 

smaller seeds (e.g. Davis et al. 2009, Carpenter and Boutin 2010). Species vary in their responses 

and sensitivity. Also, some species can recover from early injuries caused by drift rates of 

glufosinate, and other species appear to recover but exhibit reproductive abnormalities later 

(Carpenter and Boutin 2010). In fact, in their study of the responses of several crop and wild 

species to sublethal glufosinate[-ammonium] applications, Carpenter and Boutin (2010) found 

that “reproductive outputs” were a more sensitive measure than vegetative injury for more than 

 15 US EPA. 2012. White Paper in Support of the Proposed Risk Assessment Process for Bees. Submitted to the 

FIFRA Scientific Advisory Panel for Review and Comment. September 11 - 14, 2012. Prepared in collaboration 

with Health Canada Pest Management Regulatory Agency (PMRA) and California Department of Pesticide 

Regulation. Available online at http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2012-0543-0004.
20 of 21

half of the plants tested. They conclude that plants in their natural habitats might be even more 

sensitive.”

EFED Response:

For each registered herbicide active ingredient (a.i.), including glufosinate-ammonium, the 

Agency requires Tier II (concentration-response) phyotoxicity data on both seedling emergence 

and vegetative vigor endpoints. The data evaluate the potential effects of both foliar application 

and root uptake across a range of treatment rates, including rates below the maximum labeled 

application rate. Depending on the specific endpoints affected, either the inhibition 

concentration (IC25) or lethal concentration (LC25) for the most sensitive species is used for risk 

estimation for non-listed species of terrestrial plants. For listed species of terrestrial plants, the 

Agency uses the no observed adverse effect level (NOAEL) in risk estimation. Therefore, the 

preliminary risk assessment (PRA) for glufosinate-ammonium already considers potential effects 

of exposure to glufosinate-ammonium at levels below the “full application rate.”

The study by Carpenter and Boutin (2010)16 will be considered as an additional line of evidence 

in the characterization of the potential risk of glufosinate-ammonium use to non-target plants. 

The data in the Carpenter and Boutin 2010 study are however not suitable for use in quantitative 

risk estimation because they do not present IC25/LC25 values, and because the individual data are 

not available to the Agency. However, as noted by the Center for Food Safety, incorporation of 

this information is not expected to change the existing PRA’s conclusion of risk to non-target 

plants under certain scenarios.

CFS Comment 7, page 20:

“In addition to impacts from glufosinate[-ammonium] drift and runoff, we ask that EPA take into 

account the potential toxicity of glufosinate-resistant crops, after applications of glufosinate[-

ammonium], to listed species that might eat corn, soybean, canola and cotton leaves, roots, 

stems, or flower parts. Migrating birds, for example, eat parts of the corn plant. Bees consume 

corn, soybean, canola and cotton pollen; and soybean, canola and cotton nectar, and other insects 

also utilize pollen, leaves, roots, and other plant parts. Crop detritus washes into wetlands where 

it is consumed by aquatic organisms, and is also broken down in soil by detritivores, and may 

result in different dissipation patterns than those described by EPA (EFED at 23). These crops 

are planted widely, thus many species are potentially impacted. Thus the levels and toxicity of 

glufosinate[-ammonium] and its metabolites within glufosinate-resistant plant tissues need to be 

considered in the complete listed species assessment that is part of this registration review.”

EFED Response:

The residue decline data that were used in this assessment were based on applications to 

genetically modified (GMO) crops carrying the glufosinate-resistance trait. The dietary toxicity 

data used for risk estimation are based on direct treatment of a feed source with a known quantity 

 16 Carpenter, D., and C. Boutin. 2010. Sublethal effects of the herbicide glufosinate ammonium on crops and wild 

plants: Short-term effects compared to vegetative vigor and plant reproduction. Ecotoxicology 19: 1322 – 1336.
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of glufosinate-ammonium. EPA has not received any data which would suggest that toxicity 

from consumption of GMO crops treated with glufosinate-ammonium differs from consumption 

of non-GMO plants treated with glufosinate-ammonium.