Soybean Development and Yield as Affected by Three Postemergence Herbicides

Field experiments were conducted during 3 yr at four locations in Illinois and three locations in Iowa to evaluate the influence of soybean [Glycine max (L.) Merr.] planting date and postemergence herbicide application timing on soybean injury and grain yield. Glyphosate [N-(phosphonomethyl)glycine] at 1120 g a.e. ha^-1 did not cause visual soybean injury or reduce yield. Acifluorfen [5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid] at 420 g a.i. ha^-1, and imazethapyr [2-[4,5-hydro-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl] -5-ethyl-3-pyridinecarboxylic acid] at 70 g a.i. ha^-1 caused visual soybean injury at 5 to 7 and 21 to 24 d after herbicide application (DAA). Overall soybean injury was greater with the late planting date compared with the early planting date. Chlorosis and stunting at 5 to 7 DAA was greater from acifluorfen and imazethapyr at the early (V2-V3 soybean stage) compared with the late (V5-V6 soybean stage) application timing. Imazethapyr and acifluorfen reduced soybean leaf area index by 5.7 to 14.3% and soybean height by 4.1 to 8.3% at 21 to 24 DAA. Soybean yield was reduced 1.5 and 2.1% by acifluorfen and imazethapyr, respectively, compared with the noherbicide/weed-free plots. Soybean yield averaged across herbicide treatment and application timing was reduced 11% with the late planting date compared with the early planting date.