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-1 (H) under bar-imadazol-2-yl]-5)-methyl -3-pyridinecarboxylic acid], nicosulfuron [2- [[[[(4,6-dimethoxy-2-pyrimidi nyl)amino]carbonyl] amino]sulfonyl]-<(N,N)under bar>-dimethyl-3-pyridine-ca rboxamide], and primisulfuron [methyl 2-[[[[[4,6-bis(difluoromethoxy)-2-pyr imidinyl]amino]carbonyl] sulfonyl]benzoate] applied postemergence to johnso ngrass (Sorghum halepense L.) the previous year. Glyphosate at 0.75 Ib ae/a cre, nicosulfuron at 0.031 Ih ai/acre, primisulfuron at 0.035 Ib ai/acre, n icosulfuron (0.016 Ib/acre) tank-mixed with primisulfuron (0.018 Ib/acre), nicosulfuron (0.031 Ib/acre) tank-mixed with primisulfuron (0.035 Ib/acre), or imazameth at 0.064 Ib ai/acre were applied postemergence to 18- to 20-i n. johnsongrass regrowth 2 to 3 wk after mowing. Glyphosate provided the mo st consistent johnsongrass control (85% or greater) 8 wk after treatment (W AT). Tank-mixing nicosulfuron and primisulfuron did not increase control wh en 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 f resh 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 fol lowing year when applied to nonstressed johnsongrass; however, treatments a pplied to moisture stressed johnsongrass did not provide control the follow ing year.