In Brazil, most of the areas planted with cotton are located in the cerrado regions of Mato Grosso and Bahia, in naturally acidic soil. The objective of this work was to study the influence of soil acidity on the persistence of trifloxysulfuron-sodium and pyrithiobac-sodium, using a bioindicator as detection technique. Four experiments were conducted simultaneously: two with trifloxysulfuron-sodium (7.5 and 15 g ha-1), and two with pyrithiobac-sodium (70 and 140 g ha-1), using a completely randomized design with four replications in a factorial scheme (3x8+3). Treatments were composed by three types of soil (at pH levels 4.2, 4.9 and 5.5), eight sowing dates for the bio-indicator (0, 30, 60, 90, 120, 150, 180, and 210 DAA), and three control treatments, without herbicide application. The persistence of trifloxysulfuron-sodium at doses of 7.5 g ha-1 was not influenced by the pH values. However, at a dose of 15 g ha-1 the highest increase in the persistence of biological activity was observed in soil with higher pH. When pyrithiobac-sodium was applied to the soil with lower pH, at doses of 70 and 140 g ha-1, a longer time period was needed for persistence to reduce. But at 210 DAS there were no differences in the persistence of pyrithiobac-sodium (70 g ha-1) between the different pH values. However, at the dose of 140 g ha-1 of pyrithiobac-sodium, the soil with the highest pH showed the highest persistence of this herbicide.
bio-indicator; carryover; ALS inhibitors; soil pH; residual activity
