Sethoxydim Risk Assessment

The primary hazards to non-target terrestrial plants are associated with unintended direct 
deposition or spray drift.  Unintended direct spray will result in an exposure level equivalent to the 
application rate.  At least some plants that are sprayed directly with sethoxydim at or near the 
recommended range of application rates will be damaged.  Based on the AgDRIFT model, no 
more than 0.0058 of the application rate would be expected to drift 100 m offsite after low boom 
ground applications.  The AgDrift model is discussed further in Section 4.2.3.2. 
In order to encompass a wide range of field conditions, GLEAMS simulations were conducted for 
clay, loam, and sand at annual rainfall rates from 5 to 250 inches.  Under arid conditions (i.e., 
annual rainfall of about 10 inches or less), there is no or very little runoff.  Under these conditions, 
degradation, not dispersion, accounts for the decrease of sethoxydim concentrations in soil.  At 
higher rainfall rates, plausible offsite movement of sethoxydim results in runoff losses that range 
from about negligible up to about 0.5 of the application rate, depending primarily on the amount 
of rainfall rather than differences in soil type. 
Exposures to aquatic species are impacted by the same factors that influence terrestrial plants 
except the directions of the impact are reversed.  In other words, in very arid environments 
substantial contamination of water is unlikely.  In areas with increasing levels of rainfall, 
exposures to aquatic organisms are more likely to occur.  The anticipated concentrations in 
ambient water encompass a very broad range, 0.000094 to 0.003 mg/L, depending primarily on 
differences in rainfall rates. 
4.2.2.  Terrestrial Animals. 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 this exposure assessment, estimates of oral exposure are expressed in the same units as the 
available toxicity data (i.e., oral LD
50
 and similar values).  As in the human health risk assessment, 
these units are usually expressed as mg of agent per kg of body weight and abbreviated as mg/kg 
body weight.  For dermal exposure, the units of measure usually are expressed in mg of agent per 
cm
2
 of surface area of the organism and abbreviated as mg/cm
2
.  In estimating dose, however, a 
distinction is made between the exposure dose and the absorbed dose. The exposure dose is the 
amount of material on the organism (i.e., the product of the residue level in mg/cm
2
 and the 
amount of surface area exposed), which can be expressed either as mg/organism or mg/kg body 
weight.  The absorbed dose is the proportion of the exposure dose that is actually taken in or 
absorbed by the animal. 
For the exposure assessments discussed below, general allometric relationships are used to model 
exposure.  In the biological sciences, allometry is the study of the relationship of body size or 
mass to various anatomical, physiological, or pharmacological parameters (e.g., Boxenbaum and 
D'Souza 1990).  Allometric relationships take the general form: 
y = aW
x 
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where W is the weight of the animal, y is the variable to be estimated, and the model parameters 
are a and x. 
For most allometric relationships used in this exposure assessment, x ranges from approximately 
0.65 to 0.75.  These relationships dictate that, for a fixed level of exposure (e.g., levels of a 
chemical in food or water), small animals will receive a higher dose, in terms of mg/kg body 
weight, than large animals. 
Estimates of exposure are given for both a small and a large mammal as well as a small and a large 
bird. For many compounds, allometric relationships for interspecies sensitivity to toxicants 
indicate that for exposure levels expressed as mg toxicant per kg body weight (mg/kg body 
weight), large animals, compared with small animals, are more sensitive. 
As discussed in Sections 3.1.2 and 3.1.3, the limited data on sethoxydim do suggest that larger 
mammals, specifically the dog, appear to be more sensitive to sethoxydim than smaller mammals 
(i.e., rats and mice) but the data are not adequate to support the development of quantitative 
allometric relationships for toxicity.  There are no data to assess species sensitivity in small and 
large birds. 
The exposure assessments for terrestrial animals are summarized in Table 4-1.  As with the human 
health exposure assessment, the computational details for each exposure assessment presented in 
this section are provided in the attached worksheets (worksheets F01 through F14). 
4.2.2.1.  Direct Spray  –  In the broadcast application of any herbicide, wildlife species may be 
sprayed directly.  This scenario is similar to the accidental exposure scenarios for the general 
public discussed in section 3.2.3.2.  In a scenario involving exposure to direct spray, the extent of 
dermal contact depends on the application rate, the surface area of the organism, and the rate of 
absorption. 
For this risk assessment, three groups of direct spray exposure assessments are conducted.  The 
first, which is defined in worksheet F01, involves a 20 g mammal that is sprayed directly over one 
half of the body surface as the chemical is being applied.  The range of application rates as well as 
the typical application rate is used to define the amount deposited on the organism.  The absorbed 
dose over the first day (i.e., a 24-hour period) is estimated using the assumption of first-order 
dermal absorption.  In the absence of any data regarding dermal absorption in a small mammal, 
the estimated absorption rate for humans is used (see section 3.1.7).  An empirical relationship 
between body weight and surface area (Boxenbaum and D’Souza 1990) is used to estimate the 
surface area of the animal.  The estimates of absorbed doses in this scenario may bracket plausible 
levels of exposure for small mammals based on uncertainties in the dermal absorption rate of 
sethoxydim. 
Other, perhaps more substantial, uncertainties affect the estimates for absorbed dose.  For 
example, the estimate based on first-order dermal absorption does not consider fugitive losses 
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