Dicamba resistant (DR) cropping technology has increased dicamba use, resulting in observation of dicamba off-target-movement (OTM). Volatility is one form of this movement. Tank mixtures and environmental conditions impact the volatile behavior of dicamba following application. Research was conducted in 2018, 2019, and 2020 to further assess and understand volatility mitigation by understanding tank-mix effects and utility of irrigation on volatility mitigation. Low tunnel and humidome methodology were used to analyze impact of tank mixtures and irrigation on dicamba volatility. Data suggest tank mixing encapsulated chloroacetamide formulations can mitigate volatility when comparing identical active ingredients formulated as emulsifiable concentrates. Tank-mixed glyphosate increases dicamba volatility regardless of salt form, with dimethylamine salt of glyphosate having the most volatile effect. Manipulation of environmental conditions can also assist in mitigation efforts when applicable through use of irrigation. Increasing amount of irrigation applied following dicamba application has a positive effect on mitigation.

Experiments were conducted to evaluate efficacy of dicamba, glufosinate, and glyphosate on Palmer amaranth (Amaranthus palmeri S. Wats.). Residual control was evaluated after dicamba was applied alone and in combination with fomesafen, fluometuron, acetochlor, and prometryn. Postemergence efficacy of dicamba, glufosinate, and glyphosate on different size Palmer amaranth was also evaluated. In addition, combinations of dicamba, glufosinate, and glyphosate were evaluated for efficacy on Palmer amaranth as well as spray coverage and spray droplet size as affected by various spray nozzles. Lastly, tolerance to dicamba, glufosinate, and glyphosate of cotton cultivars containing Bollgard II® XtendFlex™ technology was examined. Dicamba exhibited preemergence activity on Palmer amaranth; however, activity was heavily dependent on rainfall. Postemergence applications of dicamba increased control of Palmer amaranth. Spray nozzle selection influenced spray coverage and droplet size. Tolerance of cultivars containing Bollgard II® XtendFlex™ technology was over 90% at the end of the year regardless of herbicide.

In 2017, dicamba formulations were labeled for PRE and POST applications and utilized in soybean to control herbicide-resistant weed species. Dicamba-tolerant soybean cultivars were grown in proximity to those representing other herbicide-resistant technologies, creating the potential for problems with off-target movement. Field studies conducted in 2017 and 2018 in Stoneville, MS, characterized the soybean response to exposure to sub-lethal rates of different dicamba formulations and evaluated the performance of soybean cultivars representing different soybean maturity groups following multiple exposures to a sub-lethal rate of dicamba. Other field experiments in 2018 evaluated the performance of soybean following a single exposure to sub-lethal rates of dicamba at different growth stages (Rate and Timing Study) and characterized soybean response to multiple exposures of a sub-lethal rate of dicamba at different growth stages (Multiple Exposures Study).

Master of Science / Department of Agronomy / Phillip W. Stahlman / Kochia [Kochia scoparia (L.) Schrad.] is an invasive weed that is common in cropland, pastures and rangeland, rights-of-way, and disturbed areas throughout the western and northern United States and southern Canada. This species aggressively competes with crops, especially in no-till cropping systems, and has evolved resistance to multiple herbicide modes of action. Thus, it has become highly problematic and is difficult to manage. Roundup Ready 2 Xtend™ (Monsanto Co.) soybeans with resistance to dicamba herbicide are expected to be commercialized in 2016, and will offer a new management practice for controlling kochia and other susceptible broadleaf weeds in soybeans. Objectives of this research were to (1) determine whether greenhouse-grown plants from various kochia populations from the central Great Plains differ in susceptibility to postemergence-applied dicamba; (2) compare preemergence versus postemergence control of kochia with dicamba in a greenhouse environment; and (3) investigate various management practices in a systems approach to control kochia in soybeans. GR[subscript]50 values (dose required to reduce plant biomass by 50%) indicated at least an 8-fold difference among 11 kochia populations in susceptibility to postemergence-applied dicamba. Additionally, dicamba at 210 g ha[superscript]-1 applied preemergence caused 95, 88 and 84% mortality and reduced plant biomass (fresh wt.) of the most susceptible and two least susceptible kochia populations from a previous dicamba dose-response study by 99, 68 and 60%, respectively. In comparison, <10% of kochia plants from those populations died and biomass was reduced only 39, 15 and 7%, respectively, when dicamba was applied postemergence. Field experiments demonstrated that preplant conventional tillage followed by nine different in-crop herbicide treatments, and shallow early-spring tillage followed by preplant herbicides (reduced-till) along with the same in-crop herbicides provided greater kochia control than three no-till systems involving early preplant herbicide treatments followed by the same in-crop herbicides. However, despite greater kochia control with the tillage-based systems in 2013, soybean yields were less compared to the three no-till systems. Consequently, in some years the most effective kochia control practices may not result in the highest soybean yields.

New transgenic crops are currently being developed which will be tolerant to dicamba and 2,4-D herbicides. This technology could greatly benefit producers who are impacted by weed species that have developed resistance to other herbicides, like glyphosate-resistant Palmer Amaranth. Adoption of this new technology is likely to be rapid and widespread which will lead to an increase in the amount of dicamba and 2,4-D applied each season. It is well-documented that these herbicides are very injurious to soybeans, cotton, tomatoes, and most other broadleaf crops, and their increased use brings along increased chances of physical spray drift onto susceptible crops. Because of these risks, research is being conducted on new herbicide formulation/spray nozzle combinations to determine management options which may minimize physical spray drift.

Dicamba drift is a common issue and has been known to cause injury and yield reduction in a wide range of susceptible crops. A study was established to evaluate the effects of a simulated drift on the plant growth and yield of two pumpkin varieties (Cucurbita pepo ‘Magic Wand’ and C. moschata ‘Autumn Buckskin’) during 2019 and 2020 at the Southern Illinois University Horticulture Research Center in Carbondale. Six rates of dicamba were applied to simulate a mid-post-emergence application made in soybean which drifted onto pumpkins at two growth stages, 8-leaf and 12-leaf. These two pumpkin growth stages would be the approximate size of pumpkins when a mid-post-emergence application would typically be made in soybean. The 12-leaf growth stage also corresponds with Simulated drift rates were targeted at 1/1026, 1/513, 1/256, 1/128, 1/64, 1/32 of a 0.56 kg ae ha-1 rate, corresponding to 0.00056, 0.00112, 0.00224, 0.00448, 0.00896, and 0.01792 kg ae ha-1. Actual dicamba deposition was measured used filter papers (surface area = 176.1 cm2) in each treatment and these measured exposure amounts were used in correlational analyses with pumpkin injury, growth, and yield responses. Pumpkin plants did not develop chlorosis or necrosis at any point of the growing season with any of the dicamba application treatments. However, some injury and stunting were observed on pumpkin plants, regardless of drift rate, although pumpkin plant growth and responses were minimal at the drift rates evaluated. Our results indicated that typical dicamba drift rates cause minimal dicamba injury on pumpkin plants at 8- and 12-leaf growth stages with negligible effects on resulting yields. Pumpkin plant injury and stunting would most likely be observed at dicamba drift rates > 1 μg per 176.1 cm2 of leaf area, which converts to 0.62 g per ha. Pumpkins receiving dicamba drift at different growth stages appear to be less sensitive than some other crops, but a drift event at higher rates than those evaluated in this study may impact plant growth and yield. It is important to note that other pumpkin varieties or other Cucurbita species not evaluated in this study may be more susceptible. Producers using dicamba in soybean rotations that are near pumpkin fields should spray under proper wind, temperature, and humidity conditions to best mitigate the occurrence of any drift events. Nomenclature: 3,6-dichloro-2-methloxybenzoic acid, Cucurbita pepo, C. moschata, dicamba, pumpkin.

Herbicides are a common tool in weed control. With the introduction of genetically modified herbicide-tolerant crops, there has been a dramatic increase in the use of particular herbicides. Herbicides contaminate the environment and food and feed and can come into contact with non-target organisms, especially bacteria. Salmonella enteric serovar Typhimurium, which is a human and animal pathogen, was chosen to investigate if the commercial formulations of three herbicides – Kamba, 2,4-D and Roundup are toxic to bacteria and whether sub-lethal concentrations cause a response to antibiotics. In addition, earlier work demonstrating an effect of salicylic acid on antibiotic response was reconfirmed in this study. The herbicides were toxic to S. typhimurium at concentrations above the manufacturers recommended application rates. A key finding of this study was that when S. typhimurium was grown in sub-lethal concentrations of the herbicides, it demonstrated a change in its susceptibility to various antibiotics. Kamba and 2,4-D caused increased tolerance of chloramphenicol, tetracycline, ampicillin and ciprofloxacin and increased sensitivity to kanamycin. Exposure to Roundup caused increased sensitivity to chloramphenicol and tetracycline and increased tolerance towards kanamycin and ciprofloxacin. Roundup had no measureable affect on ampicillin susceptibility. The minimum concentrations of herbicides that induced an antibiotic response were within the recommended application rates. Furthermore, the minimum 2,4-D concentration that induced tetracycline, chloramphenicol and ampicillin tolerance was at or below the maximum residue limits set for food and feed commodities. Simultaneous exposure to an herbicide and an antibiotic was necessary for the induction of antibiotic tolerance. In addition, the effect of the herbicide on the antibiotic response was faster than the lethal effect of the antibiotics. Kamba induced chloramphenicol, tetracycline, ampicillin and ciprofloxacin tolerance was maintained in the absence of Kamba once tolerance was induced by simultaneous exposure to Kamba and antibiotic. The emergence of antibiotic tolerance is an important health issue that may compromise treatment of serious bacterial infections. The widespread use of herbicides in agricultural, urban and domestic settings increases the number of bacteria that are exposed to herbicides. The tolerance induced by the herbicides may increase the frequency of antibiotic tolerant strains, increase the chance of co-exposure to antibiotics, and increase the potential for failure to treat bacterial infections as a result.

Collinsville, Illinois is the leading producer of horseradish is the nation. The river bottom geography surrounding Collinsville, Illinois near St. Louis, Missouri is a high-production area for horseradish. The development of soybean technologies resistant to dicamba or 2,4-D may allow horseradish growers to gain control of troublesome weeds, such as Palmer amaranth (Amaranthus palmeri) or volunteer horseradish, but could pose risks as well. Drift of these two herbicides or carryover to horseradish could cause severe injury and possible crop loss. While synthetic auxin-tolerant soybean may also allow growers to control volunteer horseradish, herbicide efficacy may differ depending on the volunteer horseradish variety. These risks and benefits could affect the adoption rate of these new soybean technologies in horseradish production areas. A field trial was established in 2015 in Edwardsville, IL and 2016 Medora, IL to simulate drift of both dicamba and 2,4-D onto horseradish. Applications were made in horseradish to mimic drift of a mid-post emergence application in soybean onto the horseradish crop. Plants were monitored for injury and stand, height, and yield reductions throughout the season. Individual roots were evaluated post-harvest. Overall, 2,4-D caused more injury at all rates when compared to dicamba. Horseradish growers may see yield reductions if rates at or greater than 1/1000X of a field rate of 2,4-D drift onto their fields. Not planting horseradish near a 2,4-D-tolerant soybean field, as well as reading the herbicide labels and following application requirements, should help growers prevent serious injury and yield loss. On the other hand, rates of 2,4-D at or above a full field rate offered complete control of all plants; therefore, growers who struggle with persistent volunteer horseradish could rotate to a 2,4-D-tolerant soybean and gain needed control of those plants. Field experiments were conducted in 2014, 2015, and 2016 to investigate the impact of dicamba residues following applications in a dicamba-tolerant soybean crop on horseradish planted the following season. Carryover trials were conducted as two-year rotations of soybean followed by horseradish in Collinsville, Illinois. Multiple rates of dicamba were applied at several timings in dicamba-tolerant soybean and the crop was monitored for injury. The following season horseradish was planted and monitored for injury and stand, height, and yield reductions. No injury or reductions were observed with any treatment in either year, potentially indicating a lack of dicamba remaining in the soil. Horseradish plant stand counts, height as well as yields were not reduced when compared to the nontreated. Results from this experiment suggest that rotating from dicamba-tolerant soybean to horseradish should pose no threat to horseradish. Greenhouse experiments were carried out in 2016 in three separate runs. Each run consisted of three replications of five varieties of horseradish, 604, 788, 9705, Hungarian, and V7E3, and two rates of dicamba, glyphosate, and dicamba plus glyphosate . Plants were sprayed when at least one plant in each pot had reached a height of 17 to 23 cm. Horseradish was then rated for injury at 3, 7, 14, 21, and 28 days after treatment (DAT). Heights were also taken at 0, 14, and 28 DAT. At 28 DAT plants were harvested, weighed and place in a dryer for 72 hours and weighed again. The lowest level of injury at 28 DAT was with variety 604. The control of horseradish roots is critical to ensure the plant is killed completely and does not return the following season as a volunteer. The concerns associated with auxin-tolerant crops can be mitigated with proper management of herbicides and crop locations. While off target movement of 2,4-D may cause damage to a horseradish crop, it could be used as a herbicide to control volunteer horseradish. Additionally, if a grower chooses to use a dicamba-tolerant soybean variety, they may have the choice to use a dicamba plus glyphosate premix which will also give good control of volunteer horseradish with little concerns of dicamba carryover to the subsequent horseradish crop. Capitalizing on the strengths and weaknesses of each technology will help horseradish growers manage many weeds and facilitate the production of this important specialty crop.

Doctor of Philosophy / Department of Agronomy / Mithila Jugulam / Kochia (Kochia scoparia (L.) Schrad), one of the most troublesome weeds in the North American Great Plains, including Kansas (KS), has become a significant concern in croplands ever since the evolution and spread of glyphosate resistance in this weed. Dicamba, an important synthetic auxin herbicide, is a useful substitute for managing glyphosate-resistant (GR) broadleaf weeds. As a result of extensive and intensive use, kochia populations have also developed resistance to dicamba. However, the precise mechanism(s) of dicamba resistance in kochia is still unknown. In the first part of this dissertation, the physiological, biochemical and genetic basis of dicamba resistance in dicamba-resistant (DR) kochia from KS was investigated. The results suggest that the mechanism of dicamba resistance in this kochia is not due to decreased absorption, reduced translocation or enhanced detoxification of dicamba. In contrary, reduced translocation of dicamba was found to contribute to the dicamba resistance in DR kochia from Colorado (CO). Further investigation of DR kochia from KS revealed a possible role of single nucleotide polymorphism (SNP) in TIR1 (the receptor gene of auxin) in the dicamba resistance evolution. Genetic analyses of data from inheritance studies demonstrated that an incomplete dominant nuclear gene controls the dicamba resistance in kochia from KS. Also, it was found that the genes controlling dicamba resistance in kochia from KS and CO are not linked. Similarly, although, GR and DR traits were found to be controlled by two distinct single dominant genes, they appear to co-exist in many kochia populations from KS. Nonetheless, these two genes were also found not to be linked. The second part of this dissertation focused on the development of reliable tools for the management of DR and/or GR kochia. The following experiments were conducted under greenhouse and field conditions in KS: a) the effect of temperature stress on the efficacy of dicamba or glyphosate; b) efficacy of dicamba and glyphosate when applied in combination; and c) efficacy of dicamba when used as pre-emergence (PRE) herbicide. The results suggest that the efficacy of both dicamba and glyphosate on kochia can be improved when applied at cooler temperature conditions. Also, it was found that the dicamba and glyphosate tank-mix should not be recommended to manage kochia, especially DR kochia, due to significant antagonistic interaction when applied in combination. On the other hand, application of dicamba as PRE compared to the postemergence application, was found to improve kochia control including DR kochia. Overall, this dissertation provided several novel outcomes both in basic and applied aspects of dicamba resistance in kochia.

The effects of sublethal drift rates and carryover of glyphosate and dicamba into the next generation of seed potato cultivars Atlantic and Dakota Pearl are unknown. The objective of this research is to determine the impact of sublethal glyphosate and dicamba rates on mother and daughter chipping potato plants. Field studies were conducted in 2018 and 2019 in Oakes, ND. Herbicides were sprayed at the tuber initiation stage and consisted of dicamba (0, 20, and 99 g ae ha-1) and glyphosate (0, 40, and 197% g ae ha-1). During the year of application (2018), the combination of glyphosate at 197 g ha-1 and dicamba at 99 g ha-1 resulted in a 40% yield reduction compared to the non-treated in both cultivars. In 2019, the daughter tubers from mother tubers that were treated with glyphosate (23%) experienced a 16% reduction in marketable yield in both cultivars.