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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.

The first steps in the development of dicamba tolerant transgenic plants were conducted by Sandoz Crop Protection in Palo Alto, CA and then contractually followed up at the University of Nebraska. The subsequent development by Monsanto of soybean and cotton varieties that would tolerate post-emergent application of dicamba substantially changed the use patterns of dicamba in the United States. Transgenic dicamba-tolerant (DT) seeds were first approved in 2016 in the United States, although the post-emergent use of dicamba was not legal that year. In 2017 and 2018, there was substantial market penetration of DT soybean and cotton seeds into the market and the occurrence of dicamba off-target movement (OTM) was highly variable across the United States. The driving force behind these new seed traits was the widespread failure of glyphosate to control broadleaf weeds effectively, especially those from the Conyza and Amaranthus genera. Herbicide research and development in the United States has historically involved both industry and academic weed scientists usually operating in a symbiotic and mutually respectful relationship, although there may have been disagreements at times about some aspects of herbicides and their development. The relatively recent introduction of DT varieties and legal post-emergent dicamba in the United States was a dynamic time for weed control, and adoption of DT crops and subsequent OTM of dicamba greatly changed the working relationships between academic scientists and representatives from Monsanto and some other private companies. Perhaps the greatest change was the lack of access of research materials that would be available to academic scientists for evaluation prior to the retail sale of those materials. Historically, academic scientists would have the ability to evaluate various new technologies and provide objective, independent comments on their potential utility prior to commercialisation. Monsanto largely restricted access to the DT seeds or new herbicide formulations. There are some states that have long-established policies that they will not recommend a new herbicide technology unless they have examined it under their specific field conditions. For example, University of Arkansas researchers would not recommend the use of post-emergent dicamba on DT crops when it first became legal to use. Monsanto responded by filing legal challenges of various types against the University of Arkansas faculty, including 64 exhibits of various legal aspects. Many other states had varying degrees of restrictions placed upon their research efforts, and most scientists had to sign various forms of confidentiality agreements to obtain access to the research material from Monsanto. There were two major differences in the dicamba labels when first introduced in 2017 and then again two years later. The first major difference was an entire section on herbicide resistance confirmation validation and management that was clearly stated on the label. The authors of this paper remind the readers that the reason we have dicamba being used is because of the wholesale failure of glyphosate resistance management systems. The golden era of weed control, essentially 100% weed control with no crop injury, was lost due to the complete lack of any resistance management strategies. The US EPA added label language hopefully to avoid evolution of resistance in weeds from happening again with essentially any new herbicide. Many herbicide users expect all new labels to include resistance management language. The second obvious change to the label was the duration of validity, which was normally many years in previous pesticide registrations. The first dicamba label, which came out in 2017, was valid for only two years. The next label which came out in 2019, was only valid for five years, although there is widespread speculation on the future of that label.

Junglerice has continued to expand its range as a serious weed pest in Tennessee cotton. Both glyphosate resistance and herbicide antagonism have been documented as possible causes for poor control. Approximately 15% of junglerice populations in Tennessee have been found to be glyphosate resistant. In addition, dicamba tank mixtures with glyphosate and/or clethodim have been reported to reduce junglerice control. Due to poor in-crop control, starting clean has taken on added importance when trying to control junglerice. Therefore, research was conducted to determine the best herbicide burndown methods utilizing clethodim, dicamba, glufosinate, glyphosate, or paraquat. Paraquat alone or in tank-mixtures with glyphosate or clethodim provided poor control (< 50%). Likewise, glufosinate alone or in tank-mixture with glyphosate or clethodim provided poor control (< 35%). A dicamba + glyphosate, glufosinate + clethodim, or paraquat + clethodim application provided poor junglerice control. Regardless of which herbicides were initially applied, making a follow-up application of glyphosate or glyphosate + clethodim two weeks later provided optimal control of junglerice. In Tennessee, a glyphosate + clethodim application at 14 days before planting is recommended to control junglerice, other grasses and some broadleaf weeds, followed by paraquat at-planting to control remaining weed species.

Seed retention, and ultimately seed shatter, are extremely important for the efficacy of harvest weed seed control (HWSC) and are likely influenced by various agroecological and environmental factors. Field studies investigated seed-shattering phenology of 22 weed species across three soybean [Glycine max (L.) Merr.]-producing regions in the United States. We further evaluated the potential drivers of seed shatter in terms of weather conditions, growing degree days, and plant biomass. Based on the results, weather conditions had no consistent impact on weed seed shatter. However, there was a positive correlation between individual weed plant biomass and delayed weed seed–shattering rates during harvest. This work demonstrates that HWSC can potentially reduce weed seedbank inputs of plants that have escaped early-season management practices and retained seed through harvest. However, smaller individuals of plants within the same population that shatter seed before harvest pose a risk of escaping early-season management and HWSC.

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. Nomenclature: junglerice; Echinochloa colona (L.) Link

Glufosinate is used widely to control glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats.) in cotton (Gossypium hirsutum L.). Previous research has shown weed control with several herbicides, including glufosinate, can be affected by application time of day. The response sometimes has been attributed to diurnal leaf movement (leaf orientation) in the weeds. The objectives of our research were to determine the influence of time of day of glufosinate application on Palmer amaranth control and to determine if the response was related to diurnal leaf orientation. Field experiments in five states evaluated Palmer amaranth control with glufosinate applied at nine intervals ranging from 1 h before sunrise to 6 h after sunrise and nine intervals ranging from 6 h before sunset to 1 h after sunset. Greatest Palmer amaranth control was achieved with glufosinate applied 2 h after sunrise to 1 h before sunset. Ammonium sulfate, added only to glufosinate 1 h before sunrise or 1 h after sunset treatments, did not improve control. Leaf angles of Palmer amaranth and velvetleaf (Abutilon theophrasti L.) were determined in a greenhouse at 1 h before light, 2 h after light, mid-day, 2 h before dark, and 1 h after dark. Leaves of velvetleaf oriented downward during the dark periods but time of day had no effect on leaf orientation of Palmer amaranth. These results demonstrate the need for sunlight for optimum glufosinate efficacy on Palmer amaranth.

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. Nomenclature: Clethodim; dicamba; glyphosate; junglerice [Echinochloa colona (L.) Link]; cotton (Gossypium hirsutum L.); soybean [Glycine max (L.) Merr.]

Potential effectiveness of harvest weed seed control (HWSC) systems depends upon seed shatter of the target weed species at crop maturity, enabling its collection and processing at crop harvest. However, seed retention likely is influenced by agroecological and environmental factors. In 2016 and 2017, we assessed seed-shatter phenology in 13 economically important broadleaf weed species in soybean [Glycine max (L.) Merr.] from crop physiological maturity to 4 wk after physiological maturity at multiple sites spread across 14 states in the southern, northern, and mid-Atlantic United States. Greater proportions of seeds were retained by weeds in southern latitudes and shatter rate increased at northern latitudes. Amaranthus spp. seed shatter was low (0% to 2%), whereas shatter varied widely in common ragweed (Ambrosia artemisiifolia L.) (2% to 90%) over the weeks following soybean physiological maturity. Overall, the broadleaf species studied shattered less than 10% of their seeds by soybean harvest. Our results suggest that some of the broadleaf species with greater seed retention rates in the weeks following soybean physiological maturity may be good candidates for HWSC. U.S. Department of Agriculture, Agricultural Research Service Areawide program; HATCH Program of the National Institute of Food and Agriculture; U.S. Department of AgricultureUnited States Department of Agriculture (USDA) Published version We would like to thank the staff and students at each university for helping conduct this research, specifically Kreshnik Bejleri, Sheri Heard, John Sanders, Barbara Scott, Annie Klodd, Zach Schaefer, Russ Garetson, Vitor Damiao, Matheus Martins, Camille Werner, Bruno Flaibam, and Camila Grassmann, and each institute's research and experimental stations. The authors would also like to thank the U.S. Department of Agriculture, Agricultural Research Service Areawide program for funding and the HATCH Program of the National Institute of Food and Agriculture and the U.S. Department of Agriculture for providing partial funding. No conflicts of interest have been declared.

BACKGROUND Protoporphyrinogen IX oxidase 2 (PPO2) inhibitors are important for the management of glyphosate- and acetolactate synthase-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.]. The evolving resistance to PPO inhibitors is of great concern. We surveyed the evolution of resistance to fomesafen in the US Mid-south and determined its correlation with the known functional PPO2 target-site mutations (TSM). RESULTS The 167 accessions analyzed were grouped into five categories, four resistant (147) and one susceptible (20). Arkansas accessions constituted 100% of the susceptible group while the Missouri accessions comprised 60% of the most resistant category. The majority of Mississippi accessions (88%) clustered in the high-survival-high-injury category, manifesting an early-stage resistance evolution. One hundred and fifteen accessions were genotyped for four known TSMs; 74% of accessions carried at least one TSM. The most common single TSM was ΔG210 (18% of accessions) and the predominant double mutation was ΔG210 + G399A (17%). Other mutations are likely less favorable, hence are rare. All TSMs were detected in three accessions. Further examination revealed that 9 and two individuals carried G399A + G210 and G399A + R128G TSM in the same allele, respectively. The existence of these combinations is supported by molecular modeling. CONCLUSIONS Resistance to PPO inhibitors is widespread across the Mid-southern USA. Highly resistant field populations have plants with multiple mutations. G399A is the most prone to co-occur with other ppo2 mutations in the same allele. Mutation at R128 in the configuration of the PPO2 catalytic domain restrains the co-occurrence of R128G with ΔG210, making ΔG210 + G399A the most plausible, tolerable functional mutation combination to co-occur in the same ppo2 allele.

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

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.Nomenclature: Acetochlor; flumioxazin; pyroxasulfone; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats.; waterhemp, Amaranthus tuberculatus (Moq) Sauer; cereal rye, Secale cereale L.; hairy vetch, Vicia villosa Roth; soybean, Glycine max (L.) Merr.

But your snobbiness, unless you persistently root it out like the bindweed it is, sticks by you till your grave. – George Orwell The real danger in a garden came from the bindweed. That moved underground, then surfaced and took hold. Strangling plant after healthy plant. Killing them all, slowly. And for no apparent reason, except that it was nature. – Louise Penny

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

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.Nomenclature: acifluorfen; fomesafen; glufosinate; lactofen; metribuzin; paraquat; Palmer amaranth; Amaranthus palmeri S. Watson

Primroses, The spring may love them; Summer knows but little of them. Foresight. William Wordsworth, 1819Ring-ting! I wish I were a Primrose A bright yellow Primrose, blowing in the Spring! Wishing. William Allingham“The snowdrop and primrose our woodlands adorn, and violets bathe in the wet o' the morn.” Robert Burns

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.

In recent years, the use of cover crops has increased in U.S. crop production systems. An important aspect of successful cover crop establishment is the preceding crop and herbicide program, because some herbicides have the potential to persist in the soil for several months. Few studies have been conducted to evaluate the sensitivity of cover crops to common residual herbicides used in soybean production. The same field experiment was conducted in 2016 in Arkansas, Illinois, Indiana, Missouri, Tennessee, and Wisconsin, and repeated in Arkansas, Illinois, Indiana, Mississippi, and Missouri in 2017 to evaluate the potential of residual soybean herbicides to carryover and reduce cover crop establishment. Herbicides applied during the soybean growing season included acetochlor; acetochlor plus fomesafen; chlorimuron plus thifensulfuron; fomesafen; fomesafen plus S-metolachlor followed by acetochlor; imazethapyr; pyroxasulfone; S-metolachlor; S-metolachlor plus fomesafen; sulfentrazone plus S-metolachlor; sulfentrazone plus S-metolachlor followed by fomesafen plus S-metolachlor; and sulfentrazone plus S-metolachlor followed by fomesafen plus S-metolachlor followed by acetochlor. Across all herbicide treatments, the sensitivity of cover crops to herbicide residues in the fall, from greatest to least, was forage radish = turnip > annual ryegrass = winter oat = triticale > cereal rye = Austrian winter pea = hairy vetch = wheat > crimson clover. Fomesafen (applied 21 and 42 days after planting [(DAP]); chlorimuron plus thifensulfuron and pyroxasulfone applied 42 DAP; sulfentrazone plus S-metolachlor followed by fomesafen plus S-metolachlor; and sulfentrazone plus S-metolachlor followed by fomesafen plus S-metolachlor followed by acetochlor caused the highest visual ground cover reduction to cover crop species at the fall rating. Study results indicate cover crops are most at risk when following herbicide applications in soybean containing certain active ingredients such as fomesafen, but overall there is a fairly low risk of cover crop injury from residual soybean herbicides applied in the previous soybean crop. Nomenclature: Acetochlor; chlorimuron; fomesafen; imazethapyr; pyroxasulfone; S-metolachlor; sulfentrazone; thifensulfuron; annual ryegrass, Lolium multiflorum Lam.; Austrian winter pea, Pisum sativum L.; cereal rye, Secale cereal L.; crimson clover, Trifolium incarnatum L.; hairy vetch, Vicia villosa Roth; turnip, Brassica campestris L.; soybean, Glycine max (L.) Merr; forage radish, Raphanus sativus L.; triticale, Triticosecale rimpaui; winter wheat, Triticum aestivum L; winter oat, Avena sativa L.

Glufosinate remains an important postemergence (POST) herbicide for controlling glyphosate-resistant Amaranthus palmeri (S. Wats) in auxin-tolerant trait systems. Although visual injury from glufosinate applications to WideStrike cultivars is expected, concerns have been raised on the visual injury noted in XtendFlex cultivars, particularly with sequential late-POST glufosinate applications or when S-metolachlor is tank-mixed with glufosinate. Field trials were established in Jackson, TN during 2015 and 2017 and in Huntersville, TN during 2016. Herbicide treatments included an untreated; one, two, and three sequential applications of glufosinate; and glufosinate + S-metolachlor followed by (FB) glufosinate FB glufosinate + S-metolachlor. Applications began 40 days after planting and sequential applications were made every 10 to 14 days. Cultivars included DP 1522 B2XF (DeltaPine, Bayer CropScience, St. Louis, MO), PHY 333 WRF (Phytogen, Corteva Agriscience, Indianapolis, IN), and ST 4946 GLB2 (Stoneville, BASF Corp., Florham Park, NJ). Visual injury ratings varied across timing, treatment, cultivar, and site-year. Three sequential applications of glufosinate with two applications of S-metolachlor caused 7 to 20%, 2 to 15%, and 1 to 8% injury 10 days after the last application in PHY 333 WRF, DP 1522 B2XF, and ST 4946 GLB2, respectively. Cultivar lint yield and fiber quality did not vary by herbicide treatment. Producers who apply glufosinate should expect increasing visual injury from LibertyLink to XtendFlex to WideStrike cultivars, with a sharp increase in visual injury from XtendFlex to WideStrike cultivars; however, sequential, labeled applications of glufosinate with or without two applications of S-metolachlor will likely not impact yields of LibertyLink, WideStrike, or XtendFlex cultivars.

Protoporphyrinogen IX oxidase (PPO)–inhibiting herbicides (WSSA Group 14) have been used in agronomic row crops for over 50 yr. Broadleaf weeds, including glyphosate-resistant Palmer amaranth, have been controlled by this herbicide site of action PRE and POST. Recently, Palmer amaranth populations were reported resistant to PPO inhibitors in 2011 in Arkansas, in 2015 in Tennessee, and in 2016 in Illinois. Historically, the mechanism for this resistance involves the deletion of a glycine at position 210 (ΔG210) in a PPO enzyme encoded by the PPX2 gene; however, the ΔG210 deletion did not explain all PPO inhibitor–resistant Palmer amaranth in Tennessee populations. Recently, two new mutations within PPX2 (R128G, R128M) that confer resistance to PPO inhibitors were identified in Palmer amaranth. Therefore, research is needed to document the presence and distribution of the three known mutations that confer PPO inhibitor resistance in Tennessee. In 2017, a survey was conducted in 18 fields with Palmer amaranth to determine whether resistance existed and the prevalence of each known mutation in each field. Fomesafen was applied at 265 g ai ha–1 to Palmer amaranth infestations within each field to select for resistant weeds for later analysis. Where resistance was described (70% of surviving plants), the ΔG210 mutation was detected in 47% of resistant plants. The R128G mutation accounted for 42% of resistance, similar to the frequency of the ΔG210 mutation. The R128M mutation was less frequent than the other two mutations, accounting for only 10% of the resistance. All mutations detected in this study were heterozygous. Additionally, no more than one of the three PPX2 mutations were detected in an individual surviving plant. Similar to previous research, about 70% of PPO resistance was accounted for by these three known mutations, leaving about 30% of resistance not characterized in Tennessee populations. Survivors not showing the three known PPO mutations suggest that other resistance mechanisms are present.

Protoporphyrinogen IX oxidase 2 (PPO2) inhibitors are important for the management of glyphosate- and acetolactate synthase-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.]. The evolving resistance to PPO inhibitors is of great concern. We surveyed the evolution of resistance to fomesafen in the US Mid-south and determined its correlation with the known functional PPO2 target-site mutations (TSM).The 167 accessions analyzed were grouped into five categories, four resistant (147) and one susceptible (20). Arkansas accessions constituted 100% of the susceptible group while the Missouri accessions comprised 60% of the most resistant category. The majority of Mississippi accessions (88%) clustered in the high-survival-high-injury category, manifesting an early-stage resistance evolution. One hundred and fifteen accessions were genotyped for four known TSMs; 74% of accessions carried at least one TSM. The most common single TSM was ΔG210 (18% of accessions) and the predominant double mutation was ΔG210 + G399A (17%). Other mutations are likely less favorable, hence are rare. All TSMs were detected in three accessions. Further examination revealed that 9 and two individuals carried G399A + G210 and G399A + R128G TSM in the same allele, respectively. The existence of these combinations is supported by molecular modeling.Resistance to PPO inhibitors is widespread across the Mid-southern USA. Highly resistant field populations have plants with multiple mutations. G399A is the most prone to co-occur with other ppo2 mutations in the same allele. Mutation at R128 in the configuration of the PPO2 catalytic domain restrains the co-occurrence of R128G with ΔG210, making ΔG210 + G399A the most plausible, tolerable functional mutation combination to co-occur in the same ppo2 allele.

Background Commercialization of dicamba-resistant soybean and cotton and subsequent post-emergence applications of dicamba contributed to at least 1.4 and 0.5 million hectares of dicamba-injured soybean in the United States in 2017 and 2018, respectively. This research was initiated to identify environmental factors that contribute to off-target dicamba movement. A survey was conducted following the 2017 growing season to collect information from dicamba applications that remained on the target field and those where dicamba moved. Weather and environmental data surrounding applications were collected and used to identify factors that reduce the likelihood of off-target movement. Soil pH was one factor identified in the model, and field experiments were conducted in 2018 and 2019 to validate the model. Three commercially-available dicamba formulations and one formulation currently in development were applied to soil at five distinct pH values. Sensitive soybean was used as a bioassay plant to detect dicamba volatilization. Results Wind speeds the day of and following application, nearest water source to the field, soybean production acreage in the county, and soil pH were identified as factors that influence the likelihood for off-target movement. In the field study, when dicamba was applied to pH-adjusted soil and placed under low tunnels for 72 h, dicamba volatility increased when soil pH decreased as the model predicted. Dicamba choline, which is not commercially available, had reduced volatility compared to other formulations tested. Conclusion Results of this study identified specific factors that contribute to successful and unsuccessful dicamba applications and should be considered prior to applications. © 2020 Society of Chemical Industry.

The triazines are one of the most widely used herbicide classes ever developed and are critical for managing weed populations that have developed herbicide resistance. These herbicides are traditionally valued for their residual weed control in more than 50 crops. Scientific literature suggests that atrazine, and perhaps others-triazines, may no longer remain persistent in soils due to enhanced microbial degradation. Experiments examined the rate of degradation of atrazine and two other triazine herbicides, simazine and metribuzin, in both atrazine-adapted and non-history Corn Belt soils, with similar soils being used from each state as a comparison of potential triazine degradation. In three soils with no history of atrazine use, thet1/2of atrazine was at least four times greater than in three soils with a history of atrazine use. Simazine degradation in the same three sets of soils was 2.4 to 15 times more rapid in history soils than non-history soils. Metribuzin in history soils degraded at 0.6, 0.9, and 1.9 times the rate seen in the same three non-history soils. These results indicate enhanced degradation of the symmetrical triazine simazine, but not of the asymmetrical triazine metribuzin.

Recently, several incidents of glyphosate failure on junglerice [Echinochloa colona(L.) Link] have been reported in the midsouthern United States, specifically in Mississippi and Tennessee. Research was conducted to measure the magnitude of glyphosate resistance and to determine the mechanism(s) of resistance to glyphosate inE. colonapopulations from Mississippi and Tennessee. ED50(dose required to reduce plant growth by 50%) values for a resistant MSGR4 biotype, a resistant TNGR population, and a known susceptible MSGS population were 0.8, 1.62, and 0.23 kg ae ha−1of glyphosate, respectively. The resistance index calculated from the these ED50values indicated that the MSGR4 biotype and TNGR population were 4- and 7-fold, respectively, resistant to glyphosate relative to the MSGS population. The absorption patterns of [14C]glyphosate in the TNGR and MSGS populations were similar. However, the MSGS population translocated 13% more [14C]glyphosate out of the treated leaf compared with the TNGR population at 48 h after treatment.EPSPSgene sequence analyses of TNGRE. colonaindicated no evidence of any point mutations, but several resistant biotypes, including MSGR4, possessed a single-nucleotide substitution of T for C at codon 106 position, resulting in a proline-to-serine substitution (CCA to TCA). Results from quantitative polymerase chain reaction analyses suggested that there was no amplification of theEPSPSgene in the resistant populations and biotypes. Thus, the mechanism of resistance in the MSGR population (and associated biotypes) is, in part, due to a target-site mutation at the 106 loci of theEPSPSgene, while reduced translocation of glyphosate was found to confer glyphosate resistance in the TNGR population.

Herbicide resistance is ‘wicked’ in nature; therefore, results of the many educational efforts to encourage diversification of weed control practices in the United States have been mixed. It is clear that we do not sufficiently understand the totality of the grassroots obstacles, concerns, challenges, and specific solutions needed for varied crop production systems. Weed management issues and solutions vary with such variables as management styles, regions, cropping systems, and available or affordable technologies. Therefore, to help the weed science community better understand the needs and ideas of those directly dealing with herbicide resistance, seven half-day regional listening sessions were held across the United States between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide resistance management. The major goals of the sessions were to gain an understanding of stakeholders and their goals and concerns related to herbicide resistance management, to become familiar with regional differences, and to identify decision maker needs to address herbicide resistance. The messages shared by listening-session participants could be summarized by six themes: we need new herbicides; there is no need for more regulation; there is a need for more education, especially for others who were not present; diversity is hard; the agricultural economy makes it difficult to make changes; and we are aware of herbicide resistance but are managing it. The authors concluded that more work is needed to bring a community-wide, interdisciplinary approach to understanding the complexity of managing weeds within the context of the whole farm operation and for communicating the need to address herbicide resistance.

Seven half-day regional listening sessions were held between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide-resistance management. The objective of the listening sessions was to connect with stakeholders and hear their challenges and recommendations for addressing herbicide resistance. The coordinating team hired Strategic Conservation Solutions, LLC, to facilitate all the sessions. They and the coordinating team used in-person meetings, teleconferences, and email to communicate and coordinate the activities leading up to each regional listening session. The agenda was the same across all sessions and included small-group discussions followed by reporting to the full group for discussion. The planning process was the same across all the sessions, although the selection of venue, time of day, and stakeholder participants differed to accommodate the differences among regions. The listening-session format required a great deal of work and flexibility on the part of the coordinating team and regional coordinators. Overall, the participant evaluations from the sessions were positive, with participants expressing appreciation that they were asked for their thoughts on the subject of herbicide resistance. This paper details the methods and processes used to conduct these regional listening sessions and provides an assessment of the strengths and limitations of those processes.

Palmer amaranth resistance to protoporphyrinogen oxidase (PPO)-inhibiting herbicides has become an increasing problem to producers throughout the southeast region of the United States. Traditionally, these herbicides can be used as foliar-applied and soil-applied in glyphosate resistant (GR) cropping systems to control GR Palmer amaranth. Heavy reliance on PPO herbicides has contributed to the increased selection for PPO inhibitor-resistant (PPO-R) Palmer amaranth biotypes. Dose response greenhouse research was conducted to evaluate the efficacy of soil-applied flumioxazin, fomesafen, saflufenacil and sulfentrazone on a known susceptible (S) and resistant (R) Palmer amaranth biotype. Both R and S populations reached maximum germination at 14 d after treatment (DAT). The data from this study suggests complete control (100%) was achieved for the S biotype at 35 d after treatment (DAT) with all herbicides. The R biotype showed difference among herbicide treatments with flumioxazin and saflufenacil having similar responses in control and fomesafen and sulfentrazone resulting in less control of the R Palmer amaranth biotypes. The calculated relative resistance factor ranged from 3.5 to 6.0, and averaged 5X for the four herbicides. This research indicated that the PPO-R population was still responsive to all tested herbicides, but a low level of resistance was present.Nomenclature: Flumioxazin; fomesafen; saflufenacil; sulfentrazone; Palmer amaranth, Amaranthus palmeri S. Wats

Soybean consultants from Arkansas, Louisiana, southeast Missouri, Mississippi, and Tennessee were surveyed in 2016 to assess weed management practices and the prevalence of herbicide-resistant weeds in midsouthern U.S. soybean production. The consultants surveyed represented 13%, 28%, 8%, 16%, and 5% of the total soybean area planted in Arkansas, Louisiana, southeast Missouri, Mississippi, and Tennessee, respectively. Of the total scouted area, 78% of the consultants said their growers planted glyphosate-resistant soybean in 2016, with 18% planting glufosinate-resistant (LibertyLink®), primarily due to familiarity with and cost of the technology. Although 94% of the consultants determined that glufosinate was most effective on killing Palmer amaranth, the primary concern associated with controlling herbicide-resistant weeds was the associated cost, followed by return profit and time constraints. Palmer amaranth, morningglory species, horseweed, barnyardgrass, and Italian ryegrass were the five most problematic weeds in soybean across the five states. Palmer amaranth was the most problematic and important weed in each state individually. The increased concern (77% of consultants) with this species was attributed to the rising concern with and occurrence of protoporphyrinogen oxidase-resistant Palmer amaranth. Consultants were of the opinion that more research was needed on cover crops and the new traited technologies in order to improve weed management in soybean.Nomenclature: Glufosinate; glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; horseweed, Conyza canadensis (L.) Cronq.; Italian ryegrass, Lolium perenne L. ssp. multiflorum (Lam.) Husnot; morningglory, Ipomoea spp.; Palmer amaranth, Amaranthus palmeri S. Wats.; soybean, Glycine max (L.) Merr.

A study was conducted to evaluate the response of glyphosate- and dicamba-tolerant (GDT) soybean and weed control from cover crop different termination intervals before and after soybean planting. Cover crop biomass was highest when terminated at planting, decreased with the 7- and 14-d preplant (DPP) and day-after-planting (DAP) timings, and again at the 14 DPP and DAP timings. Glyphosate dicamba provided total control of cover crops by 21 DAP. Cover crop termination timing did not influence soybean population or yield. Palmer amaranth control at the 21 and 28 d after termination (DAT) was 97% to 99%. Differences in Palmer amaranth control were not detected among herbicide programs or termination intervals at the end of season rating, and all treatments provided ≥97% control. Although differences in Palmer amaranth control were not apparent at the end of the season, the delay in cover crop affected the number of days until 10-cm Palmer amaranth was present. When utilizing a wheat hairy vetch cover crop in DGT soybeans, producers should delay cover crop termination until 11 to 14 DPP and make at least one POST application of glyphosate dicamba an additional herbicide mode of action (MOA) to maximize Palmer amaranth control and soybean yields.Nomenclature: Glyphosate; dicamba; fomesafen; Palmer amaranth, Amaranthus palmeri (S.) Wats.; hairy vetch, Vicia villosa Roth.; wheat, Triticum aestivum L.; soybean, Glycine max (L.) Merr.

A study to evaluate the effect of application time of day (TOD) on the efficacy of five burndown herbicides was conducted in Alabama and Tennessee. Treatments of 2,4-D, dicamba, glufosinate, paraquat, and saflufenacil were applied at sunrise, midday, or sunset to a native population of horseweed and analyzed separately. Control of glyphosate-resistant (GR) horseweed with 2,4-D, dicamba, glufosinate, and saflufenacil was greatest from the midday application. Percentage of living horseweed counts for all of these herbicides followed a similar pattern. Control from paraquat was lowest at the midday timing and greatest from the sunset application with surviving horseweed plant populations reflecting those control ratings. Application TOD significantly affected all of the herbicides in this research. Applications of 2,4-D, dicamba, glufosinate, and saflufenacil are more efficacious when applied during the middle portion of the day, while paraquat is more efficacious when applied at sunset for maximum horseweed control.

BACKGROUND Resistance to herbicides that inhibit protoporphyrinogen oxidase (PPO) is a widespread and growing problem for weed managers across the midwestern and midsouthern United States. In Amaranthus spp., this resistance is known to be conferred by a glycine deletion at the 210th amino acid (ΔG210) in PPO2. Preliminary analysis indicated that the ΔG210 mutation did not fully account for observed resistance to PPO inhibitors in two Amaranthus palmeri populations from Tennessee and one from Arkansas. RESULTS Sequencing PPX2 cDNA from six resistant plants uncovered two new mutations at the R98 site (R98G and R98M), a site previously found to endow PPO-inhibitor resistance in Ambrosia artemisiifolia. Sequencing of this region from additional plants sprayed with 264 g fomesafen ha−1 showed the presence of one or both R98 mutations in a subset of the resistant plants from all three populations. No plants sensitive to fomesafen contained either mutation. A derived cleaved amplified polymorphic sequence (dCAPS) assay to test for the presence of these mutations in A. palmeri was developed. CONCLUSION Two new mutations of PPX2 (R98G, R98M) likely confer resistance to PPO-inhibitors in A. palmeri, and can be rapidly identified using a dCAPS assay. © 2017 Society of Chemical Industry

Field experiments were conducted to evaluate the integration of cover crops and POST herbicides to control glyphosate-resistant Palmer amaranth in cotton. The winter-annual grasses accumulated the greatest amount of biomass and provided the most Palmer amaranth control. The estimates for the logistic regression would indicate that 1540 kg ha−1would delay Palmer amaranth emerging and growing to 10 cm by an estimated 16.5 days. The Palmer amaranth that emerged in the cereal rye and wheat cover crop treatments took a longer time to reach 10 cm compared to the hairy vetch and crimson clover treatments. POST herbicides were needed for adequate control of Palmer amaranth. The glufosinate-based weed control system provided greater control (75% vs 31%) of Palmer amaranth than did the glyphosate system. These results indicate that a POST only herbicide weed management system did not provide sufficient control of Palmer amaranth, even when used in conjunction with cover crops that produced a moderate level of biomass. Therefore, future recommendations for GR Palmer amaranth control will include integrating cover crops with PRE herbicides, overlaying residual herbicides in-season, timely POST herbicide applications, and hand weeding in order to achieve season-long control of this pest.

Background Atrazine (ATZ) has been a key herbicide for annual weed control in corn, with both a soil and post-emergence vegetation application period. Although enhanced ATZ degradation in soil with a history of ATZ use has been reported, the extent and rate of degradation in the US Corn Belt is uncertain. We show that enhanced ATZ degradation exists across much of the country. Results Soils from 15 of 16 surveyed states had enhanced ATZ degradation. The average ATZ half-life was only 2.3 days in ATZ history soils, compared with an average 14.5 days in soils with no previous ATZ use, meaning that ATZ degrades an average 6 times faster in soils with previous ATZ use. Conclusion When ATZ is used for several years, enhanced degradation will undoubtedly change the way ATZ is used in agronomic crops and also its ultimate environmental fate. © 2017 Society of Chemical Industry.

A commercial sprayer operated at a field speed of 24 km h-1 simultaneously applied glufosinate-ammonium through seven spray tip treatments spaced along a 30.5 m boom for measured foliar deposits of herbicide in 35 cm tall Palmer amaranth weeds and spray deposits on foliar-mounted water-sensitive paper (WSP). The experiment followed one that found increased herbicide deposits for dual tips with an adjacent, fore-aft mount, downward-pointed pre-orifice tip (Extremely Coarse) operated with blended pulse-width modulation (bPWM) and a pre-orifice tip (Fine) operated constant (non-bPWM) under moderate ambient wind velocities from 3.1 to 4.1 m s-1. Additional dual-tip treatments were added to the dual-tip configuration for the current experiment to expand droplet Coarseness and to add dual tips operated constant to isolate bPWM effects. Tested treatments in common with the previous experiment included the original dual-tip bPWM and non-bPWM combination, Y-adapter fore-aft-mounted pre-orifice tips with diverging spray patterns both operated bPWM, and an air-induction extended-range tip operated constant. Palmer amaranth weeds, total spray volume rate of 93.5 L ha-1, sprayer speed of 24 km h-1, and test methods were similar between studies, except for negligible wind in the current experiment. Conditions were clear and sunny during spraying without indicators of a stable atmosphere. Overall mean glufosinate-ammonium deposits recovered from leaves were greatest for dual-tips operated constant at reduced droplet size (Very Coarse and Fine) due to reduced required tip size operated without bPWM, and for increased droplet size for Y-adapter-mounted pre-orifice tips (Extremely Coarse and Coarse) operated with bPWM, resulting in overall mean glufosinate-ammonium leaf deposits of 15.9 and 15.0 µg a.i. cm-2, respectively. The combination of dual tips at reduced droplet size or the Y-adapter fore-aft spray pattern divergence of bPWM tips coupled with high sprayer speed enhanced droplet interception by Palmer amaranth plants under negligible wind conditions, since the collected deposits, even without summed integration over foliage height, significantly exceeded the applied rate of 8.2 µg a.i. cm-2. An air-induction extended-range tip non-bPWM (Very Coarse) provided the next highest mean in overall glufosinate-ammonium deposit. One increased-droplet size dual-tip, pre-orifice tip bPWM and non-bPWM (Ultra Coarse and Coarse) resulted in a mean deposit that was not significantly different from the air-induction extended-range tip operated non-bPWM. Other dual-tip combinations with bPWM and non-bPWM, including the original dual-tip configuration in the previous study, resulted in significantly reduced mean herbicide deposits. Considering all tested tips, advantages of bPWM depended on spray tip droplet size classifications and Y-mounted fore-aft divergence of spray patterns. Overall mean WSP spot deposits were greatest for reduced droplet size (Very Coarse and Fine) dual pre-orifice tips operated non-bPWM, corresponding with the highest numerical overall mean of glufosinate-ammonium deposit. This correspondence of highest spot deposits and highest mean glufosinate-ammonium deposit also occurred in the previous study. Increased Palmer amaranth control correlated with increased glufosinate-ammonium deposit and decreased volume median diameter (Dv0.5) determined with WSP electronic scans, with the air-induction extended-range tip operated constant and the Y-adapter pre-orifice tip operated as bPWM providing the highest weed control. Overall mean WSP spot deposits ranged from 42.3 to 81.1 spots cm-2, compared to 14.0 to 47.0 spots cm-2 previously reported for similar spray conditions, with spot deposits attributed to negligible wind versus wind, respectively. Thus, the spray environment, particularly wind, exhibited effects on nozzle tip comparisons for foliar deposition and may offer some rationale for the conflicting published data beyond the examined treatments. Keywords: Application technology, Blended pulse-width modulation, Herbicide, Herbicide resistance, Nozzle, Spray deposition, Water-sensitive paper, Weed.

A field study was conducted in 2014 and 2015 in Arkansas, Illinois, Indiana, Ohio, Tennessee, Wisconsin, and Missouri to determine the effects of tillage system and herbicide program on season-long emergence of Amaranthus species in glufosinate-resistant soybean. The tillage systems evaluated were deep tillage (fall moldboard plow followed by (fb) one pass with a field cultivator in the spring), conventional tillage (fall chisel plow fb one pass with a field cultivator in the spring), minimum tillage (one pass of a vertical tillage tool in the spring), and no-tillage (PRE application of paraquat). Each tillage system also received one of two herbicide programs; PRE application of flumioxazin (0.09 kg ai ha−1) fb a POST application of glufosinate (0.59 kg ai ha−1) plus S-metolachlor (1.39 kg ai ha−1), or POST-only applications of glufosinate (0.59 kg ha−1). The deep tillage system resulted in a 62, 67, and 73% reduction in Amaranthus emergence when compared to the conventional, minimum, and no-tillage systems, respectively. The residual herbicide program also resulted in an 87% reduction in Amaranthus species emergence compared to the POST-only program. The deep tillage system, combined with the residual program, resulted in a 97% reduction in Amaranthus species emergence when compared to the minimum tillage system combined with the POST-only program, which had the highest Amaranthus emergence. Soil cores taken prior to planting and herbicide application revealed that only 28% of the Amaranthus seed in the deep tillage system was placed within the top 5-cm of the soil profile compared to 79, 81, and 77% in the conventional, minimum, and no-tillage systems. Overall, the use of deep tillage with a residual herbicide program provided the greatest reduction in Amaranthus species emergence, thus providing a useful tool in managing herbicide-resistant Amaranthus species where appropriate.Nomenclature: Flumioxazin; glufosinate; metolachlor; paraquat; Amaranthus; soybean, Glycine max (L.) Merr.En 2014 y 2015, se realizo un estudio de campo en Arkansas, Illinois, Indiana, Ohio, Tennessee, Wisconsin, y Missouri para determinar los efectos del sistema de labranza y el programa de herbicidas sobre la emergencia de especies de Amaranthus a lo largo de la temporada de crecimiento en soja resistente a glufosinate. Los sistemas de labranza evaluados fueron labranza profunda (arado de vertedera en el otono seguido por (fb) un pase de cultivador de campo en la primavera), labranza convencional (arado de cincel en el otono seguido de un pase de cultivador de campo en la primavera), labranza minima (un pase de una herramienta de labranza vertical en la primavera), y cero labranza (aplicacion PRE de paraquat). Cada sistema de labranza tambien recibio uno de dos programas de herbicidas; aplicacion PRE de flumioxazin (0.09 kg ai ha−1) fb glufosinate POST (0.59 kg ai ha−1) mas S-metolachlor (1.39 kg ai ha−1), o solo aplicaciones POST de glufosinate (0.59 kg ha−1). El sistema de labranza profunda resulto en una reduccion de 62, 67, y 73% en la emergencia de Amaranthus cuando se comparo con los sistemas de labranza convencional, minima, y cero, respectivamente. El programa con un herbicida residual tambien resulto en una reduccion de 87% en la emergencia de especies Amaranthus al compararse con el programa de solo herbicidas POST. El sistema de labranza profunda, combinado con el programa residual, resulto en una reduccion de 97% en la emergencia de especies de Amaranthus cuando se comparo con el sistema de labranza minima combinado con el programa de solo herbicidas POST, el cual tuvo la mayor emergencia de Amaranthus. Muestras de suelo tomadas antes de la siembra y la aplicacion de herbicidas revelaron que en el sistema de labranza profunda solamente 28% de las semillas de Amaranthus fueron localizadas en los 5 cm de suelo superiores del perfil del suelo comparado con 79, 81, y 77% en los sistemas de labranza convencional, minima, y cero. En general

Five nozzle tip treatments included nozzle tip designs and selected use of blended pulse-width modulation (bPWM), with tip selections based on a maxim of reduced spray drift potential, either through droplet size classification or spray entrainment of dual sprays. Spray nozzle technologies prioritized for this study included bPWM of pre-orifice tips in various configurations of single and dual tips, and air-induction tips operated in constant (non-bPWM) mode. A self-propelled sprayer equipped with a 30.5 m boom and bPWM applied glufosinate-ammonium herbicide simultaneously through the five different spray nozzle tip treatments. Ambient wind veleocities ranged from 3.1 to 4.1 m s-1 during application. Herbicide deposits on leaves and water-sensitive paper (WSP) coverage, spot density, and droplet size characteristics at top and middle canopy locations in Palmer amaranth were used to compare nozzle tip treatments. Treatment 1 (pre-orifice tips, bPWM and non-bPWM) produced significantly greater herbicide deposits than all tips except Treatment 2 (pre-orifice tip, bPWM). The latter was not significantly different from all other tips. The numerically lowest deposit level, based on the overall means, was produced by Treatment 5 (air-induction deflector tip, non-bPWM), although the leaf deposit was not significantly different from most tips. Deposits of glufosinate-ammonium for top canopy locations were greater than middle canopy location deposits for a given nozzle tip treatment. Summation of deposits across locations sometimes exceeded the application rate of 4.5 μg cm-2, attributed to spray cloud momentum due to the application speed of 24 km h-1. The addition of more weed plants or crop plants could possibly alter spray cloud momentum or serve as interceptors of droplets at the expense of deposit levels on Palmer amaranth. Glufosinate-ammonium leaf deposits were inversely proportional to and significantly correlated with canopy coverage of the ground, which supported this hypothesis. The highest mean WSP coverage occurring at top location for Treatment 1 was significantly greater than the mean WSP coverage at top locations for Treatment 4 (air-induction extended-range tip, non-bPWM) and Treatment 5, and greater than mean WSP coverages for all tip treatments at middle locations based on p-level observations. The highest mean WSP spot deposit for the top location of Treatment 1 (57.9 cm-2) was significantly greater than the mean WSP spot deposits for all other tip treatments and canopy locations. A wide range of droplet sizes resulting from the entrainment of a Fine spray into an Extremely Coarse spray may have contributed to high values of coverage and spot deposit for Treatment 1. In conclusion, use of a pre-orifice nozzle tip with bPWM plus a pre-orifice nozzle tip in constant (non-bPWM) mode that produced contrasting droplet sizes for spray entrainment between the two tips provided a means to increase glufosinate-ammonium deposits on leaves and increase coverage and spot density on WSP. On the other hand, Y-adapter mounted pre-orifice tips with bPWM and separate air-induction nozzle tips (extended-range and deflector designs) operated as non-bPWM applications did not result in high levels of glufosinate-ammonium deposits on leaves and did not result in high levels of coverage and spot density on WSP.

Palmer amaranth is a very problematic weed that has evolved resistance to several classes of herbicides, including 5-enolypyruvylshikimate-3-phosate synthase–inhibiting herbicides and photosystem II–inhibiting herbicides. In recent years, corn producers have had difficulty controlling large Palmer amaranth (> 20 cm) in corn > 30 cm whether it be due to environmental conditions or management failures. Palmer amaranth management in corn this tall is made even more challenging because atrazine is not labeled POST in corn > 30 cm tall. Therefore, a study was conducted in 2013 and 2014 in Jackson, TN, to evaluate herbicide programs in corn > 30 cm tall for the control of glyphosate-resistant Palmer amaranth > 20 cm tall. Treatments consisted of herbicides applied alone and in mixtures with dicamba plus diflufenzopyr. Herbicides were applied POST to corn between the V5 and V6 growth stages. Dicamba plus diflufenzopyr 28 d after application controlled Palmer amaranth > 87%. The herbicides alone or in combinations applied as tank mixtures did not improve control (< 76%) over dicamba plus diflufenzopyr alone. There were no grain-yield differences among treatments because of Palmer amaranth control. This was likely due to the Palmer amaranth competition having already affected corn yield by the V5 to V6 corn growth stages.

Pigweeds are among the most abundant and troublesome weed species across Midwest and mid-South soybean production systems because of their prolific growth characteristics and ability to rapidly evolve resistance to several herbicide sites of action. This has renewed interest in diversifying weed management strategies by implementing integrated weed management (IWM) programs to efficiently manage weeds, increase soybean light interception, and increase grain yield. Field studies were conducted across 16 site-years to determine the effectiveness of soybean row width, seeding rate, and herbicide strategy as components of IWM in glufosinate-resistant soybean. Sites were grouped according to optimum adaptation zones for soybean maturity groups (MGs). Across all MG regions, pigweed density and height at the POST herbicide timing, and end-of-season pigweed density, height, and fecundity were reduced in IWM programs using a PRE followed by (fb) POST herbicide strategy. Furthermore, a PRE fb POST herbicide strategy treatment increased soybean cumulative intercepted photosynthetically active radiation (CIPAR) and subsequently, soybean grain yield across all MG regions. Soybean row width and seeding rate manipulation effects were highly variable. Narrow row width (≤ 38 cm) and a high seeding rate (470,000 seeds ha−1) reduced end-of-season height and fecundity variably across MG regions compared with wide row width (≥ 76 cm) and moderate to low (322,000 to 173,000 seeds ha−1) seeding rates. However, narrow row widths and high seeding rates did not reduce pigweed density at the POST herbicide application timing or at soybean harvest. Across all MG regions, soybean CIPAR increased as soybean row width decreased and seeding rate increased; however, row width and seeding rate had variable effects on soybean yield. Furthermore, soybean CIPAR was not associated with end-of-season pigweed growth and fecundity. A PRE fb POST herbicide strategy was a necessary component for an IWM program as it simultaneously managed pigweeds, increased soybean CIPAR, and increased grain yield.

Glyphosate-resistant (GR) weeds, especially GR Palmer amaranth, are very problematic in cotton-producing areas of the midsouthern region of the United States. Growers rely heavily on PRE residual herbicides to control Palmer amaranth since few effective POST options exist. Interest in integrating high-residue cover crops with existing herbicide programs to combat GR weeds has increased. Research was conducted in 2013 and 2014 in Tennessee to evaluate GR Palmer amaranth control when integrating cover crops and PRE residual herbicides. Cereal rye, crimson clover, hairy vetch, winter wheat, and combinations of one grass plus one legume were compared with winter weeds without a cover crop followed by fluometuron or acetochlor applied PRE. Biomass of cover crops was determined prior to termination 3 wk before planting. Combinations of grass and legume cover crops accumulated the most biomass (> 3,500 kg ha−1) but by 28 d after application (DAA) the cereal rye and wheat provided the best Palmer amaranth control. Crimson clover and hairy vetch treatments had the greatest number of Palmer amaranth. These cereal and legume blends reduced Palmer amaranth emergence by half compared to non–cover-treated areas. Fluometuron and acetochlor controlled Palmer amaranth 95 and 89%, respectively, at 14 DAA and 54 and 62%, respectively, at 28 DAA. Cover crops in combination with a PRE herbicide did not adequately control Palmer amaranth.

Two of the most problematicAmaranthusspecies in soybean production today are tall waterhemp and Palmer amaranth. This study determined the percentage of tall waterhemp and Palmer amaranth seed that was retained by the weed at soybean maturity to assess the likelihood of using at-harvest weed seed control tactics for soil seedbank management. Palmer amaranth plants were collected from fields in Arkansas, Tennessee, Illinois, Missouri, and Nebraska, and tall waterhemp plants were collected from fields in Nebraska, Missouri, Wisconsin, and Illinois. Collected plants were assessed for at-harvest weed seed retention in 2013 and 2014. Within 1 wk of soybean maturity,Amaranthusplants were harvested and the loose soil and debris beneath the plants were swept into a pan with a hand broom to collect any shattered seed. Percent seed retention ranged from 95 to 100% for all states both years, regardless of species. There was a strong correlation between weed biomass (g) and total seed production (no. plant−1) in that the larger the plant, the more seeds it produced. However, there was no correlation between percent seed retention and weed biomass, which indicates that regardless of plant size and likely time of emergence, seed retention is high at the time of crop maturity. Overall, this study demonstrated that there is great opportunity for Palmer amaranth and tall waterhemp seed capture or destruction at soybean harvest. It is likely that nearly all of the seeds produced for bothAmaranthusspecies passes through the combine during harvest to be returned to the soil seedbank. Thus, there is continued need for research focused on developing and testing harvest weed seed control tactics that aim at reducing the soil seedbank and lowering risks for evolution of herbicide resistance.

Palmer amaranth and waterhemp have become increasingly troublesome weeds throughout the United States. Both species are highly adaptable and emerge continuously throughout the summer months, presenting the need for a residual PRE application in soybean. To improve season-long control ofAmaranthusspp., 19 PRE treatments were evaluated on glyphosate-resistant Palmer amaranth in 2013 and 2014 at locations in Arkansas, Indiana, Nebraska, Illinois, and Tennessee; and on glyphosate-resistant waterhemp at locations in Illinois, Missouri, and Nebraska. The twoAmaranthusspecies were analyzed separately; data for each species were pooled across site-years, and site-year was included as a random variable in the analyses. The dissipation of weed control throughout the course of the experiments was compared among treatments with the use of regression analysis where percent weed control was described as a function of time (the number of weeks after treatment [WAT]). At the mean (i.e., average) WAT (4.3 and 3.2 WAT for Palmer amaranth and waterhemp, respectively) isoxaflutole +S-metolachlor + metribuzin had the highest predicted control of Palmer amaranth (98%) and waterhemp (99%). Isoxaflutole +S-metolachlor + metribuzin,S-metolachlor + mesotrione, and flumioxazin + pyroxasulfone had a predicted control ≥ 97% and similar model parameter estimates, indicating control declined at similar rates for these treatments. Dicamba and 2,4-D provided some, short-lived residual control ofAmaranthusspp. When dicamba was added to metribuzin orS-metolachlor, control increased compared to dicamba alone. Flumioxazin + pyroxasulfone, a currently labeled PRE, performed similarly to treatments containing isoxaflutole or mesotrione. Additional sites of action will provide soybean growers more opportunities to control these weeds and reduce the potential for herbicide resistance.

Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Wats) is redefining row crop weed management in the southeast United States due to its widespread distribution, high competitive ability, copious seed production, and resilience against standard weed management programs. Herbicides alone are failing to provide adequate control of GR Palmer amaranth; thus, use of conventional tillage is increasing in the Southeast in order to control GR Palmer amaranth. Cultural practices consistent with maintenance of conservation tillage were evaluated to determine if they could suppress weeds in cotton (Gossypium hirsutum L.). An on-farm research and demonstration project was initiated in the fall of 2009 in Alabama, Georgia, South Carolina, and Tennessee to address well-founded concerns that conservation tillage systems were at risk because of GR Palmer amaranth. The research continued in certain states for two additional years. Cultural practices contrasted in the conservation tillage system were a no-till planted high-residue cover crop, one-time fall inversion tillage followed by the planting of a high-residue cover crop, and winter fallow only without a cover crop. Cover crop biomass yields varied from 570 to 6,790 kg ha−1 (509 to 6,063 lb ac−1) depending on location, type of cover crop, planting date, and environment. Fall inversion tillage increased cover crop biomass at all locations, likely due to preparation of a favorable seed bed for seedling establishment or alleviation of soil compaction or both. One-time fall inversion tillage reduced the number of GR Palmer amaranth escapes compared with the other treatments. Where GR Palmer amaranth densities were relatively low (approximately 1,000 plants ha−1 [405 plants ac−1] or fewer), there were few differences in the number of GR Palmer amaranth escapes among treatments. Where GR Palmer amaranth densities were relatively high (18,000 plants ha−1 [7,284 plants ac−1] or greater), winter fallow systems had higher Palmer amaranth densities escaping weed management programs compared to either cover crop system. The number of Palmer amaranth escapes declined exponentially as a function of cover crop biomass regardless of tillage. For sites with two years of data, no year by treatment interaction was detected, indicating that relative GR Palmer amaranth escape densities were sustained in each treatment for two seasons. Trends in cotton yields were the opposite of those for Palmer amaranth escapes. High-residue cover crops tended to suppress Palmer amaranth and increase cotton lint yields. However, no cultural management system consistently netted greater returns than other systems across locations and years.