Rigid ryegrass (Lolium rigidum Gaudin) is the most problematic weed in Australia, with evolved resistance to multiple herbicide sites of action. Selection pressure by cinmethylin (Group 30, a fatty-acid thioesterase inhibitor) has been limited, because few populations have been exposed to the herbicide since its introduction in 2019. In this study, we examined the sensitivity of L. rigidum populations to this new herbicide. From a screening of almost 500 field populations in 2020, 28 potentially resistant populations were further investigated in a dose–response experiment. Seedlings from five populations surviving treatments of 250 or 375 g ai ha−1 cinmethylin were grown to maturity and seeds were harvested. The level of resistance found among the five putative-resistant parental populations of L. rigidum was negligible. In one population, one round of selection with cinmethylin resulted in a 2-fold increase in the lethal dose causing 50% mortality in the progeny population, although this dose was still only one-sixth of the recommended field rate of cinmethylin. Having a unique site of action, cinmethylin is a viable preemergence herbicide option to control existing multiple-resistance populations of L. rigidum. Comprehensive field monitoring and recurrent selection studies under controlled environmental conditions are needed to better ascertain the risk of L. rigidum evolving a high level of resistance to cinmethylin, although current data suggest that this risk is relatively low.

Italian ryegrass control is one of the most significant limitations in wheat production in the United States today. Resistance to Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Groups 1, 2, and 9 in Arkansas have further complicated postemergence control, whereas residual herbicides still show effective weed control. One problem is the potential of HRAC/WSSA Group 15 herbicides to injure wheat when applied preemergence, indicating the need for a herbicide safener. A series of experiments were conducted in Fayetteville, AR, to evaluate crop tolerance and Italian ryegrass control using a capsule suspension (CS) formulation of S-metolachlor in conjunction with fenclorim-treated wheat. Experiments were conducted as a two-factor factorial with S-metolachlor applied at three rates (0.37, 0.74, and 1.12 kg ai ha–1) and a microencapsulated formulation of acetochlor at 1.05 kg ai ha–1, and three rates of a fenclorim seed treatment at 0, 0.5, and 2.0 g ai kg–1 of seed. Separate experiments utilized either a preemergence (PRE) or a delayed-preemergence (DPRE) application timing. In both experiments, S-metolachlor at 0.74 and 1.12 kg ai ha–1 provided 77% to 96% control of Italian ryegrass by preharvest, whereas acetochlor only provided 49% to 72% control. Visible wheat injury from PRE applications ranged from 7% to 49% for all treatments 21 d after treatment (DAT), with a reduction in injury when fenclorim-treated wheat was used for both the 0.74 and 1.12 kg ai ha–1 rate of S-metolachlor. In the DPRE experiments, wheat injury ranged from 5% to 16% 21 DAT with no noticeable safening from the presence of fenclorim at any herbicide rate. The results of these experiments indicate that a DPRE application using a CS formulation of S-metolachlor would be more favorable for producers to mitigate the potential for injury to wheat while providing Italian ryegrass control. Additionally, at the DPRE application timing, fenclorim is unnecessary for S-metolachlor to be safely applied at the rates evaluated.

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.

Field studies were conducted in North Carolina to determine the critical period of weed control (CPWC) for Italian ryegrass in winter wheat. Soft red winter wheat was planted in late fall in 2017 and 2018 in no-till fields near Salisbury, NC. Treatments consisted of allowing weeds to grow from crop emergence for different intervals until removal (“weedy”), maintaining “weed-free” conditions from crop emergence for the same intervals, and then letting the weeds emerge and compete with the crop for the duration of the season. In 2017, weed removal occurred in 2-wk intervals from crop emergence up to 18 wk after crop emergence (WAE) and 3-wk intervals up to 18 WAE in 2018. Additional biological measurements, including Italian ryegrass density and height, were collected at 6, 12, and 18 WAE to characterize the effect of crop-weed interactions on the CPWC and weed populations. Nonlinear regression analysis was conducted to relate the timing of weed removal and yield loss. The analysis was carried out using growing degree days (GDDs) accumulated at corresponding WAE. Italian ryegrass density ranged from 292 to 824 plants m−2, which created intense competitive conditions with wheat. In the absence of weed control, yield loss surpassed 60%. Using 5% yield loss as an accepted threshold, the CPWC for Italian ryegrass in no-till planted wheat was estimated to be from 1,100 to 1,900 GDD. This relatively short period makes it possible to reduce weed control intensity if control actions are properly timed.

North Carolina growers have long struggled to control Italian ryegrass, and recent research has confirmed that some Italian ryegrass biotypes have become resistant to nicosulfuron, glyphosate, clethodim, and paraquat. Integrating alternative management strategies is crucial to effectively control such biotypes. The objectives of this study were to evaluate Italian ryegrass control with cover crops and fall-applied residual herbicides and investigate cover crop injury from residual herbicides. This study was conducted during the fall/winter of 2021–22 in Salisbury, NC, and fall/winter of 2021–22 and 2022–23 in Clayton, NC. The study was designed as a 3 × 5 split-plot in which the main plot consisted of three cover crop treatments (no-cover, cereal rye at 80 kg ha−1, and crimson clover at 18 kg ha−1), and the subplots consisted of five residual herbicide treatments (S-metolachlor, flumioxazin, metribuzin, pyroxasulfone, and nontreated). In the 2021–22 season at Clayton, metribuzin injured cereal rye and crimson clover 65% and 55%, respectively. However, metribuzin injured both cover crops ≤6% in 2022–23. Flumioxazin resulted in unacceptable crimson clover injury of 50% and 38% in 2021–22 and 2022–23 in Clayton and 40% in Salisbury, respectively. Without preemergence herbicides, cereal rye controlled Italian ryegrass by 85% and 61% at 24 wk after planting in 2021–22 and 2022–23 in Clayton and 82% in Salisbury, respectively. In 2021–22, Italian ryegrass seed production was lowest in cereal rye plots at both locations, except when it was treated with metribuzin. For example, in Salisbury, cereal rye plus metribuzin resulted in 39,324 seeds m–2, compared to ≤4,386 seeds m–2 from all other cereal rye treatments. In 2022–23, Italian ryegrass seed production in cereal rye was lower when either metribuzin or pyroxasulfone were used preemergence (2,670 and 1,299 seeds m–2, respectively) compared with cereal rye that did not receive an herbicide treatment (5,600 seeds m–2). cereal rye (Secale cereale L.) and crimson clover (Trifolium incarnatum L.)

Widespread resistance to selective postemergence herbicides has led to increased use of preemergence herbicides to control rigid ryegrass (Lolium rigidum Gaudin), the major weed of southern Australian cropping systems. Seeds of L. rigidum are dormant at maturity, leading to staggered germination across the growing season and avoidance of pre-sowing knockdown herbicides by the later-germinating cohorts. Although it is well known that this selects for higher seed dormancy in intensively cropped areas, there is less information on whether dormant seeds respond differently to preemergence herbicides applied at sowing. To address this, seeds of field-collected L. rigidum populations were divided into dormant and nondormant (afterripened) subsamples and treated with sublethal rates of three preemergence herbicides in order to monitor seedling emergence and seed persistence over 6 mo. The presence of prosulfocarb and pyroxasulfone eliminated the nearly 4-fold increase in seedling emergence that typically results from afterripening, while trifluralin was partially inhibitory. In all treatments, the proportion of viable seeds remaining in the soil after 6 mo was negligible (≤3% of the viable seeds originally sown) for both the dormant and nondormant seeds. Application of radiolabeled herbicides to soil and seeds showed that the herbicides persisted in the seed tissue for longer than in the bulk soil. Therefore, the presence of dormant L. rigidum seeds in the soil seedbank is unlikely to result in cohorts that can avoid preemergence herbicides.

Italian ryegrass is a troublesome weed species commonly found across the United States. In North Carolina, biotypes resistant to herbicides from Groups 1, 2, and 9 have been confirmed. In fall 2020, multiple growers reported unsatisfactory control of Italian ryegrass after sequential burndown applications of paraquat in the Southern Piedmont region of the state. The objectives of this study were to confirm the presence of a paraquat-resistant Italian ryegrass biotype in the state through a whole-plant dose–response bioassay and to characterize the response of Italian ryegrass accessions from the same region to commonly used burndown herbicides. Greenhouse studies were conducted at the North Carolina State University weed science laboratories to evaluate the response of three putative paraquat-resistant Italian ryegrass biotypes (B, H, SB) and four putative susceptible biotypes (S1, S2, S3, and S4) to paraquat rates ranging from 52.5 to 26,880 g ai ha−1 and the response of 38 accessions to clethodim (271 g ai ha−1), glyphosate (1,260 g ae ha−1), glufosinate (880 g ai ha−1), nicosulfuron (34 g ai ha−1), and paraquat (840 g ai ha−1). The effective paraquat dose required to reduce biomass by 50% (GR50) for the putative paraquat-resistant biotypes ranged from 570 to 1,729 g ai ha−1, equivalent to 19- to 58-fold more resistant to paraquat compared to the average GR50 of susceptible biotypes. This study confirms the presence of paraquat-resistant Italian ryegrass in North Carolina. Results from the accessions study reveal that 29% of biotypes tested were resistant to paraquat, all of which also exhibited resistance to glyphosate and nicosulfuron. Additionally, a wide distribution of multiple herbicide–resistant biotypes was observed in the Southern Piedmont region, with 97% and 74% of accessions tested resistant to ≥1 and ≥2 sites of action, respectively.

A field study was conducted twice in Elizabeth, MS, at on-farm sites in 2010–11 and 2011–12, and twice in 2012–13 at Mississippi State University’s Delta Research and Extension Center in Stoneville, MS, to evaluate glyphosate-resistant (GR) Italian ryegrass control and crop response to fall treatments followed by postemergence herbicide treatments in winter and/or spring. Italian ryegrass was controlled ≥92% and 61% following S-metolachlor and tillage 77 d after fall treatments (DA-FT), respectively. S-metolachlor fall treatment provided 33% greater control than clethodim winter treatment at 21 d after winter treatments (DA-WT). Tillage fall treatment followed by (fb) clethodim winter treatment fb paraquat spring treatment provided similar control (93%) to treatments containing S-metolachlor fall treatment fb a winter or spring herbicide treatment (≥93%) 24 d after spring treatments (DA-ST). Greatest soybean and corn density and yield were also observed following programs containing S-metolachlor fall treatment. Sequential postemergence herbicide treatments were not required to increase corn and soybean density and yield when S-metolachlor was used as a fall treatment. Growers have the best opportunity to maximize GR Italian ryegrass control when S-metolachlor fb a winter or spring herbicide treatment is used.

Italian ryegrass is a major weed in winter cereals in the south-central United States. Harvest weed seed control (HWSC) tactics that aim to remove weed seed from crop fields are a potential avenue to reduce Italian ryegrass seedbank inputs. To this effect, a 4-yr, large-plot field study was conducted in College Station, Texas, and Newport, Arkansas, from 2016 to 2019. The treatments were arranged in a split-plot design. The main-plot treatments were (1) no narrow-windrow burning (a HWSC strategy) + disk tillage immediately after harvest, (2) HWSC + disk tillage immediately after harvest, and (3) HWSC + disk tillage 1 mo after harvest. The subplot treatments were (1) pendimethalin (1,065 g ai ha−1; Prowl H2O®) as a delayed preemergence application (herbicide program #1), and (2) a premix of flufenacet (305 g ai ha−1) + metribuzin (76 g ai ha−1; Axiom®) mixed with pyroxasulfone (89 g ai ha−1; Zidua® WG) as an early postemergence application followed by pinoxaden (59 g ai ha−1; Axial® XL) in spring (herbicide program #2). After 4 yr, HWSC alone was significantly better than no HWSC. Herbicide program #2 was superior to herbicide program #1. Herbicide program #2 combined with HWSC was the most effective treatment. The combination of herbicide program #1 and standard harvest practice (no HWSC; check) led to an increase in fall Italian ryegrass densities from 4 plants m−2 in 2017 to 58 plants m−2 in 2019 at College Station. At wheat harvest, Italian ryegrass densities were 58 and 59 shoots m−2 in check plots at College Station and Newport, respectively, whereas the densities were near zero in plots with herbicide program #2 and HWSC at both locations. These results will be useful for developing an improved Italian ryegrass management strategy in this region.

Herbicide resistance is an increasing issue in many weed species, including rigid ryegrass (Lolium rigidum Gaudin); a major weed of winter cropping systems in southern Australia. Recently, this weed has also been found in summer crops in the southeastern region of Australia. Effective control of this herbicide-resistant weed across southeastern Australia requires alternative management strategies. These strategies can be informed by analyses on the interaction of germinable seeds with their regional environments and by identifying the differences between populations of varying herbicide-resistance levels. In this study, we explore how various environmental factors differentially affect the seed germination and seedling emergence of three L. rigidum populations, including one glyphosate-resistant population (GR), one glyphosate-susceptible population (GS), and one population of unknown resistance status (CC04). Germination was greater than 90% for all populations at each temperature regime, except 15/5 C. Populations germinated at a lower rate under 15/5 C, ranging from 74% to 87% germination. Salt stress had a similar effect on the germination of all populations, with 0% germination occurring at 250 mM salt stress. Population GS had greater tolerance to osmotic stress, with 65% germination at −0.4 MPa compared with 47% and 43% germination for CC04 and GR, respectively; however, germination was inhibited at −0.8 and −1.6 MPa for all populations. All populations had lower germination when placed in complete darkness as opposed to alternating light/dark. Germination in darkness was lower for CC04 (69%) than GR (83%) and GS (83%). Seedling emergence declined with increasing burial depth with the lowest emergence occuring at 8 cm (37%) when averaged over the populations. These results indicate that L. rigidum can survive under a range of environmental variables and that the extent of survival differs based on population; however, there was no difference based on herbicide-resistance status.

Italian ryegrass has become a problematic weed in hazelnut orchards of Oregon because of the presence of herbicide-resistant populations. Resistant and multiple-resistant Italian ryegrass populations are now the predominant biotypes in Oregon; there is no information on which herbicides effectively control Italian ryegrass in hazelnut orchards. Six field studies were conducted in commercial orchards to evaluate Italian ryegrass control with POST herbicides. Treatments included flazasulfuron, glufosinate, glyphosate, paraquat, rimsulfuron, and sethoxydim applied alone or in selected mixtures during early spring when plants were in the vegetative stage. Treatment efficacy was dependent on the experimental site. The observed range of weed control 28 d after treatment was 13% to 76% for glyphosate, 1% to 72% for paraquat, 58% to 88% for glufosinate, 16% to 97% for flazasulfuron, 8% to 94% for rimsulfuron, and 25% to 91% for sethoxydim. Herbicides in mixtures improved control of Italian ryegrass compared to single active ingredients based on contrast analysis. Herbicides in mixture increased control by 27% compared to glyphosate, 18% to rimsulfuron, 15% to flazasulfuron, 19% to sethoxydim, and 12% compared to glufosinate when averaged across all sites, but mixture did not always improve reduction of ground coverage or of biomass. This complex site-specific response highlights the importance of record-keeping for efficient herbicide use. Glufosinate is an effective option to manage Italian ryegrass. However, the glufosinate-resistant biotypes documented in Oregon may jeopardize this practice. Nonchemical weed control options are needed for sustainable weed management in hazelnuts.

Rigid ryegrass is a problematic weed species in winter crops and winter fallow; however, recently, this weed species has been observed in summer crops and fallow. These observations warrant the evaluation of different postemergence herbicides for its control. Outdoor pot studies were conducted during the spring and summer of 2021 to 2022 to determine the performance of POST herbicides on two summer-emerging (S3 and S6) and two winter-emerging (W3 and W8) accessions of rigid ryegrass. Across all accessions, butroxydim, clethodim, paraquat, and paraquat + amitrole at the field rate provided complete control of rigid ryegrass. Both summer-emerging accessions were found to be resistant to the field rates of glufosinate (750 g ai ha−1), glyphosate (454 g ae ha−1), haloxyfop (52 g ai ha−1), and pinoxaden (30 g ai ha−1). The S6 accession had the highest dose required for a 50% reduction in biomass for these herbicides (glufosinate 1,120 g ai ha−1, glyphosate 1,210 g ae ha−1, haloxyfop 140 g ai ha−1, and pinoxaden 55 g ai ha−1). This summer-emerging accession (S6) was also resistant to iodosulfuron. All four accessions were found susceptible to imazamox + imazapyr (a commercial mixture) and mesosulfuron. The study provides the first evidence of poor control of summer-emerging accessions of rigid ryegrass with different herbicides. Multiple-herbicide-resistant summer-emerging rigid ryegrass accessions would be a challenge to the production of summer crops (e.g., cotton and sorghum) as well as winter crops that rely on weed-free summer fallows for soil moisture retention; therefore these accessions warrant diversified management strategies.

Narrow-windrow burning (NWB) is a form of harvest weed seed control in which crop residues and weed seeds collected by the combine are concentrated into windrows and subsequently burned. The objectives of this study were to determine how NWB will 1) affect seed survival of Italian ryegrass in wheat and Palmer amaranth in soybean and 2) determine whether a relationship exists between NWB heat index (HI; the sum of temperatures above ambient) or effective burn time (EBT; the cumulative number of seconds temperatures exceed 200 C) and the post-NWB seed survival of both species. Average soybean and wheat windrow HI totaled 140,725 ± 14,370 and 66,196 ± 6224 C, and 259 ± 27 and 116 ± 12 s of EBT, respectively. Pre-NWB versus post-NWB germinability testing revealed an estimated seed kill rate of 79.7% for Italian ryegrass, and 86.3% for Palmer amaranth. Non-linear two-parameter exponential regressions between seed kill and HI or EBT indicated NWB at an HI of 146,000 C and 277 s of EBT potentially kills 99% of Palmer amaranth seed. Seventy-six percent of soybean windrow burning events resulted in estimated Palmer amaranth seed kill rates greater than 85%. Predicted Italian ryegrass seed kill was greater than 97% in all but two wheat NWB events; therefore, relationships were not calculated. These results validate the effectiveness of the ability of NWB to reduce seed survival, thereby improving weed management and combating herbicide resistance.