Hard Water Reduces 2,4-D Efficacy on Dandelion (Taraxacum officinale) and Broadleaf Plantain (Plantago major).

The herbicide 2,4-D is a common ingredient in postemergence broadleaf herbicides labeled for use in turf. It is a weak acid herbicide and may bind to cations present in hard water used as herbicide carrier. The objectives of this research were 1) to determine if 2,4-D efficacy is influenced by divalent cations, namely calcium (Ca), magnesium (Mg), manganese (Mn) and zinc (Zn), in spray solution, and 2) to determine if adding ammonium sulfate (AMS) to the spray solution can overcome antagonism. Experiments were conducted with 2,4-D dimethylamine to evaluate the effect of cation solutions on herbicide efficacy on dandelion (Taraxacum officinale) and broadleaf plantain (Plantago major). Three experimental runs were conducted in a greenhouse. Control of broadleaf plantain was reduced when 2,4-D was applied in a Ca solution in all three runs and by a Mg solution in runs 2 and 3. Likewise, broadleaf plantain leaf coverage, shoot and root weights decreased when AMS was added to the 2,4-D + Ca solution in run 1 and 3 and the Mg solution in run 3. Similarly, dandelion control from 2,4-D was reduced by Ca and Mg solutions on most rating dates in the absence of AMS. Likewise, dandelion leaf coverage, shoot and root weights decreased when AMS was added to Ca and Mg solutions. Some 2,4-D antagonism from Mn was noticed in runs 2 and 3 even when tank-mixed with AMS. There was no 2,4-D antagonism from Zn. In summary, divalent cations can antagonize weak acid herbicides and reduce weed control. Furthermore, AMS will overcome this antagonism when Ca and Mg are the primary cations in solution. Thus, broadleaf weed control in turf with 2,4-D amine can be reduced from hard water, especially water sources containing high Ca and Mg concentrations such as well and municipal water or Mn fertilizers.