Critical time of weed removal in glyphosate-resistant Glycine max

Field experiments were conducted in 1996 and 1997 to determine the effect o f the rate and time of glyphosate application on weed emergence, survival, biomass, and Glycine max yield in reduced-tillage (RT) and no-tillage (NT? glyphosate-resistant G. max planted in rows spaced 18 (narrow-row) and 76 c m (wide-row). Glyphosate was applied at 0.42, 0.63, and 0.84 kg ae ha(-1) a t V2, V4, R1, and R4 growth stages. On separate plots, 0.84 kg ha(-1) glyph osate was applied at each growth stage with hand weeding, A weed-free check was maintained with preemergence imazethapyr plus metolachlor supplemented with hand weeding, and a nontreated check was included. Weed population de nsity before glyphosate application ranged from 239 to 606 plants m(-2) in RT and 33 to 500 plants m(-2) in NT systems. Setaria faberi and Chenopodium album were the predominant species. Weed control efficacy and crop yield w ere influenced more by application time than by glyphosate rate. Glyphosate applied at V2, V4, and R1 gave season-long control of weeds in 18-cm rows. In 76-cm rows, glyphosate applied at V2, V4, and Ri gave almost complete c ontrol of weeds, but broadleaf weeds emerged after application at V2. The c ritical time of weed removal, the time beyond which weed competition reduce d G. max yield by 3% or more compared to the weed-free check, was at R1 and V4 in 18-cm RT G, max in 1996 and 1997, respectively, and at V2 in 76-cm R T G. max in both years. The predicted critical time of weed removal in 18- and 76-cm NT G. max was R1 and V4, respectively, in 1996 and Ri in 1997. Th is research showed that there was variation in the onset of the critical ti me of weed removal between tillage systems, as well as within tillage syste ms across years. The results indicate a single glyphosate application can p revent yield loss in narrow-row, glyphosate-resistant G. mau under favorabl e conditions, but application timing becomes more critical in wide rows bec ause the critical period of weed removal occurs earlier. Late-emerging weed s may warrant a second glyphosate application in wide-row G. max.