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Adsorption, leaching, and dissipation of pyroxasulfone and two chloroacetamide herbicides




Westra, Eric P., author
Barbarick, Ken, advisor
Shaner, Dale, committee member
Khosla, Raj, committee member
Schwartz, Howard, committee member

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Pyroxasulfone is a new pyrazole herbicide that controls weeds by inhibiting very long chain fatty acid synthesis. This mechanism of action places pyroxasulfone in the Weed Science Society of America (WSSA) group 15 or Herbicide Resistant Action Committee (HRAC) group K3 herbicides that include s-metolachlor and dimethenamid-p. Dimethenamid-p and s-metolachlor are referred to in literature as acetamide, acetanilide, chloroacetamide, or chloroacetanilide herbicides. In this thesis, these two herbicides are referred to as chloroacetamide herbicides based on the HRAC classification. The soil interactions of pyroxasulfone were evaluated and compared to s-metolachlor and dimethenamid-p to better understand how pyroxasulfone will behave under various field conditions Pyroxasulfone was compared with these two standard herbicides because of their similar mechanisms of action, use patterns, potential for use in similar cropping systems, and similar weed control spectrums. Sorption coefficients were determined for 25 different soils to evaluate relative differences in binding among pyroxasulfone, dimethenamid-p, and s-metolachlor. Across all soil types, the relative order of binding was pyroxasulfone=dimethenamid-p < s-metolachlor. Pyroxasulfone and dimethenamid-p were not statistically different in terms of their binding; however, s-metolachlor binding was statistically greater than both dimethenamid-p and pyroxasulfone. For all three herbicide, organic matter was the only soil property which was highly and significantly correlated to herbicide adsorption; all other soil properties correlated with herbicide adsorption could be explained by the correlation of OM and those soil properties. Based on the water solubility of these three herbicides, we expected the order of binding to be dimethenamid-p < s-metolachlor < pyroxasulfone. This study displayed the unique characteristics of pyroxasulfone in that it has the lowest water solubility of the three herbicides, yet sorption coefficient values indicate that pyroxasulfone is only loosely adsorbed by soil. Reduced soil binding along with a higher unit of activity makes pyroxasulfone a potent herbicide that provides comparable weed control when applied as low as one-eighth of typical application rates for other chloroacetamide herbicides. Field studies were conducted in 2009 and 2010 at two contrasting field sites to evaluate the dissipation and movement of pyroxasulfone and s-metolachlor in the top 30 cm of the soil profile. The site at the horticultural farm (HORT) has a Nunn clay loam soil (Argiustoll), whereas the site at the Limited Irrigation Farm (LIRF) has an Olney fine sandy loam soil (Haplargid). Dissipation half-lives (DT50) were the shortest at the Hort farm site with the heavier textured clay loam soil and increased moisture content. The LIRF site with lighter textured sandy loam soil and decreased moisture resulted in extended DT50 values and more variation between replicates for both herbicides. Across both years and field sites, pyroxasulfone DT50s were approximately twice as long as for s-metolachlor. The extended half-life of pyroxasulfone suggests that it would provide longer weed control compared to s-metolachlor. Herbicide movement in the top 30 cm of the soil profile was greater at the LIRF site which suggests that movement was influenced by soil type to a greater extent than irrigation amount, since the LIRF site received much less total irrigation yet had the most movement downward in the profile. In general, pyroxasulfone moved downward in the profile to a greater extent than s-metolachlor. Observed herbicide movement confirmed sorption coefficient data that shows that pyroxasulfone is bound less to the soil compared to metolachlor and, hence, is more available in the soil solution where the herbicide movement is influenced by mass flow. Extended observed half-lives, reduced soil binding, and increased unit activity indicate that pyroxasulfone is a potent inhibitor of very long chain fatty acid (VLCFA) biosynthesis that can provide comparable weed control for longer periods of time at reduced use rates when compared to commonly used chloroacetamide herbicides.


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