Quantifying Dicamba Volatility under Field Conditions: Part I, Methodology

Elucidating Factors Contributing to Dicamba Volatilization by Characterizing Chemical Speciation in Dried Dicamba-Amine Residues

Dicamba is a semivolatile herbicide that has caused widespread unintentional damage to vegetation due to its volatilization from genetically engineered dicamba-tolerant crops. Strategies to reduce dicamba volatilization rely on the use of formulations containing amines, which deprotonate dicamba to generate a nonvolatile anion in aqueous solution. Dicamba volatilization in the field is also expected to occur after aqueous spray droplets dry to produce a residue; however, dicamba speciation in this phase is poorly understood. We applied Fourier transform infrared (FTIR) spectroscopy to evaluate dicamba protonation state in dried dicamba-amine residues. We first demonstrated that commercially relevant amines such as diglycolamine (DGA) and n,n-bis(3-aminopropyl)methylamine (BAPMA) fully deprotonated dicamba when applied at an equimolar molar ratio, while dimethylamine (DMA) allowed neutral dicamba to remain detectable, which corresponded to greater dicamba volatilization. Expanding the amines tested, we determined that dicamba speciation in the residues was unrelated to solution-phase amine pKa, but instead was affected by other amine characteristics (i.e., number of hydrogen bonding sites) that also correlated with greater dicamba volatilization. Finally, we characterized dicamba-amine residues containing an additional component (i.e., the herbicide S-metolachlor registered for use alongside dicamba) to investigate dicamba speciation in a more complex chemical environment encountered in field applications.

Evaluation of dicamba volatilization when mixed with glyphosate using imazethapyr as a tracer

Expansion of dicamba-resistant crops increased the frequency of off-target movement issues, especially in the midsouthern United States. Six field trials were conducted over two growing seasons with the purpose to determine the contribution of volatilization and physical suspension of particles to the off-target movement of dicamba when applied with glyphosate and imazethapyr - a non-volatile herbicide used as a tracer for physical off-target movement. Applications included dicamba at 560 g ha-1, glyphosate at 1260 g ha-1, and imazethapyr at 105 g ha-1. Applicators include glyphosate with dicamba to increase the spectrum of weed control from these applications; however, this addition increases potential for dicamba volatilization. Following application of the mixture, air samplers were placed in the field to collect dicamba and imazethapyr. Results showed there was at least 50 times more dicamba than imazethapyr detected even though the dicamba:imazethapyr ratio applied was 5.3:1. Dicamba was detected in the treated area and the off-site locations and all intervals of air sampling, ranging from 126 to 5990 ng. No more than 37.5 ng of imazethapyr was detected during the first 24-h after application (HAA) inside the treated area. Imazethapyr was only detected in 9 of the 20 sampling combinations during these experiments, and most of these detections (6) occurred during the first 24 HAA and inside the treated area. While some movement from the suspension of particles occurred based on the detection of imazethapyr in air samples, results show that most dicamba detection was due to the volatilization of the herbicide.

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

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.

Alginate Nanohydrogels as a Biocompatible Platform for the Controlled Release of a Hydrophilic Herbicide

The large-scale application of volatile and highly water-soluble pesticides to guarantee crop production can often have negative impacts on the environment. The main loss pathways are vapor drift, direct volatilization, or leaching of the active substances. Consequently, the pesticide can either accumulate and/or undergo physicochemical transformations in the soil. In this scenario, we synthesized alginate nanoparticles using an inverse miniemulsion template in sunflower oil and successfully used them to encapsulate a hydrophilic herbicide, i.e., dicamba. The formulation and process conditions were adjusted to obtain a unimodal size distribution of nanohydrogels of about 20 nm. The loading of the nanoparticles with dicamba did not affect the nanohydrogel size nor the particle stability. The release of dicamba from the nanohydrogels was also tested: the alginate nanoparticles promoted the sustained and prolonged release of dicamba over ten days, demonstrating the potential of our preparation method to be employed for field application. The encapsulation of hydrophilic compounds inside our alginate nanoparticles could enable a more efficient use of pesticides, minimizing losses and thus environmental spreading. The use of biocompatible materials (alginate, sunflower oil) also guarantees the absence of toxic additives in the formulation.

Evaluation of a Stable Isotope-Based Direct Quantification Method for Dicamba Analysis from Air and Water Using Single-Quadrupole LC-MS

Dicamba is a moderately volatile herbicide used for post-emergent control of broadleaf weeds in corn, soybean, and a number of other crops. With increased use of dicamba due to the release of dicamba-resistant cotton and soybean varieties, growing controversy over the effects of spray drift and volatilization on non-target crops has increased the need for quantifying dicamba collected from water and air sampling. Therefore, this study was designed to evaluate stable isotope-based direct quantification of dicamba from air and water samples using single-quadrupole liquid chromatography–mass spectrometry (LC–MS). The sample preparation protocols developed in this study utilize a simple solid-phase extraction (SPE) protocol for water samples and a single-step concentration protocol for air samples. The LC–MS detection method achieves sensitive detection of dicamba based on selected ion monitoring (SIM) of precursor and fragment ions and relies on the use of an isotopically labeled internal standard (IS) (D₃-dicamba), which allows for calculating recoveries and quantification using a relative response factor (RRF). Analyte recoveries of 106–128% from water and 88–124% from air were attained, with limits of detection (LODs) of 0.1 ng mL⁻¹ and 1 ng mL⁻¹, respectively. The LC–MS detection method does not require sample pretreatment such as ion-pairing or derivatization to achieve sensitivity. Moreover, this study reveals matrix effects associated with sorbent resin used in air sample collection and demonstrates how the use of an isotopically labeled IS with RRF-based analysis can account for ion suppression. The LC–MS method is easily transferrable and offers a robust alternative to methods relying on more expensive tandem LC–MS/MS-based options.

Quantifying Dicamba Volatility under Field Conditions: Part II, Comparative Analysis of 23 Dicamba Volatility Field Trials

This study summarizes 23 field trials (over six geographic locations, with each trial composed of a separate field site and an application event) for quantifying the postapplication volatilization of dicamba from fields treated with an array of dicamba-containing formulations and tank adjuvants at an application rate of 0.56 or 1.12 kg dicamba acid equivalents (a.e.) per hectare (0.5 or 1.0 lb dicamba a.e. per acre). The data span 3 years of testing over a range of locations, field types, and environmental conditions. The aerodynamic and the integrated horizontal flux methodologies were employed (and then averaged) for estimating the vertical flux from the field for periods extending to approximately 72 h post application. In all cases, the vertical flux peaked within 24 h of application and then decayed to much lower levels by day 3. Total volatile losses among all formulations and conditions ranged from 0.023 ± 0.003 to 0.302 ± 0.045% of the applied dicamba (median = 0.08%). Analysis of the recorded meteorological and soil conditions for each field trial failed to identify any single soil or weather parameter as a dominant driver of total volatile losses. Air concentrations of dicamba observed in the course of these trials were all below the no observed adverse effect concentration for conventional soybean plant height or yield, indicating that air concentrations directly above or outside of the dicamba-treated area would not cause a reduction in plant height or yield of conventional soybean.