Russian thistle is one of the most important broadleaf weeds in the semiarid U.S. Pacific Northwest. It consumes soil water after wheat harvest, compromising the yield of the following crop. The objectives of this work were to determine the impact of post–wheat harvest herbicide application timing on Russian thistle control and of stubble height on Russian thistle postharvest control and plant dispersal. For the first objective, experiments were conducted at the Columbia Basin Agricultural Research Center, Adams, OR (CBARC), and the Lind Dryland Research Station, Lind, WA (LDRS), in 2020 and 2021. Herbicides evaluated included paraquat, glyphosate, and either bromoxynil + pyrasulfotole (CBARC) or bromoxynil + metribuzin (LDRS). The different post–wheat harvest application timings were 24 h and 1, 2, and 3 wk after harvest. For the second objective, two stubble heights (short and tall) were compared for their impact on control at CBARC and in a production field near Ione, OR. Paraquat provided the greatest control in all scenarios, with no differences in application timings or stubble height. Impacts of application timings were not clear for glyphosate or bromoxynil mixtures. For glyphosate treatments, control in short stubble was 11% greater than in tall stubble in both years. Control was also greater in short stubble for the bromoxynil + pyrasulfotole application in 2020. However, Russian thistle plant dispersal was greater in short stubble at both locations. At CBARC, plant dispersal in short stubble was 58%, compared to 18% in tall stubble. Near Ione, plant dispersal in flattened stubble was 88%, compared to 43% in nonflattened short stubble. Leaving tall stubble at harvest should be considered to reduce Russian thistle plant dispersal if the infestation is going to be left untreated after harvest; otherwise, short stubble might result in better Russian thistle control when using systemic herbicides, such as glyphosate.

Late-season control of Palmer amaranth in postharvest wheat stubble is important for reducing the seedbank. Our objectives were to evaluate the efficacy of late-season postemergence herbicides for Palmer amaranth control, shoot dry biomass, and seed production in postharvest wheat stubble. Field experiments were conducted at Kansas State University Agricultural Research Center near Hays, KS, during 2019 and 2020 growing seasons. The study site had a natural seedbank of Palmer amaranth. Herbicide treatments were applied 3 wk after wheat harvest when Palmer amaranth plants had reached the inflorescence initiation stage. Palmer amaranth was controlled by 96% to 98% 8 wk after treatment and shoot biomass as well as seed production was prevented when paraquat was applied alone or when mixed with atrazine, metribuzin, flumioxazin, 2,4-D, sulfentrazone, pyroxasulfone + sulfentrazone, or flumioxazin + metribuzin, and with glyphosate + dicamba, glyphosate + 2,4-D, saflufenacil + 2,4-D, glufosinate + dicamba + glyphosate, and glufosinate + 2,4-D + glyphosate. Palmer amaranth was controlled by 89% to 93% with application of glyphosate, glufosinate, dicamba + 2,4-D, saflufenacil + atrazine, and saflufenacil + metribuzin resulting in Palmer amaranth shoot biomass of 15 to 56 g m−2 and production of 1,080 to 7,040 seeds m−2. Palmer amaranth control was less than 86% with application of dicamba, 2,4-D, dicamba + atrazine, and saflufenacil resulting in Palmer amaranth shoot biomass of 38 to 47 g m−2 and production of 3,110 to 6,190 seeds m−2. Palmer amaranth was controlled 63% and 72%, shoot biomass was 178 and 161 g m−2, and seed production was 35,180 and 39,510 seeds m−2, respectively, with application of 2,4-D + bromoxynil + fluroxypyr, and bromoxynil + pyrasulfotole + atrazine. Growers should use these effective postemergence herbicide mixes for Palmer amaranth control to prevent seed prevention postharvest in wheat stubble.

Horseweed is a competitive summer or winter annual weed that produces up to 230,000 small seeds per plant that are capable of traveling more than 500 km via wind. Giant ragweed is a tall, highly competitive summer annual weed. Glyphosate-resistant (GR) horseweed and GR giant ragweed pose significant challenges for producers in the United States and Ontario, Canada. It is thought that an integrated weed management (IWM) system involving herbicide rotation is required to control GR biotypes. Halauxifen-methyl is a new selective broadleaf POST herbicide registered for use in cereal crops; there is limited information on its efficacy on horseweed and giant ragweed. The purpose of this research was to determine the efficacy of halauxifen-methyl applied POST, alone and in a tank mix, for the control of GR horseweed and GR giant ragweed in wheat across southwestern Ontario. For each weed species, an efficacy study consisting of six field experiments was conducted over a 2-yr period (2018, 2019). At 8 wk after application (WAA), halauxifen-methyl, fluroxypyr/halauxifen-methyl, fluroxypyr/halauxifen-methyl + MCPA EHE, fluroxypyr + MCPA ester, 2,4-D ester, clopyralid, and pyrasulfotole/bromoxynil + ammonium sulfate controlled GR horseweed >95%. Fluroxypyr and MCPA provided only 86% and 37% control of GR horseweed, respectively. At 8 WAA, fluroxypyr, fluroxypyr/halauxifen-methyl, fluroxypyr/halauxifen-methyl + MCPA EHE, fluroxypyr + MCPA ester, fluroxypyr/halauxifen-methyl + MCPA EHE + pyroxsulam, 2,4-D ester, clopyralid, and thifensulfuron/tribenuron + fluroxypyr + MCPA ester controlled GR giant ragweed 87%, 88%, 90%, 94%, 96%, 96%, 98%, and 93%, respectively. Halauxifen-methyl and pyroxsulam provided only 45% and 28% control of GR giant ragweed, respectively. Halauxifen-methyl applied alone POST in the spring controlled GR horseweed but not GR giant ragweed in winter wheat.

Double-crop grain sorghum after winter wheat harvest is a common cropping system in the southern plains region. Palmer amaranth is a troublesome weed in double-crop grain sorghum in Kansas. Populations resistant to various herbicides (e.g., atrazine, glyphosate, metsulfuron, pyrasulfotole) have made Palmer amaranth management even more difficult for producers. To evaluate control of atrazine-resistant and atrazine-susceptible Palmer amaranth in double-crop grain sorghum, we assessed 14 herbicide programs, of which 8 were PRE only and 6 were PRE followed by (fb) POST applications. Visible ratings of Palmer amaranth control were taken at 3 and 8 wk after planting (WAP) grain sorghum. PRE treatments containing very-long-chain fatty acid (VLCFA)–inhibiting herbicides provided 91% control of atrazine-resistant Palmer amaranth 3 WAP, and reduced weed density 8 WAP compared to atrazine-only PRE treatments. PRE fb POST treatments, especially those that included VLCFA-inhibiting herbicides, provided greater control (71% to 93%) of both atrazine-resistant and atrazine-susceptible Palmer amaranth, respectively, at 8 WAP compared to PRE treatments alone (59% to 79%). These results demonstrated the utility of VLCFA-inhibiting herbicides applied PRE and in a layered PRE fb POST approach in controlling atrazine-resistant Palmer amaranth, as well as the importance of an effective POST application following residual PRE herbicides for controlling both atrazine-resistant and atrazine-susceptible Palmer amaranth in double-crop grain sorghum.

In the southeastern United States, growers often double-crop soft red winter wheat with peanut. In some areas, tobacco is also grown as a rotational crop. Pyrasulfotole is a residual POST-applied herbicide used in winter wheat, but information about its effects on rotational crops is limited. Winter wheat planted in autumn 2014 was treated at Feekes stage 1 or 2 with pyrasulfotole at 300 or 600 g ai ha−1. Wheat was terminated by glyphosate at Feekes stage 3 to 4. Peanut was planted via strip tillage, while tobacco was transplanted into prepared beds after minimal soil disturbance. Peanut exhibited no differences in stand establishment, growth, or yield, and tobacco stand, growth, and biomass yields were not different from the nontreated control for any pyrasulfotole rate or treatment timing.

Two putative glyphosate-resistant (GR) Russian-thistle accessions were collected from fallow fields (wheat-fallow rotation): one from Choteau County, MT (MT-R), and a second from Columbia County, WA (WA-R) in summer/fall of 2015. Greenhouse and outdoor/field whole-plant dose-response studies were conducted to confirm and characterize the levels of glyphosate resistance in these GR accessions relative to known glyphosate-susceptible accessions (MT-S and WA-S from MT and WA, respectively). Based on GR50 values of the progeny plants, the MT-R accession exhibited 4.5-fold and 5.9-fold resistance to glyphosate relative to the MT-S accession under greenhouse and outdoor conditions, respectively. The WA-R accession showed 3.0- to 5.0-fold resistance relative to the WA-S accession in greenhouse experiments, and 1.9- to 7.5-fold resistance in multi-site field experiments. In a separate greenhouse study on alternative POST herbicides to control GR Russian-thistle, bicyclopyrone plus bromoxynil, bromoxynil plus fluroxypyr, bromoxynil plus pyrasulfotole, bromoxynil plus MCPA, paraquat alone, paraquat plus metribuzin, saflufenacil alone, saflufenacil plus 2,4-D, and 2,4-D plus bromoxynil plus fluroxypyr provided effective control (≥95%) and shoot dry weight reduction (up to 98%) of GR accessions. This research confirms the first global case of field-evolved GR Russian-thistle. Best management practices (BMPs); including alternative, effective herbicide programs (based on multiple mechanisms of action highlighted in this study) need immediate implementation to prevent further spread of GR or evolution of multiple HR Russian-thistle populations in this region.

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 (I50 values). On the basis of shoot dry weight response (GR50 values), 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.

Field experiments were conducted in 2011 through 2013 at the MSU Southern Agricultural Research Center near Huntley, MT, to evaluate the effectiveness of various PRE and POST herbicide programs for kochia control in the absence of a crop. PRE herbicides labeled for corn, grain sorghum, soybean, wheat/barley, and/or in chemical fallow were applied at recommended field-use rates. Acetochlor + atrazine, S-metolachlor + atrazine + mesotrione, and sulfentrazone applied PRE provided ≥91% control of kochia at 12 wk after treatment (WAT). Metribuzin, metribuzin + linuron, and pyroxasulfone + atrazine PRE provided 82% control at 12 WAT. PRE control with acetochlor + flumetsulam + clopyralid, pyroxasulfone alone, and saflufenacil + 2,4-D was ≤23% at 12 WAT. Paraquat + atrazine, paraquat + linuron, and paraquat + metribuzin controlled kochia ≥98% at 5 WAT. POST control with bromoxynil + fluroxypyr, paraquat, tembotrione + atrazine, and topramezone + atrazine treatments averaged 84% at 5 WAT, and did not differ from glyphosate. Control with POST-applied bromoxynil + pyrasulfotole, dicamba, diflufenzopyr + dicamba + 2,4-D, saflufenacil, saflufenacil + 2,4-D, saflufenacil + linuron was 67 to 78% at 5 WAT. Because of the presence of kochia resistant to acetolactate synthase-inhibiting herbicides at the test site, cloransulam-methyl was not a viable option for kochia control. In a separate greenhouse study, kochia accessions showed differential response to the POST herbicides (labeled for corn or soybean) tested. Tembotrione + atrazine, topramezone + atrazine, lactofen, or fomesafen effectively controlled the glyphosate-resistant kochia accession tested. Growers should utilize these effective PRE- or POST-applied herbicide premixes or tank mixtures (multiple modes of action) to control herbicide-resistant kochia accessions in the field. PRE herbicides with 8 wk of soil-residual activity on kochia would be acceptable if crop competition were present; however, a follow-up herbicide application may be needed to obtain season-long kochia control in the absence of crop competition.

A study was conducted in summer fallow fields near Davenport, WA, and Pendleton, OR, in 2007 and 2008 to evaluate the POST weed control efficacy of herbicide treatments applied with a light-activated, sensor-controlled (LASC) sprayer compared to the broadcast application of glyphosate. The LASC application of glyphosate alone (at all rates) and in mixture with pyrasulfotole plus bromoxynil or 2,4-D had weed control (≥ 88%) and dry weight (≤ 6% of control) similar to the broadcast application of glyphosate across locations and years. Tumble pigweed and prickly lettuce control with bromoxynil, 2,4-D, or carfentrazone plus dicamba, was 12 to 85% less than glyphosate applied alone with LASC or broadcast sprayer. Overall, none of the tested alternate herbicides was promising enough to replace glyphosate under present conditions.

Field experiments were conducted in grain sorghum at five locations in Kansas in 2009 and 2010, to evaluate the efficacy and crop safety of early- to mid-POST (EMPOST) and late-POST (LPOST) applications of premixed pyrasulfotole and bromoxynil (PYRA&BROM) in tank mix combinations with atrazine or atrazine plus 2,4-D ester or dicamba compared to bromoxynil plus atrazine. PYRA&BROM at 244 or 300 g ai ha−1 plus atrazine at 560 g ai ha−1 applied EMPOST controlled pigweed species (Palmer amaranth, tumble pigweed, and redroot pigweed), kochia, velvetleaf, common sunflower, ivyleaf morningglory, and common lambsquarters 93% or greater. Puncturevine control among three locations ranged from 85 to 99%. Control of most weed species was not improved by increasing PYRA&BROM rate from 244 to 300 g ha−1 or by tank mixing 2,4-D or dicamba with PYRA&BROM plus atrazine. However, ivyleaf morningglory control was improved at the LPOST timing by adding 2,4-D or dicamba at 140 g ae ha−1. In no instance did any PYRA&BROM treatment provide greater weed control than bromoxynil plus atrazine at 281 + 560 g ha−1 when applied EMPOST, but in most instances PYRA&BROM treatments were more effective than bromoxynil plus atrazine when applied LPOST. Generally, PYRA&BROM treatments were more effective when applied EMPOST than LPOST, especially when 2,4-D or dicamba was added. PYRA&BROM plus atrazine treatments caused foliar bleaching in sorghum within 7 ± 3 d after treatment, but recovery was complete within 3 to 4 wk and grain yields were not reduced. Tank mixing dicamba with PYRA&BROM and atrazine occasionally reduced visible crop response compared to PYRA&BROM plus atrazine. Our results indicate that PYRA&BROM plus atrazine with or without 2,4-D or dicamba selectively controls several troublesome broadleaf weeds in grain sorghum. Foliar bleaching of sorghum leaves can occur but the symptoms are transient, and grain yields are not likely to be reduced.

This greenhouse experiment examined the response of homozygous susceptible and acetolactate synthase (ALS) inhibitor–resistant plants from six Canadian kochia accessions with the Pro197 or Trp574 mutation to six alternative herbicides of different sites of action. The null hypothesis was ALS-inhibitor–resistant and –susceptible plants from within and across accessions would respond similarly to herbicides of different sites of action. This hypothesis was accepted for all accessions except that of MBK2 with the Trp574 mutation. Resistant plants of that accession were 80, 60, and 50% more sensitive than susceptible plants to pyrasulfotole, mesotrione (hydroxyphenylpyruvate dioxygenase [HPPD] inhibitors), and carfentrazone (protoporphyrinogen oxidase [PPO] inhibitor), respectively. However, no differential dose response between resistant and susceptible plants of this kochia accession to bromoxynil, fluroxypyr, or glyphosate was observed. A previous study had found marked differences in growth and development between resistant and susceptible plants of this accession, but not of the other accessions examined in this experiment. Negative cross-resistance exhibited by resistant plants of accession MBK2 to PPO and HPPD inhibitors in this experiment may be a pleiotropic effect related to the Trp574 mutation.

Palmer amaranth is a difficult-to-control broadleaf weed that infests corn and soybean fields in south-central and southwestern Nebraska and several other states in the United States. The objectives of this research were to confirm triazine and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-resistant Palmer amaranth in Nebraska and to determine sensitivity and efficacy of POST-applied corn herbicides for control of resistant and susceptible Palmer amaranth biotypes. Seeds from a putative HPPD-resistant Palmer amaranth biotype from Fillmore County, NE were collected from a seed corn production field in fall 2010. The response of Palmer amaranth biotypes to 12 rates (0 to 12×) of mesotrione, tembotrione, topramezone, and atrazine was evaluated in a dose–response bioassay in a greenhouse. On the basis of the values at the 90% effective dose (ED90) level, the analysis showed a 4- to 23-fold resistance depending upon the type of HPPD-inhibiting herbicide being investigated and susceptible biotype used for comparison. This biotype also had a 9- to 14-fold level of resistance to atrazine applied POST. Results of a POST-applied herbicide efficacy study suggested a synergistic interaction between atrazine and HPPD-inhibiting herbicides that resulted in > 90% control of all Palmer amaranth biotypes. The resistant biotype had a reduced sensitivity to acetolactate synthase inhibiting herbicides (halosulfuron and primisulfuron), a photosystem-II inhibitor (bromoxynil), and a protoporphyrinogen oxidase inhibitor (fluthiacet-methyl). Palmer amaranth biotypes were effectively controlled (≥ 90%) with glyphosate, glufosinate, and dicamba, whereas 2,4-D ester provided 81 to 83% control of the resistant biotype and > 90% control of both susceptible biotypes.