Sethoxydim Risk Assessment

4.4. RISK CHARACTERIZATION 
4.4.1. Overview. 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 
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. 
4.4.2. Terrestrial Organisms 
4.4.2.1. Terrestrial Animals– The quantitative risk characterization for terrestrial animals is 
summarized in Table 4-3.  These hazard quotients are calculated by dividing the exposure 
assessments summarized in Table 4-1 by the toxicity values given in  Table 4-2. 
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). 
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For the honey bee, the hazard quotient is based on the acute NOAEL of 107 mg/kg (BASF 1982). 
Even at the upper range of exposure associated with a direct spray, the hazard quotient is below 
the level of concern by a factor of about 2 – i.e., 1÷0.56 
•
 1.79).  Thus, there is no basis for 
expecting mortality in bees directly sprayed with sethoxydim.  The study by Agnello et al. (1986) 
in coleoptera suggests that applications of sethoxydim at rates of 5-6 lbs/acre might have an effect 
on the life cycle of some beetles.  The effect noted in this study, however, was a slight  increase in 
days to pupation but an increase in both the number of eggs masses as well as total number of 
eggs produced by beetles feeding on sethoxydim treated plants relative to beetles feeding on 
untreated plants.  Thus, it is not clear that this would be regarded as an adverse effect.  In any 
event, the application rate used in the Agnello et al. (1986) study is substantially higher than that 
used in Forest Service programs. 
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.  As with the human health risk assessment, this characterization of 
risk must be qualified.  Sethoxydim has been tested in only a limited number of species and under 
conditions that may not well represent populations of free-ranging non-target animals. Given the 
very large number of nontarget terrestrial animal species and the limited requirements for and 
capacity to test nontarget species, this limitation is common to virtually all ecological risk 
assessments.  Notwithstanding this limitation, the available data are sufficient to assert that no 
adverse effects can be anticipated in terrestrial animals from the use of this compound in Forest 
Service programs. 
4.4.2.2. Terrestrial Plants– The quantitative risk characterizations for terrestrial plants are 
summarized in Worksheet G04 for the offsite movement of sethoxydim in runoff and Worksheet 
G05 for offsite movement of sethoxydim by drift and wind erosion. 
The runoff estimates are based on GLEAMS modeling using three different soils (clay, loam, and 
sand) at annual rainfall rates of 5 to 250 inches and using the highest application rate that the 
Forest Service is considering, 0.0624 lb/acre.  The toxicity index is based on the pre-emergence 
NOAEL of 0.059 lb/acre for the most sensitive species - i.e., rye grass - and the NOAEL of 0.235 
to the most tolerant species (corn).  Based on these indices of toxicity, some sensitive species 
could be effected in areas with annual rainfall rates of 50 inches and higher.  Tolerant plant species 
are not likely to be affected by off-site runoff of sethoxydim under any conditions. 
Hazard quotients for offsite drift (Worksheet G05) are based on the NOAEL value of 0.006 
lb/acre for sensitive species (corn) as well as the NOAEL of 0.03 lb/acre for several tolerant 
species.  As discussed in Section 4.2.2.4, the estimates for offsite drift encompass plausible 
exposures attributable to wind erosion.  For relatively tolerant species, there is no indication that 
sethoxydim is likely to result in damage at distances as close as 25 feet from the application site. 
For sensitive species, there is a modest excursion about the NOAEL (a hazard quotient of 1.2) at 
25 feet offsite but not at distances of 50 feet or greater. 
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