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 canons 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 dH(2)O, 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 ANIS 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 ANIS with dicamba increased control of both redroot pigweed and common lambsquarters in the dH(2)O, Mg, and Mn solutions.
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