Petri dish bioassays, based on root response of corn grown in soil and perlite, were used to study the activity, adsorption, and leaching of pyrithiobac in a clay, loam, and a clay loam soils containing 2.3, 1.4, and 1.3% organic matter, respectively. Both bioassays indicated that activity of pyrithiobac (reduction of corn root length) increased with increasing herbicide concentration, but in a nonlinear manner, particularly at higher concentrations. Activity of pyrithiobac was similar in clay loam and loam soils, but was lower in clay soil. Adsorption distribution coefficients (Kd) for the clay, loam, and clay loam soils were 0.56, 0.10, and 0.24, respectively. Pyrithiobac leached through all three soils, and biologically available herbicide was detected below 30 cm in all soils; however, the amount leached through the clay soil was lower than that leached through the other two soils. Field persistence of pyrithiobac applied preplant incorporated (PPI), preemergence (PRE), or postemergence (POST) at 68, 102, or 136 g ha−1 was similar in loam and clay loam soils, but was more persistent in clay soil. Pyrithiobac applied POST in clay and loam soils was more persistent than that applied PPI or PRE; however, in clay soil field persistence of POST pyrithiobac was similar with that applied PPI or PRE. Biologically available residues were not detected in 0- to 10-cm soil depth 120 d after any herbicide treatment applied either PPI or PRE in all soils, but this was not the case for pyrithiobac applied POST in the loam soil. Adsorption of pyrithiobac was very low in all three soils, and this was the reason for its increased mobility even below 30-cm depth in all soils. The field persistence of pyrithiobac was generally less than one growing season. However, some pyrithiobac may have moved deeper in the soil and could be harmful to rotational crops after plowing or through capillary movement upward.
Nomenclature: Pyrithiobac; corn, Zea mays L. ‘Pioneer Costanza’; cotton, Gossypium hirsutum L. ‘Stoneville 474’, ‘Hazera Vered’, ‘Deltapine Vulcano’.