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.
Quantitative
assessment of the volatility of field applied herbicides
requires orchestrated sampling logistics, robust analytical methods,
and sophisticated modeling techniques. This manuscript describes a
comprehensive system developed to measure dicamba volatility in an
agricultural setting. Details about study design, sample collection,
analytical chemistry, and flux modeling are described. A key component
of the system is the interlaboratory validation of an analytical method
for trace level detection (limit of quantitation of 1.0 ng/PUF) of
dicamba in polyurethane foam (PUF) air samplers. Validation of field
sampling and flux methodologies was conducted in a field trial that
demonstrated agreement between predicted and directly measured dicamba
air concentrations at a series of off-target locations. This validated
system was applied to a field case study on two plots to demonstrate
the utility of these methods under typical agricultural conditions.
This case study resulted in a time-varying volatile flux profile,
which showed that less than 0.2 ± 0.05% of the applied dicamba
was volatilized over the 3-day sampling period.
Long‐term simulations of agricultural watersheds have often been done assuming constant land use over time, but this is not a realistic assumption for many agricultural regions. This paper presents the soil and water assessment tool (SWAT)‐Landuse Update Tool (LUT), a standalone, user‐friendly desktop‐based tool for updating land use in the SWAT model that allows users to process multi‐year land use data. SWAT‐LUT is compatible with several SWAT model interfaces, provides users with several options to easily prepare and incorporate land use changes (LUCs) over a simulation period, and allows users to incorporate past or emerging land use categories. Incorporation of LUCs is expected to provide realistic model parameterization and scenario simulations. SWAT‐LUT is a public domain interface written in Python programming language. Two applications at the Fort Cobb Reservoir Experimental Watershed located in Oklahoma and pertinent results are provided to demonstrate its use. Incorporating LUCs related to implementation of recommended conservation practices over the years reduced discharge, evapotranspiration, sediment, total nitrogen, and total phosphorus loads by 59%, 9%, 68%, 53%, and 88%, respectively. The user’s manual is included in this article as Supporting Information. The SWAT‐LUT executable file and an example SWAT project with three land use rasters and the user’s manual are available at the United States Department of Agriculture‐Agricultural Research Service Grazinglands Research Laboratory website under Software. Editor’s note: This paper is part of the featured series on Optimizing Ogallala Aquifer Water Use to Sustain Food Systems. See the February 2019 issue for the introduction and background to the series.
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.
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