How water quality affects herbicide efficiency

Recently, I read a chemistry quote that said, “You either have to be part of the solution, or you’re going to be part of the precipitate”. Now, I know that’s a play on Eldridge Cleaver’s famous words (You either have to be part of the solution, or you’re going to be part of the problem), but when mixing herbicides with spray water, it’s a good idea to think about it that way. As far back as the 1990s, research has demonstrated that the solubility and efficacy of some herbicides can be adversely affected by the quality of water used to dilute them.

So, what are the key parameters to watch out for?

The pH measures the level of acidity or alkalinity. A pH of 7.0 is neutral. If the pH is lower than 7.0, it is acidic, higher than 7.0, it is alkaline. Herbicides such as glyphosate, 2,4-D, dicamba, and many others become negatively charged at alkaline pH. In this condition, they are more susceptible to being tied up by positively charged ions such as those of Calcium (Ca²⁺), Magnesium (Mg²⁺), and iron (Fe²⁺, Fe³⁺), and to form complexes that are not easily absorbed by the plant, thus reducing the effectiveness of the herbicide. Extreme water pH levels can also reduce the solubility of some herbicides or cause the herbicides to break down faster, and not be as effective.

Water hardness is caused by positively charged ions of mostly calcium and magnesium, and sometimes iron and sodium. These positively charged ions can bind to negatively charged herbicides such as glyphosate and 2,4-D amine thereby reducing herbicide efficacy.

Bicarbonate is known to reduce the activity of 2,4-D amine and the “dim” group of herbicides – tralkoxydims (such as Achieve), sethoxydims (such as Poast), and clethodims (such as Centurion and Select). Bicarbonate levels as low as 500 parts per million can interfere with herbicide effectiveness.

Cleanliness/turbidity refers to how much suspended matter is in the water. Soil and organic matter particles can bind onto and reduce the effectiveness of herbicides such as glyphosate, dicamba, diquat, bromoxynil and paraquat. These particles can also block spray nozzles and affect the delivery of the product.

Cost of herbicides can be substantial, and using water of poor quality will reduce the return on investment. Besides, anything that lessens the efficiency of herbicides can lead to poor weed control and significant yield loss.  Moreover, having herbicides tied up by cations in water will result in lower-than-recommended rates of herbicides and possible herbicide resistance. So, it is important to test spray water to determine its suitability for herbicide dilution. Where laboratory services are difficult to access, a good place to start is to test the electrical conductivity (EC) of the water using a conductivity meter. The EC standardized at 25 ⁰C gives a useful approximation of the Total Dissolved Solids (TDS). If EC is less than 500 µS/cm, it is unlikely that the efficacy of herbicides will be affected. Where EC is higher than 500 µS/cm further tests are necessary to confirm the culprit ions.

If your water source is of poor quality, adding water conditioners can be helpful. Some conditioners can modify the pH of the spray water to better suit the specific herbicide. Some of these products can also bind up the problematic cations so that herbicides don’t get tied up. Be sure to talk with your local SynergyAG rep for the right options.

-Ikenna Mbakwe, PhD, PAg
Head of Research
SynergyAG