Spray mixture pH as affected by dicamba, glyphosate, and spray additives

Thomas C Mueller,Lawrence E Steckel

doi:10.1017/wet.2019.40

Abstract

AbstractThe pH of spray mixtures is an important attribute that affects dicamba volatility under field conditions. This report examined the effect of different components added to water sources that ranged in initial pH from 4.6 to 8.4. Commercial products were used, which include formulations of dicamba, glyphosate, the drift retardant Intact, ammonium sulfate (AMS), and several pH modifiers. Adding BAPMA salt of dicamba always increased the mixture pH, whereas diglycolamine + VaporGrip® (DGA+VG) had a mixed response. The addition of AMS decreased pH slightly (usually <0.5 pH unit), whereas the addition of potassium salt of glyphosate (GLY-K) always decreased the measured pH (from 1.0 to 2.1 pH units). A substantial pH change could have profound effects on dicamba volatility. Moreover, the 1.0 to 2.1 pH units would not be consistent with the registrant’s report stating that GLY-K decreased mixtures with DGA+VG pH by only 0.2 to 0.3 units. The drift retardant Intact had no effect on pH. There was no difference in resultant pH when comparing K salt and isopropylamine (IPA) salts of glyphosate. Spray carrier volume, ranging from 94 to 187 L ha–1, had only a minor effect on measured pH after the addition of various spray components. The addition of selected pH modifiers raised the pH above 5.0, which is a critical value according to the latest dicamba application labels. The order of mixing of various pH modifiers, including AMS, had only limited effect on measured spray solution pH.

Highlights

Dicamba is an ionizable organic acid, and pH affects its behavior
The pH of the spray mixture of dicamba has been implicated as an important factor in potential off-target movement, with solutions lower than pH 5.0 being associated with more off-target movement (Hemminghaus et al 2017; MacInnes 2017)
We found few refereed publications on spray pH, de Carvalho et al (2009) reported that an increase in glyphosate concentration promoted gradual acidification of the spray solution, which stabilized at a pH of 4.5

Summary

Introduction

Dicamba is an ionizable organic acid, and pH affects its behavior. The preponderance of the literature on dicamba and pH effects relates to either degradation using various Fenton reactions (Huston and Pignatello 1999) or the pH effect of sorption phenomena onto various solid matrices, such as clays (Carrizosa et al 2001). The pH of the spray mixture of dicamba has been implicated as an important factor in potential off-target movement, with solutions lower than pH 5.0 being associated with more off-target movement (Hemminghaus et al 2017; MacInnes 2017). No published literature is available reporting the pH of spray mixtures of recently introduced dicamba formulations and the effect of adding various additives to those dicamba mixtures. Monsanto introduced a diglycolamine (DGA) salt of dicamba that includes an acetic acid:acetate pH modifier, referred to in this article as DGAþVG (Hemminghaus et al 2017; MacInnes 2017; Werle et al 2018). BASF introduced the N,N-Bis-(3-aminopropyl) methylamine salt (BAPMA) for use in dicamba-resistant crops. Extensive label restrictions have been placed upon users to attempt to reduce off-target movement of dicamba from either particle or vapor drift (Anonymous 2018a, Anonymous 2018b)

Methods

Results

Conclusion