Experiments were initiated to determine the amount of time required for postemergence herbicides to render yellow nutsedge physiologically noncompetitive. The rate of net CO₂ assimilation (A_N) was chosen as the response variable to describe competitiveness. Specifically, the time required after herbicide treatment for A_N to drop to 50% of that of the untreated control (A_N50) was determined. A_N50 values for halosulfuron, imazapic, glyphosate, and MSMA were 1.6, 2.1, 3.2, and 3.3 d, respectively. An A_N50 value was not calculated for bentazon because A_N rapidly decreased below 50% but recovered to > 50% by 9 d after treatment (DAT). Stomatal conductance to water vapor (g_s) declined similarly with A_N over time for halosulfuron, imazapic, and glyphosate treatments. However, g_s of MSMA-treated plants was near 95% of the untreated control, whereas A_N declined to 35% 11 DAT. At 11 DAT, all aboveground biomass was removed, and plants were returned to the greenhouse, and regrowth was determined after an additional 14 d. Yellow nutsedge regrowth for halosulfuron, imazapic, glyphosate, and MSMA was below 5% of the untreated control and was not statistically different. However, regrowth from bentazon was 44% of the control. Therefore, bentazon was the least effective herbicide tested, whereas halosulfuron and imazapic were most effective for yellow nutsedge control.

Nomenclature: Bentazon; glyphosate; halosulfuron; imazapic; MSMA; yellow nutsedge, Cyperus esculentus L. CYPER.