Influence of Dicamba and Dicamba plus Glyphosate Combinations on the Control of Glyphosate-Resistant Waterhemp (Amaranthus rudis)

A total of four field experiments were conducted over a 2-yr period (2011 and 2012) near Mokane and Moberly, Missouri, to determine the control of glyphosate-resistant (GR) waterhemp with dicamba and glyphosate applied alone or as a tankmix combination. In one experiment, dicamba was applied at 0.14, 0.28, 0.42, and 0.56 kg ae ha(-1) with or without 0.86 kg ae ha(-1) glyphosate to GR waterhemp plants 7.5, 15, and 30 cm in height. In a second experiment, sequential treatments of dicamba or dicamba plus glyphosate were applied 4, 7, and 14 d after the initial herbicide treatment to plants measuring either 7.5 or 23 cm in height. Control of GR waterhemp ranged from 7 to 62%, 11 to 40%, and 8 to 30% when applied to 7.5-, 15-, and 30-cm plants, respectively. Control of 7.5-cm GR waterhemp increased by 16 to 36%, and biomass reduction increased by 29 to 52% in response to 0.14, 0.28, 0.42, and 0.56 kg ha(-1) dicamba plus glyphosate compared to these same rates of dicamba alone. When sequential dicamba-containing treatments were averaged across all treatments and application timings, GR waterhemp control ranged from 46 to 47 %, and biomass reduction ranged from 55 to 66 %. No differences in control were observed based on the timing of the sequential herbicide treatment. However, in terms of GR waterhemp biomass reduction, sequential treatments applied 4 d after the initial treatment reduced GR waterhemp biomass more than sequential treatments applied 14 d after the initial treatment. Results from these experiments indicate that, in the absence of crop competition, a single treatment of dicamba up to 0.56 kg ha(-1) provides less than 62% control of GR waterhemp, and sequential dicamba plus glyphosate treatments targeting 7.5 cm plants are required to achieve at least 72% control.