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-methylethyl)-5-oxo-1H- imadazol-2-yl]-5)-methyl-3-pyridinecarboxylic acid], nicosulfuron [2- [[[(4,6-dimethoxy-2-pyrimidinyl)amino] carbonyl] amino]sulfonyl]-N,N-dimethyl-3- pyridinecarboxamide], and primisulfuron [methyl 2-[[[4,6-bis (difluromethoxy)-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.