Palmer amaranth is native to the United States, but was discovered in 2015 in Brazil Palmer amaranth populations in Brazil were very difficult to control using glyphosate, which resulted in many changes to standard weed management practices A genotyping assay was used to confirm that the population detected in Mato Grosso State, Brazil, was correctly identified as Palmer amaranth and that it was not tall waterhemp Greenhouse dose—response curves and shikimate accumulation assays showed that the Brazilian population was highly resistant to glyphosate, with an LD50 value (3,982 g glyphosate ha-1) more than twice the typical use rates and very little shikimate accumulation at 1 mM glyphosate concentrations in a leaf-disk assay The Brazilian population was also resistant to sulfonylurea and imidazolinone acetolactate synthase (ALS) inhibitor herbicides The resistance mechanisms in the Brazilian population were identified as increased EPSPS gene copy number for glyphosate resistance (between 50- and 179-f

/pdf/multiple-resistance-to-glyphosate-and-acetolactate-synthase-2w5o8pvwsf.pdf

Multiple Resistance to Glyphosate and Acetolactate Synthase Inhibitors in Palmer Amaranth (Amaranthus palmeri) Identified in Brazil

https://espace.library.uq.edu.au/view/UQ:411530/UQ411530_OA.pdf

Weed management in cotton (Gossypium hirsutum L.) through weed-crop competition: a review

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...

Evaluating Cover Crops and Herbicides for Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri ) Control in Cotton

Timing of Cover Crop Termination: Management Considerations for the Southeast

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.

High residue cover crops alone or with strategic tillage to manage glyphosate-resistant palmer amaranth (amaranthus palmeri) in Southeastern cotton (gossypium hirsutum)

Conservation agriculture (CA) practices are threatened by glyphosate-resistant Palmer amaranth. Integrated control practices including PRE herbicides and high-residue CA systems can decrease Amaranthus emergence. Field experiments were conducted from autumn 2006 through crop harvest in 2009 at two sites in Alabama to evaluate the effect of integrated weed management practices on Amaranthus population density and biomass, cotton yield, and economics in glyphosate-resistant cotton. Horizontal strips included four CA systems with three cereal rye cover crop seeding dates and a winter fallow (WF) CA system compared to a conventional tillage (CT) system. Additionally, vertical strips of four herbicide regimes consisted of: broadcast, banded, or no PRE applications of S-metolachlor (1.12 kg ai ha 21 ) followed by (fb) glyphosate (1.12 kg ae ha 21 ) applied POST fb layby applications of diuron (1.12 kg ai ha 21 ) plus MSMA (2.24 kg ai ha 21 ) or the LAYBY application alone. Early-season Amaranthus density was reduced in high-residue CA in comparison to the CA WF systems in 2 of 3 yr. Amaranthus densities in herbicide treatments that included a broadcast PRE application were lower at three of five sampling dates compared to banding early-season PRE applications; however, the differences were not significant during the late season and cotton yields were not affected by PRE placement. Highresidue conservation tillage yields were 577 to 899 kg ha 21 more than CT, except at one site in 1 yr when CT treatment yields were higher. CA utilizing high-residue cover crops increased net returns over CT by $100 ha 21 or more 2 out of 3 yr at both locations. High-residue cover crop integration into a CA system reduced Amaranthus density and increased yield over WF systems; the inclusion of a broadcast PRE application can increase early-season Amaranthus control and might provide additional control when glyphosate-resistant Amaranthus populations are present. Nomenclature: Diuron; glyphosate; MSMA; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats.; cotton, Gossypium hirsutum L; rye, Secale cereale L.

https://bioone.org/journals/Weed-Technology/volume-26/issue-3/WT-D-11-00127.1/Herbicide-and-Cover-Crop-Residue-Integration-for-span-classgenus-speciesAmaranthus/10.1614/WT-D-11-00127.1.pdf

Herbicide and Cover Crop Residue Integration for Amaranthus Control in Conservation Agriculture Cotton and Implications for Resistance Management

Conservation tillage, after being widely adopted in the past few decades, is now threatened by the development of herbicide resistant weeds. Of the 157.7 million ha of cropland across the United States (U.S.) in 2012, approximately 127.5 million ha were harvested. Roughly 44% of total cropland area used conservation tillage practices, including no-tillage. In 1989, there were approximately 0.34 ha of conservation tillage for every hectare of conventional tillage, including reduced tillage. By 2012, the ratio had grown to 1.64 ha of conservation tillage for every hectare of conventional tillage. New technology, such as herbicide-resistant crops, greatly helped to increase the adoption rate of conservation tillage practices, yet current herbicide-resistant weed challenges are a real and expanding threat to conservation tillage. Produc- ers are faced with difficult management decisions regarding herbicide-resistant weeds. Despite many of these ha currently being under United States Department of Agriculture (USDA)– Natural Resources Conservation Service (NRCS) conservation program contracts, the difficulty of controlling herbicide-resistant weeds places many ha at risk of being converted to higher-intensity tillage systems. The shift to higher-intensity tillage facilitates burial of small weed seed and/or the increase of pre-emergence and pre-plant incorporated herbicide activ- ity to control problematic weeds, especially in dry-land crop production. However, in the past decade in the U.S., there has been a renewed interest in cover crops and their management for both carbon sequestration and agronomic purposes. Start- ing in the late-1990s, scientists at the USDA – Agriculture Research Service (ARS) National Soil Dynamics Laboratory in Auburn, Alabama initiated research using a Brazilian style cover crop management technique utilizing a roller-crimper, in conjunction with herbicides, to terminate mature cover crops. High residue cover crops are increasingly being incorporated into integrated pest management recommendations to help alleviate herbicide resistance selection pressure through their weed suppressive characteristics.

Conservation tillage under threat in the United States.

The increased adoption of conservation tillage and organic weed control practices in vegetable production requires more information on the role of various cover crops in integrated weed control, tomato quality, and yield. Two conservation-till‐ age systems utilizing crimson clover and cereal rye as winter cover crops were com‐ pared to a conventional black polythene mulch system, with or without organic weed management options, for weed control, tomato yield, and profitability. All cover crops were terminated with a mechanical roller/crimper prior to planting. Or‐ ganic weed control treatments included: 1) flaming utilizing a one burner hand torch, 2) PRE application of corn gluten, 3) PRE application of corn gluten followed by flaming, or 4) intermittent hand weeding as needed. A non-treated control and a standard herbicide program were included for comparison. The herbicide program consisting of a PRE application of S-metolachlor (1.87 kg a.i./ha) followed by an ear‐ ly POST metribuzin (0.56 kg a.i. /ha) application followed by a late POST applica‐ tion of clethodim (0.28 kg a.i./ha). In general, high-residue clover and cereal rye cover crops provided substantial suppression of Palmer amaranth, large crabgrass, and yellow nutsedge. Across systems, minimum input in high-residue systems pro‐ vided the highest net returns above variable costs compared to organic herbicide treatments that are costly and provide marginal benefit.

/pdf/organic-weed-control-and-cover-crop-residue-integration-1mh7z8payk.pdf

Organic Weed Control and Cover Crop Residue Integration Impacts on Weed Control, Quality, Yield and Economics in Conservation Tillage Tomato-A Case Study

Agricultural Management Options for Climate Variability and Change: Conservation Tillage (AE486)
This 4-page fact sheet focuses on the use of conservation tillage in crop production systems as a strategy to minimize the risks associated with climate variability and change and to improve resource-use efficiency. Written by Kip Balkcom, Leah Duzy, Daniel Dourte, and Clyde Fraisse, and published by the UF Department of Agricultural and Biological Engineering, June 2012.
http://edis.ifas.ufl.edu/ae486

https://journals.flvc.org/edis/article/download/119876/117883

Agricultural Management Options for Climate Variability and Change: Conservation Tillage

Previous research has indicated that conservation tillage is a viable option for suc‐ cessful peanut production; however, interactions between cover crop residues and peanut growth are not fully understood. Additional information is needed about the effects of varying levels of cover crop biomass on peanut growth and develop‐ ment. Level of cover crop residue may also affect the preemergence herbicide activi‐ ty through interception and efficacy of weed suppression. The objectives of this peanut research were to determine if varying amounts of cover crop biomass would affect peanut growth, herbicide interception, or weed control. This research also aimed to determine if cover crop management practices (rolling or standing cover) would affect herbicide interception rates. The study consisted of a rye (Secale cereale L.) cover crop planted at three different dates as well as a fallow treatment at two locations: Dawson, GA, and Headland, AL. Pendimethalin was applied PRE at 1 kg ai/ha across the entire area just prior to planting of the Georgia 03-L peanut variety. Soil samples were collected at three different dates after planting for high-pressure liquid chromatography (HPLC) analysis to determine pendimethalin levels. Peanut yields differed only between location regardless of cover crop residue level with the Headland, Alabama, site averaging 4,272 kg/ha and the Dawson, Georgia, site aver‐ aging 2,247 kg/ha. Pendimethalin extraction from soil samples indicated no differ‐ ence in herbicide recovery between winter fallow systems compared to treatments with cover crops. Weed control ratings taken at 21 and 45 days after planting (DAP) showed greater weed suppression for cover crop systems for an extended period of time when higher levels of cover crop biomass are present. Results of this experi‐ ment indicate the inclusion of cover crops in a conservation-tilled peanut system can be a successful alternative to winter fallow systems without reducing peanut yield or herbicide efficacy.

/pdf/peanut-performance-and-weed-management-in-a-high-residue-4ectv69si4.pdf

Peanut performance and weed management in a high-residue cover crop system

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