Italian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] is one of the most troublesome weeds worldwide. Lolium multiflorum is also a grass seed crop cultivated on 50,000 ha in Oregon, where both diploid and tetraploid cultivars are grown. For this work, we will refer to the species as L. multiflorum, since the common names annual ryegrass and Italian ryegrass both refer to the same species. A survey was conducted to understand the distribution and frequency of L. multiflorum and its susceptibility to selected herbicides used in its control. The herbicides selected were clethodim, glufosinate, glyphosate, mesosulfuron-methyl (mesosulfuron), paraquat, pinoxaden, pyroxsulam, quizalofop-P-ethyl (quizalofop), pronamide, flufenacet + metribuzin, and pyroxasulfone. The ploidy levels of the populations were also tested. A total of 150 fields were surveyed between 2017 and 2018, of which 75 (50%) had L. multiflorum present. Herbicide-resistant populations were documented in 88% of the 75 populations collected. The most frequent resistances were to acetyl-CoA carboxylase (ACCase), acetolactate synthase (ALS), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitors, and combinations thereof. Multiple resistance and cross-resistance, found in 75% of the populations, were the most frequent patterns of resistance. Paraquat-resistant biotypes were confirmed in six orchard crop populations for the first time in Oregon. Herbicide resistance was spatially clustered, with most cases of resistance in the northern part of the surveyed area. Populations resistant to ALS and ACCase inhibitors were prevalent in wheat (Triticum aestivum L.) fields. Multiple resistance was positively correlated with plant density. Tetraploid feral populations were identified, but no cases of herbicide resistance were documented. This is the first survey of herbicide resistance and ploidy diversity in L. multiflorum in western Oregon. Resistant populations were present across the surveyed area, indicating that the problem is widespread.

Italian ryegrass is one of the most troublesome weeds worldwide because of the rapid evolution of herbicide resistance in this species. Oregon tall fescue seed production requires high seed purity, demanding good control of Italian ryegrass. The necessity to control herbicide-resistant Italian ryegrass and maintain tall fescue seed purity created interest in new chemical management options. The objectives of this study were to assess the effects of synthetic auxin herbicides on seed viability of Italian ryegrass biotypes and the feasibility of this management strategy for use in tall fescue seed production. Eight treatments of synthetic auxin herbicides were applied to Italian ryegrass and tall fescue at two growth stages (boot and anthesis): dicamba (1.0 and 2.2 kg ae ha−1), 2,4-D (1.1 and 2.2 kg ae ha−1), aminopyralid (0.5 kg ae ha−1), dicamba + 2.4-D (0.8 + 1.1 kg ae ha−1), 2.4-D + clopyralid (1.1 + 0.3 kg ae ha−1), and halauxifen-methyl + florasulam (0.4 kg ae ha−1 + 0.4 kg ai ha−1). Aminopyralid applied at boot and anthesis stages of Italian ryegrass reduced seed viability. Aminopyralid treatments reduced seed viability and weight of Italian ryegrass more than 50% compared to the control. Four biotypes from different locations in western Oregon with different types of herbicide resistance were sprayed, and differences in aminopyralid effect among Italian ryegrass biotypes were documented. Aminopyralid reduced the speed of germination by 1 to 2 d. Aminopyralid treatments had a greater effect when applied at the anthesis stage and had a greater negative impact on tall fescue. Tall fescue plants were more susceptible to aminopyralid, so this management practice is not feasible for tall fescue seed production. Future studies are needed to understand the physiological mechanisms involved in the reduced seed viability and to define an optimum aminopyralid rate for different Italian ryegrass biotypes.

The adoption of chemical fallow rotations in Pacific Northwest dryland winter wheat production has caused a weed species composition shift in which scouringrush has established in production fields. Thus, there has been interest in identifying herbicides that effectively control scouringrush in winter wheat–chemical fallow cropping systems. Field experiments were established in growers’ fields near Reardan, WA, in 2014, and The Dalles, OR, in 2015. Ten herbicide treatments were applied to mowed and nonmowed plots during chemical fallow rotations. Scouringrush stem densities were quantified the following spring and after wheat harvest at both locations. Chlorsulfuron plus MCPA-ester resulted in nearly 100% control of scouringrush through wheat harvest. Before herbicide application, mowing had no effect on herbicide efficacy. We conclude chlorsulfuron plus MCPA-ester is a commercially acceptable treatment for smooth and intermediate scouringrush control in winter wheat–chemical fallow cropping systems; however, the lack of a positive yield response when scouringrushes were controlled should factor into management decisions.

Many Italian ryegrass populations in Oregon are resistant to more than one herbicide; therefore, the resistance patterns of these populations must be determined to identify alternative herbicides for management. Two suspected resistant Italian ryegrass populations (R2 and R4) survived flufenacet plus metribuzin applications under typical winter wheat production conditions. Populations R2 and R4 were resistant to clethodim, pinoxaden, quizalofop, mesosulfuron-methyl, flufenacet, but not to acetochlor, dimethenamid-p, metolachlor, pyroxasulfone, imazapyr, sulfometuron, or glyphosate. R4 was resistant to diuron, but R2 was not. The estimated flufenacet doses required for 50% growth reduction (GR50) were 438 g ai ha−1 (R2) and 308 g ai ha−1 (R4). Both populations were controlled by pyroxasulfone at rates greater than 15 g ai ha−1. An Asp-2078-Gly substitution in the ACCase gene was found in both populations, while an Ile-2041-Asn was found only in the R4 population. A Ser-264-Gly substitution in psbA gene was found in the R4 population. These mutations previously have been reported to provide resistance to ACCase and photosynthetic inhibitors, respectively. No resistance mutations were identified in the acetolactate synthase (ALS) gene of either population. The addition of the P450 inhibitor, chlorpyrifos, increased the injury resulting from mesosulfuron-methyl on both resistant populations providing indirect evidence that the ALS resistance may be metabolic. Multiple herbicide-resistant Italian ryegrass populations were identified in this study with both target site and nontarget site based mechanisms likely involved. However, several herbicides were identified including pyroxasulfone, a herbicide in the same group as flufenacet, which could be used to control these two populations.

Glyphosate-tolerant spring wheat currently is being developed and most likely will be the first major genetically engineered crop to be marketed and grown in several areas of the northern Great Plains of the United States. The public has expressed concerns about environmental risks from glyphosate-tolerant wheat. Replacement of traditional herbicide active ingredients with glyphosate in a glyphosate-tolerant spring wheat system may alter ecological risks associated with weed management. The objective of this study was to use a Tier 1 quantitative risk assessment methodology to compare ecological risks for 16 herbicide active ingredients used in spring wheat. The herbicide active ingredients included 2,4-D, bromoxynil, clodinafop, clopyralid, dicamba, fenoxaprop, flucarbazone, glyphosate, MCPA, metsulfuron, thifensulfuron, tralkoxydim, triallate, triasulfuron, tribenuron, and trifluralin. We compared the relative risks of these herbicides to glyphosate to provide an indication of the effect of glyphosate when it is used in a glyphosate-tolerant spring wheat system. Ecological receptors and effects evaluated were avian (acute dietary risk), wild mammal (acute dietary risk), aquatic vertebrates (acute risk), aquatic invertebrates (acute risk), aquatic plants (acute risk), nontarget terrestrial plants (seedling emergence and vegetative vigor), and groundwater exposure. Ecological risks were assessed by integrating toxicity and exposure, primarily using the risk quotient method. Ecological risks for the 15 herbicides relative to glyphosate were highly variable. For risks to duckweed, green algae, groundwater, and nontarget plant seedling emergence, glyphosate had less relative risk than most other active ingredients. The differences in relative risks were most pronounced when glyphosate was compared with herbicides currently widely used on spring wheat.

Acetyl-coenzyme A carboxylase (ACCase)–resistant Italian ryegrass is one of the most difficult-to-control weeds in United States wheat-production systems. Seed was collected from a suspected ACCase-resistant Italian ryegrass population in a winter wheat field with a history of ACCase-inhibitor herbicide use. This study investigated cross-resistance patterns in this Italian ryegrass population. Resistance was identified to the commercial dose of the ACCase herbicides pinoxaden, clethodim, sethoxydim, and clodinafop. Partial chloroplastic ACCase sequences revealed aspartate-to-glycine or isoleucine-to-asparagine substitutions at positions 2078 or 2041 in individuals of the resistant population. This is the first report, to our knowledge, of Asp-2078-Gly and Ile-2041-Asn substitutions in ACCase-resistant Italian ryegrass in the United States. Associating the occurrence of resistance alleles with resistance to specific active ingredients provides a better understanding of ACCase cross-resistance in Italian ryegrass and possibly options for its control.

Management of Italian ryegrass in cereal-based cropping systems continues to be a major production constraint in areas of the United States, including the soft white winter wheat producing regions of the Pacific Northwest. Pyroxasulfone is a soil-applied herbicide with the potential to control broadleaf and grass weed species, including grass weed biotypes resistant to group 1, 2, and 7 herbicides, in several crops for which registration has been completed or is pending, including wheat, corn, sunflower, dry bean, and soybean. Field experiments were conducted from 2006 through 2009 near Corvallis, OR, to evaluate the potential for Italian ryegrass control in winter wheat with applications of pyroxasulfone. Application rates of PRE treatments ranged from 0.05 to 0.15 kg ai ha−1. All treatments were compared to standard Italian ryegrass soil-applied herbicides used in winter wheat, including diuron, flufenacet, and flufenacet + metribuzin. Visual evaluations of Italian ryegrass and ivyleaf speedwell control and winter wheat injury were made at regular intervals following applications. Winter wheat yields were quantified at grain maturity. Ivyleaf speedwell control was variable, and Italian ryegrass control following pyroxasulfone applications ranged from 65 to 100% and was equal to control achieved with flufenacet and flufenacet + metribuzin treatments and greater than that achieved with diuron applications. Winter wheat injury from pyroxasulfone ranged from 0 to 8% and was most associated with the 0.15–kg ha−1 application rate. However, this early-season injury did not negatively impact winter wheat yield. Pyroxasulfone applied at the application rates and timings in these studies resulted in high levels of activity on Italian ryegrass and excellent winter wheat safety. Based on the results, pyroxasulfone has the potential to be used as a soil-applied herbicide in winter wheat for Italian ryegrass management and its utility for management of other important grass and broadleaf weeds of cereal-based cropping systems should be evaluated.

Mayweed chamomile seeds were collected from six different fields across the Pacific Northwest. All populations (each collection site was considered a population) were suspected to have some level of acetolactate synthase (ALS) resistance. Greenhouse and laboratory studies were conducted to determine if these populations were resistant to three different classes of ALS inhibitors: sulfonylureas (SU), imidazolinones (IMI), and triazolopyrimidines (TP). A whole-plant dose–response and in vitro ALS activity studies confirmed cross-resistance to thifensulfuron + tribenuron/chlorsulfuron (SU), imazethapyr (IMI), and cloransulam (TP); however, resistance varied by herbicide class and population. Two ALS isoforms of the ALS gene (ALS1 and ALS2) were identified in mayweed chamomile; however, only mutations in ALS1 were responsible for resistance. No mutations were found in ALS2. Sequence analysis of the partial ALS gene identified four point mutations at position 197 (Pro197 to Leu, Gln, Thr, or Ser) in the resistant populations. This study demonstrates genotypic variation associated with cross-resistance to ALS inhibitors within and between populations.

Meadowfoam seed meal (MSM), a by-product after oil extraction, has potential uses for crop growth enhancement or weed control. The herbicidal effect of MSM is the result of a secondary metabolite, glucosinolate glucolimnanthin (GLN). Field evaluations were conducted using concentrations of 3, 5, and 7% by weight and two forms (nonactivated and activated) of MSM applied as soil amendments. No injury was observed on lettuce transplanted 7 d after MSM incorporation in 2011. Activated MSM at 7% reduced weed emergence up to 71%. Lettuce leaf N content was at least 8.5-fold greater in MSM treatments compared to the untreated control. Greater soil nitrate levels correlated with greater weed biomass in MSM-amended plots. Isothiocyanate, a potent herbicidal compound, was detected in soil incorporated with 7% activated MSM. In 2012, 2.86 g m−2 of activated MSM, applied as a split or single dose, was evaluated for weed control efficacy and crop injury response. The split MSM application provided weed control similar to that from the single MSM application. The split and single MSM applications inhibited spiny sowthistle emergence more than 95% compared to the untreated control. A single application of activated MSM as a PRE soil amendment suppressed weeds and increased lettuce yield.

Greenhouse studies were conducted to evaluate the influence of selected adjuvants on glyphosate efficacy on yellow nutsedge and tuber production. Glyphosate was applied at 0, 0.25, 0.43, 0.87, 1.26 (1× rate), and 1.74 kg ae ha−1 at 31 d after yellow nutsedge was planted. Each rate was mixed with one of the following adjuvants: ammonium sulfate (AMS), AMS plus nonionic surfactant (NIS), or AMS plus an experimental adjuvant (W-7995) plus NIS. Plants were evaluated for injury and for the number and size of tubers produced. Dose–response curves based on log-logistic models were used to determine the effective glyphosate rate plus adjuvant that provided both 90% effective dose (ED90) for yellow nutsedge injury and reduced tuber production. Addition of NIS to glyphosate plus AMS resulted in the greatest yellow nutsedge injury at 28 d after treatment (DAT). Addition of the experimental adjuvant plus NIS resulted in injury similar to NIS alone. The ED90 for injury at 28 DAT was 2.12 kg ha−1 with glyphosate plus AMS and NIS compared with 2.18 kg ha−1 for W-7995 plus NIS and 3.06 kg ha−1 with AMS alone. The ED90 rates with different adjuvants represent 168%, 173%, and 243% of the highest glyphosate rate (1.26 kg ha−1) labeled for application on many glyphosate-resistant crops. However, the estimated ED90 to reduce small, medium, large, and total tubers were 1.60, 1.50, 1.63, and 1.66 kg ha−1, respectively. Increases in labeled rates of glyphosate may be required to reduce yellow nutsedge tuber production in field conditions. Use of lower glyphosate rates should be discouraged because it may increase tuber production and exacerbate yellow nutsedge expansion in infested fields.

Uncontrolled kochia plants that regrow after small-grain harvest can produce substantial numbers of seeds. An average of 4,100 seeds per plant were produced between harvest (late July to mid August) and the first killing frost (late September) at three locations in Montana. Field experiments were conducted to determine the optimal timing of postharvest herbicide applications to prevent kochia from producing viable seeds. Herbicide treatments were applied at three timings from late August to mid September. The most effective treatments were glyphosate (631 g/ha) and paraquat (701 g/ha) applied at the second application timing (late August to early September). These treatments reduced kochia seed production by 92% or greater at each site. Kochia regrowth by this time had sufficient leaf area for herbicide absorption, but few viable seed had been produced. Herbicide treatments at the first and third application timings were generally less effective and more variable in reducing kochia seed production. Sulfentrazone (157 g/ha) and 2,4-D (561 g/ha) were not as effective at reducing seed production as other herbicide treatments.

The transition period to certified organic production can present a significant weed management challenge for growers. Organic certification requires that prohibited fertilizers and pesticides must not have been used for 36 mo before harvest of the first organic crop. Understanding how organic management practices and initial weed seed-bank densities affect weed population dynamics during the transition period may improve weed management efficacy and adoption of organic practices. We examined how tillage systems (full or reduced) and cover crop species planted during the first transition year (rye or a mixture of timothy and red clover) affect the seedling densities of three common annual weed species, common lambsquarters, velvetleaf, and foxtail spp., during the 3-yr transition period. Weed seeds were applied in a one-time pulse at the beginning of the study at three densities, low, medium, and high (60, 460, and 2,100 seeds m−2, respectively), and cumulative seedling densities of each species were assessed annually. Treatment factors had variable and species-specific effects on weed seedling densities. In general, the full-tillage system, with an initial cover crop of timothy and red clover, resulted in the lowest density of weed seedlings following seed-bank augmentation. There was little consistent association between the initial densities of applied weed seeds in the weed seed bank at the start of the transition and weed seedling densities at the end of the transition period. This suggests that when multiple crop and weed cultural management practices are employed during the organic transition period, initial failures in weed management may not necessarily lead to persistent and intractable annual weed species management problems following organic certification.

Organic farming is one of the fastest growing agricultural sectors worldwide, and funds to support research and extension activities that address the needs of organic producers are becoming more widely available in the USA. Solutions to problems in ecologically complex systems, such as organic farming systems often exceed the expertise of individual investigators or single disciplines. Further, the complex nature of ecological and social interactions within systems-based agricultural research requires not only more emphasis on information exchange but also synthesis between multidisciplinary teams of academic researchers and organic farmers. Accordingly, federal grant agencies that support organic agriculture research increasingly require that projects encompass multiple academic disciplines, multiple functions (research, outreach, education), and the participation of stakeholders for the ultimate purpose of the integration of knowledge. Many researchers, educators and administrators at land grant universities (LGUs) remain inexperienced in multidisciplinary, multifunctional and participatory research. Using post-completion project interviews of the project investigators on an organic transition project, we identified eight factors that affected the integration of knowledge from a farmer advisory board and the conduct of our multidisciplinary, participatory organic transition project. The first five factors include shared values, balance in technical competence, institutional capacity for research, team capacity for problem solving and institutional resistance. The research team also identified three other factors that evoked confusion and divergence during the project, and include the ambiguity of power and control of knowledge, the proposed experimental plan and terms of team engagement. We considered participatory elements of the project according to Biggs’ linear typology of participation, but found more appropriate Neef and Neubert's position that a linear scale of participatory approach is an inadequate framework for helping agricultural scientists to decide on whether and in which phases they want to, can and should incorporate participatory elements into their research projects. From these findings, we conclude with critical issues for academic research and extension teams to consider during the development and before conduct of these types of projects. We also offer recommendations for LGUs and other research institutions, and funding organizations, to facilitate multidisciplinary, multifunctional, participatory research.