Field studies were conducted in 1998 and 1999 to evaluate the efficacy of glufosinate on Palmer amaranth, redroot pigweed, and common waterhemp at different growth stages in soybean planted at early, normal, and late dates. At 2, 4, and 8 wk after treatment, common waterhemp control was greater than Palmer amaranth and redroot pigweed control with single glufosinate applications of 410 g ai/ha at 2- to 5-, 7- to 10-, or 15- to 18-cm Amaranthus height or with two sequential applications of 293 g/ha at 2- to 5-cm height and 2 wk later. Only the sequential applications of 410 and 293 g/ha resulted in more than 80% control of Palmer amaranth and redroot pigweed, but all five treatments controlled common waterhemp more than 80%. All glufosinate treatments reduced the dry weight of all Amaranthus species by more than 65%. However, the sequential applications resulted in the greatest dry weight reductions.
Nomenclature: Glufosinate; common waterhemp, Amaranthus rudis Sauer #3 AMATA; Palmer amaranth, Amaranthus palmeri S.Wats. # AMAPA; redroot pigweed, Amaranthus retroflexus L. # AMARE; soybean, Glycine max (L.) Merr.
Additional index words: Environmental conditions, herbicide-tolerant soybean, postemergence herbicide.
Abbreviations: ALS, Acetolactate synthase (EC 188.8.131.52); OM, organic matter; RH, relative humidity; WAT, weeks after treatment.