Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) and Waterhemp (Amaranthus rudis) Management in Dicamba-Resistant Soybean (Glycine max)

Abstract

Field experiments were conducted across two locations during 2011 and 2012 to evaluate herbicide options for the control of glyphosate-resistant (GR) giant ragweed and GR waterhemp in dicamba-resistant (DR) soybean. All herbicide treatments provided 91 to 100% control of GR giant ragweed 3 wk after treatment (WAT). Flumioxazin plus dicamba plus glyphosate applied preplant provided greater control and density reduction of GR giant ragweed than flumioxazin plus 2,4-D plus glyphosate. When flumioxazin plus dicamba plus glyphosate were applied preplant, the addition of dicamba to glyphosate at either the early-postemergence (EPOST) or mid-postemergence (MPOST) timing provided greater control and density reduction of GR giant ragweed than glyphosate alone. Regardless of the preplant treatment, delay of EPOST dicamba to the MPOST timing did not influence GR giant ragweed control or density reduction. In the GR waterhemp experiment, dicamba plus glyphosate applied sequentially provided 88 to 89% control and 90% density reduction at the EPOST and MPOST timings compared to only 24% control and 42% density reduction in response to glyphosate applied sequentially. Control and GR waterhemp density reduction did not improve with the addition of acetochlor to either the EPOST or late-postemergence (LPOST) timings. Flumioxazin plus chlorimuron applied PRE followed by dicamba plus glyphosate or dicamba plus glyphosate plus acetochlor provided greater control of GR waterhemp than glyphosate plus fomesafen or glyphosate alone applied EPOST. Results from this research indicate that dicamba applied once or sequentially and when timed appropriately to match the biology of the weed species can be utilized as a component of an integrated program for the management of GR weeds like giant ragweed and waterhemp in DR soybean.