Sethoxydim Risk Assessment

toxicity study of Poast in birds is required.  Studies on the toxicity of Poast to birds were not 
encountered in the studies submitted to the U.S. EPA as of January, 2001, the time that the search 
of U.S. EPA’s CBI files was conducted in the preparation of this risk assessment. 
Bryceland et al. (1997) summarize an avian reproduction study by Beavers (1996) in which a 
decrease in the number of hatchlings was observed at dietary sethoxydim concentrations of 100 
ppm and 500 ppm in mallard ducks.  This study is summarized in the U.S. EPA/OPP ecological 
risk assessment of sethoxydim (Bryceland et al.  1997) but was not encountered in a search of the 
U.S. EPA CBI files.  This is the lowest effect level for birds.  Another study in bobwhite quail 
(Munk 1996) reports no effects on reproductive parameters at a dietary concentration of 1000 
ppm.  This study is also summarized in Bryceland et al. (1997). 
4.1.2.3. Terrestrial Invertebrates– Two studies are available on the toxicity of sethoxydim to 
bees: a direct contact study and a spray study, both presented in BASF (1982).  The direct 
contact study is a standard study required by the U.S. EPA for the registration of pesticides.  In 
this study, the compound is dissolved in acetone or some other appropriate vehicle and applied to 
the thorax of groups of bees (typically 1 to 7 day old animals at 50 animals per dose).  The study 
on sethoxydim, however, is not described in detail in BASF (1982).  The only information 
reported is that the direct application of 10 µg sethoxydim/bee (BAS 9052) was not toxic. 
Similarly, the spray study summarized in BASF (1982) is not described in detail.  The summary 
simply indicates that a solution of sethoxydim was sprayed on the bees at a concentration 
simulating exposure to “the maximum recommended use rate”, 2000 ppm (mg/L).  The field 
concentrations that may be used in Forest Service programs are estimated to range from 0.56 to 9 
mg/mL with a typical value of 3.59 mg/mL.  These correspond to concentrations of 560 to 9000 
ppm (mg/L) with a typical value of 3,590 ppm. 
The only other available information on the toxicity of sethoxydim to terrestrial invertebrates 
comes from the publication by Agnello et al. (1986).  In this study, Poast was applied at a rate of 
31-38 L/ha to soybean and lima bean plants.  This application is equivalent to about 8.2-10 
gallons/ha [0.2642 L/gallon] or 3.3-4 gallons/acre [2.471 acres/ha] which is in turn equivalent to 
5-6 lbs/acre  [see Table 2-3, 0.25 gallons/acre = 0.375 lb/acre, 1 gallon/acre = 1.5 lbs/acre]. 
Mexican bean beetle larvae (Epilachna varivestis, Coleoptera:Coccinellidae) were then reared on 
the treated plants.  The only potentially adverse effects noted were a slight increase in days to 
pupation (19.1 days in treated animals compared to 17.7 days in control animals).  In addition, 
there were significant increases in both the number of egg masses as well as total number of eggs 
produced by beetles feeding on sethoxydim treated plants relative to beetles feeding on untreated 
plants.  While a delay in pupation would be regarded generally as an adverse effect, the increases 
in the number of eggs and egg masses would not necessarily be regarded as adverse. 
4.1.2.4. Terrestrial Plants (Macrophytes) – Two types of toxicity studies on terrestrial plants are 
typically required by the U.S. EPA for the registration of herbicides: Tier I and Tier II.  Tier I 
studies are seedling emergence studies and typically involve exposure of seeds in a petri dish and 
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the measurement of the proportion of seeds that emerge after treatment with the herbicide at 
various application rates (mg compound/cm
2
 surface area).  Tier II studies involve both seedling 
emergence assays and assays referred to as vegetative vigor.  In the seedling emergence assay for 
Tier II, exposure is through contaminated soil - i.e., the compound is incorporated into the soil 
and the response of seedlings planted in the contaminated soil is observed.  The vegetative vigor 
assay involves direct foliar spray of the growing plant with subsequent measurements of plant 
growth and survival. 
Both sets of studies have been conducted on sethoxydim.  An early study (Ludwig 1980) was 
identified in the EPA/FIFRA files and a full text copy of this study has been reviewed.  In the U.S. 
EPA’s RED chapter on the ecological effects of sethoxydim (Bryceland et al.  1997), additional 
and more recent Tier I and Tier II studies are described. 
Ludwig (1980) conducted a Tier II study on Poast.  The study specifically states that Poast and 
not just sethoxydim was assayed.  The emergence assays consisted of the incorporation of Poast 
into soil at initial concentrations that mimic application rates of 0.2, 0.4, 0.8, 1.5, and 3.0 lb/acre. 
The species tested in this assay included tomatoes, lettuce, cabbage, carrots, onions, cucumbers, 
corn, soybeans, oats, and perennial ryegrass.  The plants were observed for up to 120 days after 
application.  Oats and ryegrass were severely damaged over the full course of the study at all dose 
rates - i.e., a NOAEL was not determined.  Corn was damaged at day 20 by all doses but 
recovered by day 29 at the dose rates of 0.2 and 0.4 lb/acre. 
In the postemergence application, Poast was sprayed on to the growing plants, again at 
application rates of 0.2, 0.4, 0.8, 1.5, and 3.0 lb/acre.  No effects were seen on tomatoes, carrots, 
lettuce, and onions.  Severe adverse effects were observed at all dose levels for ryegrass, oats, and 
corn. The toxicity of Poast to cabbage, soybeans, and cucumbers was between these most and 
least sensitive plant groups (Ludwig 1980). 
The more recent Tier II study summarized in Bryceland et al. (1997) is consistent with the earlier 
study by Ludwig (1980) but used lower application rates that defined a NOAEL.  The most 
sensitive species in the pre-emergence assay was ryegrass (NOAEL of 0.059 lb/acre and an EC
25 
of 0.065 lb/acre) followed by oats (NOAEL of 0.059 lb/acre and an EC
25
 of 0.197 lb/acre). The 
least sensitive species was corn (NOAEL of 0.235 lb/acre and an EC
25
 of 0.418 lb/acre).  In the 
vegetative vigor (postemergence) assay, corn was most sensitive in terms of the NOAEL 
(NOAEL of 0.0074 lb/acre and an EC
25
 of 0.021 lb/acre) and ryegrass was most sensitive in terms 
of the EC
25
 (NOAEL of 0.025 lb/acre and an EC
25
 of 0.019 lb/acre). 
4.1.2.5. Terrestrial Microorganisms– The effect of sethoxydim on mixed bacterial populations in 
sandy loam have been assayed by Roslycky (1987).  At soil concentrations of 50 ppm (µg/g), no 
remarkable effects were noted.  At 1000 ppm, however, substantial but transient increases were 
noted in the population of actinomycete and various bacteria and slight decreases were noted in 
the population of various fungi.  In a separate series of pure culture studies in artificial media, a 
slight inhibition of oxygen consumption was observed at concentrations as low as 1 ppm (µg/mL) 
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