Effect of carrier pH on Accent activity

While products intended to modify the pH of water used as a carrier for herbicides have been promoted in the past, we have taken the position that there usually is no need for these products under typical Iowa conditions.  This recommendation was based on the fact that most pH modifiers were promoted to solve a problem that didn't exist - alkaline hydrolysis of the herbicide in the spray tank.  A recent paper in Weed Technology (Green, J. M. and W. R. Cahill.  2003. Enhancing the biological activity of nicosulfuron with pH adjusters.  Weed Technol. 17:338-345.) discusses how the activity of Accent can be improved under certain conditions by increasing the pH of the carrier.  The research of Green and Cahill was inspired by the work of Dr. John Nalewaja at North Dakota State University who studied high pH adjuvants.

To understand how pH influences herbicide performance a basic understanding of chemistry is useful.  The sulfonylurea herbicides (Accent, Classic, Harmony, etc.) are in a class of compounds known as weak acids.  Weak acids are compounds that partially ionize in aqueous solution.  The amount of ionization that occurs is dependent upon the pH of the water.   The higher the pH, the more ionization that occurs.  The water solubility of sulfonylureas increases as they ionize, thus the herbicides dissolve more readily when mixed in water with a high pH.   With Accent, the solubility is 360 ppm at a pH of 5, 12,200 ppm at a pH of 6.9, and 29,200 ppm at pH 8.8.  It is generally believed that herbicides have to be fully dissolved to be absorbed through waxy plant cuticles.

Green and Cahill evaluated whether modifying the water pH with a pH adjuster would affect weed control with Accent.  The adjuster raised the pH from less than 5 to above 8, regardless of the spray additive used, and this increase in pH greatly enhanced the solubility of Accent in the spray solution (Table 1).  In almost all situations the pH adjuster improved the performance of Accent on large crabgrass and common cocklebur (it should be noted that neither of these species are listed on the Accent label as being controlled).   The effect of increasing pH was much greater on cocklebur than crabgrass, with the pH adjuster increasing control by at least 30% when applied with an appropriate spray additive.  The authors found that selection of appropriate additives was critical to fully take advantage of the enhanced solubility gained by raising the water pH (data not shown).  These adjuvants would be classified as 'Basic Blends' by the ASTM.

Table 1.  Influence of spray additives and acidifiers on water pH and Accent activity.   Accent was applied at 0.25 oz/A on crabgrass and at 1.0 oz/acre for cocklebur.   All treatments included 2% AMS.¹

¹Adapted from Green and Cahill.  2003.  Weed Technol. 17:338-345.
²Numbers in red are significantly different than same treatment without an acidifier.

The authors concluded that increasing the pH of the herbicide carrier could increase Accent solubility and biological activity under specific conditions.  The conditions where a benefit  might be seen include:  1) a difficult to control weed species, 2) high use rates of Accent, 3) low spray volumes, and 4) situations where the spray mixture has a low pH.  However, it is interesting to note that the current Accent label states not to include products that will change the pH of the spray solution.

So would there be a benefit in Iowa to adding pH adjusters to increase water pH when using Accent?  Tom Glanville, Department of Agricultural and Biosystems Engineering, reports that most Iowa well water  will reach an equilibrium pH of 7-8 shortly after being pumped.  Water from certain Iowa geologic formations may have a pH of 6-7 due to dissolved CO₂ when first extracted, but the CO₂ escapes quickly into the atmosphere when placed in an open tank.  Thus, for most applications in Iowa there is no need to use a product to raise water pH.  

I suspect that the publication of the Weed Technology article will stimulate the promotion of spray additives intended to modify the pH of the spray solution.  Several commercial products already are marketed for this purpose, but most products I'm familiar with are designed to lower the pH rather than increase it as evaluated in this research.   This is probably why the Accent label restricts use of pH modifiers.   Before purchasing any additive, make sure there is a true need and benefit to the product.

Addendum:  After this article was posted I received a note from Clark Brenzil (Provincial Weed Control Specialist, Saskatchewan) about an interesting situation in Canada.  Apparently after Monsanto introduced Sundance (Maverick in the U.S.) for controlling grasses in wheat they had significant problems with residues remaining in the spray tank and causing injury to other crops.  The active ingredient of Sundance is sulfosulfuron, a sulfonylurea closely related to nicosulfuron.  The problem of sprayer residues was largely resolved by supplying growers with ammonium hydroxide when they purchased Sundance.  The ammonium hydroxide was added to the tank prior to adding the herbicide in order to increase the water pH.  This increased the solubility of sulfosulfuron and reduced the amount of residues that remained in the tank following spraying, therefore reducing problems with damage to other crops when using the sprayer.

Acknowledgement:  Jerry Green, DuPont, provided excellent editorial comments on this article (as well as on earlier articles on this website that have discussed adjuvants).  His tolerance of my general lack of  basic chemistry knowledge is greatly appreciated (I once knew the stuff, just haven't used it enough the past 25 years to retain it).

Prepared by Bob Hartzler, extension weed management specialist