The utility of biological control for weed management in agroecosystems will increase with a greater understanding of the relationships between common weed and granivore species. Giant foxtail is an introduced, summer annual grass weed that is common throughout the United States and problematic in numerous crops. Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) is a common, native, omnivorous carabid beetle with a range that overlaps giant foxtail. In 2004 and 2005, H. pensylvanicus was captured from farm fields in Centre County, PA, and subjected to laboratory feeding trials to test the preference of giant foxtail and other species on predation by H. pensylvanicus. Weed species seed preference experiments that included “Choice” and “No Choice” treatments were conducted using giant foxtail, common lambsquarters, and velvetleaf. When given a choice amongst the three weed species, H. pensylvanicus preferred giant foxtail and common lambsquarters seeds equally compared to velvetleaf seeds. When given the choice, H. pensylvanicus preferred newly dispersed giant foxtail seeds over field-aged seeds. Phenology of giant foxtail seed shed relative to H. pensylvanicus activity density was also quantified in field experiments in 2005 and 2006. Giant foxtail seed rain was determined by collecting shed seeds from August through October using pan traps. Activity density of H. pensylvanicus was monitored for 72-h periods using pitfall traps from June to October. Peak activity density of H. pensylvanicus occurred at the onset of giant foxtail seed shed in both years; however, giant foxtail seed shed peaked approximately 30 to 50 d after H. pensylvanicus activity density. Future research should focus on management practices that enhance and support H. pensylvanicus populations later in the growing season to maximize suppression of giant foxtail and other weeds that shed palatable seeds later in the season.

Nicosulfuron provides POST weed control in corn, pastures, and grassy roadsides, and has potential for use in fine turfgrass. The objective of this research was to evaluate tolerance, absorption, translocation, and metabolism of nicosulfuron in annual bluegrass and four turfgrass species. In greenhouse experiments, relative tolerance of grasses to nicosulfuron (35, 70, or 140 g ai ha−1) from high to low was bermudagrass = zoysiagrass > tall fescue > creeping bentgrass > annual bluegrass. In laboratory experiments, grasses had similar foliar and root absorption of 14C-nicosulfuon. Annual bluegrass and creeping bentgrass averaged 80% greater radioactivity per unit dry mass in shoots than bermudagrass following root uptake of 14C-nicosulfuron, but other species were similar to these grasses. At 72 h after treatment (HAT), annual bluegrass metabolized 36% of absorbed 14C-nicosulfuron, which was less than bermudagrass, tall fescue, and zoysiagrass that metabolized 47 to 58%. Creeping bentgrass metabolism of nicosulfuron was similar to annual bluegrass. Tall fescue had similar levels of metabolism to bermudagrass and zoysiagrass, averaging 67%, at 168 HAT but produced fewer metabolites. Overall, turfgrass tolerance to nicosulfuron is associated with relative herbicide concentrations in shoots and differential species metabolism.

Nikolai Ivanovich Vavilov was an early 20th century Russian plant scientist who was killed by Joseph Stalin in 1943 for his adherence to basic genetic principles. Vavilov is well known within plant breeding and plant evolutionary biology circles, yet the science of Vavilov is just as important to the field of weed science. Specifically, Vavilov proposed that certain weeds adapted to weed control practices to survive in prehistorical agrarian societies. Most would refer to this adaption as crop mimicry, but the term “Vavilovian mimicry” is more apt. Vavilovian mimicry requires three factors: a model—the crop or desirable plant; a mimic—the weed; and an operator—the discriminating agent, possibly human, animal, or machine. In a modern context, it is proposed that weed adaptation to herbicide applications be included as a form of Vavilovian mimicry, with the acknowledgement that the operator is the herbicide. In this context, Vavilovian mimicry is the adaption of the weed mimic to be perceived by the operator as visually, physically, or biochemically indistinguishable from the crop model. This review will cover the history and legacy of Vavilov in a condensed version in the hope that weed scientists will hold this individual in high regard in our future endeavors and begin to acknowledge Vavilov as one of the first scientists to propose that weeds can mimic the attributes of crops.

Long-term control of leafy spurge with glyphosate requires multiple applications because the plant reproduces vegetatively from abundant underground adventitious buds, referred to as crown and root buds. Determining the molecular mechanisms involved in controlling vegetative reproduction in leafy spurge following foliar glyphosate treatment could identify limiting factors or new targets for manipulation of plant growth and development in invasive perennial species. Thus, we treated leafy spurge plants with 0 or 2.24 kg ai ha−1 glyphosate to determine its impact on selected molecular processes in crown buds derived from intact plants and plants decapitated at the soil surface 7 d after glyphosate treatment. New shoot growth from crown buds of foliar glyphosate-treated plants was significantly reduced compared with controls after growth-inducing decapitation, and had a stunted or bushy phenotype. Quantification of a selected set of transcripts involved in hormone biosynthesis and signaling pathways indicated that glyphosate had the most significant impact on abundance of ENT-COPALYL DIPHOSPHATE SYNTHETASE 1, which is involved in a committed step for gibberellin biosynthesis, and auxin transporters including PINs, PIN-LIKES, and ABC TRANSPORTERS. Foliar glyphosate treatment also reduced the abundance of transcripts involved in cell cycle processes, which would be consistent with altered growth patterns observed in this study. Overall, these results suggest that interplay among phytohormones such as auxin, ethylene, and gibberellins affect vegetative growth patterns from crown buds of leafy spurge in response to foliar glyphosate treatment.

Field, greenhouse, and laboratory experiments were conducted to investigate resistance to glyphosate in tropical sprangletop biotypes (Lv8 and Lv9) collected in Persian lime from Veracruz, Mexico. Assays to determine the dose required to reduce seedling fresh weight by 50% indicated a resistance factor (RF) of 4.9 and 3.2 for biotypes Lv8 and Lv9, respectively; whereas the LD50 showed a RF of 4.4 and 3.3 for biotypes Lv8 and Lv9, respectively. On the other hand, the RFs using whole plant dose–response assays were lower (RF of 3 for Lv8 and 2.3 for Lv9). The susceptible biotype (LvS) accumulated 5.5 and 11.8 times more shikimate than biotypes Lv8 and Lv9, respectively, at 96 h after treatment (HAT). In field experiments, alternatives to glyphosate-resistant tropical sprangletop management were identified. Indaziflam + glufosinate and paraquat + diuron provided over 80% control of in-field populations of tropical sprangletop at 60 d after treatment (DAT). These results confirmed the first reported case of glyphosate-resistant tropical sprangletop.

Populations of rigid ryegrass with resistance to glyphosate have started to become a problem on fence lines of cropping fields of southern Australian farms. Seed of rigid ryegrass plants that survived glyphosate application were collected from two fence line locations in Clare, South Australia. Dose–response experiments confirmed resistance of these fence line populations to glyphosate. Both populations required 9- to 15-fold higher glyphosate dose to achieve 50% mortality in comparison to a standard susceptible population. The mechanism of resistance in these populations was investigated. Sequencing a conserved region of the gene encoding 5-enolpyruvyl-shikimate-3-phosphate synthase identified no differences between the resistant and susceptible populations. Absorption of glyphosate into leaves of the resistant populations was not different from the susceptible population. However, the resistant plants retained significantly more herbicide in the treated leaf blades than did the susceptible plants. Conversely, susceptible plants translocated significantly more herbicide to the leaf sheaths and untreated leaves than the resistant plants. The differences in translocation pattern for glyphosate between the resistant and susceptible populations of rigid ryegrass suggest resistance is associated with altered translocation of glyphosate in the fence line populations.

Experiments were conducted in laboratory and screenhouse to determine the effects of light, temperature, salt and water stress, seed burial depth, and amount of rice residue on seed germination and seedling emergence of wild bushbean and to evaluate the response of that weed to commonly available POST herbicides in the Philippines. Germination (81 to 85%) was not influenced by light; however, it was higher at 30/20 C (88%) than at 25/15 C (82%) and 35/25 C (80%) day/night temperatures. The sodium chloride and osmotic potential required for 50% inhibition of maximum germination were 149 mM and −0.28 MPa, respectively. Seedling emergence (81%) was similar for seeds placed on the soil surface and for seeds buried up to 4 cm deep. No seedlings emerged from a burial depth of 8 cm. Rice residue applied on the soil surface up to 8 t ha−1 did not affect seedling emergence (72 to 80%) of wild bushbean. The herbicide bentazon provided 100% control of wild bushbean when applied at the three- to five-leaf stages. However, to achieve 100% control by 2,4-D and metsulfuron-methyl + chlorimuron-ethyl, herbicides had to be applied at the three-leaf stage. The information gained from this study can help in developing components of integrated weed management programs for wild bushbean. A deep-tillage operation to bury weed seeds below their maximum depth of emergence and early application of an effective POST herbicide can help in managing this weed in rice fields.

Dithiopyr provides PRE and early POST control of smooth crabgrass, but POST efficacy is often inconsistent on tillered plants. Experiments were conducted to evaluate the interaction of temperature and growth stage on dithiopyr efficacy, absorption, translocation, and metabolism in smooth crabgrass. In greenhouse experiments, I50 (predicted rate to induce 50% injury) measured < 0.14, 0.14, and 0.15 kg ha−1 at low temperatures (average 23 C) for multi-leaf, one-tiller, and multi-tiller smooth crabgrass, respectively, while I50 measured < 0.14, 0.88, and > 2.24 kg ha−1 at high temperatures (average 32 C), respectively. Multi-tiller (three to five tillers) smooth crabgrass absorbed more root applied 14C-dithiopyr than multi-leaf (three to four leaves) and one-tiller plants, but specific radioactivity (Bq mg−1) was two to three times greater in multi-leaf plants compared to tillered plants. Smooth crabgrass treated at 15/10 C (day/night) had ≈ two times greater specific radioactivity following root applied 14C-dithiopyr than at 30/25 C. Radioactivity distribution to shoots from root applications measured 43, 30, and 20% of the total absorbed for multi-leaf, one-tiller, and multi-tiller plants, respectively. Smooth crabgrass had two times more foliar absorption of 14C-dithiopyr at 15/10 than 30/25 C while 14C losses were greater at 30/25 than 15/10 C. Smooth crabgrass metabolism of 14C-dithiopyr was ≈ two times greater at 30/25 than 15/10 C, and multi-leaf plants averaged 10 to 20% more metabolism than tillered plants at 7 d after treatment. Results suggest differential absorption, translocation, and metabolism may contribute to dithiopyr efficacy on smooth crabgrass at various growth stages, but use under high temperatures (30/25 C) could increase losses from volatilization, reduce foliar absorption, and increase metabolism compared to cooler temperatures (15/10 C).

Horseweed is a surface-germinating ruderal facultative winter annual. The ruderal nature is a key adaptive characteristic that implicates emergence timing as an important recruitment factor. Experiments were established at three sites in southern Ontario, Canada, from 2009 to 2012 to determine the possible effect of emergence timing of horseweed on plant number, fecundity, and flowering timing. Emerged seedlings were tagged in 0.25-m2 plots in five 2-wk cohorts in the fall and spring of each experimental season. Each plot was followed though until the plants contained within each plot completed their life cycle. Generally, spring-emerging plants were found to flower earlier than fall-emerging plants, but with fall emergence there were higher plant densities in August each season compared with spring emergence. Overall, there was no difference in fecundity between spring- or fall-emerging cohorts, but when cohorts were parsed beyond just spring or fall emergence, we found that plants emerging in early fall and early spring were more fecund and flowered earlier than plants emerging in late fall and late spring. Disturbance (tilled versus not-tilled) significantly affected emergence levels but not emergence timing. The differences in performance among emergence cohorts are likely due to spatial or temporal density-dependent growth advantages. These results show that spring-emerging cohorts of horseweed, especially early spring–emerging cohorts, should not be discounted when considering the weediness of this species, and this may hold true for other facultative winter annual weeds as well.

Junglerice is one of the world's most problematic C4 grass weeds present in dry-seeded rice in many countries. A screenhouse study was conducted to determine the effect of four rice planting densities (0, 100, 200, and 400 plants m−2) and four nitrogen (N) rates (0, 50, 100, and 150 kg ha−1) on the growth and morphological plasticity of junglerice. Junglerice plant height was reduced by 15 to 35%, tiller number by 54 to 77%, leaf number by 61 to 85%, leaf area by 69 to 90%, leaf biomass by 63 to 88%, stem biomass by 70 to 92%, and inflorescence biomass by 66 to 94% at rice planting densities ranging from 100 to 400 plants m−2 relative to the junglerice plants grown alone. However, all these growth parameters increased with increasing N rates. Junglerice biomass increased by 125 to 472%, whereas rice biomass increased by 122 to 285% with the application of 50 to 150 kg N ha−1. Additional N favored junglerice biomass production relative to rice. Rice crop interference (200 to 400 plants m−2) reduced junglerice growth and biomass and overshaded the junglerice plants when no N was applied. Increasing N application resulted in taller plants and higher biomass of junglerice, while it reduced root-shoot weight ratio. These results suggest that increasing N rate increased the competitive ability of the junglerice over rice regardless of crop planting density. Information generated in this study could be useful in devising appropriate combinations of planting density and fertilizer management strategies for cultural junglerice management, particularly in situations where junglerice species are more responsive than rice to N.

Perennial ryegrass is overseeded in bermudagrass and Kentucky bluegrass to improve turf quality, but selective control may be warranted for transition back to monostand turfgrass. Flucarbazone–sodium controls perennial ryegrass in bermudagrass and Kentucky bluegrass, but the physiological basis of selectivity has received limited investigation. Greenhouse and laboratory experiments were conducted to evaluate efficacy, absorption, translocation, and metabolism of flucarbazone–sodium in these grasses. Flucarbazone–sodium reduced perennial ryegrass shoot mass from the nontreated an average ≈ 22 times and 3 times more than bermudagrass and Kentucky bluegrass at 4 wk after treatment, respectively. In laboratory experiments, foliar and root absorption of 14C–flucarbazone–sodium were similar among species. Bermudagrass distributed ≈ 25% more foliar-absorbed 14C to nontreated shoots than Kentucky bluegrass and perennial ryegrass. From root applications, all grasses averaged 84% distribution of 14C to shoots. Bermudagrass and Kentucky bluegrass metabolized 100% and 74% of 14C–flucarbazone–sodium at 1 d after treatment (DAT), whereas perennial ryegrass metabolism measured 44, 58, and 65% at 1, 3, and 7 DAT, respectively. Bermudagrass, Kentucky bluegrass, and perennial ryegrass had 4, 4, and 2 metabolites after 7 d, respectively. Results suggest differential metabolism of flucarbazone–sodium is attributed to selectivity for controlling perennial ryegrass in bermudagrass and Kentucky bluegrass.

This research was aimed at understanding how far and how fast glyphosate-resistant (GR) Palmer amaranth will spread in cotton and the consequences associated with allowing a single plant to escape control. Specifically, research was conducted to determine the collective impact of seed dispersal agents on the in-field expansion of GR Palmer amaranth, and any resulting yield reductions in an enhanced GR cotton system where glyphosate was solely used for weed control. Introduction of 20,000 GR Palmer amaranth seed into a 1-m2 circle in February 2008 was used to represent survival through maturity of a single GR female Palmer amaranth escape from the 2007 growing season. The experiment was conducted in four different cotton fields (0.53 to 0.77 ha in size) with no history of Palmer amaranth infestation. In the subsequent year, Palmer amaranth was located as far as 114 m downslope, creating a separate patch. It is believed that rainwater dispersed the seeds from the original area of introduction. In less than 2 yr after introduction, GR Palmer amaranth expanded to the boundaries of all fields, infesting over 20% of the total field area. Spatial regression estimates indicated that no yield penalty was associated with Palmer amaranth density the first year after introduction, which is not surprising since only 0.56% of the field area was infested with GR Palmer amaranth in 2008. Lint yield reductions as high as 17 kg ha−1 were observed 2 yr after the introduction (in 2009). Three years after the introduction (2010), Palmer amaranth infested 95 to 100% of the area in all fields, resulting in complete crop loss since it was impossible to harvest the crop. These results indicate that resistance management options such as a “zero-tolerance threshold” should be used in managing or mitigating the spread of GR Palmer amaranth. This research demonstrates the need for proactive resistance management.

The role of weed biomass in the nutrient balance of agro-ecosystems remains poorly understood. To measure the rate of decomposition and nutrient release of common weeds, litterbag methodology was employed using waterhemp and giant foxtail desiccated by glyphosate at heights of 10, 20, 30, and 45 cm in two southern Illinois soybean fields. Losses were then expressed as a decay constant (k) regressed over time according to the single exponential decay model. Concentrations of the recalcitrant cell wall components (cellulose, hemicellulose, and lignin) were generally greatest as weed height (maturity) increased in giant foxtail compared with waterhemp. Sixteen weeks after desiccation by glyphosate, 10-cm waterhemp and giant foxtail detritus had lost 10 and 12% more mass, respectively, compared to the 45-cm height of each species. Decomposition rates revealed mass loss was highest for 10-cm waterhemp (kD = 0.022) and lowest for 45-cm giant foxtail (kD = 0.011) and this process was negatively correlated to the overall amount of cell wall constituents (r = −0.73). Nutrient release rates followed a similar trend in that both shorter (younger) weeds and waterhemp liberated nutrients more readily. Across all tested plant material, K was the nutrient most rapidly released, whereas, Ca was the most strongly retained nutrient.

One method that appears promising for the treatment of Eurasian watermilfoil in areas of high water exchange is the use of herbicide-impregnated granules. Experiments were conducted using liquid triclopyr-triethylamine and granules impregnated with triclopyr-triethylamine to test this theory. Uniform, multistemmed Eurasian watermilfoil plants were selected for these experiments. Plants were treated in clear acrylic cylinders containing 7 L of water with 0.5 mg/L triclopyr as the liquid triethylamine plus 20 kBq 14C-triclopyr or blank granules impregnated with triclopyr triethylamine plus 20 kBq of 14C-triclopyr. Plants were harvested 6, 12, 24, 48, 96, and 192 h after treatment (HAT) and the radioactivity in the apical meristems, remaining shoot and root was determined with sample oxidation and liquid scintillation spectroscopy. There were no significant differences in overall herbicide absorption by Eurasian watermilfoil following liquid and granular triclopyr treatments; however, differences were observed between plant parts. Apical meristems accumulated the most radioactivity, whereas roots accumulated very little radioactivity following liquid treatment. Granular applications resulted in 7.5 times more radioactivity in the Eurasian watermilfoil roots then the liquid triclopyr application; therefore, long-term control of well-established Eurasian watermilfoil plants could improve with granular applications, especially in areas where rapid herbicide dilution could be an issue.

Chinese sprangletop, a problematic weed in aerobic rice fields, has developed resistance to various groups of herbicides. In search of natural herbicides, the phytotoxic effects of napiergrass extracts on Chinese sprangletop were investigated. Phytotoxicity-directed extraction and fractionation of the culm plus leaves of napiergrass led to the isolation and identification of three major compounds: 2,4-di-tert-butylphenol (2,4-DTBP), cis-9-octadecenoic methyl ester (methyl oleate), and phthalic acid, mono-(2-ethylhexyl) ester (MEHP). These compounds showed different degrees of inhibition against the tested bioassay species. 2,4-DTBP was the most potent of the three compounds and completely inhibited the germination of Chinese sprangletop at the concentration of 0.5 g L−1, followed by MEHP, where 62% inhibition of germination was obtained. In contrast, methyl oleate showed only slight inhibition (< 10%) of germination. A soil bioassay further demonstrated that 2,4-DTBP is a strong inhibitor of root growth and completely prevented root growth of Chinese sprangletop at an application rate as low as 0.60 kg ai ha−1. Under aerobic conditions, 2,4-DTBP at 2.4 kg ai ha−1 reduced the emergence and shoot fresh weight of Chinese sprangletop by more than 60%, with negligible effect on root and shoot growth of aerobic rice seedlings, suggesting that 2.4 kg ai ha−1 2,4-DTBP is the most suitable rate to control Chinese sprangletop without injuring rice seedlings. Reduction in shoot height of rice plant was evident at 0 d after sowing across 2,4-DTBP rates. However, the rice plants became less susceptible with increasing growth stages. The present findings imply that 2,4-DTBP may potentially be developed as a PRE soil-applied natural herbicide for control of Chinese sprangletop and perhaps other weeds in aerobic rice system.

Since its discovery in 2005, glyphosate-resistant Palmer amaranth has become a major problem for many farmers in the southern United States. One mechanism of resistance found in a Georgia population of glyphosate-resistant Palmer amaranth is amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene throughout the genome, with some resistant plants containing and expressing more than 100 EPSPS genes. Such high numbers of EPSPS genes and protein production could result in a fitness cost to resistant plants due to (1) metabolic cost of overproduction of this enzyme and (2) disruption of other genes after insertion of the EPSPS gene. A greenhouse experiment was set up to investigate differences in growth and reproduction between glyphosate-susceptible and -resistant Palmer amaranth plants. Measurements included growth rate, plant height/volume ratio, final biomass, photosynthetic rate, inflorescence length, pollen viability, and seed set. This study found no significant fitness costs for plants with the resistance trait. This study also provided a clear example of how controlling for genetic background is important in fitness cost studies and how potentially misleading results can be obtained if only a few fitness traits are measured. These results indicate that glyphosate-resistant Palmer amaranth plants with high EPSPS gene copy numbers are likely to persist in field populations, even in the absence of glyphosate, potentially leading to long-term loss of glyphosate as a control option for Palmer amaranth.

The implementation of a successful glyphosate resistance management strategy requires a simple and cost-effective method for detecting resistance in key weeds. To date, however, glyphosate resistance is still routinely confirmed via laborious and time consuming whole-plant pot assays using seeds collected at the end of the growing season. Here, we describe a simple, early-season bioassay for detecting evolved glyphosate resistance in grass and broadleaf weeds. It involves transplanting suspected glyphosate resistant seedlings alongside known sensitive and resistant standards into agar containing informative rates of herbicide and recording percentage survival 14 d after plating. The method was validated using sensitive and resistant populations of Lolium, Eleusine, Conyza, and Amaranthus species encompassing the main glyphosate resistance mechanisms, namely, impaired translocation, EPSPS gene duplication, and mutations. The whole plant pot and agar-based seedling tests generated comparable resistance indices in dose-response assays and percentage survival at discriminating glyphosate rates. The method was applied successfully to detect resistance in a rigid ryegrass population collected from a French vineyard well before glyphosate was applied in the field for the current season. Additionally, the test was shown to be highly transferable to several other grass and broadleaf weeds that have evolved resistance to glyphosate. One major attribute of the method is that it is capable of detecting resistance regardless of the mechanism involved. In addition to being very simple, quick and, cost-effective, it allows determination of glyphosate resistance in weeds prior to field application. It thus offers the opportunity for an informed choice of herbicides for effective weed control.

Littleseed canarygrass is a troublesome grass weed in wheat fields in Iran. Predicting weed emergence dynamics can help farmers more effectively control weeds. In this work, four nonlinear regression models (beta, three-piece segmented, two-piece segmented, and modified Malo's exponential sine) were compared to describe the cardinal temperatures for the germination of littleseed canarygrass. Two replicated experiments were performed with the same temperatures. An iterative optimization method was used to calibrate the models and different statistical indices (mean absolute error [MAE], coefficient of determination [R2], intercept and slope of the regression equation of predicted vs. observed hours to germination) were applied to compare their performance. The three-piece segmented model was the best model to predict the germination rate (R2 = 0.99, MAE = 0.20 d, and coefficient of variation 1.01 to 4.06%). Based on the model outputs, the base, the lower optimum, the upper optimum, and the maximum temperatures for the germination of littleseed canarygrass were estimated to be 4.69, 22.60, 29.62, and 38.13 C, respectively. The thermal time required to reach 10, 50, and 90% germination was 31.98, 39.26 and 45.55 degree-days, respectively. The cardinal temperatures depended on the model used for their estimation. Overall, the three-piece segmented model was better suited than the other models to estimate the cardinal temperatures for the germination of littleseed canarygrass.

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.