Conversations about the Future of Dicamba: The Science Behind Off-Target Movement

Abstract

Dicamba is an important herbicide for controlling post-emergent resistant weeds in soybean farming. Recently, the scientific community and general public have further examined off-target transport mechanisms (e.g., spray drift, volatilization, and tank contamination) and the visual responses of soybeans to ultralow dicamba concentrations. This paper synthesizes key chemical concepts and environmental processes associated with dicamba formulations, transport mechanisms, drift measurements, and plant responses. This paper proposes additional areas of research and actions to increase our understanding and communicate the science findings, which should provide farmers with more robust tools and practices for sustainable dicamba use.

Impact of volatility reduction agents on dicamba and glyphosate spray solution pH, droplet dynamics, and weed control

“…Dicamba is a weak acid (pKa of 1.87) with most products formulated as dicamba salts of organic amines 12 . Once dissolved in water these dicamba salts of organic amines dissociate into dicamba anion and counterion with the dicamba anion able to combine with any available proton in solution to form the volatile dicamba acid according to the Henderson‐Hasselbalch equation relating a weak acid degree of dissociation to the pKa of the acid and the pH of the solution 8 . The VRAs increase the pH of spray solutions by reducing protons number and availability with some containing acetate buffer and others being pH buffers 8 .…”

“…7 Factors known to favor dicamba off-target movement include dicamba formulations, spray solutions/tank-mix partners, solution pH, droplet size, droplet velocity, and environmental conditions during and following applications. [8][9][10][11] Different dicamba formulations exist with the volatile dicamba acid 12 formulated as a salt with either organic amine bases [dimethylamine (DMA) or diglycolamine (DGA)] or with a metal cation (such as sodium) that neutralizes the dicamba acid and decreases volatility. 8 DMA dicamba salt is more volatile than the DGA dicamba salt.…”

“…[8][9][10][11] Different dicamba formulations exist with the volatile dicamba acid 12 formulated as a salt with either organic amine bases [dimethylamine (DMA) or diglycolamine (DGA)] or with a metal cation (such as sodium) that neutralizes the dicamba acid and decreases volatility. 8 DMA dicamba salt is more volatile than the DGA dicamba salt. 13 The commercialization of dicamba-resistant crops was accompanied by the development of lower volatility formulations namely XtendiMax® with VaporGrip® technology (Bayer Crop Science, St. Louis, MO), Tavium® with VaporGrip® technology (Syngenta Crop Protection, Greensboro, NC), and Engenia® (BASF, Research Triangle Park, NC).…”

“…Dicamba off‐target movement became a major concern 3–5 following the rise in within‐season applications of the herbicide 6 with approximately 1.5 million hectares of soybean fields injured in 2017 7 . Factors known to favor dicamba off‐target movement include dicamba formulations, spray solutions/tank‐mix partners, solution pH, droplet size, droplet velocity, and environmental conditions during and following applications 8–11 …”

The consequences of synthetic auxin herbicide on plant-herbivore interactions

“…Most current knowledge of the direct effects of nontarget, synthetic auxin exposure on plants comes from studies of crop plants with a focus on non-transgenic soybean [14,15,44], which like other members of the Fabaceae [45], is highly susceptible to synthetic auxins [46]. In fact, due to its sensitivity, recent landscape level dicamba drift events have been captured using soybean as the 'canary in a coal mine'.…”