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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION

MEMORANDUM

Date: March 29, 2012

Subject: Glufosinate Ammonium. Report of the Residues of Concern Knowledgebase 

Subcommittee (ROCKS). 

PC Code: 128850 DP Barcode: D397644

Decision No.: 424395 Registration No.: NA

Petition No.: NA Regulatory Action: Section 3 Registration

Risk Assessment Type: Single Chemical Aggregate Case No.: NA

TXR No.: NA CAS No.: 77182-82-2

MRID No.: NA 40 CFR: 180.437

From: Ideliz Negrón-Encarnación, ROCKS Alternate Secretary

Health Effects Division (7509P)

Through: Christine L. Olinger, ROCKS Co-Chair

Edward J. Scollon, ROCKS Co-Chair

Health Effects Division (7509P)

To: Glufosinate Ammonium Risk Assessment Team

Risk Assessment Branch V

Health Effects Division (7509P)

The ROCKS met on January 11, 2012 to discuss the residues of concern for the herbicide 

glufosinate ammonium in drinking water. The residue of concern recommendation was 

subjected to electronic review to consider a subsequent ToxSAC report (02/21/2012).

Team Members Attended:

John Doherty, Chuck Peck, William Donovan, Mike Metzger, and Marietta Echeverria

ROCKS Members Attended:

Christine Olinger, Edward Scollon, Ray Kent, Richard Loranger, Elizabeth Holman, William 

Irwin, Leung Cheng, George Kramer, Reuben Baris, Dennis McNeilly and Ideliz Negrón

Material Reviewed:

Briefing Materials: Glufosinate Ammonium ROCKS Submission
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Background

The residues of concern of the herbicide glufosinate ammonium in plants, rotational crops, 

livestock and drinking water were previously evaluated (Txt# 0050727, T. Bloem, 05/09/2002). 

The risk assessment team solicited the ROCKS a re-evaluation of the residues of concern in 

drinking water. Supplemental toxicity information for water degradates was provided for the 

ROCKS consideration. Based on input provided by the ToxSAC (02/21/2012), the 

recommendation from the ROCKS meeting held on January 11, 2012 was amended. The 

amended decision memo was circulated electronically to the ROCKS committee on February 28, 

2012 for comments.

Committee Recommendation

Summary of Degradates to be included in the Risk Assessment

Matrix Residues included in Risk Assessment Residues included in Tolerance 

Expression

Drinking Water Glufosinate Ammonium 1,2 Not Applicable

1 Glufosinate ammonium is butanoic acid, 2-amino-4-(hydroxymethylphosphinyl)-monoammonium salt. Although 

glufosinate ammonium will be dissociated in water, for risk assessment purposes it is described as the salt in order to 

aggregate the residues in drinking water with those in food where the concentration is expressed in terms of the salt.

2 Since MPP is likely less toxic than the parent compound and should not be aggregated with the parent, if MPP 

EDWCs are not significantly greater than those for glufosinate, the risk assessment for the parent will be protective 

of any toxicity associated with exposure to MPP in drinking water and a quantitative risk assessment for MPP will 

not be required.

Summary of Briefing Materials Considered

Glufosinate ammonium [ammonium-DL-homoalanin-4-yl-(methyl)-phosphinate] is a nonselective foliar herbicide that acts by inhibiting glutamine synthetase. In addition to the 

complete toxicity database for the parent compound, subchronic toxicity and developmental 

studies are available for the environmental degradates MPP, MPA and NAG (structures are 

shown in Appendix 1). MPA is at least 100 fold less potent than glufosinate ammonium based 

upon the lack of effects in a 90-day study in rats. Rabbit developmental studies are also 

available for GA, NAG, and MPP to aid in relative potency comparisons. NAG and MPP were 

5- to 10-fold less potent than the parent; however, the maternal effects noted were different than 

those observed than the parent glufosinate ammonium.

Glufosinate ammonium is readily miscible in water. Glufosinate will not volatilize significantly 

due to a low vapor pressure, and is moderately mobile. Glufosinate is considered stable to 

photolysis in water but degrades in soil due to photolysis with a half-life of 17 days. The 

metabolism studies showed that glufosinate degrades in aerobic soil and aqueous environments, 

with soil half-lives ranging from 8.5 to 23 days, and aquatic half-lives ranging from 11 to 87

days. Glufosinate in the anaerobic soil environment degraded with a measured half-life of 37 

days in one study, but the chemical was persistent in the anaerobic aquatic environment, with a 

measured half-life of 415 days in another study. 
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MPP was a major degradate in the soil photolysis study, reaching a maximum of 60.4% (Day

30). MPP was also a major degradate in all metabolism studies with the exception of the 

anaerobic aquatic study. In aerobic systems, maximum MPP values ranged from 26 to 55% in 

soil and 17 to 80% in aquatic environments. In anaerobic soil, MPP reached a maximum of 42% 

at the end of the study (Day 60).

MPA was a major or minor degradate in all of metabolism studies with the exception to the 

anerobic aquatic metabolism. In aerobic systems, maximum MPA values ranged from 3 to 28% 

in soil and 7 to 20% in aquatic environments. When MPA was a major degradate, it tended to 

form at the end of the study, supporting the supposition that it forms as a result of MPP 

degradation. In anaerobic soil and soil photolysis, MPA was a minor degradate.

NAG was major degradate of photolysis in soil, reaching maximum of 17.7% (Day 4). It was a 

minor degradate in one of two aerobic aquatic studies and not detected in the other. In addition, 

it was not detected in the anaerobic aquatic study, and aerobic and anaerobic soil studies.

Rationale

MPA: A 90-day rat feeding study showed no effects at the highest dose tested which is about 

100-fold higher than the NOAEL of the parent; therefore, MPA is considered less toxic than the 

parent. Even though it is a major degradate in some studies, considering its much lower toxicity

than the parent, ROCKS recommends its exclusion as a residue of concern in drinking water.

NAG: Based on the rabbit developmental studies NAG can be considered slightly less toxic than 

the parent. It was only observed as a major degradate during photolysis in soil; therefore, its 

exposure is significantly lower to that of the parent in drinking water. The ROCKS recommends 

its exclusion as a residue of concern in drinking water. 

MPP: The ROCKS noted that MPP may have different toxicity than the parent and is likely to 

have a lower toxicity but not so low that it could be definitely excluded from consideration. 

After the ROCKS meeting, the team consulted with Health Effects Division ToxSAC regarding 

the toxicity of MPP (J. Kidwell, 02/21/2012). The consensus of the ToxSAC was that

glufosinate ammonium, the parent, and MPP, the degradate, show different toxicities, such that 

they should not be aggregated. Moreover, the Environmental Fate and Effects Division have

indicated that the acute concentrations of MPP are not likely to be significantly greater than that 

of glufosinate in drinking water. However, given the minimal fate data available for MPP that 

indicates MPP does not degrade in aerobic aquatic environments, it is unclear if this will be true 

for chronic concentrations of MPP and glufosinate. Since MPP is likely less toxic than the parent 

compound and should not be aggregated with the parent, if MPP EDWCs are not significantly

greater than those for glufosinate, the risk assessment for the parent will be protective of any

toxicity associated with exposure to MPP in drinking water. Therefore, a quantitative risk 

assessment for MPP in drinking water will not be required.
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Appendix 1. Glufosinate Ammonium and it major degradates in water.

Common Name Chemical Name Structure

Glufosinate 

ammonium (parent)

(HOE 039866)

IUPAC name: ammonium-DL-homoalanin4-yl-(methyl)-phosphinate 

CAS name: butanoic acid, (±)-2-amino-4-

(hydroxymethylphosphinyl)-, 

monoammonium salt

P

NH2

O

OH

O

-O CH3

NH4+

MPP

(HOE 061517)

CAS Name: 3-methylphosphinico-propionic 

acid

P

HO

O CH3

OH

O

MPA

(HOE 064619)

CAS Name: 2-methylphosphinico-acetic acid

P

HO

O CH3

O

OH

NAG

(HOE 085355/HOE 

099730)

CAS Name: 2-acetamido-4-

methylphosphinico-butanoic acid

P

HO

O CH3

O

OH

NH

CH3

O