Sethoxydim Risk Assessment

substantial contamination of water is unlikely.  In areas with increasing levels of rainfall, 
exposures to aquatic organisms are more likely to occur.  The anticipated concentrations in 
ambient water encompass a very broad range, 0.000094 to 0.003 mg/L, depending primarily on 
differences in rainfall rates. 
Dose-Response Assessment – A summary of all toxicity values used in this risk assessment is 
given in Table 4-2.  For terrestrial mammals, the dose-response assessment is based on the same 
data as the human health risk assessment (i.e., an estimated chronic NOAEL of 9 mg/kg/day and 
an acute NOAEL of 180 mg/kg/day.  For birds, a chronic NOAEL of 10 mg/kg bw/day is used 
from a subchronic feeding study that assayed for both signs of systemic toxicity as well as 
reproductive capacity.  The potential effects of acute exposures of birds are characterized using an 
acute NOAEL of 500 mg/kg/day.  For terrestrial invertebrates, the dose-response assessment is 
based on a study in honey bees in which a dose of 107 mg/kg bw caused no apparent adverse 
effects. 
Sethoxydim is a herbicide that causes adverse effects in a variety of target and non-target plant 
species.  In general, grasses are much more sensitive to sethoxydim than broad-leaved plants.  For 
exposures associated with direct sprays or drift, NOAELs for sensitive and tolerant species are 
0.006 lbs/acre and 0.03 lbs/acre, respectively.  With respect to soil contamination, the NOAEL for 
sensitive species is 0.059 lbs/acre and the NOAEL for tolerant species is 0.235 lbs/acre. 
Sethoxydim has a low order of acute toxicity to fish and aquatic invertebrates, with LC
50
 values of 
1.2 and 2.6 mg/L, respectively.  Aquatic macrophytes are much more sensitive to sethoxydim than 
fish or invertebrates.  For aquatic plants, a NOAEL of 0.25 mg/L is used to assess the 
consequences of sethoxydim exposure. 
Risk Characterization – None of the hazard quotients for mammals or birds approach a level of 
concern, even at the upper limit of exposure.  For sethoxydim, further refinement of the exposure 
assessment would have little impact on the risk characterization because the hazard quotients are 
below a level of concern by factors of at least 10 for acute exposure scenarios (a large mammal 
consuming vegetation) and about 7 for chronic exposure scenarios (a large bird consuming 
vegetation at the application site).  The more plausible scenarios involving off-site exposures have 
hazard quotients below a level of concern by factors of about 385 (large bird) to 50,000 (small 
mammal).  The simple verbal interpretation of this quantitative risk characterization for terrestrial 
animals is similar to that of the human health risk assessment: the weight of evidence suggests that 
no adverse effects in terrestrial animals are plausible using typical or even very conservative worst 
case exposure assumptions. 
For terrestrial plants, runoff may present a risk to some sensitive species.  The extent to which this 
effect might be observed in the field is likely to depend on a number of site specific conditions, 
particularly how the runoff is distributed in areas adjacent to the application site.  For sensitive 
species in areas with high rates of rainfall, the hazard quotients are slightly above unity - e.g., the 
highest hazard quotient is about 3.  In arid environments - i.e., annual rainfall rates of about 15 
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inches per year or less - very little runoff of sethoxydim would occur and risks to any nontarget 
plant species would be minimal and below the level of concern.  Drift, including dispersion of 
contaminated soil by wind, does not appear to present a major hazard to nontarget plant species. 
Hazard quotients for offsite drift indicate that sethoxydim is not likely to result in damage at 
distances as close as 25 feet from the application site.  For sensitive species, the hazard quotient 
exceeds unity at 25 feet but not at 50 feet. 
There is no indication that fish, aquatic invertebrates, or aquatic plants are likely to be exposed to 
concentrations of sethoxydim that will result in toxic effects, although the upper range of the 
hazard quotient for aquatic plants – i.e., 0.75) approaches a level of concern.  A major limitation 
of this risk characterization for aquatic animals is the lack of any chronic toxicity studies on fish or 
aquatic invertebrates. 
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1.  INTRODUCTION  
This document provides risk assessments for human health effects and ecological effects to 
support an assessment of the environmental consequences of using sethoxydim in Forest Service 
programs.  The USDA Forest Service uses the herbicide, sethoxydim, in its vegetation 
management programs.  The USDA Forest Service plans on using only one commercial 
formulation, Poast. 
This document has four chapters, including the introduction, program description, risk assessment 
for human health effects, and risk assessment for ecological effects or effects on wildlife species. 
Each of the two risk assessment chapters has four major sections, including an identification of 
the hazards associated with sethoxydim, an assessment of potential exposure to this compound, an 
assessment of the dose-response relationships, and a characterization of the risks associated with 
plausible levels of exposure.  These are the basic steps recommended by the National Research 
Council of the National Academy of Sciences (NRC 1983) for conducting and organizing risk 
assessments. 
This is a technical support document and it addresses some specialized technical areas. 
Nevertheless an effort was  made to ensure that the document can be understood by individuals 
who do not have specialized training in the chemical and biological sciences.  Certain technical 
concepts, methods, and terms common to all parts of the risk assessment are described in plain 
language in a separate document (SERA 2000).  Some of the more complicated terms and 
concepts are defined, as necessary, in the text. 
In the preparation of this risk assessment, literature searches of Poast and sethoxydim were 
conducted in the open literature using AGRICOLA and TOXLINE as well as the U.S. EPA CBI 
files.  In addition to these standard literature searches, additional sources of information were 
used including U.S. EPA/OPP (1998a) pesticide tolerances for sethoxydim, the U.S. EPA 
ecological risk assessment on sethoxydim (Bryceland et al.  1997), the IRIS entry for this 
compound (U.S. EPA/IRIS 1989), as well as the EXTOXNET review of this compound 
(Extoxnet 2000).  The Forest Service funded a review of this compound - i.e., a chemical 
background statement - in 1989 (Sczerzenie et al.  1989) and this review was also consulted. 
The search of U.S. EPA’s FIFRA/CBI files indicated that there is a complete set of standard 
studies conducted for this compound - i.e., a total of 184 submissions.  While many of these 
studies were conducted to support the initial registration of sethoxydim, a substantial number of 
studies were conducted and submitted to EPA after 1989, the date of the last Forest Service 
review of sethoxydim.  Full text copies of the most relevant CBI studies [n=93] were kindly 
provided by the U.S. EPA Office of Pesticide Programs.  The studies were reviewed, and 
synopses of the most relevant studies are included in the appendices to this document.  In several 
areas of concern, the U.S. EPA review by Bryceland et al. (1997) discusses studies that were 
apparently submitted to U.S. EPA but studies that did not appear in the searches of the CBI files. 
When the data described by Bryceland et al. (1997) results in more conservative dose-response 
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