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U OF M EXTENSION SERVICE
Joseph Ikley, NDSU weed management specialist, and Greg Dahl, director of adjuvant education, CPDA, joined UMN Extension IPM educator Eric Yu to talk about how water quality influences herbicide efficacy. This was an episode of the 2026 Strategic Farming: Let’ s talk crops! webinar series.
With the frequency of herbicide resistant weed species on the rise, it is more important that we take action to ensure that herbicides work as well as possible. Spray water sources vary, from private wells, to chlorinated, city water, to pond or river water and knowing the quality of your spray water source can help inform
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whether there is a need to add an adjuvant to the spray mixture to ensure that active ingredients are readily absorbed into plant tissue.
Water hardness, carbonate concentration, turbidity and pH can all impact the ability of some herbicide active ingredients to stay in solution for plant absorption without being inactivated by binding to negatively or positively charged mineral ions in spray water. Water hardness, or the concentration( often in parts per million) of positively charged minerals( cations) such as calcium( Ca2 +), sodium( Na +), potassium( K +) and iron( Fe2 + or Fe3 +) can easily bind to weak acid herbicide active ingredients( such as glyphosate) that have
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a negative charge in solution, rendering them unable to be taken up by plant tissues. When there is a critical mass of cations in a spray water solution, when the water from the solution evaporates, mineral ions bound to herbicides form crystals, oftentimes incapable of being absorbed by the plant. Leading to Dahl’ s memorable catch phrase,“ plants don’ t drink rocks”.
Not using an adjuvant required for an active ingredient to be taken up by plants can reduce the ability to control problematic weeds. Research from Kansas State University compared applying glyphosate alone resulted in between 40 and 73 % control of velvetleaf and volunteer sorghum, sunflower and corn.
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But adding the adjuvant ammonium sulfate( AMS) improved control by 17 to 32 % and adding a surfactant to the glyphosate / AMS mixture added an additional 4 to 5 % control. There are numerous adjuvants that can lower pH, but as Ikley states,“ the quick story about hardness and conditioning water, is that sometimes AMS is all that is needed”. For additional information about spray water quality, water testing, herbicide absorption, and herbicide efficacy with and without the addition of adjuvants, a recording of the session is now available to view on YouTube at z. umn. edu / StrategicFarmingRecordings. |