EFFECTS OF PREEMERGENCE HERBICIDES ON THE COMPETITIVENESS OF SELECTED WEEDS

Experiments were conducted to evaluate the effects of preemergence herbicides on the growth characteristics and competitiveness of sicklepod and common cocklebur to soybean. Alachlor was applied at 1.1 or 2.2 kg ai ha-1 and metribuzin at 0.2 or 0.4 kg ai ha-1 in glasshouse and field experiments. Alachlor at 2.2 kg ha-1 reduced sicklepod dry weight and leaf area 4 weeks after planting. Sicklepod water use was initially lowered by both rates of alachlor while soybean and common cocklebur water use was initially lowered by metribuzin at 0.4 kg ha-1. Sicklepod and common cocklebur fresh weights were reduced at soybean harvest by the high rates of alachlor and metribuzin. Soybean competing with sicklepod injured by 1.1 or 2.2 kg ha-1 of alachlor produced yields greater than soybean competing with untreated sicklepod. Soybean yields were not improved when metribuzin was used on common cocklebur.