Characterization of Triazine-Resistant Giant Foxtail (Setaria faberi) and its Control in No-tillage Corn (Zea mays)

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Abstract
Triazine-resistant giant foxtail was identified in 1984 near Delta, PA. In field studies conducted from 1985 to 1987, preemergence applications of atrazine with cyanazine or simazine provided poor (≤60%) season-long control of this annual grass in no-tillage corn. Best season-long preemergence control was obtained with metolachlor or microencapsulated formulations of alachlor or EPTC. Postemergence applications of cyanazine or tridiphane + atrazine + crop oil provided poor giant foxtail control. Postdirected applications of paraquat resulted in fair (≥70%) control of giant foxtail through midseason. In greenhouse studies, triazineresistant (R) giant foxtail tolerated preemergence applications of atrazine or simazine at dosages to 9.0 kg ai/ha. Triazine-sensitive (S) giant foxtail was injured by 2.2 kg/ha and higher rates of atrazine and simazine. In laboratory studies, the I50 for inhibition by atrazine of photoinduced electron transport in thylakoids isolated from S and R biotypes was determined to be 0.24 and 205 μM, respectively. The differential sensitivity was paralleled by simazine. However, the limited solubility of simazine prevented determination of an I50 value with thylakoids from the R biotype.