Interaction of Pyrithiobac and Graminicides for Weed Control in Cotton (Gossypium hirsutum)1

Field studies were conducted from 1996 through 1998 to evaluate control of broadleaf signalgrass and slender amaranth by clethodim, fluazifop-P-butyl, and fluazifop-P-butyl plus fenoxaprop-P-ethyl applied alone, in combination with pyrithiobac, or sequentially with pyrithiobac. Broadleaf signalgrass control with graminicides alone was 75 to 100%. Although broadleaf signalgrass control with clethodim was not reduced by pyrithiobac, control with fluazifop-P-butyl and fluazifop-P-butyl plus fenoxaprop-P-ethyl was reduced by pyrithiobac. Pyrithiobac controlled slender amaranth 85 to 100% when applied alone. However, slender amaranth control by pyrithiobac was inconsistent when pyrithiobac was applied in mixture or sequentially with graminicides. A reduction in slender amaranth control may have resulted from a combination of reduced graminicide efficacy and interference caused by broadleaf signalgrass with slender amaranth (pyrithiobac alone). Cotton yields were highest when pyrithiobac was applied 24 h after graminicides. Nomenclature: Clethodim; fenoxaprop-P-ethyl; fluazifop-P-butyl; pyrithiobac; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash #3 BRAPP; slender amaranth, Amaranthus gracilis Desf. # AMAVI; cotton, Gossypium hirsutum L. ‘Deltapine 50’, ‘Paymaster H-1215’, ‘DP5690RR’. Additional index words: Amaranthus gracilis, AMAVI, antagonism, Brachiaria platyphylla, BRAPP, herbicide interaction. Abbreviations: POST, postemergence; PRE, preemergence; WAT, weeks after treatment.