Interaction of graminicides applied in combination with pyrithiobac

Field experiments were conducted from 1992 through 1994 to determine the effect of selected graminicides when applied in combination with pyrithiobac for johnsongrass control in cotton. Quizalofop-P, clethodim, sethoxydim, and fluazifop-P were applied at rates recommended for rhizome johnsongrass control with or without pyrithiobac at 70 g ae/ha. At 14 DAT johnsongrass control without pyrithiobac was more than 85% for all graminicides all years except sethoxydim in 1993. When graminicides were applied in combination with pyrithiobac, johnsongrass control was 86% or less and was lower than when each respective graminicide was applied alone. At 21 DAT, similar differences were apparent. Seed cotton yield, determined only one year, was not affected by reductions in johnsongrass control when either quizalofop or clethodim were used in combination with pyrithiobac. Pyrithiobac applied alone controlled johnsongrass 20 to 44% 14 DAT depending on year.