UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF CHEMICAL SAFETY AND POLLUTION PREVENTION 

PC Code: 122009 

DP Barcode: 422361 

Date: October 24, 2014 

MEMORANDUM 

Subjects: Review ofBCS-C060720 (Degradate ofMesosulfuron-methyl) Vascular Aquatic 

Plant Toxicity Study (MRID 4944 7202) in Support of Registration Review 

To: 

From: 

and 

EFED Response to Registrant's Data Waiver Request for Vascular Aquatic Plant 

Toxicity Testing with BCS-CV14885, a Degradation Product ofMesosulfuronmethyl 

Jolene Trujillo, Chemical Review Manager 

Jill Bloom, Team Leader 

Linda Arrington, Branch Chief 

Risk Management and Implementation Branch V 

Pesticide Re-Evaluation Division (7508P) 

Michael Lowit, Ph.D., Ecologist ~ · / /J ~ li~Y/f Meghan Radtke, Ph.D., Acting RAPL ~ 'fo /.) '111'1 , . / .. 

Sujatha Sankula, Ph.D., Branch Chief rtYk ~ /.u. Q ·-~k ~f/-UMA, Environmental Risk Branch I . ·vg . - r· ~ -0"\) 

Environmental Fate and Effects Division (7507P) 

Review of BCS-C060720 (De gradate of Mesosulfuron-methyl) Vascular Aquatic Plant Toxicity 

Study 

The Environmental Fate and Effects Division (EFED) evaluated a study ofBCS-C060720 

(degradate ofmesosulfuron-methyl) toxicity to Lemna gibba, an aquatic vascular plant, in 

support of Registration Review (Table 1). Please refer to the attached DER for additional 

details. 

T a bl e 1 . St u d v S urn mary 

Study Test Species MRID Study Summary Classification 

850.4400 Frond number yield, Frond number growth rate, 

(Aquatic Frond area yield, Frond area growth rate 

vascular plant ICso > 13 mg ai/L 

toxicity) Lemna gibba 49447202 Acceptable LOAEC > 13 mg ai/L 

NOAEC 2': 13 mg ai/L 

Degradate 

(98.3% purity) Endpoints affected: none 
2 

Data Waiver Request for Vascular Aquatic Plant Toxicity Testing with BCS-CV14885 

EFED reviewed a waiver request from the registrant, Bayer Crop Science AG, in response to the 

data call-in (DCI) issued as part of the registration review of mesosulfuron-methyl. The 

registrant is requesting a waiver for vascular aquatic plant toxicity testing (OCSPP 850.4400; 

Lemna spp.) with BCS-CV14885, a degradation product of mesosulfuron-methyl based on the 

available Lemna gibba toxicity data for BCS-CO60720, a structurally similar compound. 

In the waiver request the registrant asserts that toxicity to Lemna within the sulfonylurea 

herbicide class is dependent on the intactness of the methyl ester function of the phenyl ring 

moiety and that toxicity typically decreases when the methyl ester is hydrolyzed to a carboxylic 

acid. BCS-CV14885 (a carboxylic acid) is structurally related to BCS-CO60720 (the methyl 

ester variant of BCS-CV14885); both compounds are categorized into a carbamoyl-guanidine 

type structural class. The registrant recently submitted an aquatic vascular plant toxicity study 

for BCS-CO60720 (MRID 49447202, review presented above). This acceptable study showed 

no toxicity at the maximum application rate (NOAEC ≥ 13 mg ai/L), suggesting that BCSCO60720 is at least four orders of magnitude less toxic than mesosulfuron-methyl to Lemna 

gibba (NOAEC = 0.00019 mg ai/L; MRID 45386310). Therefore, BCS-CV14885 is likely 

much less toxic than mesosulfuron-methyl given its structural similarity to BCS-CO60720 and 

that carboxylic acids are generally less toxic than methyl ester variants within a structure class. 

In the absence of BCS-CV14885 toxicity data, EFED will assume that BCS-CV14885 and BCSCO60720 are equally toxic to aquatic vascular plants. 

EFED recommends waiving vascular aquatic plant toxicity testing (OCSPP 850.4400; Lemna

spp.) with BCS-CV14885 given the lack of toxicity of BCS-CO60720 (the methyl ester variant 

of BCS-CV14885) compared to mesosulfuron-methyl.