Mobility of the herbicide pyrithiobac through intact soil columns

The relative mobility of pyrithiobac [sodium 2-chloro-6-(4,6- dimethoxypyrimidin-2-ylthio)benzoate], a new herbicide used for postemergence control of broadleaf weeds in cotton (Gossypium hirsutum), was evaluated and compared against that of bromide (Br^-) tracer on four soils representative of cotton-growing regions using intact soil columns under saturated flow conditions. Pyrithiobac breakthrough curves were asymmetrical in shape with significant tailing and displaced to the left of I pore volume in the Houston Black clay (fine, montmorillonitic, thermic Udic Pellustert), Orelia fine sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualfs), and ships silty clay (veryfine, mixed, thermic Udic Chromustert) soils. Breakthrough of pyrithiobac in the Hidalgo sandy loam soil (fine-loamy, mixed, hyperthermic Typic CalciustoII) was delayed and more symmetrical, with peak pyrithiobac concentration reached after 1.2 pore volumes. The immobile pore water (IPW) fractions estimated from the Br^- breakthrough curves ranged from 20 to 87% of total pore water. The IPW values demonstrated that soils with the greatest amount of IPW (Ships with IPW = 87.3%) exhibited the most rapid movement of pyrithiobac (peak concentration after 0.04 pore volume). The experimentally determined pyrithiobac breakthrough curves confirmed the high mobility of this herbicide in these alkaline and predominantly smectitic soils. These results indicate that pyrithiobac mobility was influenced by soil type and preferential flow processes when leached through intact soil columns.