Field experiments were conducted at four locations (Larissa, Halkidona, Thessaloniki, and Halastra) in Greece to evaluate weed and cotton response to various pyrithiobac rates applied preplant incorporated (PPI), preemergence (PRE), or postemergence (POST). Pyrithiobac applied PPI or PRE at 0.068, 0.102, or 0.136 kg ai/ha controlled black nightshade, pigweeds, and common purslane at Larissa. However, pyrithiobac applied PRE at Thessaloniki and Halkidona was more effective against black nightshade and pigweeds than pyrithiobac applied PPI. Pyrithiobac applied PPI or PRE at 0.068 or 0.102 kg/ha did not control common lambsquarters at Thessaloniki. Weed control with trifluralin plus fluometuron applied PPI and alachlor plus fluometuron applied PRE at Larissa was slightly lower than that obtained with pyrithiobac. At Halkidona, trifluralin plus fluometuron applied PPI and alachlor plus fluometuron applied PRE provided weed control similar to that obtained with pyrithiobac. But at Thessaloniki, these treatments provided better weed control than pyrithiobac. Furthermore, pyrithiobac applied early postemergence (EPOST), midpostemergence, or in sequential systems controlled black nightshade and pigweeds, but it resulted in fair to good control of common purslane, velvetleaf, and common cocklebur. None of the POST treatments controlled common lambsquarters. Fluometuron EPOST controlled black nightshade, common lambsquarters, and common purslane ≥70, 86, and 67%, respectively. Fluometuron EPOST did not control pigweeds, velvetleaf, and common cocklebur. Cotton treated with pyrithiobac, regardless of method of application, yielded similar to the weed-free control. Cotton treated with pyrithiobac PPI at the highest rate (0.136 kg/ ha) yielded less at Halkidona, although adverse effects after its application were not visually apparent. Yield of cotton treated with herbicides was similar, with no difference among treatments.
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 (K d) 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.
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