Influence of Water Hardness on 2,4-D Efficacy in Field and Greenhouse Weed Control Experiments.

Herbicides are an important tool in any agricultural system where weeds threaten production or reduce crop function. One of the earliest herbicides created was 2,4-D, which is a synthetic auxin, weak acid herbicide used to control broadleaf weeds in turf and grass systems.  Water quality is an important factor known to influence the efficacy of weak acid herbicides, especially glyphosate.  Recent studies have demonstrated that hard water can also antagonize 2,4-D dimethylamine formulations.  The obejective of our field and greenhouse experiments is to evaluate the influence of water hardness levels on the control of broadleaf weeds  by 2,4-D dimethylamine. A greenhouse experiment evaluating the influence of water hardness levels (0, 100, 200, 400, 800 ppm CaCO₃) on 2,4-D efficacy when applied at 1.60 kg ae ha^-1 found that dandelion (Taraxacum officinale G. H. Weber ex Wiggers) and horseweed (Conyza canadensis L.) control by 2,4-D was reduced at 800 ppm CaCO₃ and that this antagonism was overcome by the addition of 20.4 g L^-1 ammonium sulfate into the tank-mixture prior to application. Field experiments were variable, but in general revealed 2,4-D efficacy was improved when ammonium sulfate was added to various hard water sources. In summary, high levels of hard water antagonizes 2,4-D dimethylamine in field and greenhouse experiments and this antagonism can be overcome by adding ammonium sulfate to spray tank-mixtures.