Sethoxydim Risk Assessment

Table 3-7: Summary of risk characterization for the general public 
1
 . 
Chronic RfD 
0.09 
mg/kg/day 
Sect. 3.3.3.  
Acute RfD 
0.6 
mg/kg/day 
Sect. 3.3.3.  
Target 
Hazard Quotient 
Worksheet 
Scenario 
Central 
Lower 
Upper 
Acute/Accidental Exposures 
Direct spray, entire body 
Child 
2e-02 
2e-03 
1e-01 
D01a 
Direct spray, lower legs 
Woman 
2e-03 
2e-04 
1e-02 
D01b 
Dermal, contaminated 
Woman 
1e-03 
1e-04 
4e-03 
D02 
vegetation 
Contaminated fruit 
Woman 
6e-03 
2e-03 
1e-01 
D03 
Contaminated water, spill 
Child 
3e-01 
3e-02 
1.3 
D05 
Contaminated water, stream 
Child 
8e-03 
1e-04 
4e-02 
D06 
Consumption of fish, 
Man 
1e-02 
2e-03 
3e-02 
D08a 
general public 
Consumption of fish, 
Man 
6e-02 
9e-03 
1e-01 
D08b 
subsistence populations 
Chronic/Longer Term Exposures 
Contaminated fruit 
Woman 
2e-03 
6e-04 
4e-02 
D04 
Consumption of water 
Man 
8e-05 
4e-07 
2e-04 
D07 
Consumption of fish, 
general public 
Man 
3e-06 
2e-08 
5e-06 
D09a 
Consumption of fish, 
subsistence populations 
Man 
2e-05 
2e-07 
4e-05 
D09b 
1
 Hazard quotient is the level of exposure divided by the provisional RfD then rounded to one or two significant 
decimal places or digits. See Worksheet E01 for summary of exposure assessments.  Hazard quotients >0.5 and 
<1.5 are shown to two significant digits.  All others are rounded to one significant decimal place or integer. 
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4.  ECOLOGICAL RISK ASSESSMENT  
4.1.  HAZARD IDENTIFICATION 
4.1.1.  Overview. Data used in the  human health risk assessment to identify the toxicity of 
sethoxydim and Poast to humans can also be used to identify potential toxic effects in wildlife 
mammalian species.  In mammals, the major effects of sethoxydim as well as Poast appear to be 
related to neurologic effects and the major signs of toxicity in mammals include lacrimation, 
salivation, incontinence,  ataxia, tremors, and convulsions.  Based on studies in mice, rats, and 
dogs, larger mammals appear to be more sensitive than smaller mammals.  Because relatively few 
studies are available to support this apparent relationship, quantitative estimates of inter-species 
differences in sensitivity are not developed.  Instead, the assumption is made that wildlife species 
may be as sensitive to sethoxydim as the most sensitive species on which data are available - i.e., 
the dog.  Based on acute toxicity studies, sethoxydim and Poast appear to be about equally toxic 
to mammals. 
The U.S. EPA/OPP (1998a) classified sethoxydim as practically non-toxic to birds and this 
assessment is supported by standard toxicity studies on sethoxydim in ducks and quail.  No acute 
toxicity studies on the formulated product – i.e., Poast – are available and the U.S. EPA has 
indicated that such studies will need to be conducted. 
Relatively little information is available of the toxicity of sethoxydim to terrestrial invertebrates. 
A standard acute toxicity study in bees indicates that direct applications of 10 µg sethoxydim/bee 
are not toxic and this value is used quantitatively in the risk assessment as a NOAEL.  There is a 
published study on effects in beetle larvae that suggests that Poast is relatively non-toxic at 
application rates higher than those planned by the Forest Service. 
Standard pre-emergence and post-emergence toxicity studies have been conducted on a number 
of terrestrial plant species and these studies are adequate for assessing the potential damage to 
non-target plant species posed by runoff or drift. 
Unlike the case with mammals, Poast is much more toxic to aquatic species than sethoxydim. 
Poast contains 74% petroleum solvent and only 18 % sethoxydim.  While somewhat speculative, 
it appears that the acute toxicity of Poast to aquatic species may be attributable almost exclusively 
to the solvent rather than to sethoxydim. 
4.1.2.  Toxicity to Terrestrial Organisms. 
4.1.2.1. Mammals– As summarized in the human health risk assessment (Section 3.1), there are 
several standard toxicity studies in experimental mammals that were conducted as part of the 
registration process.  Just as these studies are used in the human health risk assessment to identify 
the potential toxic hazards associated with exposures to sethoxydim or Poast to humans, they can 
also be used to identify potential toxic effects in wildlife mammalian species. 
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The major effects of sethoxydim as well as Poast appear to be related to neurologic effects and 
the major signs of toxicity in mammals include lacrimation, salivation, incontinence,  ataxia, 
tremors, and convulsions (Section 3 and Appendix 1).  The mechanism of action of sethoxydim in 
mammals, however, is unclear.  One published study (Yamano and Morita 1995) has reported that 
sethoxydim uncouples mitochondrial oxidative phosphorylation in vitro at concentrations of 10
-3 
M. 
Because toxicity data in mammals are available in only three species of experimental mammals 
(mice, rats, and dogs), the use of these data to assess the potential hazards to large number of 
diverse mammalian wildlife species is an uncertain process.  One approach to this process involves 
identifying patterns of toxicity in mammals of various sizes (i.e., allometric relationships as 
discussed in SERA 2000, Section 3.2.).  The acute oral LD
50
 values for sethoxydim in mice, rats, 
and dogs range from about 2,500 mg/kg to 6,000 mg/kg (Section 3.1.2).  While this is not a 
particularly wide range, a comparison of gavage oral LD
50
 values in mice and rats suggests that 
rats are somewhat more sensitive to sethoxydim than are mice.  The only acute LD
50
 data in dogs 
involve dosing by gelatin capsule rather than gavage and the oral LD
50
 is in the range 2500-5000 
mg/kg, encompassing the values for mice and rats.  The use of a different method of 
administration in dogs from that used in mice and rats complicates the interpretation of any 
allometric relationship in species sensitivity.  In chronic studies, however, dogs do appear to be 
the most sensitive species, with the lowest observed adverse effect level (LOAEL) in the range of 
17.5 to 19.9 mg/kg/day in a one-year feeding study.  In contrast, the LOAEL in mice from a two-
year feeding study is about 44 mg/kg/day (Takaori et al.  1981, Appendix 1).  While a chronic 
LOAEL has not been identified in rats, the highest 2-year dietary NOAEL is 18 mg/kg/day 
(Burdock et al.  1981, Appendix 1), again indicating that dogs are more sensitive than either mice 
or rats. 
Thus, the limited available data appear to suggest that larger mammals, such as dogs, are more 
sensitive to sethoxydim than smaller mammals such as mice and rats.  Because relatively few 
studies are available to support this apparent relationship, quantitative estimates of inter-species 
differences in sensitivity are not developed.  Instead, the assumption is made that wildlife species 
may be as sensitive to sethoxydim as the most sensitive species on which data are available - i.e., 
the dog. 
4.1.2.2. Birds– Both acute and subchronic reproductive toxicity studies have been conducted on 
mallard ducks and bobwhite (Appendix 2).  These studies are required by the U.S. EPA for 
pesticide registration and were submitted to the U.S. EPA during the registration process. 
Consistent with the gavage studies in rats (Section 3.1 and  Appendix 1), the acute toxicity of 
sethoxydim to birds appears to be low, with no mortality observed after single gavage doses as 
high as 2000 mg/kg and no effects on reproductive performance at dietary concentrations of up to 
1000 ppm (approximately 100 to150 mg/kg assuming food consumption of 10% to15% of body 
weight per day).  The U.S. EPA/OPP (1998a) classified sethoxydim as practically non-toxic to 
birds.  However, based on the higher acute toxicity of Poast to mammals when expressed as 
sethoxydim equivalents (Section 3.1.2), the U.S. EPA/OPP (1998a) indicated that an acute 
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