Sethoxydim Risk Assessment

(Roslycky 1987, Table 1, p. 415).  Various species and strains of Azotobacter were much less 
sensitive to sethoxydim, with inhibition of growth and nitrogen fixation in liquid shake cultures at 
concentrations of 5,000 ppm (Roslycky (1991). 
Reichad et al. (1997) assayed the effects of various herbicides on a plant pathogen, Sclerotinia 
trifoliorum, which caused stem rot in alfalfa and other legumes.  The pathogen was cultured in 
dextrose agar.  At a concentration of 10 µg/mL (10 ppm), sethoxydim inhibited mycelial growth. 
At 1000 µg/mL (1000 ppm), sclerotium weight was reduced. 
4.1.3.  Aquatic Organisms. 
4.1.3.1. Fish– Standard toxicity bioassays to assess the effects of sethoxydim on fish are 
summarized in Appendix 3.  The acute static LC
50
 values for technical grade sethoxydim range 
from 170 to 265 ppm (mg/L) in bluegill sunfish and rainbow trout, respectively (BASF 1982). 
The formulated product, Poast, however, is much more toxic with LC
50
 values of 2.6 ppm in 
bluegill sunfish and 1.2 ppm in rainbow trout (Bowman and Howell 1991a,b). 
The higher toxicity (lower LC
50
 values) of Poast compared to sethoxydim is probably attributable 
to the presence of naphtha solvent in Poast.  As summarized in Section 2, Poast contains 74% 
petroleum solvent (approximately 740,000 mg/L) but only 18 % sethoxydim (1.5 lbs per gallon or 
680.5 g/3.785 L or 179,789 mg/L).  Information on the aquatic toxicity of the specific solvent 
used in Poast has not been encountered in the literature.  A related solvent (Stoddard Solvent) has 
LC
50
 values of 0.5 to 5.0 ppm (mg/L) (Anon. 1996). 
As detailed by Finney (1971, p. 233), the toxicity of a mixture under the assumption of 
concentration addition may be calculated as: 
.
M
 = 
.
1
 / (
B
1
 + 
B
2 
(
.
1
/
.
2
)) 
where 
.
 is some measure of uniform toxicity, such as the LC
50
 and 
B
i
 is the proportion of the i
th 
agent in the mixture.  Taking 200 ppm as the approximate LC
50
 for sethoxydim (
.
2
) and 0.5 to 5.0 
ppm range for the plausible LC
50
 values for the petroleum solvent (
.
1
) in Poast and using the 
proportions of 0.74 for the solvent  (
B
1
) and 0.18 for sethoxydim (
B
2
)), the estimated LC
50
 value 
for Poast would range from about 0.67 ppm to 6.8 ppm using the above equation, with 
sethoxydim itself making virtually no contribution to the toxicity of the mixture - i.e., the term 
B
1
(
.
1
/
.
2
) ranges from 0.0025 to 0.025.  Thus, the acute toxicity of Poast to fish may be 
attributable almost exclusively to the solvent rather than to sethoxydim.  Further, because the 
toxic components in the solvent are volatilized and/or sorbed to sediments, the apparent acute 
toxic potency of the solvent and thus of Poast may overestimate the potential effects in the 
environment (Anon. 1996).  This is considered quantitatively in the exposure and dose response 
assessments (Sections 4.2 and 4.3) as well as the risk characterization (Section 4.4). 
4.1.3.2. Amphibians– Neither the published literature nor the U.S. EPA files include data 
regarding the toxicity of sethoxydim to amphibian species. 
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4.1.3.3. Aquatic Invertebrates– Standard toxicity bioassays have been conducted to assess the 
effects of sethoxydim and Poast on an aquatic invertebrate, Daphnia magna.  As with fish, the 
acute toxicity of sethoxydim (LC
50
 = 78.1 ppm) is less than that of Poast (LC
50
 = 2.6 ppm) by a 
factor of about 30.  Sethoxydim is less toxic to daphnids than fish by a factor of about two to 
three [170 to 265 ppm ÷ 78.1 ppm 
.
 2.2 to 3.4].  The LC
50
 values of Poast to daphnids and fish 
are virtually identical (1.2 ppm to 2.6 ppm). 
The U.S. EPA has asked for a life cycle study in daphnids for both sethoxydim and Poast 
(Bryceland et al.  1997).  These studies were not encountered in the search of the EPA files 
conducted as part of this risk assessment (in January 2001). 
4.1.3.4. Aquatic Plants– Standard toxicity bioassays to assess the effects of sethoxydim on 
aquatic plants were submitted to the U.S. EPA in support of the registration of sethoxydim and 
are summarized in Appendix 3.  The most sensitive species on which data are available is the 
aquatic macrophyte, Lemna gibba (duckweed), with an NOEC of < 0.56 ppm (Hughes 1980a). 
Since the reported effect consisted of an increase rather than a decrease in growth, however, it is 
not clear that the study in Lemna gibba constitutes an adverse effect.  Unicellular algae are less 
sensitive with LC
50
 values greater than 5.6 ppm (Hughes 1980a,b). 
In a general screening study involving a variety of different herbicides, Schrader et al. (1998) have 
reported that sethoxydim has no inhibitory effect on Oscillatoria chalybea, a cyanobacterium that 
produces an unpleasant odor in water, at concentrations of up to 1 mM (327.5 ppm). 
4.1.3.5. Other Aquatic Microorganisms– U.S. EPA files do not include data regarding the 
toxicity of sethoxydim to other aquatic microorganisms and no studies on this group have been 
encountered in the open literature. 
4.2.  EXPOSURE ASSESSMENT 
4.2.1. Overview.  Terrestrial animals might be exposed to any applied herbicide from direct spray, 
the ingestion of contaminated media (vegetation, prey species, or water), grooming activities, or 
indirect contact with contaminated vegetation.  In acute exposure scenarios and under the 
assumption of 100% dermal absorption, the highest exposures for small terrestrial vertebrates will 
occur after a direct spray and could reach up to about 7 mg/kg under typical exposure conditions 
and up to about 9 mg/kg under more extreme conditions.  Other routes of exposure, like the 
consumption of contaminated water or contaminated vegetation, generally will lead to much 
lower levels of exposure.  In chronic exposure scenarios, the maximum estimated daily doses for a 
small vertebrate is 0.006 mg/kg/day.  Based on general relationships of body size to body volume, 
larger vertebrates will be exposed to lower doses and smaller animals, like insects, will be exposed 
to much higher doses under comparable exposure conditions.  Because of the apparent low 
toxicity of sethoxydim to animals, the rather substantial variations in the exposure assessments 
have little impact on the assessment of risk to terrestrial animals. 
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