Sethoxydim Risk Assessment

4.3.2.3. Terrestrial Invertebrates – As discussed in Section 4.1.2.3, a standard bioassay was 
conducted on the toxicity of sethoxydim to honey bees (BASF 1982).  At the highest dose tested, 
10 µg/bee, mortality was observed.  Using a body weight of 0.093 g for the honey bee 
(USDA/APHIS 1993), the 10 µg/bee dose corresponds to 107 mg/kg bw [0.010 mg/0.000093 
kg].  This value will be used in the risk characterization for assessing effects on terrestrial 
invertebrates.  Given the large number of species of terrestrial invertebrates, the use of this single 
study on a single species obviously leads to uncertainty in the risk assessment.  The BASF (1982) 
study is also used by U.S. EPA as the toxicity benchmark for terrestrial invertebrates and 
sethoxydim is classified by U.S. EPA as “practically non-toxic to bees” (Bryceland et al.  1997). 
The study by Agnello et al. (1986) on toxicity to bean beetle larvae cannot be used quantitatively 
in the dose-response assessment but is discussed further in the risk characterization. 
4.3.2.4. Terrestrial Plants (Macrophytes)– As discussed in Section 4.1.2.4, two sets 
pre-emergence and post-emergence studies are available on the toxicity of sethoxydim to 
nontarget plants, an early study by Ludwig (1980) and a more recent study summarized by 
Bryceland et al. (1997).  Based on the summary provided by Bryceland et al. (1997) the most 
sensitive species in the pre-emergence assay is ryegrass, with a NOAEL of 0.059 lb/acre and an 
EC
25
 of 0.065 lb/acre.  The most tolerant species was corn, with a NOAEL of 0.235 lb/acre and 
an EC
25
 of 0.418 lb/acre.  In the post-emergence (vegetative vigor) assay, ryegrass is also the 
most sensitive species, with a NOAEL of 0.006 lb/acre and an EC
25
 of 0.025 lb/acre.  The most 
tolerant species was oats, with a NOAEL of 0.03 lb/acre and an EC
25
 of 0.0313 lb/acre. [Note 
that the NOAEL’s are experimental doses whereas the EC
25
 values are estimates based on the 
experimental data.  This accounts for the similarity between some of the NOAEL values and EC
25 
estimates.] 
The U.S. EPA (Bryceland et al. 1997) use EC
25
 values for characterizing risks to terrestrial plants. 
For this risk assessment, the NOAEL values will be used because this approach is more closely 
related to the hazard index used to characterize risk to terrestrial animals.  The results of the 
post-emergence assays will be applied to scenarios involving drift and the pre-emergence value 
will be applied to scenarios involving inadvertent soil contamination by runoff. 
For pre-emergence exposures, the NOAEL of 0.059 lb/acre (ryegrass) will be used to characterize 
risk to sensitive species and the NOAEL of 0.235 lb/acre (corn) will be used to characterize risk 
to tolerant species.  For post-emergence exposures, the NOAEL of 0.006 lb/acre (ryegrass) will 
be used to characterize risk to sensitive species and the NOAEL of 0.03 lb/acre (oats) will be used 
to characterize risk to tolerant species. 
4.3.2.5. Terrestrial Microorganisms– As discussed in section 4.1.2.5, no information is available 
on the toxicity of sethoxydim to terrestrial microorganisms.  Thus, no dose-response assessment 
for this group is possible. 
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4.3.3.  Aquatic Organisms. 
4.3.3.1. Animals– As discussed in Sections 4.1.3.1 and 4.1.3.3, the formulated product, Poast, is 
much more toxic to aquatic species than the active ingredient, technical grade sethoxydim.  As 
detailed in Appendix 3, acute LC
50
 values for technical grade sethoxydim are on the order of 100 
to 300 mg/L and 78.1 mg/L for daphnids.  For Poast, however, the acute LC
50
 values (expressed 
as concentrations of sethoxydim in water) are about 1 to 3 mg/L for fish and 2.6 mg/L for 
daphnids.  Thus, exposures to Poast are about 100 times more hazardous than exposures to 
technical grade sethoxydim.  Consequently, the U.S. EPA/OPP (Bryceland et al.  1997) based all 
of the acute toxicity benchmarks on sethoxydim concentrations associated with exposures to 
Poast.  This essentially considers to the extent possible the influence of the inerts in Poast on the 
overall toxicity to aquatic species and the same approach will be used in the current risk 
assessment. 
The U.S. EPA (Bryceland et al. 1997) use acute exposure concentrations of 1.2 mg/L for fish 
(based on results in trout) and 2.6 mg/L for aquatic invertebrates (based on results in daphnids). 
As noted in Appendix 3, the confidence interval for trout is 1.0-1.7 mg/L and the corresponding 
interval for daphnids is 2.0-3.3 mg/L.  Thus, while the differences are not substantial, the 
distinction between fish and aquatic invertebrates maintained by U.S. EPA seems justified and the 
values of 1.2 mg/L for fish and 2.6 mg/L for aquatic invertebrates will be used in this risk 
assessment to characterize risk. 
No chronic studies are available on the toxicity of sethoxydim (technical grade or formulated 
product) to any aquatic animals.  Thus, no dose-response assessment for aquatic exposures can be 
made. 
4.3.3.2. Aquatic Plants– Aquatic macrophytes and algae appear to be somewhat more sensitive 
to sethoxydim than fish or invertebrates.  The studies identified in the U.S. EPA/CBI files, 
summarized in Appendix 3, suggest NOAEL values of less than 0.56 mg/L, with  Lemna gibba 
(an aquatic macrophyte commonly known as duckweed) more sensitive than algae.  Bryceland et 
al. (1997) reference additional  studies not encountered in the search of the EPA/CBI files that 
identify EC
50
 values of >0.281 mg/L for duckweed and >0.25 mg/L for algae.  These values are 
very similar and, for this risk assessment, the lower value of 0.25 mg/L will be used to 
characterize risks for aquatic plants.  Based on the data from Appendix 3, it appears that adverse 
effects in Lemna gibba and perhaps other aquatic macrophytes could be expected at 
concentrations of 0.56 mg/L. 
4.3.3.3. Aquatic Microorganisms– As with terrestrial microorganisms, no data are available on 
the toxicity of sethoxydim to aquatic microorganisms other than algae and a separate 
dose-response assessment cannot be made for this group. 
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