Dicamba or 2,4-D will be used POST for the control of weeds in soybean when dicamba- or 2,4-D-resistant soybean are commercialized. The active ingredients of both herbicides are weak acids in solution and may bind to cations present from hard water used as herbicide carrier or from foliar fertilizers added to spray solutions. The objectives of this research were (1) to determine if the efficacy of dicamba or 2,4-D are influenced by divalent cations, namely calcium (Ca), magnesium (Mg), manganese (Mn), and zinc (Zn), in the spray solution, and (2) to determine if adding ammonium sulfate (AMS) to the spray solution can overcome antagonism. The factorial study included five cation solutions (deionized water [dH2O], Ca at 590 mg L−1, Mg at 630 mg L−1, Mn at 4.97 L ha−1, and Zn at 2.33 L ha−1), two herbicide treatments (dicamba or 2,4-D), and two water conditioner treatments (without or with AMS at 20.37 g L−1). Treatments were applied to common lambsquarters, horseweed, and redroot pigweed. Control of horseweed and redroot pigweed increased when AMS was added to the 2,4-D treatments, irrespective of cation solution. Control of common lambsquarters was increased when AMS was added to 2,4-D for only the Ca and Mn cation solution. In contrast to the results obtained with 2,4-D, control of horseweed with dicamba was not influenced by cation solution. Tank-mixing AMS with dicamba increased control of both redroot pigweed and common lambsquarters in the dH2O, Mg, and Mn solutions.

With the forthcoming release of the 2,4-D- and dicamba-resistance traits stacked with either glyphosate or glufosinate resistance, the use of 2,4-D or dicamba alone or in tank mix with glyphosate or glufosinate likely will increase the control of glyphosate-resistant weeds in soybean. There also is an increasing trend among soybean growers to apply POST herbicides in combination with fungicides, insecticides, and fertilizers to reduce trips over the field. Greenhouse experiments were conducted during 2011 and 2012 to evaluate glyphosate or glufosinate applications with growth regulator herbicides and other agrochemicals for the control of glyphosate-resistant horseweed and glyphosate-resistant common lambsquarters. In most cases, glyphosate or glufosinate application with 2,4-D or dicamba provided 80% or more control of glyphosate-resistant horseweed and glyphosate-resistant common lambsquarters. These studies demonstrate that performance of glufosinate alone and with agrochemicals was poor on glyphosate-resistant common lambsquarters. However, no differences in glyphosate-resistant common lambsquarters biomass were noted among treatments including glufosinate alone, glufosinate plus growth regulator herbicides, and glufosinate plus growth regulator herbicides plus agrochemicals. The agrochemicals lambda-cyhalothrin, manganese, and pyraclostrobin did not affect weed control by glyphosate or glufosinate combinations with growth regulator herbicides. Visible soybean injury was noted at 1 wk after treatment (WAT) only when glufosinate was applied with lambda-cyhalothrin or pyraclostrobin but no differences in visible injury were seen with these combinations at 3 WAT.

Field studies were conducted in central Missouri and central Kansas to evaluate the crop tolerance and efficacy of various combinations of atrazine, flufenacet + isoxaflutole, flumetsulam + clopyralid, isoxaflutole, andS-metolachlor applied PPI or PRE in conventional-till corn. Application technique did not influence crop injury in Kansas. In Missouri, greater crop injury was observed with treatments containing isoxaflutole when applied PPI vs. PRE. Application technique influenced giant foxtail, ivyleaf morningglory, large crabgrass, Palmer amaranth, and common waterhemp control. In dry years, control of these weeds was usually either same or greater with PPI than it was with PRE treatments. In years with average to above average precipitation, isoxaflutole provided greater control as a PRE application than as a PPI application. Palmer amaranth and common waterhemp control was usually greater with atrazine, isoxaflutole, andS-metolachlor applied PRE than it was applied PPI. Differences in control of all weeds between PPI and PRE applications were less obvious with two or three herbicides compared with treatments with a single herbicide. In general, the corn yield was greater with most of the treatments having two, three, or four herbicides than it was with treatments having a single herbicide, which was due to better weed control with the tank-mixtreatments.

Experiments were conducted in 2008, 2009, and 2010 to determine the influence of water source as carrier and other agrochemicals on glyphosate efficacy and physicochemical compatibility. Glyphosate efficacy was not affected by most water sources, when compared with deionized water, although response was not consistent across all weed species, including cereal rye, common lambsquarters, common ragweed, goosegrass, Italian ryegrass, large crabgrass, Palmer amaranth, tall morningglory, and wheat. Control by glyphosate was not negatively affected when coapplied with cloransulam-methyl, dicamba, flumioxazin, pyrithiobac-sodium, thifensulfuron-methyl plus tribenuron-methyl, trifloxysulfuron-sodium, and 2,4-D but was affected by acifluorfen and glufosinate. Calcium, manganese, and zinc solutions consistently reduced weed control by glyphosate, whereas boron seldom affected efficacy. Compared with deionized water, Italian ryegrass control was affected by water sources when applied at seedling and jointing stages more so than at tillering and heading growth stages. Calcium, manganese, and zinc reduced control regardless of growth stage. Precipitates were not produced when glyphosate was applied with the water sources or fertilizer solutions. However, transient precipitates developed when glyphosate was coapplied with cloransulam-methyl, flumioxazin, thifensulfuron-methyl plus tribenuron-methyl, and trifloxysulfuron-sodium but not when coapplied with acifluorfen, dicamba, glufosinate, pyrithiobac-sodium, and 2,4-D. Solution pH ranged from 4.11 to 5.60 after glyphosate was added, regardless of solution pH before glyphosate addition.

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