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.