Date of Award
Doctor of Philosophy (PhD)
Due to the increasing reports of glyphosate resistant weeds, genetically modified (GM) glyphosate tolerant crops have been supplemented with crops resistant to alternative herbicides such as dicamba (3,6-dichloro-2-methoxybenzoic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid). Since the introduction of dicamba and 2,4-D tolerant crops in the U.S., there has been a substantial increase in reports of off-target dicamba and 2,4-D movement and damage (i.e., drift) to non-tolerant vegetation and crops. The off-target damage has been documented by U.S. news sources and increased complaints to State Departments of Agriculture. These complaints have been especially high for dicamba, with the U.S. Environmental Protection Agency reporting that over 3,400 complaints and damage to 1.1 million acres of crops occurred in 2021 due to dicamba drift. This dissertation focused on determining and addressing current research gaps related to dicamba and 2,4-D off-target drift.The first dissertation objective was to establish the current state of dicamba and 2,4-D’s use and the changing regulations controlling their application. While other pathways (e.g., spray drift) exist that lead to off-target drift of dicamba and 2,4-D, field studies indicate that volatilization occurs for the semi-volatile dicamba and 2,4-D, even when new low-volatility formulations containing new amine salts are used. Why volatilization continues despite the use of low-volatility amine salts remains an open question that we explored in the next three objectives. The second objective aimed to measure dicamba volatilization from dicamba free acid and amine salt formulations at different temperatures, amine ratios, and in the presence of glyphosate. Due to the heightened number of complaints and regulatory scrutiny for dicamba compared to 2,4-D, we focused on dicamba and its three different high use dicamba-amine formulations (i.e., formulations including dimethylamine (DMA), diglycolamine (DGA), and n,n-bis-(3-aminopropyl)methylamine (BAPMA)). We discovered that dicamba volatilization from dried dicamba-amine residues was lessened when amines with more hydrogen bonding moieties were used. Our findings also suggested that the addition of glyphosate, which is included in one dicamba formulation, increased dicamba volatilization from dried dicamba-amine residues, although this effect was lessened by amines with multiple hydrogen bonding moieties. The third objective explored the fate of the amine from dicamba, 2,4-D, and glyphosate amine salts. Since amines used in herbicide formulations possess a higher vapor pressure than dicamba, 2,4-D, and glyphosate, amines could volatilize themselves. Amine volatilization is important because of its potential impact to herbicide volatilization and atmospheric chemistry. We found that two commonly used amines (i.e., DMA and isopropylamine (IPA)) volatilized from aqueous salt solutions and dried residues when combined with dicamba, 2,4-D, and glyphosate, the latter of which is the most used herbicide in the world. We also found that the amines included in low-volatility dicamba-amine formulations, DGA and BAPMA, also volatilized from dried residues, which could help explain why dicamba volatilization occurs from low-volatility formulations. The final objective was to quantify proton transfer between dicamba and amine in dried dicamba-amine residues. Similar to the aqueous phase, proton transfer between dicamba and amine could be incomplete in the dried residue, leaving dicamba in the more volatile neutral state. We utilized Fourier Transform Infrared (FTIR) spectroscopy to determine the relative extent of proton transfer in dicamba-amine salt residues. Our FTIR results indicated dicamba-amine residues with higher extents of proton transfer had lower dicamba volatility. Among eight different dicamba residues prepared, the number of hydrogen bonding moieties present on the amine correlated with a higher degree of proton transfer from dicamba. Overall, our work contributed to the understanding of dicamba and amine volatilization from dicamba-amine dried residues. Specifically, we (1) detailed the state of the regulatory and scientific literature related to dicamba and 2,4-D applications and drift, (2) determined that dicamba volatility increased when combined with an amine with fewer hydrogen bonding moieties as well as glyphosate, (3) measured, to the best of our knowledge, the first instance of amine volatilization from herbicide-amine salts, and (4) assessed using FTIR spectroscopy that higher extents of proton transfer in dicamba-amine residues correlated with lower dicamba volatility and number of hydrogen bonding sites on the amine.
Kimberly M. Parker
Brent J. Williams, Jay R. Turner, Marcus Foston, Alexander S. Bradley,
Available for download on Friday, April 26, 2024