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Smooth scouringrush is an herbaceous perennial with an extensive underground rhizome system that has invaded no-till dryland production fields in the inland Pacific Northwest. The objective of this field study was to determine whether there were any short- or long-term benefits to tank-mixing chlorsulfuron + metsulfuron with glyphosate for smooth scouringrush control. Field studies were conducted at three sites across eastern Washington from 2020 to 2024. Glyphosate was applied during fallow periods at 0, 1,260, 2,520, and 3,780 g ae ha −1 with and without chlorsulfuron + metsulfuron applied at 21.9 + 4.4 g ai ha −1 . Smooth scouringrush stem density was evaluated 1, 2, and 3 yr after treatment. Chlorsulfuron + metsulfuron provided excellent control of smooth scouringrush (<5 plants m −2 ) for the first 2 yr at all three sites, and there was no observed benefit of tank-mixing with glyphosate. This continued to be the case 3 yr after treatment at two of the sites, but at one site, adding glyphosate at 2,520 or 3,780 g ha −1 to chlorsulfuron + metsulfuron decreased stem density compared to chlorsulfuron + metsulfuron applied alone. For treatments containing glyphosate only, the greatest efficacy 3 yr after treatment was achieved at the highest application rate of 3,780 g ha −1 . Although no short-term benefit was observed in adding glyphosate to chlorsulfuron + metsulfuron for smooth scouringrush control, at one of three sites the duration of control was increased by at least 1 yr with the addition of glyphosate at a rate of 2,520 g ha −1 or more and an organosilicone surfactant as tank-mix partners.

The water-limited environment of the semiarid Central Great Plains may not produce enough cover crop biomass to generate benefits associated with cover crop use in more humid regions. There have been reports that cover crops grown in mixtures produce more biomass with greater water use efficiency than single-species plantings. This study was conducted to determine differences in cover crop biomass production, water use efficiency, and residue cover between a mixture and single-species plantings. The study was conducted at Akron, CO, and Sidney, NE, during the 2012 and 2013 growing seasons under both rainfed and irrigated conditions. Water use, biomass, and residue cover were measured and water use efficiency was calculated for four single-species cover crops (flax [Linum usitatissimum L.], oat [Avena sativa L.], pea [Pisum sativum ssp. arvense L.Poir], rapeseed [Brassica napus L.]) and a 10-species mixture. The mixture did not produce greater biomass nor exhibit greater water use efficiency than the single-species plantings. The slope of the water-limited yield relationship was not significantly greater for the mixture than for single-species plantings. Waterlimited yield relationship slopes were in the order of rapeseed < flax < pea < mixture < oat, which was the expected order based on previously published biomass productivity values generated from values of glucose conversion into carbohydrates, protein, or lipids. Residue cover was not generally greater from the mixture than from single-species plantings. The greater expense associated with a mixture is not justified unless a certain cover crop forage quality is required for grazing or haying.

Rush skeletonweed is an invasive weed in winter wheat (WW)/summer fallow (SF) rotations in the low to intermediate rainfall areas of the inland Pacific Northwest. Standard weed control practices are not effective, resulting in additional SF tillage or herbicide applications. The objective of this field research was to identify herbicide treatments that control rush skeletonweed during the SF phase of the WW/SF rotation. Trials were conducted near LaCrosse, WA, in 2017–2019 and 2018–2020, and near Hay, WA, in 2018–2020. The LaCrosse 2017–2019 trial was in tilled SF; the other two trials were in no-till SF. Fall postharvest applications in October included clopyralid, clopyralid plus 2,4-D, clopyralid plus 2,4-D plus chlorsulfuron plus metsulfuron, aminopyralid, picloram, and glyphosate plus 2,4-D. Spring treatments of clopyralid, aminopyralid, and glyphosate were applied to rush skeletonweed rosettes. Summer treatments of 2,4-D were applied when rush skeletonweed initiated bolting. Plant density was monitored through the SF phase in all plots. Picloram provided complete control of rush skeletonweed through June at all three locations. Fall-applied clopyralid, clopyralid plus 2,4-D, and clopyralid followed by 2,4-D in summer reduced rush skeletonweed through June at the two LaCrosse sites but were ineffective at Hay. In August, just prior to WW seeding, the greatest reductions in rush skeletonweed density were achieved with picloram and fall-applied clopyralid at the two LaCrosse sites. No treatments provided effective control into August at Hay. Wheat yield in the next crop compared to the nontreated control was reduced only at one LaCrosse site by a spring-applied aminopyralid treatment, otherwise no other reductions were found. Long-term control of rush skeletonweed in WW/SF may be achieved by a combination of fall application of picloram, after wheat harvest, followed by an effective burn-down treatment in August prior to WW seeding. Nomenclature: 2; 4-D; aminopyralid; chlorsulfuron; clopyralid; glyphosate; metsulfuron; picloram; rush skeletonweed; Chondrilla juncea L.; winter wheat; Triticum aestivum L.

Rush skeletonweed is an aggressive perennial weed that establishes itself on land in the Conservation Reserve Program (CRP), and persists during cropping following contract expiration. It depletes critical soil moisture required for yield potential of winter wheat. In a winter wheat/fallow cropping system, weed control is maintained with glyphosate and tillage during conventional fallow, and with herbicides only in no-till fallow. Research was conducted for control of rush skeletonweed at two sites in eastern Washington, Lacrosse and Hay, to compare the effectiveness of a weed-sensing sprayer and broadcast applications of four herbicides (aminopyralid, chlorsulfuron + metsulfuron, clopyralid, and glyphosate). Experimental design was a split-plot with herbicide and application type as main and subplot factors, respectively. Herbicides were applied in the fall at either broadcast or spot-spraying rates depending on sprayer type. Rush skeletonweed density in May was reduced with use of aminopyralid (1.1 plants m–2), glyphosate (1.4 plants m–2), clopyralid (1.7 plants m–2), and chlorsulfuron + metsulfuron (1.8 plants m–2) compared with the nontreated check (2.6 plants m–2). No treatment differences were observed after May 2019. There was no interaction between herbicide and application system. Area covered using the weed-sensing sprayer was, on average, 52% (P < 0.001) less than the broadcast application at the Lacrosse location but only 20% (P = 0.01) at the Hay location. Spray reduction is dependent on foliar cover in relation to weed density and size. At Lacrosse, the weed-sensing sprayer reduced costs for all herbicide treatments except aminopyralid, with savings up to US$6.80 per hectare. At Hay, the weed-sensing sprayer resulted in economic loss for all products because of higher rush skeletonweed density. The weed-sensing sprayer is a viable fallow weed control tool when weed densities are low or patchy. Nomenclature: Aminopyralid; chlorsulfuron; clopyralid; glyphosate; metsulfuron; rush skeletonweed, Chondrilla juncea L.; winter wheat, Triticum aestivum L.

Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] has become a major annual weed in wheat (Triticum aestivum L.) production systems in the inland Pacific Northwest. With large genetic variability and abundant seed production, L. perenne ssp. multiflorum has developed globally 74 documented cases of herbicide resistance covering 8 different mechanisms of action. Harvest weed seed control (HWSC) systems were introduced in Australia in response to the widespread evolution of multiple herbicide resistance in rigid ryegrass (Lolium rigidum Gaudin) and wild radish (Raphanus raphanistrum L.). The efficacy of these systems for any given weed species is directly related to the proportion of total seed retained by that species at harvest time. From 2017 to 2020, ten L. perenne ssp. multiflorum plants were collected from three different slope aspects at each location in Washington, USA. Collections were initiated in each field when it was visually apparent that seed fill was nearly complete, and seed shatter had not yet occurred. Collection continued at near-weekly intervals until the fields were harvested. The number of filled florets on a spikelet was used to assess the degree of seed shatter over time. Seed shatter at harvest was 67% of the total number of florets on each spikelet. Seed shatter was closely aligned with wheat kernel development in both spring and winter wheat. The high percentage of L. perenne ssp. multiflorum seeds that are shattered by harvest may make HWSC less effective than for L. rigidum in Australia; however, seeds with the greatest biomass tend to not shatter before harvest, which may increase the efficacy of HWSC for managing the soil seedbank. Strategies like planting earlier-maturing wheat cultivars could help HWSC be more effective by having wheat harvest begin earlier, when more L. perenne ssp. multiflorum seeds are still on the mother plant.

Smooth scouringrush is a creeping perennial with a high silica content in stems that may impede herbicide uptake. Smooth scouringrush has become a troublesome weed in no-till cropping systems across eastern Washington. In previous field studies, glyphosate provided inconsistent control of smooth scouringrush. The objective of this study was to determine if the addition of an organosilicone surfactant to glyphosate would improve the efficacy and consistency of control through stomatal flooding. To test this hypothesis, glyphosate was applied at three field sites at 3.78 kg ae ha–1 alone, with an organosilicone surfactant (OS1 or OS2), an organosilicone plus nonionic surfactant blend, or an alcohol-based surfactant applied during the day or at night. Stem counts were recorded 1 yr after herbicide applications. Five of the six effective treatments observed across the three study sites included organosilicone surfactant or an organosilicone plus nonionic surfactant blend. At two sites, when there was a difference in efficacy between application times; daytime applications were more effective than nighttime applications. These results support the hypothesis of stomatal flooding as a likely mechanism for enhanced efficacy of glyphosate with the addition of an organosilicone surfactant. However, at one site, the treatments containing organosilicone surfactant were more efficacious when applied at night than during the day. At this site, high daytime temperatures and low relative humidity may have resulted in rapid evaporation of spray droplets. The addition of an organosilicone surfactant to glyphosate is recommended for smooth scouringrush control, and daytime treatments are preferred but should be applied when temperatures and humidity are not conducive to rapid droplet drying. Further research is necessary to confirm that stomatal flooding is responsible for improved glyphosate efficacy. Nomenclature: Glyphosate; smooth scouringrush; Equisetum laevigatum A. Braun

Crop production systems in the water-limited environment of the semiarid central Great Plains may not have potential to profitably use cover crops because of lowered subsequent wheat (Triticum asestivum L.) yields following the cover crop. Mixtures have reportedly shown less yield-reducing effects on subsequent crops than single-species plantings. This study was conducted to determine winter wheat yields following both mixtures and single-species plantings of spring-planted cover crops. The study was conducted at Akron, CO, and Sidney, NE, during the 2012–2013 and 2013–2014 wheat growing seasons under both rainfed and irrigated conditions. Precipitation storage efficiency before wheat planting, wheat water use, biomass, and yield were measured and water use efficiency and harvest index were calculated for wheat following four single-species cover crops (flax [Linum usitatissimum L.], oat [Avena sativa L.], pea [Pisum sativum ssp. arvense L. Poir], rapeseed [Brassica napus L.]), a 10-species mixture, and a fallow treatment with proso millet (Panicum miliaceum L.) residue. There was an average 10% reduction in wheat yield following a cover crop compared with following fallow, regardless of whether the cover crop was grown in a mixture or in a single-species planting. Yield reductions were greater under drier conditions. The slope of the wheat water use–yield relationship was not significantly different for wheat following the mixture (11.80 kg ha–1 mm–1) than for wheat following single-species plantings (12.32–13.57 kg ha–1 mm–1). The greater expense associated with a cover crop mixture compared with a single species is not justified.

Rush skeletonweed is an invasive weed in the winter wheat–fallow production regions of the inland Pacific Northwest. The objectives of this study were to determine the dose response of rush skeletonweed to picloram applied in the fall or spring of the fallow year with either a broadcast or weed-sensing sprayer, and to evaluate injury and grain yield in the subsequent winter wheat crop from these fallow treatments. Field studies were conducted between 2019 and 2022. Fall treatments were applied at one site in 2019, and one site in 2020. Spring treatments were applied at two sites in 2021. Four picloram herbicide rates (0, 140, 280, and 560 g ae ha–1), were applied with either a weed-sensing precision applicator or with a standard broadcast spray applicator. Rush skeletonweed densities in the wheat crop following fall-applied treatments declined with increasing picloram rates at both sites. Treatments applied with the weed-sensing sprayer achieved similar efficacy to broadcast treatments with an average of 37% and 26% of the broadcast rate applied. Spring-applied broadcast treatments resulted in reduced rush skeletonweed densities in wheat with increasing picloram rates. Picloram rate had no apparent effect on rush skeletonweed density when applied in the spring with a weed-sensing sprayer; however, the weed-sensing sprayer applied just 16% and 9% of the broadcast rate. Winter wheat grain yields were not reduced by fall picloram applications. Grain yields were not reduced by spring applications of picloram with the weed-sensing sprayer; however, grain yields were reduced by spring broadcast applications of picloram at both locations, and grain yields declined as the picloram rate increased. Applying picloram in the fall of the fallow phase with a weed-sensing sprayer provides effective and economical control of rush skeletonweed with a low risk for crop injury and yield loss in the following winter wheat crop. Nomenclature: Picloram; rush skeletonweed, Chondrilla juncea L.; wheat, Triticum aestivum L.

Smooth scouringrush has invaded no-till production fields across the US Pacific Northwest. The ability of Equisetum species to take up and accumulate silica on the epidermis and in cell walls may affect herbicide uptake. The objectives of this study were to measure the silica concentration in smooth scouringrush stems over time, and to determine how time of application affects the efficacy of glyphosate for smooth scouringrush control, with and without the addition of an organosilicone surfactant (OSS). Field studies were conducted at three sites in eastern Washington from 2019 to 2021. Three herbicide treatments (no herbicide, glyphosate, and glyphosate + OSS) were applied at four application times (May, June, July, and August) in 2019 fallow. The silica content of smooth scouringrush stems increased over the course of the 2019 growing season at all three sites. In 2020, smooth scouringrush stem densities were reduced when the 2019 herbicide treatments were applied in late June (12% of no herbicide density) compared to late July (24%) or August (30%). Smooth scouringrush stem densities at all three sites, in both 2020 and 2021, were reduced in the glyphosate + OSS treatment compared to glyphosate alone. In 2021, 2 yr after herbicide application, there was no effect of application timing for the glyphosate treatment without OSS, but stem densities were reduced when glyphosate + OSS was applied in late June (1%) compared with applications in late July (26%) or late August (21%). It is not clear if the cause of reduced glyphosate efficacy with late July and late August applications is the result of increased silica content in smooth scouringrush stems over time. Maximum glyphosate efficacy on smooth scouringrush was achieved with an application in late June and with the addition of an OSS. Control of smooth scouringrush with glyphosate + OSS can be sustained for at least 2 yr after application. Nomenclature: Glyphosate; smooth scouringrush, Equisetum laevigatum A. Braun; winter wheat, Triticum aestivum L.

Recent recommendations advocating the use of cover crop mixtures instead of single-species in semi-arid environments require rigorous scientific studies. One of those stated benefits is greatly reduced water use by cover crops grown in mixtures. The objectives of this study were to characterize soil water extraction patterns and determine water use of cover crops grown in single-species plantings and in a 10-species mixture and to compare cover crop water use to evaporative water loss from no-till fallow. The study was conducted at Akron, CO, and Sidney, NE, during the 2012 and 2013 growing seasons on silt loam soils. At each location there were a dryland treatment and an irrigated treatment. Soil water contents were measured by neutron scattering and time-domain reflectometry at six depths (0.0–1.8 m, Akron) or four or five depths (to 1.2 m or 1.5 m, Sidney). There were no consistent significant differences in soil water contents or growing season crop water use with the single-species plantings compared with the 10-species mixture. Cover crop water use (216 mm) averaged 1.78 times greater than evaporative water loss (122 mm) from the no-till fallow treatment with proso millet (Panicum miliaceum L.) residue. There appears to be no evidence from data collected in this semi-arid environment, even when irrigated to simulate higher rainfall environments, to support the conclusion that cover crops grown in multi-species mixtures use water differently than single species-plantings of cover crops.