Sethoxydim Risk Assessment

from the surface of the animal and may overestimate the absorbed dose.  Conversely, some 
animals, particularly birds and mammals, groom frequently, and grooming may contribute to the 
total absorbed dose by direct ingestion of the compound residing on fur or feathers.  Furthermore, 
other vertebrates, particularly amphibians, may have skin that is far more permeable than the skin 
of most mammals (Moore 1964). 
Quantitative methods for considering the effects of grooming or increased dermal permeability are 
not available.  As a conservative upper limit, the second exposure scenario, detailed in worksheet 
F02a, is developed in which complete absorption over day 1 of exposure is assumed. 
Because of the relationship of body size to surface area, very small organisms, like bees and other 
terrestrial insects, might be exposed to much greater amounts of sethoxydim per unit body weight, 
compared with small mammals.  Consequently, a third exposure assessment is developed using a 
body weight of 0.093 g for the honey bee (USDA/APHIS 1993).  Because there is no information 
regarding the dermal absorption rate of sethoxydim by bees or other invertebrates, this exposure 
scenario, detailed in worksheet F02b, also assumes complete absorption over the first day of 
exposure. 
Direct spray scenarios are not given for large mammals.  As noted above, allometric relationships 
dictate that large mammals will be exposed to lesser amounts per unit body weight  of a 
compound in any direct spray scenario than smaller mammals.  As detailed further in Section 4.4, 
the direct spray scenarios for the small mammal are substantially below a level of concern. 
Consequently, elaborating direct spray scenarios for a large mammal would have no impact on the 
characterization of risk. 
4.2.2.2.  Indirect Contact  –  As in the human health risk assessment (see section 3.2.3.3), the 
only approach for estimating the potential significance of indirect dermal contact is to assume a 
relationship between the application rate and dislodgeable foliar residue.  The study by Harris and 
Solomon (1992) is used to estimate that the dislodgeable residue will be approximately 10 times 
less than the nominal application rate. 
Unlike the human health risk assessment in which transfer rates for humans are available, there are 
no transfer rates available for wildlife species.  As discussed in Durkin et al. (1995), the transfer 
rates for humans are based on brief (e.g., 0.5- to 1-hour) exposures that measure the transfer from 
contaminated soil to uncontaminated skin.  Species of wildlife are likely to spend longer periods 
of time, compared to humans, in contact with contaminated vegetation. 
It is reasonable to assume that for prolonged exposures a steady-state may be reached between 
levels on the skin, rates of absorption, and levels on contaminated vegetation, although there are 
no data regarding the kinetics of such a process.  The bioconcentration data on sethoxydim 
(section 3.2.3.5) as well as its high water solubility and low octanol/water partition coefficient 
suggest that sethoxydim is not likely to partition from the surface of contaminated vegetation to 
the surface of skin, feathers, or fur.  Thus, a plausible partition coefficient is unity (i.e., the 
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concentration of the chemical on the surface of the animal will be equal to the dislodgeable 
residue on the vegetation). 
Under these assumptions, the absorbed dose resulting from contact with contaminated vegetation 
will be one-tenth that associated with comparable direct spray scenarios.  As discussed in the risk 
characterization for ecological effects (section 4.4), the direct spray scenarios result in exposure 
levels far below those of toxicological concern.  Consequently, details of the indirect exposure 
scenarios for contaminated vegetation are not further elaborated in this document. 
4.2.2.3.  Ingestion of Contaminated Vegetation or Prey – Since sethoxydim will be applied to 
vegetation, the consumption of contaminated vegetation is an obvious concern and separate 
exposure scenarios are developed for acute and chronic exposure scenarios for a small mammal 
(Worksheets F04a and F04b) and large mammal (Worksheets F10, F11a, and F11b) as well as 
large birds (Worksheets F12, F13a, and F13b). 
A small mammal is used because allometric relationships indicate that small mammals will ingest 
greater amounts of food per unit body weight, compared with large mammals.  The amount of 
food consumed per day by a small mammal (i.e., an animal weighing approximately 20 g) is equal 
to about 15% of the mammal's total body weight (U.S. EPA/ORD 1989).  When applied 
generally, this value may overestimate or underestimate exposure in some circumstances.  For 
example, a 20 g herbivore has a caloric requirement of about 13.5 kcal/day.  If the diet of the 
herbivore consists largely of seeds (4.92 kcal/g), the animal would have to consume a daily 
amount of food equivalent to approximately 14% of its body weight [(13.5 kcal/day ÷ 4.92 
kcal/g)÷20g = 0.137].  Conversely, if the diet of the herbivore consists largely of vegetation (2.46 
kcal/g), the animal would have to consume a daily amount of food equivalent to approximately 
27% of its body weight [(13.5 kcal/day ÷ 2.46 kcal/g)÷20g = 0.274] (U.S. EPA/ORD 1993, 
pp.3-5 to 3-6).  For this exposure assessment, the amount of food consumed per day by a small 
mammal is estimated at about 3.6 g/day from the general allometric relationship for food 
consumption in rodents (U.S. EPA/ORD 1993, p. 3-6).  As detailed in Section 4.4, this variability 
in food consumption estimates has little impact on the characterization of risk because any 
plausible levels of exposure are far below levels of concern. 
A large herbivorous mammal is included because empirical relationships of concentrations of 
pesticides in vegetation, discussed below, indicate that grasses may have substantially higher 
pesticide residues than other types of vegetation such as forage crops or fruits (Worksheet A04). 
Grasses are an important part of the diet for some large herbivores, but small mammals do not 
consume grasses as a substantial proportion of their diet.  Thus, even though using residues from 
grass to model exposure for a small mammal is the most conservative approach, it is not generally 
applicable to the assessment of potential adverse effects.  Hence, in the exposure scenarios for 
large mammals, the consumption of contaminated range grass is modeled for a 70 kg herbivore, 
like a deer.  Caloric requirements for herbivores and the caloric content of vegetation  are used to 
estimate food consumption based on data from U.S. EPA/ORD (1993).  Details of these exposure 
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