Sethoxydim Risk Assessment

relatively small off-site area.  An objective approach for modeling these types of events was not 
available in the literature.  For this risk assessment, neither concentration nor dispersion is 
considered quantitatively. 
4.2.4.  Aquatic Organisms.  The potential for effects on aquatic species are based on estimated 
concentrations of sethoxydim in water that are identical to those used in the human health risk 
assessment (Section 3.2.3.4).  Thus, for an accidental spill, the central estimate for the 
concentration of sethoxydim in a small pond is estimated at about 2.7 mg/L with a range from 0.4 
to 6.8 mg/L (Section 3.2.3.4.1).  For longer term exposure scenarios, the expected concentrations 
of sethoxydim in ambient water range from 0.0001 to 0.003 mg/L with a central value of 0.0015 
mg/L. (Section 3.2.3.4.2). 
4.3. DOSE-RESPONSE ASSESSMENT 
4.3.1.  Overview.  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. 
4.3.2. 
Toxicity to Terrestrial Organisms. 
4.3.2.1. Mammals– As summarized in the dose-response assessment for the human health risk 
assessment (Section 3.3.3.), the acute NOAEL in experimental mammals is taken as 180 mg/kg 
with an associated LOAEL of 480 mg/kg and the chronic NOAEL is taken as 9 mg/kg/day with 
an associated LOAEL of 18 mg/kg.  For this risk assessment, these NOAEL’s will be used to 
characterize risk.  The acute NOAEL is based on reproductive toxicity - i.e., 23 day exposures on 
days 6-28 of gestation (IRDC 1980a) - and is thus a very conservative index in that many of the 
acute exposures estimated in Section 4.2 will be for much less than 23 days.. 
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The U.S. EPA/OPP (1998a) has taken a somewhat different approach.  Acute risks in mammals 
were not assessed because the acute risks to birds did not trigger concern.  For the chronic risk 
assessment, the U.S. EPA/OPP (1998a) used a dietary NOAEL of 3000 ppm.  The basis for the 
selection of the 3000 ppm NOAEL is unclear and is not specified in U.S. EPA/OPP (1998a).  As 
noted in 3.3.2, dietary concentrations of 600 ppm and 3600 ppm were classified as adverse effect 
levels (AEL’s) in dogs.  In addition, longer-term dietary concentrations of 360 ppm and 1080 
ppm have been associated with histopathologic changes in the liver of mice (Takaori et al.  1981 
as detailed in Appendix 1). 
4.3.2.2. Birds – As noted in section 4.1.2.2, sethoxydim has been classified by the U.S. EPA 
(Bryceland et al. 1997) as essentially non-toxic to birds in acute exposures.  The lowest 5-day 
dietary LD
50
 for birds is >5000 ppm (Appendix 2).  The U.S. EPA (Bryceland et al.  1997) uses 
the dietary concentration of 5000 ppm as the toxicity benchmark for the characterization of risk 
following acute exposure. 
The U.S. EPA (Bryceland et al. 1997) uses reported dietary concentrations.  This approach, 
however, may be under-protective.  Laboratory diets generally involve the use of dry food.  Dry 
laboratory chow usually has a higher caloric content than food consumed in the wild, if only 
because most food consumed in the wild has a high water content.  In addition, most reported 
concentrations of a pesticide in environmental samples are given on a wet (natural) weight rather 
than a dry (dedicated) weight basis.  Consequently, animals  tend to eat greater amounts of food 
in the wild than they do under laboratory conditions (U.S. EPA/ORD 1993).  Consequently, for a 
fixed concentration in food, ingested doses expressed as mg/kg bw/day often will be higher in free 
living animals than in laboratory animals. 
Because of these relationships, Forest Service risk assessments use doses expressed as mg/kg 
body weight for both the exposure and dose-response assessments.  As detailed in the worksheets, 
information on caloric requirements and caloric values of different foods are used to estimate the 
amount of a particular food that an animal will use. 
The studies summarized in Appendix 2 do not specify food consumption rates.  Based on average 
measured food consumption and body weight from other laboratory toxicity studies on mallard 
ducks and pheasant, the daily food consumption rates of the birds are approximately 10% to 20% 
of the body weight.  Taking a conservative value of 10%, the 5000 ppm benchmark dose used by 
U.S. EPA corresponds to a daily dose of 500 mg/kg bw and this value will be used in the current 
risk assessment as a benchmark dose for acute exposure. 
As noted in Section 4.1.2.2, the U.S. EPA/OPP (1998a) uses a LOAEL of 100 ppm for 
reproductive effects in mallard ducks as a toxicity benchmark for chronic exposures.  For the 
current risk assessment, this benchmark will be adopted and converted to a daily dose of 10 
mg/kg bw/day using the 10% food consumption estimate. 
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