Sethoxydim Risk Assessment

2.  PROGRAM DESCRIPTION  
2.1.  OVERVIEW 
Poast is a commercial formulation of sethoxydim, available from BASF, that is used by the USDA 
Forest Service.  Sethoxydim is recommended as selective postemergence herbicides for the 
control of annual or perennial grass weeds.  Poast is labeled for application to a number of 
different crops.  Poast Plus is labeled for application to alfalfa, citrus, clover, corn, cotton, 
peanuts, and soybeans  as well as for deciduous trees, non-food crop areas, and fallow lands.  The 
Forest Service will used Poast only in non-crop areas and the only use contemplated by the Forest 
Service and considered in this risk assessment involves the control of unwanted vegetation in 
nurseries. 
Poast contains a petroleum solvent (74%) that includes naphthalene (7% of the solvent).  The 
potential impact of these inert components on this risk assessment is discussed further in Sections 
3.1.9 (human health) and 4.1.3 (ecological effects). 
The most common method of application for sethoxydim in Forest Service programs will involve 
broadcast foliar applications.  Although Poast is registered for aerial applications, this application 
method will not be used in Forest Service programs.  The labeled application rates for Poast range 
from 0.09375 lb sethoxydim/acre to 0.375 lb sethoxydim/acre.  For simplicity, all application rates 
cited in this risk assessment are referenced simply as lb/acre rather than lb a.i./acre or lb a.e./acre. 
Unless otherwise specified, all such designations refer to lb a.i./acre or lb sethoxydim/acre.  For 
this risk assessment, the lower and upper limits of the application rate are taken as 0.09375 lb/acre 
to 0.375 lb/acre, respectively, based on the lower and upper limits of the labeled rates.  Based on 
the most recent use statistics from the Forest Service, the central estimate of the application rate 
is taken as 0.3 lbs/acre. 
Poast as well as many of the other commercial formulations of sethoxydim are used extensively in 
agriculture.  Based on the most recent use statistics encountered in the literature, over 1,000,000 
lbs of sethoxydim are applied to crops annually, primarily to soybeans and cotton in the mid-west. 
By comparison, the uses of sethoxydim by the Forest Service are trivial - i.e., a total of 3.8 lbs in 
1999. 
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2.2.  CHEMICAL DESCRIPTION AND COMMERCIAL FORMULATIONS 
Sethoxydim is the common name for 2-(1-(ethoxyimino)butyl)-5-(2-(ethylthio)propyl)­
3-hydroxy-2-cyclohexen-1-one.  The chemical structure of sethoxydim (Cambridge Software 
2001) is: 
Selected chemical and physical properties of sethoxydim are summarized in Table 2-1.  Additional 
information is presented in worksheet B03. 
Several commercial formulations of sethoxydim are available (Table 2-2). Only one commercial 
formulation of sethoxydim, Poast, is used in Forest Service programs.  This formulation is 
produced by BASF and contains sethoxydim as the only active ingredient.  Poast is a liquid 
formulation containing sethoxydim (18%) at a concentration of 1.5 lbs per gallon and inerts 
(82%).  Poast is recommended as a selective postemergence herbicide for the control of annual or 
perennial grass weeds.  Poast is labeled for application to a number of different crops.  A very 
similar formulation used in agriculture, Poast Plus, is labeled for application to alfalfa, citrus, 
clover, corn, cotton, peanuts, and soybeans  as well as for deciduous trees, non-food crop areas, 
and fallow lands (BASF 2000).  The Forest Service will use Poast only in non-crop areas and the 
only use contemplated by the Forest Service and considered in this risk assessment involves the 
control of unwanted vegetation in nurseries. 
The identity of the inerts has been disclosed to the U.S. EPA as part of the registration process. 
Inerts are classified by the U.S. EPA as ranging from inerts of toxicologic concern (List 1) to 
inerts of minimal concern (List 4) (U.S. EPA/OPP 1998b).  Some inerts - i.e., those listed under 
SARA Title III, Section 313 - are specified on the product material safety data sheets (BASF 
2000) and can be publicly disclosed. 
Poast contains a petroleum solvent (74%) that includes naphthalene (7% of the solvent).  Based 
on the CAS Number given on the MSDS [64742-94-5], the specific petroleum solvent is specified 
by the U.S. EPA as “solvent naphtha (petroleum) heavy aromatic” and is classified by the U.S. 
EPA as List II: Potentially Toxic with a high priority for testing (U.S. EPA/OPP 1998b).  Poast 
also contains a non-ionic emulsifier [CAS No. 9016-45-9], polyoxyethylene nonylphenol, that is 
classified by the U.S. EPA as List 4B, Inerts of Minimal Concern.  The potential impact of these 
inert components on this risk assessment is discussed further in the human health (Section 3.1.9.) 
and ecological (Section 4.1.3.) risk assessments. 
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2.3.  APPLICATION METHODS 
Sethoxydim may be applied by directed foliar, broadcast foliar, or aerial methods.  The most 
common method of application for sethoxydim in Forest Service programs will involve broadcast 
foliar applications.  Broadcast foliar ground applications will most often involve the use of a two 
to six nozzle boom mounted on a tractor or other heavy duty vehicle.  With this equipment, 
workers will typically treat 11 to 21 acres per hour, with the low end of this range representative 
of a four-wheel drive vehicle in tall grass and the upper end of the range representative of a large 
bulldozer (USDA 1989b p 2-9 to 2-10). 
In selective foliar applications, the herbicide sprayer or container is carried by backpack and the 
herbicide is applied to selected target vegetation.  Application crews may treat up to shoulder high 
brush, which means that chemical contact with the arms, hands, or face is plausible.  To reduce 
the likelihood of significant exposure, application crews are directed not to walk through treated 
vegetation.  Usually, a worker treats approximately 0.5 acre/hour with a plausible range of 
0.25-1.0 acre/hour. 
Poast is registered for aerial applications (BASF 2000).  In Forest Service programs, this 
application method will not be used and is not further considered in this risk assessment
. 
2.4.  MIXING AND APPLICATION RATES 
The labeled application rates for Poast range from 0.09375 lb sethoxydim/acre to 0.375 lb 
sethoxydim/acre (Table 2-3).  For simplicity, all application rates cited in this risk assessment are 
referenced simply as lb/acre rather than lb a.i./acre or lb a.e./acre.  Unless otherwise specified, all 
such designations refer to lb a.i./acre or lb sethoxydim/acre. 
While multiple applications of Poast may be made to various food crops, the product label for 
Poast does not include multiple applications to deciduous trees, non-food crops, and fallow land ­
i.e., the areas that would be treated in Forest Service programs. 
For this risk assessment, the lower and upper limits of the application rate are 0.09375 lb/acre to 
0.375 lb/acre, respectively, based on the lower and upper limits of the labeled rates.  In 1999, the 
most recent year for which statistics are available, the Forest Service used a total of 3.8 lbs of 
sethoxydim on 13 acres for an average application rate of about 0.3 lbs/acre (USDA/FS/FH 
2000).  This will be taken as the typical application rate for this risk assessment. 
Mixing volumes for sethoxydim vary only modestly depending on the type of vegetation to be 
treated as well as the application method.  For ground applications of Poast, 5 to 20 gallons of 
water per acre are recommended but not less than 10 gallons per acre in the western and 
southwestern United States (BASF 2000). 
For this risk assessment, the extent to which a formulation of sethoxydim is diluted prior to 
application primarily influences dermal and direct spray scenarios, both of which are dependent on 
‘field dilution’(i.e., the concentration of sethoxydim in the applied spray).  In all cases, the higher 
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