Greenhouse studies were initiated to determine the duration of time after herbicide treatment required to render johnsongrass physiologically noncompetitive. Nicosulfuron, imazapic, clethodim, and glyphosate were applied to rhizomatous johnsongrass at 35, 70, 140, and 840 g ai ha⁻¹, respectively. Net carbon assimilation, stomatal conductance, chlorophyll meter readings, and maximum (dark adapted) efficiency of photosystem II were measured. Net carbon assimilation (A_N) was assumed to be the best indicator of johnsongrass competitiveness. Johnsongrass was considered to be physiologically noncompetitive when A_N declined below 50% of that of nontreated check. From these data, it was concluded that glyphosate rendered johnsongrass noncompetitive most readily, 4.3 d after treatment, whereas no differences were detected between nicosulfuron, imazapic, or clethodim throughout the experiment. Stomatal conductance (g_s) was highly correlated to A_N and was determined to be an adequate substitute for A_N when determining johnsongrass competitiveness. It was concluded that chlorophyll meter readings and photosystem II efficiency were poor indicators of johnsongrass competitiveness.

Nomenclature: Clethodim; glyphosate; imazapic; nicosulfuron; johnsongrass, Sorghum halepense L., SORHA.