Field experiments were conducted in 2004, 2005, and 2006, at Pendleton, SC, to determine the effects of soybean canopy and tillage on Palmer amaranth emergence from sites with a uniform population of Palmer amaranth. In 2006, the effect of soybean canopy was evaluated only in no-tillage plots. Palmer amaranth emerged from May 10 through October 23, May 13 through September 2, and April 28 through August 25 in 2004, 2005, and 2006, respectively. Two to three consistent emergence periods occurred from early May through mid-July. Shallow (10-cm depth) spring tillage had minimal influence on the cumulative emergence of Palmer amaranth. Increase in light interception following soybean canopy formation was evident by early July, resulting in reduced Palmer amaranth emergence, especially in no-tillage conditions. In no-tillage plots, from 32 or 33 d after soybean emergence through senescence, Palmer amaranth emergence was reduced by 73 to 76% in plots with soybean compared with plots without soybean. Emergence of Palmer amaranth was favored by high-thermal soil amplitudes (10 to 16 C) in the absence of soybean. Of the total emergence during a season, > 90% occurred before soybean canopy closure. The seedling recruitment pattern of Palmer amaranth from this research suggests that, although Palmer amaranth exhibits an extended emergence period, cohorts during the peak emergence periods from early May to mid-July need greater attention in weed management.
Kochia [Bassia scoparia(L.) A. J. Scott] is a problematic annual broadleaf weed species in the North American Great Plains.Bassia scopariainherits unique biological characteristics that contribute to its propensity to evolve herbicide resistance. Evolution of glyphosate resistance inB. scopariahas become a serious threat to the major cropping systems and soil conservation practices in the region.Bassia scopariapopulations with resistance to four different herbicide sites of action are a concern for growers. The widespread occurrence of multiple herbicide–resistant (HR)B. scopariaacross the North American Great Plains has renewed research efforts to devise integrated weed management strategies beyond herbicide use. In this review, we aim to compile and document the growing body of literature on HRB. scopariawith emphasis on herbicide-resistance evolutionary dynamics, distribution, mechanisms of evolved resistance, agronomic impacts, and current/future weed management technologies. We focused on ecologically based, non-herbicidal strategies such as diverse crop rotations comprising winter cereals and perennial forages, enhanced crop competition, cover crops, harvest weed seed control (HWSC), and tillage to manage HRB. scopariaseedbanks. Remote sensing using hyperspectral imaging and other sensor-based technologies would be valuable for early detection and rapid response and site-specific herbicide resistance management. We propose research priorities based on an improved understanding of the biology, genetic diversity, and plasticity of this weed that will aid in preserving existing herbicide resources and designing sustainable, integrated HRB. scopariamitigation plans.
Seeds of a suspected glyphosate-resistant giant ragweed biotype from Lauderdale County, TN, were collected from a continuous cotton field in fall 2007 after plants were nonresponsive to multiple glyphosate applications. The objectives of this research were to (1) confirm resistance by quantifying the response of the putative resistant biotype to glyphosate compared to a susceptible biotype from a nonagricultural area, (2) quantify shikimate accumulation over time in both biotypes, and (3) determine the effectiveness of POST-applied herbicides labeled for use in cotton in controlling both biotypes at three growth stages. The susceptible biotype had a 50% lethal dose of 407 g ae/ha of glyphosate compared with 2,176 g/ha for the resistant biotype when treated at the four-node stage, a 5.3-fold level of resistance. The resistant biotype accumulated 3.3- to 9.8-fold less shikimate than the susceptible biotype at 1 to 7 d after treatment. The resistant biotype was less responsive to glyphosate as treatment was delayed past the two-node stage, much more than the susceptible biotype. Glufosinate, MSMA, and diuron controlled both biotypes by at least 90%, regardless of size at application. Prometryn, flumioxazin, carfentrazone-ethyl, fomesafen, and trifloxysulfuron controlled both biotypes by at least 89% when applied at the two-node stage, but control generally diminished with later application timings. Pyrithiobac was not effective in controlling either biotype, regardless of size at application. Hence, there are effective herbicide options for controlling glyphosate-resistant giant ragweed in cotton, and the resistant biotype does not appear to exhibit multiple resistances to other herbicides.
The rapid evolution and spread of glyphosate-resistant (GR) kochia in the Northern Great Plains is an increasing threat to GR cropping systems and conservation tillage practices common in this region. GR kochia accessions with 4.6- to 11-fold levels of resistance to glyphosate have recently been reported in Montana. Those GR kochia accessions were also suspected to be resistant to acetolactate synthase (ALS) inhibitors, i.e., multiple herbicide-resistant (MHR) kochia. In this research, the level of resistance to the ALS-inhibitor herbicides (sulfonylureas) and the molecular mechanisms conferring resistance to glyphosate and ALS-inhibitor herbicides in MHR kochia was investigated. On the basis of whole-plant dose–response assays, MHR kochia accessions (GIL01, JOP01, and CHES01) were 9.3- to 30-fold more resistant to premixed thifensulfuron methyl + tribenuron methyl + metsulfuron methyl than the susceptible (SUS) accession. In an in vivo leaf-disk shikimate assay, MHR plants accumulated less shikimate than the SUS plants at a discriminate dose of 100 μM glyphosate. Sequencing of the conserved region ofEPSPSrevealed no target-site mutation at Thr102or Pro106residue. MHR kochia accessions had increased relativeEPSPSgene copies (~ 4 to 10) compared with the SUS accession (single copy). Furthermore, MHR kochia accumulated higher EPSPS protein compared with the SUS plants. Resistance to the ALS-inhibitor herbicides was conferred by Pro197amino acid substitution (proline to glutamine).EPSPSgene amplification and a single target-site mutation at Pro197inALSgene confer resistance to glyphosate and ALS-inhibitor herbicides, respectively, in MHR kochia accessions from Montana. This is the first confirmation of occurrence of MHR kochia in Montana.
Herbicide-resistant kochia is an increasing concern for growers in the northwestern United States. Four suspected glyphosate-resistant (Gly-R) kochia accessions (referred to as GIL01, JOP01, CHES01, and CHES02) collected in fall 2012 from four different chemical-fallow fields in northern Montana were evaluated. The objectives were to confirm and characterize the level of glyphosate resistance in kochia accessions relative to a glyphosate-susceptible (Gly-S) accession and evaluate the effectiveness of various POST herbicides for Gly-R kochia control. Whole-plant dose–response experiments indicated that the four Gly-R kochia accessions had 7.1- to 11-fold levels of resistance relative to the Gly-S accession on the basis of percent control ratings (I50values). On the basis of shoot dry weight response (GR50values), the four Gly-R kochia accessions exhibited resistance index (R/S) ratios ranging from 4.6 to 8.1. In a separate study, the two tested Gly-R accessions (GIL01 and JOP01) showed differential response (control and shoot dry weight reduction) to various POST herbicides 21 d after application (DAA). Paraquat, paraquat + linuron, carfentrazone + 2,4-D, saflufenacil alone or with 2,4-D, and bromoxynil + fluroxypyr effectively controlled (99 to 100%) and reduced shoot dry weight (88 to 92%) of the GIL01 accession, consistent with the Gly-S kochia accession; however, bromoxynil + MCPA and bromoxynil + pyrasulfotole provided 76% control and 83% shoot dry weight reduction of the GIL01 accession and were lower compared with the Gly-S accession. The JOP01 accession exhibited lower control or shoot dry weight reduction to all herbicides tested, except dicamba, diflufenzopyr + dicamba + 2,4-D, paraquat + linuron, and bromoxynil + pyrasulfotole, compared with the Gly-S or GIL01 population. Furthermore, paraquat + linuron was the only treatment with ≥ 90% control and shoot dry weight reduction of the JOP01 kochia plants. Among all POST herbicides tested, glufosinate was the least effective on kochia. This research confirms the first evolution of Gly-R kochia in Montana. Future research will investigate the mechanism of glyphosate resistance, inheritance, ecological fitness, and alternative strategies for management of Gly-R kochia.
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