Growth room studies were conducted to determine the physiological basis of reduced glyphosate efficacy under low soil nitrogen using velvetleaf, common ragweed, and common lambsquarters as model species. Glyphosate dose–response experiments of weeds grown under low (1.5 mM) and high (15 mM) soil N were conducted. Velvetleaf and common lambsquarters grown under low N required 169 g ae ha−1 glyphosate for a significant reduction in biomass, but only 84 g ae ha−1 were required when grown under high N. However, when common ragweed was grown under low or high soil N there was no significant difference in response to glyphosate at all doses tested. The reduced glyphosate efficacy on velvetleaf and common lambsquarters under low N was primarily due to decreased herbicide translocation to the meristem. It appears that low N may decrease the net assimilation of carbon in plants, resulting in a decrease in the net export of sugars and hence glyphosate from mature leaves. Understanding the relationship between soil N and herbicide efficacy may help explain observed weed control failures with glyphosate and may contribute to our knowledge of the occurrence of weed patchiness in fields. This is the first report illustrating a physiological basis for decreased glyphosate efficacy under low soil N in selected weed species.

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.

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.