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PRODUCTION PAPERS

Soybean Development and Yield as Affected by Three Postemergence Herbicides

^a Dep. of Plant, Soil, and Gen. Agriculture, Center of Excellence for Soybean Research, Teaching, and Outreach, Southern Illinois Univ., MC 4415, Carbondale, IL 62901-4415
^b Iowa State Univ., 2104 Agronomy Hall, Ames, IA 50014
^c Crop Sciences Dep., Univ. of Illinois, Urbana, IL 61801

^* Corresponding author (bgyoung{at}siu.edu).

Received for publication December 9, 2002. 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 no-herbicide/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.

Abbreviations: a.e., acid equivalent • a.i., active ingredient • DAA, days after application • EPOST, early postemergence • LAI, leaf area index • LPOST, late postemergence