Does water quality affect herbicide performance?
by Bob Hartzler

April 4 - 2000 - Several herbicide additives are sold on the premise that they will improve herbicide performance by negating negative effects of water used as a carrier.  Two two main processes by which these alleged benefits are achieved are 1) reducing the pH of the water, or 2) encapsulating the herbicide.  This article will discuss possible effects of water quality and benefits of specialty additives to overcome these effects.

Alkaline hydrolysis is often proposed to be a problem with herbicides with water having a high pH (>7).  Alkaline hydrolysis simply is the degradation of a compound under alkaline (high pH) conditions.  The reason certain people promote this problem is that there are a few insecticides that breakdown fairly quickly when placed in alkaline water.  However, there is no evidence that this is a problem with the herbicides (and insecticides) commonly used in Iowa.  The rate of breakdown in water of several herbicides is shown in Table 1.  As would be expected, different herbicides respond differently to water pH.  Most products were relatively stable under the conditions imposed. The sulfonylurea herbicides (Beacon, Pinnacle) actually broke down more quickly under acid conditions than neutral or alkaline conditions.  Assure II was the only herbicide where it appears alkaline hydrolysis could be a potential problem.  Before concluding this is a real problem, keep in mind that no common water sources in Iowa would have a pH approaching 9.  While many sources of water may indeed be alkaline, it would be unusual to find sources where the pH is greater than 7.5.  Thus, it is unlikely that the activity of Assure II would be affected by alkaline hydrolysis unless it was allowed to sit in the spray tank for several days.  Roundup is also reported to be very stable under a wide range of acid and alkaline conditions.   Research has shown there is no need to 'protect' herbicides from alkaline hydrolysis, whether by acidifying the water carrier or encapsulating the herbicide.

Table 1.  Rate of herbicide degradation in water at pH of 5, 7 and 9.

Water pH is not the only factor that can influence herbicide activity.  Water contains many minerals such as calcium, magnesium, sodium and iron.  At high concentrations (hard water) these minerals may interact with some herbicides.  Preemergence herbicides are not affected by water hardness since the amount of minerals in the water carrier is insignificant compared to what is found in the soil.  However, minerals in the water may react with certain postemergence herbicides.  Many postemergence herbicides are a type of compound known as weak acids.  Examples of weak acid herbicides are Roundup, Banvel (and other forms of dicamba), Basagran, and Flexstar.  Weak acids have a position on the parent molecule where a salt is attached by an ionic bond (an ionic bond is formed by the electronic attraction of two molecules having opposite charges.  A covalent bond involves sharing electrons - a much stronger bond than an ionic bond). 

Water quality can influence the activity of weak acid postemergence herbicides by influencing herbicide absorption.  These herbicides are usually formulated as salts, the specific salt used in the formulation influences both the handling characteristic of the product and how readily the herbicide is absorbed.  Certain herbicides are formulated as different salts under different tradenames in order to change the properties of the product (see dicamba).  If the water used as a carrier has a high concentration of salts, some of these minerals may form complexes with the herbicide.  The new herbicide salt complex may may be absorbed less efficiently than the formulated product.  For example, Roundup Ultra contains the isopropylamine salt of glyphosate.  If water containing a high concentration of sodium is used for the carrier, the sodium in the water may replace the isopropylamine molecule on glyphosate.  This sodium salt of glyphosate may not be absorbed by plants as efficiently as the isopropylamine salt, therefore reducing the effectiveness of the herbicide.

There are a couple ways that the antagonism with hard water can be overcome.  The most common method is to add an ammonium source to the spray tank.  I'm not aware if the exact mechanism for ammonium overcoming the hard water induced antagonism has been identified, but some persons have speculated that the ammonium added to the tank binds to the herbicide, therefore preventing binding of the herbicide with other salts.  The ammonium-herbicide complex is believed to have herbicidal properties equivalent to the formulated salt of the herbicide.  This is the rationale for adding AMS to Roundup Ultra and other glyphosate products.

Antagonism due to hard water also can be overcome by reducing the pH of the water.  As the pH is reduced (made more acid), the concentration of hydrogen ions increases.  The hydrogen ions have a positive charge just like the other salts in the water, and at a certain pH these hydrogen ions will bind with the herbicide molecule.  The pKa is the pH at which 50% of a weak acid will complex with hydrogen ions found in the solution rather than with other salts present.  Thus, if the pH of the water is acidified so that it is below the pKa of the herbicide, the majority of the herbicide will bond with hydrogen ions, rather than the salts present in the water.  As with the ammonium salts that form when ammonium is added to the spray tank, the hydrogen-herbicide complex also eliminates or reduces the antagonism caused by the salts in hard water.  Thus, reducing the pH of the water can help overcome problems associated with hard water.  However, the use of ammonium sources has been proven to be more efficient than reducing water pH, thus the use of ammonium (AMS with Roundup) is the recommended practice.

In summary, under certain situations water quality may affect herbicide performance.  However, keep in mind the following facts:

1)  Most water sources in Iowa are high quality and should not pose problems due to hardness or pH,
2)  Alkaline hydrolysis is not a problem with any herbicide commonly used in Iowa,
3)  Preemergence herbicides will not be affected by water hardness, 
4)  The use of AMS with Roundup is more effective at eliminating salt antagonism than lowering water pH, and
5)  No benefits have been found for the process of 'encapsulating' herbicides in oil or any other product prior to mixing with water.

Disclaimer:  This is the second article in the past month that I've tried to explain how basic chemistry (resolved isomers) can influence herbicide activity.  I apologize if some of the descriptions seem a bit fuzzy  - it has been too many years since I took a chemistry class!!

Acknowledgement:  Much of the information in this article was adapted from an article originally prepared by Chris Boerboom, University of Wisconsin.  1995.  Effect of spray solution, mineral concentration, and ammonium additives on herbicide activity.   Wisconsin Crop Manager 2:94-97.  University of Wisconsin Extension.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University