2

Two major degradates (unknown A and unknown D) were thought to be unidentified at the time 

of the problem formulation. BCS indicated that structures (coded BCS-CV14885 and BCSCO60720) were proposed in the aerobic aquatic metabolism study (MRID 45386430). EFED 

acknowledges these structures.

Data needs were reassessed given this new information and EFED has revised its 

recommendations on the value of data on toxicity to certain aquatic organisms (see below). 

EFED does not have any toxicity data for BCS-CV14885 or BCS-CO60720 (BCS incorrectly 

indicates that MRID 45386331 contains information about herbicidal activity of these 

degradates). BCS-CV14885 and BCS-CO60720 are SU bridge-intact degradates; therefore, 

toxicity to terrestrial and aquatic plants will be assumed the same as that of mesosulfuron-methyl 

in the absence of data and a TTR approach will be used to estimate EECs. Following the same 

assessment procedures in the problem formulation, it was determined that BCS-CV14885 and 

BCS-CO60720 toxicity studies with fish, aquatic invertebrates, aquatic non-vascular plants, 

birds, and mammals are unlikely to change risk conclusions. However, the lack of BCSCV14885 and BCS-CO60720 toxicity information for aquatic vascular plants could impact risk 

conclusions (see pg 22 of problem formulation). 

In addition, a recent DER review found that the available ECM/ILV water studies (MRID 

46451701 and 46451702) did not include the major degradates of mesosulfuron-methyl in 

aquatic environments (USEPA, 2013). 

The following studies are recommended in addition to those indicated in the problem 

formulation:

• Aquatic plant growth (850.4400) - vascular plant (BCS-CV14885 and BCS-CO60720)

• ECM/ILV (water) (850.6100) - to include the major degradates observed in aquatic 

environments (AE F160459, AE F160460, AE F140584, AE F147447, AE F092944, 

BCS-CV14885 (benzoic acid with guanidine), and BCS-CO60720 (methyl ester of 

benzoic acid with guanidine))

The following study is no longer recommended:

• Aerobic Aquatic Metabolism (835.4300)

Modifications are made for the following study recommended in the problem formulation:

• ECM/ILV (soil) (850.6100) - to include the major degradates observed in soil

environments (AE F154851, AE F099095, and AE F092944). Minor degradate observed

in the soil environment are not recommended.
3

Ecological Effects

BCS Comment:

Bayer CropScience provided comments on ecological effects data gaps identified in the problem 

formulation regarding their intent to submit studies that address data gaps and refine 

uncertainties. 

EFED Response:

EFED will review submitted studies to the extent that they reduce uncertainty in the risk 

conclusions. 

Specific comment on 850.4500:

”........Bayer CropScience believes the existing study is sufficient to properly characterize the 

risk to green alga and would appreciate clarification from the Agency on the request for 

additional data”.

After review, some studies were reclassified and no longer meet a given data requirement. The 

existing study (MRID 45386306) was downgraded to Invalid because the control was 

contaminated with test substance (see DP 412289 and accompanying DER amendment). 

Therefore, submission of a new study is still recommended.

Specific comment on 850.1400 (mesosulfuron-methyl) and 850.4500 (AE F092944, AE

F099095, and AE F154851):

“BCS will submit these reports if requested under the DCI”.

Although data gaps, EFED did not recommend these studies given that they would unlikely 

change the risk conclusions based on information available at the time of the problem 

formulation. However, if submitted EFED will consider the studies to the extent that they reduce 

uncertainty in the risk conclusions.

Specific comment on terrestrial plant toxicity:

“....EPA noted that in the lack of additional data on terrestrial nontarget plant toxicity SU 

bridge-intact degradates will be assumed equal in toxicity to TGAI (mesosulfuron-methyl). BCS 

requests that the Agency review Rosinger, C. 1999 (MRID 45386331) which demonstrates > 10 

fold decrease in pre- and/or post-emergent herbicidal activity of major plant and soil degradates 

of mesosulfuron-methyl against a wide range of grass and broad-leaf plant species”.

EFED will consider all lines of evidence including MRID 45386331 in determining if SU bridgeintact degradates will be assumed equal in toxicity to mesosulfuron-methyl. A preliminary 

review of MRID 45386331 suggests that three of the bridge-intact degradates (AE F154851, AE 

F160459, and AE F160460) are less toxic than mesosulfuron-methyl to the tested plant species. 

Shortcomings of the report include a lack of information on experimental design (was it the same 
4

for mesosulfuron-methyl and the degradates) and the measure of herbicidal activity (what 

endpoints were considered). EFED is unaware of any toxicity data (terrestrial plants) for the 

other two bridge-intact degradates (BCS-CV14885 and BCS-CO60720). Data are not requested 

at this time; however, SU bridge-intact degradates will be assumed equal in toxicity to 

mesosulfuron-methyl in the absence of data.

Environmental Fate

BCS Comment (Aerobic Aquatic Metabolism):

“There are no unknown major metabolites in the submitted study. All metabolites were 

identified and described in the reports.” “....Therefore, based on the structural elucidation of 

these metabolites, the submitted report (Steinfuhrer and Zumdick, 2000; MRID 45386430) 

should fulfill the requirements and repeat of the study is not warranted.”

“Metabolite Unknown A (coded as BCS-CV14885) was synthesized and shown not to have any 

herbicidal activity (Rosinger, C., 1999; MRID 45386331).”

EFED Response:

EFED acknowledges the degradate identification data from “LC/MS-investigations” (page 55-57, 

MRID 45386430) and concurs all major metabolites were identified. EFED will revise the data 

evaluation record (DER) for MRID 45386430 to reflect the changes. An aerobic aquatic 

metabolism study is no longer recommended.

There is no toxicity information regarding Unknown A (BCS-CV14885) in MRID 45386331.

BCS Comment (Aquatic Field Dissipation):

“Bayer believes that the currently available information is adequate to characterize 

mesosulfuron-methyl's fate in aquatic systems, and in conjunction with the available aquatic 

toxicity studies, assess the potential risk to aquatic organisms. Mesosulfuron-methyl is applied at 

a very low maximum rate of 15 g a.i./ ha (0.0134 lb a.i./acre) with only one application per year, 

and is not registered for direct application to water, so it is unlikely to reach aquatic habitat at 

high concentrations. .... While mesosulfuron does exhibit some toxicity to aquatic plants, the 

potential effects on aquatic plants are well characterized (Teixeira, 2003, C036256; Hoberg, 

2009, M-329474-01-1; etc.). Although not designed as an aquatic dissipation study, the artificial 

pond study of Hoberg, 2009 did demonstrate dissipation of mesosulfuron from the water phase at 

a half-life of 39 d based on the kinetic evaluation of 7 separate outdoor ponds. .... BCS will 

submit the Hoberg (2009) study if requested under the DCI.”

“Conducting an aquatic field dissipation study is unlikely to generate information that will 

significantly change our understanding of the behavior of the molecule in the environment or its 

potential risk to aquatic organisms.”
5

EFED Response:

EFED will review the artificial pond study by Hoberg (2009) if submitted; however, EFED

reiterates that an aquatic field dissipation study is recommended to better characterize the aquatic 

fate and risk of mesosulfuron-methyl given that it is likely to enter water bodies through soil runoff, erosion, and leaching because of its persistence and mobility. 

BCS Comment (Environmental Chemistry Methods for Soil and Plant Material):

“The metabolic fate of mesosulfuron-methyl has been thoroughly investigated in numerous soils 

and sediment under aerobic and anaerobic conditions, covering a wide range of textures and 

organic carbon contents....”. “As shown above, only three metabolites (AE F154851, AE 

F099095 and AE F092944) exceeded 10% of applied radioactivity in soil and are considered 

major metabolites.”. “Therefore, Bayer proposes to develop and validate an analytical method 

containing parent and the major metabolites AE F154851, AE F099095, and AE F092944. These 

analytes will adequately define the behavior of mesosulfuron-methyl in the soil environment.”

EFED Response:

EFED recommends submission of the proposed soil analytical method containing mesosulfuronmethyl and the major soil metabolites (AE F154851, AE F099095, and AE F092944). The 

method should follow the 850.6100 guideline. EFED concurs that minor soil degradates (AE 

F160459, AE F160460, AE F140458, and AE F147447) are not necessary for the recommended 

soil study (these degradates were previously recommended in the problem formulation).

In addition, EFED recommends an aquatic analytical method (guideline 850.6100) that includes

the major degradates observed in the available aquatic studies (listed below): AE F160459, AE 

F160460, AE F140584, AE F147447, AE F092944, BCS-CV14885 (Unknown A, benzoic acid 

with guanidine), and BCS-CO60720 (Unknown D, methyl ester benzoic acid with guanidine). 

This study is recommended because a recent DER review found that the submitted ECM/ILV 

(water) studies (MRID 46451701 and 46451702) did not include the major degradates of 

mesosulfuron-methyl in aquatic environments (USEPA, 2013). 

Hydrolysis (MRID 45386418): AE F092944 (38.4%, pH 4) and AE F140584 (50.3%, pH 4)

Anaerobic Aquatic Metabolism (MRID 45386427 and MRID 45386429):

AE F160459 (25.9%,) and AE F160460 (12.6%) 

Aerobic aquatic Metabolism (MRID 45386430 and MRID 45386431):

AE F160459 (21.6%), AE F147447 (16.1%), Unknown A (U1) (6.9-22%)--revised as: benzoic 

acid with guanidine, and Unknown D (U3) (6.2-13.1%)--revised as methyl ester of benzoic acid 

with guanidine.
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BCS Comment (Potential for Bioaccumulation of Mesosulfuron-methyl in Fish):

“In Table 16 of the Preliminary Problem Formulation EPA noted a waiver was granted in 

December 2003 (DP Barcode 297232) for a fish bioaccumulation study.....this study should not 

be listed as a data gap.”

EFED Response:

A data gap means there are no data available. However, EFED acknowledges the previously 

granted data waiver request. A fish bioaccumulation study is not recommended. 

REFERENCES

Hoberg, J. (2009). Outdoor Growth Inhibition of Aquatic Plants Exposed to Mesosulfuronmethyl. Report Number EBMML001. Document Number: M-329474-01-1. Unpublished 

study performed by Bayer CropScience. 238 p.

Rosinger, C. (1999). Evaluation of Herbicidal Potential of Metabolites of AE F130060. Report 

Number: C002929. Document Number: M-185253-01-1. Unpublished study performed 

by Bayer CropScience. 10p. MRID: 45386331.

USEPA 2013. Review of Environmental Chemistry Methods (MRID 46451701) and 

Independent Lab Validation (MRID 46451702) for Mesosulfuron-methyl. DP410701. 

6pp.