Don't Forget Water Quality for Spray Mixtures

Resistance management in the weed control arena has become increasingly important–maybe the most important issue to consider when weed control programs are designed for soybean production systems.

When a herbicide fails to kill a targeted weed for which it is labeled in today’s weed control programs, the first reason offered is that the weed has become resistant to the herbicide. And in many cases, and maybe most, this is likely. After all, this phenomenon has been documented in numerous weed species in all US soybean production regions.

However, in the rush to come to this conclusion, there are other things that should at least be considered.

One is the quality or chemistry of the water that serves as the delivery system for a herbicide. After all, water is the primary carrier for herbicide applications, it makes up more than 95% of the spray volume when they are applied, and its properties can greatly influence the performance of numerous herbicides.

Below are items that can affect how the quality of the water used to mix herbicides may affect herbicide performance.

Water or spray solution pH. The pH value is a measure of acidity or alkalinity of a solution. Most herbicides perform best in water that is slightly acidic to slightly alkaline.

When mixed in water, a portion of the herbicide molecules will dissociate. The herbicide molecules that do not dissociate are more readily absorbed by foliage of the target plant. How much the herbicide dissociates depends primarily on the pH of water used to make the spray mixture.

Cations and water quality or “hardness”. The presence of cations such as K+, Ca++, Mg++, Na+, Fe++, etc. can interact with the herbicide structure to form complexes that will subsequently reduce absorption of the herbicide into the plant.

A recommended water conditioner that is both economical and effective is ammonium sulfate (AMS). AMS functions in a dual role: the sulfate ion will bind with the above cationic minerals in the spray water, and the ammonium ion binds with many herbicides to form an optimum ammonium-herbicide salt that will enhance herbicide absorption by the leaf and thus increase herbicide efficacy.

Click here for a formula from North Dakota State Univ. that can be used to calculate the amount of AMS to add to the spray mixture based on water quality tests that determine the amount of the above cations present in the water used for the spray mixture. Click here for a Purdue Univ. publication that shows an example calculation using this formula.

Carbonates and bicarbonates. These compounds will contribute to alkalinity of water, and in high concentrations (>500 ppm) can reduce herbicide efficacy, especially of grass and auxin herbicides. AMS can be used to overcome their negative effect.

Turbidity. This water property results from the presence of suspended solids, usually soil and organic matter. These particulates can bind to a herbicide in the solution and lower its efficacy. All classes of herbicides are not similarly affected.

Herbicide label. The herbicide label may give specific instructions about using additives to correct water “hardness”, using or not using surfactants or other additives to alter pH, and the optimum range of pH for the spray solution. Even if the label does not address any of these factors, it is still important to consider water quality when making a spray mixture.

So here are three things to consider doing before starting herbicide applications this spring.


  • Producers and pesticide applicators should strive to use water from the same source for each tank mix with all herbicides that are used. This is important since water from each source is unique in its chemistry and turbidity.

  • At the beginning of each season, have the water from this source tested, especially for pH, iron level, and water hardness. Then, use the results from this test to determine any additives or adjuvants that should be added to the mixture according to stipulations in the label of the chemical you are applying. Also, ask your ag pilot to test the water they use to ensure that they also add the proper additives or adjuvants to mixtures they apply on your farm.

  • If the quality of the water used for a spray solution is tested and its chemistry is known to be within acceptable limits for mixing an intended herbicide, then an occurrence of poor weed control likely did not result from poor water quality.


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Click here and here for publications from Purdue University that provide detailed discussions of how water quality can affect herbicide performance. Click here for a PowerPoint presentation from the University of Florida that provides a concise summary of water quality issues as related to spray mixtures.

I realize that verifying the quality of water that is used when making a herbicide mixture may seem like a small thing to consider. However, in the quest to ensure that effective weed control is achieved with each herbicide application, and that resistance to an applied herbicide is not automatically assumed if expected control is not achieved, it is a good idea to cover all the bases when it comes to applying herbicides.

Composed by Larry G. Heatherly, Jan. 2016, larryheatherly@bellsouth.net

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