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Cotton and soybean growers were offered new technologies in 2016, expanding in-crop herbicide options to include dicamba or 2,4-D. Within three years of commercialization, dicamba use in these crops increased ten-fold and growers began to report Palmer amaranth escapes in dicamba-tolerant production systems in western Tennessee. In 2020, Palmer amaranth seed was collected from eight Tennessee locations where growers witnessed poor control following dicamba. Greenhouse experiments were conducted to evaluate the response of these Palmer amaranth populations to dicamba. In 2021, field experiments were conducted on two tentative dicamba-susceptible populations in Georgia, on three confirmed dicamba-resistant populations in Tennessee, and on a tentative dicamba-susceptible population in Texas to evaluate cotton response following dicamba and to examine if malathion insecticide (a cytochrome P450 inhibitor) would improve weed control and not reduce cotton yield when applied in conjunction with dicamba. Palmer amaranth populations collected in 2020 survived dicamba in the greenhouse at 1, 2, and 4 times the labeled rate. There was 15 to 26% survival exhibited by five Palmer amaranth populations to the labeled dicamba rate (560 g ha-1) in the greenhouse. These findings were reinforced in the field when research on three of those populations in 2021 showed 55% control with the labeled dicamba rate and 69% control with 2 times the labeled rate. This demonstrates the dicamba resistance allele or alleles were passed between generations. This result was not consistent in the Macon County or Worth County, GA locations where malathion improved dicamba control of 15- to 38-cm tall Palmer amaranth. Cotton injury was observed when malathion was applied in combination with dicamba. These results further document the evolution of dicamba-resistant Palmer amaranth in Tennessee. Moreover, the non-reversal of resistance phenotype by malathion may suggest that the resistance mechanism is something other than metabolism.
Palmer amaranth has a long history of evolving resistance to herbicides to the point at which it has become a significant obstacle to row crop production. A survey of Palmer amaranth escapes in dicamba-resistant cotton and soybean fields in Tennessee was conducted in fall 2021 with the objective of determining whether poor control was due to environmental phenomena or the development of dicamba resistance. A greenhouse dicamba dose-response screen was conducted on 15 Tennessee accessions. Three accessions were identified with a relative resistance factor between 1.85 and 2.49, and one accession from Lauderdale County was found with a relative resistance factor of 14.25. The Lauderdale County 1 accession developed a higher dicamba resistance level than all others evaluated and can no longer be effectively controlled using dicamba. The history of Palmer amaranth escaping dicamba in the Lauderdale County 1 location from 2019 to 2021 in the field and in preliminary greenhouse screens would suggest that the dicamba resistance has passed between generations. This research documents the first findings of Palmer amaranth control failures in cotton and soybean fields due to the evolution of dicamba resistance.
Auxinic herbicides have been commonly used in production systems for broadleaf weed control for many years. One potential negative aspect to their use is their propensity to volatilize and move away from the treated area after application. This research examined three herbicide formulations and their relative amounts of vaporization following application under field conditions in Knoxville, TN, in 2017, 2018, and 2019. Herbicide treatments evaluated included 2,4-D choline, 2,4-D amine, and the diglycolamine (DGA) salt of dicamba. Ten field studies were conducted with major parameters including air sampler height (0.3 and 1.3 m) and applied surface condition (dry wheat stubble or green-plant vegetation). The relative volatility indicated by the study was that dicamba > 2,4-D choline = 2,4-D amine. Detected herbicide concentrations were numerically higher at the 0.3-m sampling height and in the green-plant surface condition. These results confirm that dicamba is more volatile than 2,4-D and that there was no difference in vapor emissions between the amine and choline salts of 2,4-D under field conditions.
Palmer amaranth (Amaranthus palmeri S. Watson) is one of the most problematic weeds in many cropping systems in the midsouthern United States because of its multiple weedy traits and its propensity to evolve resistance to many herbicides with different mechanisms of action. In Arkansas, A. palmeri has evolved metabolic resistance to S-metolachlor, compromising the effectiveness of an important weed management tool. Greenhouse studies were conducted to evaluate the differential response of A. palmeri accessions from three states (Arkansas, Mississippi, and Tennessee) to (1) assess the occurrence of resistance to S-metolachlor among A. palmeri populations, (2) evaluate the resistance level in selected accessions and their resistant progeny, (3) and determine the susceptibility of most resistant accessions to other soil-applied herbicides. Seeds were collected from 168 crop fields between 2017 and 2019. One hundred seeds per accession were planted in silt loam soil without herbicide for >20 yr and sprayed with the labeled rate of S-metolachlor (1,120 g ai ha−1). Six accessions (four from Arkansas and two from Mississippi) were classified resistant to S-metolachlor. The effective doses (LD50) to control the parent accessions ranged between 73 and 443 g ha−1, and those of F1 progeny of survivors were 73 to 577 g ha−1. The resistance level was generally greater among progeny of surviving plants than among resistant field populations. The resistant field populations required 2.2 to 7.0 times more S-metolachlor to reduce seedling emergence 50%, while the F1 of survivors needed up to 9.2 times more herbicide to reduce emergence 50% compared with the susceptible standard.
This research examined a potential nuisance aspect of the use of the volatility-reducing agent (VRA) potassium carbonate when combined with glyphosate in spray-tank mixtures. A VRA is now required to be added to dicamba applications to reduce off-target movement from volatility. When no VRA potassium carbonate was added to the spray mixture, there was no pressure buildup. The addition of VRA potassium carbonate plus glyphosate (which lowers the pH) resulted in an observed pressure buildup. Although the gas produced was not identified, it would be expected to be carbon dioxide formed by the dissolution of the carbonate anion from the VRA. Source water pH range from 3.2 to 8.2 had no effect on pressure buildup. Pressure buildup was directly related to water temperature, with a linear response to temperature when the VRA was added last; in contrast, a less direct relationship of temperature to pressure buildup existed at temperatures >30 C when the VRA potassium carbonate was added first. There was no effect on the pressure increase from adding a defoamer or a drift control agent.
BASF Corp. has developed p-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor–resistant cotton and soybean that will allow growers to use isoxaflutole in future weed management programs. In 2019 and 2020, a multi-state non-crop research project was conducted to examine weed control following isoxaflutole applied preemergence alone and with several tank-mix partners at high and low labeled rates. At 28 d after treatment (DAT), Palmer amaranth was controlled ≥95% at six of seven locations with isoxaflutole plus the high rate of diuron or fluridone. These same combinations provided the greatest control 42 DAT at four of seven locations. Where large crabgrass was present, isoxaflutole plus the high rate of diuron, fluridone, pendimethalin, or S-metolachlor or isoxaflutole plus the low rate of fluometuron controlled large crabgrass ≥95% in two of three locations 28 DAT. In two of three locations, isoxaflutole plus the high rate of pendimethalin or S-metolachlor improved large crabgrass control 42 DAT when compared to isoxaflutole alone. At 21 DAT, morningglory was controlled ≥95% at all locations with isoxaflutole plus the high rate of diuron and at three of four locations with isoxaflutole plus the high rate of fluometuron. At 42 DAT at all locations, isoxaflutole plus diuron or fluridone and isoxaflutole plus the high rate of fluometuron improved morningglory control compared to isoxaflutole alone. These results suggest that isoxaflutole applied preemergence alone or in tank mixture is efficacious on a number of cross-spectrum annual weeds in cotton, and extended weed control may be achieved when isoxaflutole is tank-mixed with several soil-residual herbicides.
Sicklepod [Senna obtusifolia (L.) H. S. Irwin & Barneby], also known as Cassia obtusifolia (L.), is an annual, herbaceous, dicotyledonous plant in the Fabaceae (Leguminosae) family, which is commonly known as the bean, legume, and pea family. The Fabaceae consist of herbs, shrubs, vines, or trees; the family has a cosmopolitan distribution with members numbering approximately 751 genera and 19,500 species (Christenhusz and Byng 2016). Characteristics of the Fabaceae include alternate, stipulate, and compound leaves. Leaflets often have pulvini (i.e., cushion-like swellings at the base of leaves that are subject to changes in turgor pressure), which are responsible for growth-independent or “sleep” movements. Another interesting anatomical feature exhibited by many species in the family is the formation of parenchymatous root nodules that are generated in association with nitrogen-fixing bacteria (Zomlefer 1994). The ovary of the Fabaceae usually develops into a dehiscent legume (e.g., pod). Although some Fabaceae may be weedy pests, others are important food crops [e.g., soybean, Glycine max (L.) Merr.] and fodder and forage plants (e.g., alfalfa, Medicago sativa L.). Some members of the Fabaceae produce valuable gums [e.g., gum arabic, Acacia senegal (L.) Britton] and dyes (e.g., indigo, Indigofera tinctoria L.), whereas others are prized as desirable ornamentals (e.g., eastern redbud, Cercis canadensis L.). Many species in the Fabaceae produce alkaloids or cyanogenic glycosides in different plant structures. Rotenone, an isoflavone insecticide, is derived from Derris eclipta (Wall.) Benth.
Junglerice is becoming more prevalent in Tennessee, Arkansas, and Mississippi row crop fields. The evolution of glyphosate-resistant (GR) junglerice populations is one reason for the increase. Another possible explanation is that glyphosate and clethodim grass activity is being antagonized by dicamba. This question has led to research to examine whether sequential applications alleviate antagonism observed with dicamba plus glyphosate and/or clethodim mixtures and determine whether sequential treatments with those herbicides at 24 h, 72 h, or 168 h can improve junglerice control. Glyphosate + clethodim applications provided >90% junglerice control. The observed levels of antagonism varied by whether the location of the test was in the greenhouse or the field, and the timing of applications. In the greenhouse, clethodim + dicamba provided excellent control, whereas in the field, the same treatment showed a greater than 30% reduction in junglerice control compared with clethodim alone. However, control was restored by using a mixture of glyphosate + clethodim without dicamba. The environment at the time of application and relative GR level of the junglerice influenced the overall control of these sequential applications. When clethodim applied first followed by dicamba at 72 h or 168 h, better control was observed compared with applying dicamba followed by clethodim. Overall, mixing glyphosate + clethodim provided the most complete junglerice control regardless of timing. These data confirm that leaving dicamba out of the spray tank will mitigate herbicide antagonism on junglerice control. These data would also indicate that avoiding dicamba and glyphosate mixtures will also improve the consistency of control with glyphosate-susceptible junglerice.
Junglerice has become a major weed in the mid-south and other areas of the United States. Glyphosate resistance has been documented in junglerice populations and is part of the reason for the increase in its prevalence. However, reduced junglerice control with glyphosate + dicamba and clethodim + dicamba mixtures has been observed in many production fields where glyphosate resistance has not yet evolved. Therefore, research was conducted to assess reduced junglerice control with glyphosate and clethodim when applied with dicamba. Adding dicamba to the spray tank with glyphosate reduced junglerice control by 27%. Adding dicamba to the spray tank with clethodim reduced junglerice control by 11%. The use of Turbo Teejet Induction (TTI) nozzles reduced junglerice control an additional 8% compared to applications with an air induction extended range (AIXR) nozzle. When a drift reduction agent (DRA) was added to dicamba mixtures with glyphosate or clethodim, junglerice control was reduced 36%. Junglerice control was similar with the glyphosate + dicamba treatment compared to the glyphosate + 2,4-D mixture. There was no interaction between nozzles and herbicide treatment. Regardless of herbicide treatment junglerice control was always lower when applied with the ultracourse TTI nozzle. Many applicators in Tennessee prefer to make one application of glyphosate + dicamba in a mixture to save time (authors’ personal experience). These results show that the addition of dicamba to glyphosate or clethodim applied with labeled nozzles and a DRA results in reduced junglerice control and should be avoided.
Junglerice has become a major weed in Tennessee cotton and soybean fields. Glyphosate has been relied on to control these accessions over the past two decades, but in recent years cotton and soybean producers have reported junglerice escapes after glyphosate + dicamba and/or clethodim applications. In the growing seasons of 2018 and 2019, a survey was conducted of weed escapes in dicamba-resistant (DR) crops. Junglerice was the most prevalent weed escape in these DR (Roundup Ready Xtend®) cotton and soybean fields in both years of the study. In 2018 and 2019, junglerice was found 76% and 64% of the time in DR cotton and soybean fields, respectively. Progeny from junglerice seeds collected during this survey was screened for glyphosate and clethodim resistance. Seventy percent of the junglerice accessions tested had an effective relative resistance factor to glyphosate of 3.1 to 8.5. In all, 13% of the junglerice accessions could no longer be effectively controlled with glyphosate. This research also showed that all sampled accessions could still be controlled with clethodim in a greenhouse environment, but less control was observed in the field. These data also suggest that another cause for the poor junglerice control is dicamba antagonism of glyphosate and clethodim activity.
Atrazine applied at planting is commonly used for weed control in corn. With global climate change causing an increase in river flooding in the United States over the past decade, producers need information to determine the best course of action in flooded fields treated with atrazine into which they wish to immediately plant soybean. Studies were designed to understand the effect of flooding on atrazine residual activity including atrazine concentration, soybean injury, and soybean yield. In 2012, soybean yield in flooded treatments was reduced by prior atrazine application. In 2014, soybean injury was <10% in all plots, and nonflooded, atrazine-treated soils had yields equal to the nontreated. Findings from this research indicated that it is possible for producers to consider replanting soybean after atrazine application, with appropriate changes to product labeling.
The evolution and widespread distribution of glyphosate-resistant broadleaf weed species catalyzed the introduction of dicamba-resistant crops that allow this herbicide to be applied POST to soybean and cotton. Applications of dicamba that are most cited for off-target movement have occurred in June and July in many states when weeds are often in high densities and at least 10 cm or taller at the time of application. For registration purposes, most field studies examining pesticide emissions are conducted using bare ground or very small plants. Research was conducted in Knoxville, TN, in the summer of 2017, 2018, and 2019 to examine the effect of application surface (tilled soil, dead plants, green plants) on dicamba emissions under field conditions. Dicamba emissions after application were affected by the treated surface in all years, with the order from least to most emissions being dead plants < tilled soil < green plant material. In fact, dicamba emissions were >300% when applied to green plants compared to other surfaces. These findings suggest that dicamba applications made to bare ground will likely underestimate what may occur under normal field use conditions when POST applications are made and the crop canopy or weed groundcover is nearly 100% green material. A potential change to enhance the accuracy of current environmental simulation models would be to increase the theoretical findings to allow for the effect of green plant material on dicamba emissions under field conditions.
Field studies were conducted in 2018 and 2019 in Arkansas, Indiana, Illinois, Missouri, and Tennessee to determine if cover-crop residue interfered with herbicides that provide residual control of Palmer amaranth and waterhemp in no-till soybean. The experiments were established in the fall with planting of cover crops (cereal rye + hairy vetch). Herbicide treatments consisted of a nontreated or no residual, acetochlor, dimethenamid-P, flumioxazin, pyroxasulfone + flumioxazin, pendimethalin, metribuzin, pyroxasulfone, and S-metolachlor. Palmer amaranth took 18 d and waterhemp took 24 d in the cover crop–alone (nontreated) treatment to reach a height of 10 cm. Compared with this treatment, all herbicides except metribuzin increased the number of days until 10-cm Palmer amaranth was present. Flumioxazin applied alone or in a mixture with pyroxasulfone were the best at delaying Palmer amaranth growing to a height of 10 cm (35 d and 33 d, respectively). The herbicides that resulted in the lowest Palmer amaranth density (1.5 to 4 times less) integrated with a cover crop were pyroxasulfone + flumioxazin, flumioxazin, pyroxasulfone, and acetochlor. Those four herbicide treatments also delayed Palmer amaranth emergence for the longest period (27 to 34 d). Waterhemp density was 7 to 14 times less with acetochlor than all the other herbicides present. Yield differences were observed for locations with waterhemp. This research supports previous research indicating that utilizing soil-residual herbicides along with cover crops improves control of Palmer amaranth and/or waterhemp.
Hophornbeam copperleaf (Acalypha ostryifolia Riddell) is an erect, herbaceous, dicot species in the Euphorbiaceae, or spurge, family that constitutes more than 200 genera and some 6,000 species (Mayfield and Webster 2013). Although the euphorbs have a cosmopolitan distribution, none are found in the Arctic (Mabberley 1997). Members of the Euphorbiaceae may be trees, shrubs, herbs (occasionally aquatic), or vines; sometimes succulent and cactus-like; and often have glands on vegetative plant parts (Mabberley 1997; Zomlefer 1994). Genera in the spurge family include Croton, Euphorbia, Ricinus, and Acalypha. Acalypha consists of 450 species that are native to both the Eastern and Western hemispheres (Zomlefer 1994). Acalypha was the name used by Hippocrates because the leaves resemble those of nettles, whereas ostryifolia alludes to the resemblance of leaves to plants in the genus Ostrya (hophornbeam trees; Burrows and Tyrl 2013; Haddock 2014; Hilty 2018). As plants mature in the fall, the leaves can turn reddish-brown, which may indicate why “copperleaf” is included in the species’ common name (Hilty 2018). Hophornbeam copperleaf is native to North America; it occurs in the United States ranging from Arizona east to Florida, north to Pennsylvania, and west to Nebraska (Anonymous 2019). It occurs in a variety of habitats including agronomic fields, cultivated areas, landscapes, roadsides, river and stream banks, thickets, pastures, and waste sites (Bryson and DeFelice 2010; Haddock 2014; Hilty 2018). This plant’s other common names include copperleaf, pineland three-seed mercury, Virginia copperleaf, hornbeam mercury, hornbeam three-seed mercury, mercury, and rough-pod copperleaf (Bryson and DeFelice 2010; Haddock 2014; Hilty 2018; Steckel 2006).
The pH of spray mixtures is an important attribute that affects dicamba volatility under field conditions. This report examined the effect of different components added to water sources that ranged in initial pH from 4.6 to 8.4. Commercial products were used, which include formulations of dicamba, glyphosate, the drift retardant Intact, ammonium sulfate (AMS), and several pH modifiers. Adding BAPMA salt of dicamba always increased the mixture pH, whereas diglycolamine + VaporGrip® (DGA+VG) had a mixed response. The addition of AMS decreased pH slightly (usually <0.5 pH unit), whereas the addition of potassium salt of glyphosate (GLY-K) always decreased the measured pH (from 1.0 to 2.1 pH units). A substantial pH change could have profound effects on dicamba volatility. Moreover, the 1.0 to 2.1 pH units would not be consistent with the registrant’s report stating that GLY-K decreased mixtures with DGA+VG pH by only 0.2 to 0.3 units. The drift retardant Intact had no effect on pH. There was no difference in resultant pH when comparing K salt and isopropylamine (IPA) salts of glyphosate. Spray carrier volume, ranging from 94 to 187 L ha–1, had only a minor effect on measured pH after the addition of various spray components. The addition of selected pH modifiers raised the pH above 5.0, which is a critical value according to the latest dicamba application labels. The order of mixing of various pH modifiers, including AMS, had only limited effect on measured spray solution pH.
Studies to evaluate the effect of application time of day (TOD) and protoporphyrinogen IX oxidase (PPO)-inhibiting herbicide–resistant Palmer amaranth on the efficacy of commonly used herbicides was conducted in Tennessee in 2017 and 2018. Treatments of fomesafen, lactofen, acifluorfen, paraquat, glufosinate, glufosinate plus fomesafen, paraquat plus fomesafen, and paraquat plus metribuzin were applied to PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) Palmer amaranth at sunrise and midday. Control of Palmer amaranth with acifluorfen, glufosinate, and glufosinate plus fomesafen was greater with the midday application. However, control of Palmer amaranth with paraquat-based treatments was greater with the sunrise application. TOD effects on PPO-inhibiting herbicides and paraquat-based treatments were more prominent for the PPO-R Palmer amaranth biotype. The TOD effect observed when applying glufosinate in early morning hours on PPO-S Palmer amaranth can be minimized by adding fomesafen to the tank mix. However, this strategy did not provide consistent performance on PPO-R Palmer amaranth. The percentages of living Palmer amaranth plants and control were greater when paraquat plus metribuzin was applied to both biotypes. These results highlight the necessity of at least two effective herbicide sites of action for POST applications intended for controlling PPO-R Palmer amaranth. In addition, the timing of herbicide applications can affect their activity in both PPO-R and PPO-S Palmer amaranth populations.
This research examined dicamba measurements following an application to soil inside a humidome. The dicamba formulations examined were the diglycolamine (DGA) and diglycolamine plus VaporGrip® (DGA+VG), both applied with glyphosate. Post-application dicamba measurements were related to ambient temperature, with more dicamba detected as the temperature increased. There also appeared to be a minimum temperature of ~15 C at which dicamba decreased to low levels. The addition of glyphosate to dicamba formulations decreased the spray mixture pH and increased the observed dicamba air concentrations. Adding glyphosate to DGA+VG increased detectable dicamba air concentrations by 2.9 to 9.3 times across the temperature ranges examined. Particle drift would not be expected to be a factor in the research, as applications were made remotely before treated soil was transported into the greenhouse. The most probable reason for the increased detection of dicamba at higher temperatures and with mixtures of glyphosate is via volatility.