Doses Of Dicamba Research Articles

Increased Absorption and Translocation Contribute to Improved Efficacy of Dicamba to Control Early Growth Stage Palmer amaranth (Amaranthus palmeri)

Abstract Rapid growth of Palmer amaranth (Amaranthus palmeri S. Watson) poses a challenge for timely management of this weed. Dose response studies were conducted in 2017 and 2018 under field and greenhouse conditions near Garden City and Manhattan, KS, respectively, to evaluate the efficacy of dicamba to control ≤10 cm, 15 cm, and 30 cm tall-Palmer amaranth that mimics three herbicide application timing: on time application (Day 0), and 1 (Day 1) and 4 days (Day 4) delay. Visual injury rating and reduction in shoot biomass (% of non-treated), and mortality were assessed at four weeks after treatment using a three- and four-parameter log-logistic model, in R software program. Increasing dicamba doses increased A. palmeri control regardless of plant height both in the field and greenhouse studies. The results suggest that delaying application one (15 cm) and four days (30 cm), resulted in a two- and 27-fold increase in the effective dose of dicamba on A. palmeri, respectively, under field conditions. However, in the greenhouse, for the same level of A. palmeri control, more than one- and two-fold increase in dicamba dose, respectively was required. Similarly, the effective dose of dicamba required for 50% reduction in A. palmeri shoot biomass (GR50) increased more than four- and eight-fold or more than one- and two-fold when dicamba application was delayed by one (15 cm) and four days (30 cm), in the field or in the greenhouse, respectively. To understand the basis of increased efficacy of dicamba in controlling early growth stage of A. palmeri, dicamba absorption and translocation studies were conducted. Results indicate a significant reduction in dicamba absorption (7%) and translocation (15%) with increase in A. palmeri height. Therefore, increased absorption and translocation of dicamba results in increased efficacy in improving A. palmeri control at early growth stage.

Dicamba-resistant kochia (Bassia scoparia) in Kansas: characterization and management with fall- or spring-applied PRE herbicides

AbstractDicamba-resistant (DR) kochia is an increasing concern for growers in the US Great Plains, including Kansas. Greenhouse and field experiments (Garden City and Tribune, KS, in the 2014 to 2015 growing season) were conducted to characterize the dicamba resistance levels in two recently evolved DR kochia accessions collected from fallow fields (wheat–sorghum–fallow rotation) near Hays, KS, and to determine the effectiveness of various PRE herbicide tank mixtures applied in fall or spring prior to the fallow year. Dicamba dose–response studies indicated that the KS-110 and KS-113 accessions had 5- to 8-fold resistance to dicamba, respectively, relative to a dicamba-susceptible (DS) accession. In separate field studies, atrazine-based PRE herbicide tank mixtures, dicamba + pendimethalin + sulfentrazone, and metribuzin + sulfentrazone when applied in the spring had excellent kochia control (85% to 95%) for 3 to 4 mo at the Garden City and Tribune sites. In contrast, kochia control with those PRE herbicide tank mixtures when applied in the fall did not exceed 79% at the later evaluation dates. In conclusion, the tested kochia accessions from western Kansas had evolved moderate to high levels of resistance to dicamba. Growers should utilize these effective PRE herbicide tank mixtures (multiple sites of action) in early spring to manage kochia seed bank during the summer fallow phase of this 3-yr crop rotation (wheat–corn/sorghum–fallow) in the Central Great Plains.

First Report of Kochia (Bassia scoparia) with Cross-Resistance to Dicamba and Fluroxypyr in Western Kansas

AbstractEvolution and rapid spread of herbicide-resistant (HR) kochia has become a significant challenge for growers in the U.S. Great Plains. The main objectives of this research were to confirm and characterize the response of putative auxinic HR (Aux-HR) kochia accessions (designated as KS-4A, KS-4D, KS-4H, KS-10A, KS-10-G, and KS-10H) collected from two different corn fields near Garden City, KS, to dicamba and fluroxypyr and to determine the EPSPS gene copy number to detect whether those accessions were also resistant to glyphosate. Single-dose experiments indicated that putative Aux-HR kochia accessions had 78% to 100% and 85% to 100% survivors when treated with dicamba (560 g ae ha−1) and fluroxypyr (235 g ae ha−1), respectively. Whole-plant dicamba dose–response studies revealed that the selected Aux-HR accessions had 2.9- to 15.1- and 3.1- to 9.4-fold resistance to dicamba relative to two susceptible accessions (MT-SUS and KS-SUS). In a separate fluroxypyr dose–response experiment, the selected Aux-HR accessions also exhibited 3.8- to 7.3- and 3.0- to 8.6-fold resistance to fluroxypyr on the basis of shoot fresh and dry weight responses, respectively. The confirmed Aux-HR kochia accessions also had 3 to 13 EPSPS gene copies relative to MT-SUS and KS-SUS accessions (each with 1 EPSPS gene copy). These results suggest that the putative Aux-HR kochia accessions from Kansas had developed moderate to high levels of cross-resistance to dicamba and fluroxypyr and low to high levels of resistance to glyphosate. This is the first confirmation of kochia accessions with cross-resistance to dicamba and fluroxypyr in Kansas. Growers should use diverse kochia control programs, including the proper use of dicamba and fluroxypyr stewardship, use of cover crops, occasional tillage, diversified crop rotations, and alternative effective herbicides to prevent further evolution and spread of Aux-HR kochia on their fields.

Glyphosate- and Dicamba-Resistant Genes Are Not Linked in Kochia (Bassia scoparia)

AbstractKochia [Bassia scoparia(L.) A. J. Scott] is one of the most troublesome weeds throughout the North American Great Plains. Herbicides such as glyphosate and dicamba have been used widely to controlB. scopariafor decades. However, manyB. scopariapopulations have evolved resistance to these herbicides due to selection. Especially, dicamba-resistantB. scopariapopulations are often also found to be glyphosate-resistant. The objective of this research was to determine whether these two herbicide resistances are linked inB. scoparia.Reciprocal crosses were performed between glyphosate- and dicamba-resistant (GDR) and glyphosate- and dicamba-susceptible (GDS)B. scopariato produce F1and F2progeny. Two F1and seven F2progeny families were screened with various doses of dicamba or glyphosate. All the F1progeny survived both dicamba and glyphosate treatments. Chi-square analyses of F2progeny suggest (1) glyphosate and dicamba resistances inB. scopariaare inherited via single, dominant nuclear genes; and (2) glyphosate- and dicamba-resistant genes are not linked. Thus, the dicamba and glyphosate resistances appear to have evolved independently due to intense selection but do not seem to spread together.

Effect of Low Doses of Dicamba Alone and in Combination with Glyphosate on Parent Soybean and Offspring

AbstractIt is well established that soybean that does not contain the dicamba-resistant (DR) trait is highly sensitive to off-target exposure to dicamba. However, there is limited information on the effect of low doses of dicamba plus glyphosate mixtures on dicamba-sensitive soybean—a mixture likely to be used on a vast acreage of dicamba/glyphosate-resistant soybean. The objective of this research was to examine leaf and pod malformation, along with height and yield effects, when dicamba, glyphosate, or a mixture of the two was applied to soybean sensitive to both dicamba and glyphosate at sublethal doses. Field applications were made at three growth stages (R1, R3, and R5) at multiple locations. Two glyphosate rates (1/64 and 1/256 of the labeled rate of 870 g ae ha−1) and two dicamba rates (1/64 and 1/256 of the labeled rate of 560 g ae ha−1) were used. Adding glyphosate to dicamba increased leaf malformation by 6% more than dicamba alone when applied at the R1 soybean growth stage. After R3 applications, pod malformation was 10% greater in treatments containing dicamba and glyphosate than dicamba alone. Applications at R5 showed minimal leaf and pod malformation. Seed from field trials was planted in the greenhouse to evaluate the offspring. The number of offspring plants showing dicamba-like symptomology was not increased with the addition of glyphosate to dicamba. Overall, injury to offspring was similar in dicamba alone and dicamba plus glyphosate treatments; however, the number of plants injured increased when parent plants were exposed to sublethal doses of dicamba at R3 and R5 compared with R1 growth-stage exposure. Vigor was reduced in dicamba-containing treatments, but not glyphosate-alone treatments. Glyphosate addition to dicamba had no effect on vigor of soybean offspring. Although there is increased injury to parent plants when glyphosate is added to dicamba, this research demonstrates that glyphosate does not contribute to the negative effects of dicamba on soybean offspring.

Soybean Response to Dicamba: A Meta-Analysis

AbstractA meta-analysis of 11 previously published field studies was conducted with the objectives being to (1) estimate the no observable effects dose (NOED) for dicamba on susceptible soybean; (2) evaluate available evidence for hormesis, or increased soybean yield in response to low doses of dicamba; (3) estimate the dose of dicamba likely to cause measurable soybean yield loss under field conditions; and (4) quantify the relationship between visible injury symptoms and soybean yield loss. All studies that included visible injury data (N=7) reported injury symptoms at the lowest nonzero dicamba dose applied (as low as 0.03 g ae ha−1), and therefore a NOED could not be estimated from the existing peer-reviewed literature. Based on statistical tests for hormesis, there is insufficient evidence to support any claim of increased soybean yield at low dicamba doses. Future research should include a range of dicamba doses lower than 0.03 g ha−1to estimate a NOED and determine whether a hormesis effect is possible at or below dicamba doses that cause visible injury symptoms. Soybean is more susceptible to dicamba when exposed at flowering (R1 to R2 stage) compared with vegetative stages (V1 to V7). A dicamba dose of 0.9 g ha−1(95% CI=0.08 to 1.7) at the flowering stage was estimated to cause 5% soybean yield loss. When exposed at vegetative stages, dicamba doses that cause less than 30% visible injury symptoms (95% CI=23 to 49%) appear unlikely to cause greater than 5% soybean yield loss; however, if soybean is exposed at flowering, visible injury symptoms greater than 12% (95% CI=8 to 16%) are likely to be associated with at least 5% soybean yield loss.

Reduced Translocation of Glyphosate and Dicamba in Combination Contributes to Poor Control of Kochia scoparia: Evidence of Herbicide Antagonism

Kochia scoparia is a troublesome weed across the Great Plains of North America. Glyphosate and dicamba have been used for decades to control K. scoparia. Due to extensive selection, glyphosate- and dicamba-resistant (GDR) K. scoparia have evolved in the USA. Herbicide mixtures are routinely used to improve weed control. Herbicide interactions if result in an antagonistic effect can significantly affect the management of weeds, such as K. scoparia. To uncover the interaction of glyphosate and dicamba when applied in combination in K. scoparia management the efficacies of different doses of glyphosate plus dicamba were evaluated under greenhouse and field conditions using GDR and a known glyphosate- and dicamba-susceptible (GDS) K. scoparia. The results of greenhouse and field studies suggest that the combination of glyphosate and dicamba application controlled GDS, but glyphosate alone provided a better control of GDR K. scoparia compared to glyphosate plus dicamba combinations. Furthermore, investigation of the basis of this response suggested glyphosate and dicamba interact antagonistically and consequently, the translocation of both herbicides was significantly reduced resulting in poor control of K. scoparia. Therefore, a combination of glyphosate plus dicamba may not be a viable option to control GDR K. scoparia.

Defining Glyphosate and Dicamba Drift Injury to Dry Edible Pea, Dry Edible Bean, and Potato

Field trials using sublethal doses of glyphosate, dicamba, or mixtures of both herbicides on dry edible pea (Pisum sativum), dry edible bean (Phaseolus vulgaris), and potato (Solanum tuberosum) were conducted at six locations to determine the injury potential if spray drift were to occur. All studies used three increasing sublethal doses of glyphosate and dicamba, which were labeled as low, medium, and high. The doses for each herbicide varied for the three crops because of expected sensitivity differences. Herbicide doses were targeted for the reproductive stage 1 with dry edible pea and dry edible bean, and at tuber initiation for potato. Visible injury 20 days after the treatment ranged from 0% to 13% for dry edible pea, 0% to 53% for dry edible bean, and 0% to 50% for potato. Compared with the nontreated, yield was least when doses included dicamba, regardless of the crop. Dry edible bean was the most sensitive crop to sublethal doses of dicamba, followed by dry edible pea and potato. Results from these six studies suggested that drift injury potential to dry edible pea, dry edible bean, and potato will be greater if a dicamba-resistant soybean (Glycine max) crop is adjacent and upwind compared with a glyphosate-resistant crop. Results also reinforce the need for diligence in the application of these herbicides in proximity to susceptible crops and the need to thoroughly clean sprayers before spraying a sensitive crop.

Recurrent Sublethal-Dose Selection for Reduced Susceptibility of Palmer Amaranth (Amaranthus palmeri) to Dicamba

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance. Therefore, selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the individuals to dicamba was evaluated, and its susceptibility to 2,4-D was characterized. Based on the greenhouse study, following three generations of dicamba selection, the dose required to cause 50% mortality increased from 111 g ae ha−1for parental individuals (P0) to 309 g ae ha−1for the P3. Furthermore, reduced susceptibility of the P3to 2,4-D was also evident. This research presents the first evidence that recurrent use of sublethal dicamba doses can lead to reduced susceptibility of Palmer amaranth to dicamba as well as 2,4-D. Here, we show that selection from sublethal dicamba doses has an important role in rapid evolution of Palmer amaranth with reduced susceptibility to auxin-type herbicides.

Baseline Sensitivity of Nebraska Waterhemp and Palmer Amaranth to Dicamba and 2,4‐D

The development of dicamba (3,6‐dichloro‐2‐methoxybenzoic acid)‐ and 2,4‐D [(2,4‐dichlorophenoxy) acetic acid]‐resistant soybean [Glycine max (L.) Merr.] will increase the frequency of applying these herbicides to control broadleaf weeds in soybean production systems. We evaluated the sensitivity of randomly selected waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif] and Palmer amaranth (Amaranthuspalmeri S. Wats) populations to a single dose of dicamba and 2,4‐D. Selected populations were then used in dicamba and 2,4‐D dose‐response experiments to evaluate: (i) visual injury at 28 d after treatment; and (ii) plant survival, biomass accumulation, and flowering at 84 DAT. Across selected populations, the estimated dicamba dose for 50% visual injury (I50) was below the recommended field rate (560 g ha−1) and ranged between 59 and 117 g ha−1 for waterhemp and 43 and 81 g ha−1 for Palmer amaranth populations. The estimated 2,4‐D dose for I50 was also below the recommended field rate (800 g ha−1) and ranged between 127 and 242 g ha−1 for waterhemp and 127 and 196 g ha−1 for Palmer amaranth. Waterhemp plants flowered with up to 560 g ha−1 dicamba and 2240 g ha−1 2,4‐D, while Palmer amaranth plants flowered with up to 280 g ha−1 dicamba, and 1120 g ha−1 2,4‐D. Individual plants will likely survive and flower following recommended field rates of dicamba (waterhemp) and 2,4‐D (both species). Close monitoring of weed populations are needed for the stewardship of available weed management technologies.Core Ideas We conducted a state‐wide baseline study of auxinic herbicide sensitivity in randomly selected waterhemp and Palmer amaranth populations. Individual plants of both species will likely survive and flower after field rates of dicamba and/or 2,4‐D. Close monitoring of weed populations and continuous update of best management practices are needed for the stewardship of available weed management technologies. This study sets a benchmark to detect shifting in sensitivities to dicamba or 2,4‐D after resistant crops are released.

Response of glyphosate-resistant soybean to dicamba spray tank contamination during vegetative and reproductive growth stages

The anticipated availability of dicamba-resistant crops will increase the potential for crop injury to non-dicamba-resistant soybean due to dicamba spray tank contamination. A total of eight field trials were conducted at various locations in Ontario, Canada during 2012–2014 to determine the response of non-dicamba-resistant soybean to dicamba spray tank contamination at 0, 0.75, 1.5, 3, 6, 15, 30, and 60 g a.e. ha−1 applied postemergence (POST) at the V2-3 (2–3 trifoliate) or R1 (1st flower) stage. At one week after treatment (WAT), dicamba applied at 0.75, 1.5, 3, 6, 15, 30, and 60 g a.e. ha−1 at V2-3 caused 12, 18, 25, 31, 43, 53, and 66% visible injury in soybean, respectively. Injury increased at 2 and 4 WAT and decreased by 8 WAT with 68% visible injury observed at the highest dose. Dicamba applied at R1 caused 23, 28, 36, 40, 48, 61, and 73% visible injury in soybean at 0.75, 1.5, 3, 6, 15, 30, and 60 g a.e. ha−1, respectively. The predicted dose of dicamba to reduce soybean seed yield 1, 5, 10, 20 or 50% was 1.1, 5.8, 11.8, 25.2, and >60 g a.e. ha−1 when applied at V2-3 and <0.75, 1.0, 2.0, 4.3, and 11.5 g a.e. ha−1 when applied at R1, respectively. Results show that dicamba spray tank contamination of as little as 0.75 g a.e. ha−1 can cause significant crop injury in non-dicamba-resistant soybean when applied during the vegetative or reproductive stages.

Effects of the herbicide dicamba on nontarget plants and pollinator visitation.

Nearly 80% of all pesticides applied to row crops are herbicides, and these applications pose potentially significant ecotoxicological risks to nontarget plants and associated pollinators. In response to the widespread occurrence of weed species resistant to glyphosate, biotechnology companies have developed crops resistant to the synthetic-auxin herbicides dicamba and 2,4-dichlorophenoxyacetic acid (2,4-D); and once commercialized, adoption of these crops is likely to change herbicide-use patterns. Despite current limited use, dicamba and 2,4-D are often responsible for injury to nontarget plants; but effects of these herbicides on insect communities are poorly understood. To understand the influence of dicamba on pollinators, the authors applied several sublethal, drift-level rates of dicamba to alfalfa (Medicago sativa L.) and Eupatorium perfoliatum L. and evaluated plant flowering and floral visitation by pollinators. The authors found that dicamba doses simulating particle drift (≈1% of the field application rate) delayed onset of flowering and reduced the number of flowers of each plant species; however, plants that did flower produced similar-quality pollen in terms of protein concentrations. Further, plants affected by particle drift rates were visited less often by pollinators. Because plants exposed to sublethal levels of dicamba may produce fewer floral resources and be less frequently visited by pollinators, use of dicamba or other synthetic-auxin herbicides with widespread planting of herbicide-resistant crops will need to be carefully stewarded to prevent potential disturbances of plant and beneficial insect communities in agricultural landscapes.

Response of Nebraska Kochia (Kochia scoparia) Accessions to Dicamba

Kochia is a troublesome weed in the western Great Plains and many accessions have evolved resistance to one or more herbicides. Dicamba-resistant soybean is being developed to provide an additional herbicide mechanism of action for POST weed control in soybean. The objective of this study was to evaluate variation in response to dicamba among kochia accessions collected from across Nebraska. Kochia plants were grown in a greenhouse and treated when they were 8 to 12 cm tall. A discriminating experiment with a single dose of 420 g ae ha−1of dicamba was conducted on 67 accessions collected in Nebraska in 2010. Visual injury estimates were recorded at 21 d after treatment (DAT) and accessions were ranked from least to most susceptible. Four accessions representing two of the most and least susceptible accessions from this screening were subjected to dose-response experiments using dicamba. At 28 DAT, visible injury estimates were made and plants were harvested to determine dry weight. An 18-fold difference in dicamba dose was necessary to achieve 90% injury (I90) between the least (accession 11) and most susceptible accessions. Approximately 3,500 g ha−1of dicamba was required in accession 11 to reach a 50% dry weight reduction (GR50). There was less than twofold variation among the three more susceptible accessions for both the I90and GR90parameters, suggesting that most kochia accessions will be similarly susceptible to dicamba. At 110 DAT, accession 11 had plants that survived doses of 35,840 g ha−1, and produced seed at doses of 17,420 g ha−1. The identification of one resistant accession among the 67 accessions screened, and the fact that dicamba doses greater than 560 g ha−1were required to achieve GR80for all accessions suggest that repeated use of dicamba for weed control in fields where kochia is present may quickly result in the evolution of dicamba-resistant kochia populations.

A Meta-Analysis on the Effects of 2,4-D and Dicamba Drift on Soybean and Cotton

Commercial introduction of cultivars of soybean and cotton genetically modified with resistance to the synthetic auxin herbicides dicamba and 2,4-D will allow these compounds to be used with greater flexibility but may expose susceptible soybean and cotton cultivars to nontarget herbicide drift. From past experience, it is well known that soybean and cotton are both highly sensitive to low-dose exposures of dicamba and 2,4-D. In this study, a meta-analysis approach was used to synthesize data from over seven decades of simulated drift experiments in which investigators treated soybean and cotton with low doses of dicamba and 2,4-D and measured the resulting yields. These data were used to produce global dose–response curves for each crop and herbicide, with crop yield plotted against herbicide dose. The meta-analysis showed that soybean is more susceptible to dicamba in the flowering stage and relatively tolerant to 2,4-D at all growth stages. Conversely, cotton is tolerant to dicamba but extremely sensitive to 2,4-D, especially in the vegetative and preflowering squaring stages. Both crops are highly variable in their responses to synthetic auxin herbicide exposure, with soil moisture and air temperature at the time of exposure identified as key factors. Visual injury symptoms, especially during vegetative stages, are not predictive of final yield loss. Global dose–response curves generated by this meta-analysis can inform guidelines for herbicide applications and provide producers and agricultural professionals with a benchmark of the mean and range of crop yield loss that can be expected from drift or other nontarget exposures to 2,4-D or dicamba.

Herbicide drift can affect plant and arthropod communities

Field edges, old fields, and other semi-natural habitats in agricultural landscapes support diverse plant communities that help sustain pollinators, predators, and other beneficial arthropods. These plant and arthropod communities may be at significant ecotoxicological risk from herbicides applied to nearby crop fields. Recent innovations in herbicide-resistant crop biotechnology may lead to major increases in the use of the herbicides dicamba and 2,4-D. These herbicides selectively affect broadleaf plants, and non-target exposures may therefore lead to a net reduction in the functional diversity and floral resources provided by semi-natural habitats. In multi-year experiments at two sites (a field edge and an old field), we exposed replicated plots to low doses of dicamba designed to simulate herbicide drift and monitored changes in plant and arthropod communities. At the field edge site, we observed a significant decline in forb cover (but not floral resources) in plots treated at doses (∼1% of the field application rate) that are substantially lower than those that have been documented to affect plant communities in previous research. We also observed declines in three herbivorous pest species (pea aphids, spotted alfalfa aphid, and potato leaf hopper), increases in one pest species (clover root curculio), and increases in beneficial seed predators (crickets) associated with dicamba exposure. In contrast, at the old field site, drift-level doses did not affect plant community structure but reduced flowering of a key species (Monarda fistulosa). Variability across sites and taxonomic groups makes it difficult to offer general conclusions about the risks of dicamba drift to plant and arthropod biodiversity. Factors including the successional age of the plant community and water stress at the time of herbicide exposure likely explain the differing responses at the two sites to simulated drift.

Direct and Indirect Effects of the Synthetic-Auxin Herbicide Dicamba on Two Lepidopteran Species

Herbicides are the most commonly applied pesticides in agroecosystems, and therefore pose potentially significant ecotoxicological risks to plants and insects. Glyphosate is the most common herbicide worldwide, and glyphosate-resistant weeds are quickly becoming serious challenges in some agroecosystems. Because of this resistance epidemic and the recent development of crops with resistance to dicamba or 2,4-D, herbicide-use patterns are likely to change. Presently, dicamba and 2,4-D cause most herbicide-drift damage to nontarget plants despite limited agricultural usage, but the effects of these synthetic auxin herbicides on insects have been poorly explored. To understand the influence of dicamba on insects, we applied several sublethal, drift-level rates of dicamba to soybean, Glycine max L., and Carduus thistle, and measured growth and survival of Helicoverpa zea (Boddie) and Vanessa cardui (L.) larvae, respectively. For thistle, we measured percent nitrogen content before and after dicamba application. We also performed direct toxicity bioassays on the two caterpillar species with several rates of dicamba. Dicamba was not directly toxic to larvae of either species, and H. zea showed no negative effects when feeding on soybeans dosed with dicamba. We did, however, detect significant negative, indirect effects of higher rates of dicamba on V. cardui larval and pupal mass, total nitrogen of thistles post application, and thistle biomass in the presence of V. cardui larvae. Notably, thistle biomass was not related to dicamba dose in absence of larvae. Our results indicate that dicamba can indirectly influence the performance of some caterpillar species, possibly by altering plant nutritional content.

Response of Nebraska Horseweed (Conyza canadensis) Populations to Dicamba

Dicamba-resistant soybeans are being developed to provide an additional herbicide mechanism-of-action for postemergence weed control in soybean. Numerous broadleaf species, including horseweed, have evolved resistance to glyphosate. It is anticipated that dicamba will be used by farmers as a primary tool to manage these weeds. Studying and understanding variability in horseweed response to dicamba will aid in developing appropriate risk management strategies to extend the utility of the dicamba-resistance technology. Horseweed plants from ten Nebraska populations were treated with one of nine doses of dicamba in greenhouse experiments. At 28 days after treatment (DAT) visual injury estimations were made and plants were harvested to determine dry weight. There was a three-fold difference in the I90 (90% visual injury estimate) between the least (638 g ha -1 ) and most (205 g ha -1 ) susceptible populations. Two plants from five populations were observed for an additional three months. No plants treated at doses above 280 g ha -1 survived to set seeds. These results suggest that maintaining use doses of 560 g ha -1 or greater may fully control horseweed populations from Nebraska and minimize the risk of

Dose Response of Glyphosate and Dicamba on Tomato (Lycopersicon esculentum) Injury

Field studies were conducted to determine the response of sublethal glyphosate and dicamba doses to processing tomato flowering loss and marketable yield. Dose–response studies for both herbicides were conducted on four commercial processing tomato lines (two different lines within each study) and plants were sprayed at either the vegetative stage or the early bloom stage. Both glyphosate and dicamba caused higher yield losses when sprayed at the early bloom stage. A 25% yield loss was observed with 8.5 and 7.5 g ae ha−1for glyphosate and dicamba, respectively, at the early bloom stage and 43.9 and 11.9 g ae ha−1for glyphosate and dicamba, respectively, at the early vegetative stage. Overall, these tomato cultivars were more sensitive to dicamba than to glyphosate. We conclude that glyphosate and dicamba drift could have serious implications on tomato yields especially if the drift occurs during flowering.

Evaluation of 2,4-D and Dicamba genotoxicity in bean seedlings using comet and RAPD assays

The present study was undertaken to evaluate genotoxic potential of two auxinic herbicides [2,4-dicholorophenoxy acetic acid (2,4-D) and 3,6-dichloro-2-methoxybenzoic acid (Dicamba)] in the roots of common bean ( Phaseolus vulgaris L.) seedlings. Two-day-old etiolated seedlings were treated with 10 ppm methyl methanesulfonate (MMS, positive control) or 0.1, 0.2, or 0.3 ppm of either 2,4-D or Dicamba. At the end of a 96 h growth period, root growth, total soluble protein content, DNA damage in individual cells (comet assay scores) and randomly amplified polymorphic DNA (RAPD) profiles were used as endpoints of genotoxicity. 2,4-D and Dicamba were clearly dose-dependent root growth inhibitors. Total soluble protein content was significantly decreased in the positive control and at high concentrations (0.2 and 0.3 ppm) of Dicamba. Soluble protein content increased significantly only at 0.3 ppm 2,4-D ( P<0.05). In the comet assay, DNA fragmentation increased in a dose-dependent manner. The diagnostic and phenetic analyzes of appeared and/or disappeared RAPD bands indicated that dose-dependent DNA polymorphism was induced by both herbicides. Genomic template stability was significantly affected at all 2,4-D and Dicamba doses tested. Overall 2,4-D and Dicamba have similar effects on DNA damage detected by comet and RAPD assays.

Corn–Velvetleaf (Abutilon theophrasti) Interference Is Affected by Sublethal Doses of Postemergence Herbicides

Injury to weeds from sublethal doses of POST herbicides may reduce the effect of weed interference on crop yield. Information on how herbicide dose influences weed mortality, growth, and seed production is needed to assess the potential benefit of applying reduced herbicide doses. Field experiments were conducted at Mead, NE, in 2001 and 2002 to quantify velvetleaf mortality, growth, and corn–velvetleaf interference in response to varying doses of three POST herbicides. Untreated velvetleaf at six densities (0, 1, 3, 6, 12, and 20 plants m−1corn row) was grown in mixture with corn to establish a baseline corn–velvetleaf interference relationship. Treated velvetleaf at a density of 20 plants m−1row received one of five doses of dicamba, halosulfuron, or flumiclorac. Untreated velvetleaf height, biomass, and seed capsule production were greater in 2002 than 2001 and declined with increasing velvetleaf density in both years. Corn yield was not affected by untreated velvetleaf in 2001, but yield loss increased with increasing velvetleaf density in 2002. Mortality of herbicide-treated velvetleaf was 56% greater in 2001 than 2002 and increased with increasing herbicide dose. Maximum height of treated velvetleaf was similar for all treatments in 2001 but declined with increasing herbicide dose in 2002. Biomass and seed production of treated velvetleaf varied among herbicides in 2002 and decreased with increasing dose. Corn yield was not influenced by velvetleaf in 2001, but yield loss in response to herbicide-treated velvetleaf declined with increasing herbicide dose in 2002. Results show that the assumption that weeds surviving herbicide application are as competitive as untreated weeds is incorrect. Reduction in growth and resource consumption by herbicide-damaged weeds reduced the negative effects of weeds on corn.