Johnsongrass Response to Postemergence Herbicides Applied the Previous Year

Field experiments were conducted in West Virginia (1992–1994) and Georgia (1995–1996) to evaluate the effects of glyphosate, imazameth [2‐[4,5‐dihydro‐4‐methyl‐4‐(1‐methyl‐ethyl)‐5‐oxo‐1H‐imadazol‐2‐yl]‐5)‐methyl‐3‐pyridinecar‐boxylic acid], nicosulfuron [2‐[[[[(4,6‐dimethoxy‐2‐pyrimidinyl)amino]carbonyl] amino]sulfonyl]‐N,N‐dimethyl‐3‐pyridinecarboxamide], and primisulfuron [methyl 2‐[[[[[4,6‐bis(difluo‐romethoxy)‐2‐pyrimidinyl]amino[carbonyl]amino] sulfonyl]benzoate] applied postemergence to johnsongrass (Sorghum halepense L.) the previous year. Glyphosate at 0.75 lb ae/acre, nicosulfuron at 0.031 lb ai/acre, primisulfuron at 0.035 lb ai/acre, nicosulfuron (0.016 lb/acre) tank‐mixed with primisulfuron (0.018 lb/acre), nicosulfuron (0.031 lb/acre) tank‐mixed with primisulfuron (0.035 lb/acre), or imazameth at 0.064 lb ai/acre were applied postemergence to 18‐ to 20‐in. johnsongrass regrowth 2 to 3 wk after mowing. Glyphosate provided the most consistent johnsongrass control (85% or greater) 8 wk after treatment (WAT). Tank‐mixing nicosulfuron and primisulfuron did not increase control when compared to nicosulfuron applied alone. Imazameth and primisulfuron did not control johnsongrass over 81% in 1993 or 1995 at 8 WAT. Glyphosate and nicosulfuron applied alone the previous year gave greater than 70% control of johnsongrass regrowth in two out of three experiments. Stem counts and fresh weights indicated that treatments reduced regrowth in 1993 but not in 1994 at 53 WAT. Glyphosate reduced stem counts and fresh weight in two out of three experiments. Most treatments reduced johnsongrass regrowth the following year when applied to nonstressed johnsongrass; however, treatments applied to moisture stressed johnsongrass did not provide control the following year.Research QuestionField corn producers who plant in fields infested with johnsongrass have used crop rotation, tillage, and nonselective herbicides for control. In the early 1990s, postemergence herbicides were developed that effectively controlled johnsongrass with over‐the‐top applications without injuring corn. Additionally, biotechnology researchers have developed corn cultivars that can withstand herbicides that injure or kill conventional cultivars. Since these herbicides are systemic, they may reduce the infestation level the following growing season. This study evaluated four herbicides for controlling johnsongrass the year following application.Literature SummaryStudies have been conducted to evaluate herbicides for controlling johnsongrass in various crops. Most studies integrate production management tools with herbicides. Rotations that include broadleaf crops allow the use of selective postemergence herbicides for controlling johnsongrass. Recent herbicide and cultivar developments make johnsongrass control in field corn possible. However, few studies evaluate herbicide impact on rhizome johnsongrass populations the following season.Study DescriptionField experiments were conducted in West Virginia (1992 through 1994) and Georgia (1995 through 1996) to evaluate the effects of glyphosate, imazameth, nicosulfuron, and primisulfuron applied postemergence to johnsongrass the previous year. The West Virginia experiment was conducted from 1992 through 1994 on a Lily loam and the Georgia experiment was conducted from 1995 through 1996 on a Cecil sandy loam. Glyphosate at 0.75 lb ae/acre, nicosulfuron at 0.031 lb ai/acre, primisulfuron at 0.035 lb ai/acre, nicosulfuron (0.016 lb/acre) tank‐mixed with primisulfuron (0.018 lb/acre), nicosulfuron (0.031 lb/acre) tank‐mixed with primisulfuron (0.035 lb/acre), or imazameth at 0.064 lb ai/acre were applied postemergence to 18‐ to 20‐in. johnsongrass regrowth 2 to 3 wk after mowing. Johnsongrass control was evaluated at 8 and 50 wk after treatment (WAT) using visual ratings where 0 = no effect and 100 = plant death. Johnsongrass stems from a randomly selected area in each plot (9 sq ft/plot) were counted (all locations) and, in the West Virginia experiment, harvested for fresh weight 53 WAT.Applied QuestionsDid the herbicide treatments effect rhizome johnsongrass growth the year following application?Glyphosate and nicosulfuron applied alone gave greater than 70% control of rhizome johnsongrass in two out of three experiments. Glyphosate was the most effective treatment (two out of three experiments) for reducing stem counts the second year. There was no advantage in tankmixing primisulfuron with nicosulfuron compared with nicosulfhon applied alone. Imazameth and primisulfuron were generally not as effective as the other treatments.Did the herbicide treatments respond differently under variable moisture conditions?Most treatments gave 74% or greater control of johnsongrass 8 WAT when applied under good growing conditions to plants that did not exhibit moisture stress. However, in 1993, treatments were applied to johnsongrass that had received very little rainfall 30 d prior to application and control was reduced. The following season, rhizome johnsongrass production was not different between the nontreated control and all herbicide treatments.