Weeds are a significant problem in crop production and their management in modern agriculture is crucial to avoid yield losses and ensure food security. Intensive agricultural practices, changing climate, and natural disasters affect weed dynamics and that requires a change in weed management protocols. The existing manual control options are no longer viable because of labor shortages; chemical control options are limited by ecodegradation, health hazards, and development of herbicide resistance in weeds. We are therefore reviewing some potential nonconventional weed management strategies for modern agriculture that are viable, feasible, and efficient. Improvement in tillage regimes has long been identified as an impressive weed-control measure. Harvest weed seed control and seed predation have been shown as potential tools for reducing weed emergence and seed bank reserves. Development in the field of allelopathy for weed management has led to new techniques for weed control. The remarkable role of biotechnological advancements in developing herbicide-resistant crops, bioherbicides, and harnessing the allelopathic potential of crops is also worth mentioning in a modern weed management program. Thermal weed management has also been observed as a useful technique, especially under conservation agriculture systems. Last, precision weed management has been elaborated with sufficient details. The role of remote sensing, modeling, and robotics as an integral part of precision weed management has been highlighted in a realistic manner. All these strategies are viable for today’s agriculture; however, site-specific selection and the use of right combinations will be the key to success. No single strategy is perfect, and therefore an integrated approach may provide better results. Future research is needed to explore the potential of these strategies and to optimize them on technological and cultural bases. The adoption of such methods may improve the efficiency of cropping systems under sustainable and conservation practices.

Overuse of acetolactate synthase (ALS)–inhibiting herbicides in rice has led to the evolution of halosulfuron-resistant rice flatsedge in Arkansas and Mississippi. Resistant accessions were cross-resistant to labeled field rates of ALS-inhibiting herbicides from four different families, in comparison to a susceptible (SUS) biotype. Resistance index of Arkansas and Mississippi accessions based on an R/S ratio of the lethal dose required for 50% plant mortality (LD₅₀) to bispyribac-sodium, halosulfuron, imazamox, and penoxsulam was ≥ 21-fold. Control of Arkansas, Mississippi, and SUS accessions with labeled field rates of 2,4-D, bentazon, and propanil was ≥ 93%. An enzyme assay revealed that an R/S ratio for 50% inhibition (I₅₀) of ALS for halosulfuron was 2,600 and 200 in Arkansas and Mississippi, respectively. Malathion studies did not reveal enhanced herbicide metabolism in resistant plants. The ALS enzyme assay and cross-resistance studies point toward altered a target site as the potential mechanism of resistance. Trp₅₇₄–Leu amino acid substitution within the ALS gene was found in both Arkansas and Mississippi rice flatsedge accessions using the Illumina HiSeq platform, which corresponds to the mechanism of resistance found in many weed species. Field-rate applications of 2,4-D, bentazon, and propanil can be used to control these ALS-resistant rice flatsedge accessions.

Nomenclature: 2,4-D; acetolactate synthase; bentazon; bispyribac-sodium; halosulfuron; imazamox; malathion; propanil; rice flatsedge, Cyperus iria L; rice, Oryza sativa L.

The rapid evolution and spread of glyphosate-resistant (GR) kochia in the Northern Great Plains is an increasing threat to GR cropping systems and conservation tillage practices common in this region. GR kochia accessions with 4.6- to 11-fold levels of resistance to glyphosate have recently been reported in Montana. Those GR kochia accessions were also suspected to be resistant to acetolactate synthase (ALS) inhibitors, i.e., multiple herbicide-resistant (MHR) kochia. In this research, the level of resistance to the ALS-inhibitor herbicides (sulfonylureas) and the molecular mechanisms conferring resistance to glyphosate and ALS-inhibitor herbicides in MHR kochia was investigated. On the basis of whole-plant dose–response assays, MHR kochia accessions (GIL01, JOP01, and CHES01) were 9.3- to 30-fold more resistant to premixed thifensulfuron methyl tribenuron methyl metsulfuron methyl than the susceptible (SUS) accession. In an in vivo leaf-disk shikimate assay, MHR plants accumulated less shikimate than the SUS plants at a discriminate dose of 100 μM glyphosate. Sequencing of the conserved region of EPSPS revealed no target-site mutation at Thr₁₀₂ or Pro₁₀₆ residue. MHR kochia accessions had increased relative EPSPS gene copies (~ 4 to 10) compared with the SUS accession (single copy). Furthermore, MHR kochia accumulated higher EPSPS protein compared with the SUS plants. Resistance to the ALS-inhibitor herbicides was conferred by Pro₁₉₇ amino acid substitution (proline to glutamine). EPSPS gene amplification and a single target-site mutation at Pro₁₉₇ in ALS gene confer resistance to glyphosate and ALS-inhibitor herbicides, respectively, in MHR kochia accessions from Montana. This is the first confirmation of occurrence of MHR kochia in Montana.

Nomenclature: Glyphosate; metsulfuron methyl; thifensulfuron methyl; tribenuron methyl; Kochia, Kochia scoparia (L.) Schrad.

Overreliance on tribenuron has resulted in resistance evolution in water starwort. This study investigates the resistance mechanisms to tribenuron in water starwort populations from China. The cytochrome P450 monooxygenase (P450) inhibitor malathion increased tribenuron sensitivity in all populations. The decrease in the amount of herbicide dose that causes 50% growth reduction (GR₅₀) for the sensitive (S) population JS24 and the resistant (R) populations JS16 and JS17 were 2.3-, 2.5-, and 4.1-fold, respectively. However, the GR₅₀ values for the R populations were still much higher than those of the S population. This observation indicates that P450-mediated enhanced metabolism is one mechanism for resistance in water starwort. The glutathione-S-transferase (GST) activity could be induced by tribenuron for all tested populations. In particular, the GST activity of JS16 is inherently greater and is more rapidly induced than that of JS17 or JS24. Resistance attributed to mutant acetolactate synthase (ALS) alleles was identified by sequence analysis for each population. Pro197Ser substitution was detected in JS16 and JS17. Molecular markers were also developed to rapidly identify resistance as well as individuals carrying the specific Pro197Ser mutation in water starwort populations. The resistance patterns experiment revealed that the R populations exhibited different levels of resistance to pyrithiobac sodium salt, florasulam, pyroxsulam, and flucarbazone-Na; however, R populations were sensitive to imazethapyr, fluroxypyr-meptyl, 2,4-D butylate, isoproturon, and diflufenican. This study establishes that either one or at least two resistance mechanisms are involved in herbicide resistance in water starwort. Moreover, these mechanisms might contribute to the different levels of resistance to tribenuron among water starwort populations.

Nomenclature:  2,4-D butylate; diflufenican; florasulam; flucarbazone-Na; fluroxypyr-meptyl; imazethapyr; isoproturon; pyrithiobac sodium salt; pyroxsulam; tribenuron; water starwort, Myosoton aquaticum (L.) Moench..

American sloughgrass is a troublesome grass weed in winter wheat fields after rice in China. Mesosulfuron-methyl failed to control American sloughgrass in Danyang County in 2012. The purpose of this research was to determine the resistance level to mesosulfuron and other herbicides in American sloughgrass and to identify the molecular basis of resistance. Dose–response experiments indicated that this population was moderately resistant to mesosulfuron-methyl (7.6-fold) and pyroxsulam (6.0-fold), highly resistant to flucarbazone-sodium (20.3-fold), fenoxaprop-p-ethyl (565.0-fold), clodinafop-proargyl (19.5-fold), and pinoxaden (45.9-fold), and susceptible to isoproturon. Part of the acetolactate sythase (ALS) gene was cloned and sequenced to confirm the molecular mechanism of resistance to ALS-inhibiting herbicides. A Pro₁₉₇Ser substitution was identified. This substitution is likely the molecular mechanism of resistance to mesosulfuron-methyl in the Danyang population in which it is cross-resistant to flucarbazone-sodium and pyroxsulam. This study established the first report of mesosulfuron-methyl resistance likely caused by a Pro₁₉₇ substitution in American sloughgrass and a potential herbicide to control this resistant weed.

Nomenclature: Clodinafop-propargyl; fenoxaprop-p-ethyl; flucarbazone-sodium; isoproturon; mesosulfuron-methyl; pinoxaden; pyroxsulam; American sloughgrass, Beckmannia syzigachne Steud; maize, Zea mays L.; rice, Oryza sativa L.; winter wheat, Triticum aestivum L.

Aminocyclopyrachlor (AMCP) is a synthetic auxin herbicide used for broadleaf weed control in pasture and rangeland. The tolerance and fate of AMCP within pertinent grass species is not well understood. Research was conducted to establish the tolerance of four grass species to AMCP application and observe their absorption, translocation, and metabolism. Results indicate that tall fescue is the most tolerant of AMCP at rates required for weed control. Bahiagrass and bermudagrass are marginally tolerant, and cogongrass is the most sensitive. Tall fescue and bahiagrass absorbed more AMCP than bermudagrass and cogongrass, but cogongrass absorption is the most rapid and complete within 2 days after treatment (DAT). Cogongrass and bermudagrass translocated the least amount out of the target area, whereas bahiagrass and tall fescue translocated the most. Radioisotope imaging revealed that tall fescue may sequester absorbed AMCP in leaf tips. This sequestering may be the basis of the greater tolerance to AMCP by tall fescue relative to the other species evaluated. No metabolism of AMCP was detected in any grass species out to 42 DAT.

Nomenclature: Aminocyclopyrachlor; bahiagrass, Paspalum notatum Flueggé; bermudagrass, Cynadon dactylon (L.) Pers.; cogongrass, Imperata cylindrica (L.) Beauv.; tall fescue, Lolium arundinaceum (Schreb.) S.J. Darbyshire.

A waterhemp population (McLean County resistant, MCR) from McLean County, Illinois is resistant to both mesotrione and atrazine by elevated rates of herbicide metabolism. Research was conducted to investigate the inheritance of these resistance traits. Resistant and sensitive plants were crossed to obtain reciprocal F₁ populations, which were then used to create pseudo-F₂ and backcross (to sensitive parent; BC_S) populations. The various populations were evaluated with whole-plant herbicide efficacy studies in a greenhouse. The responses of the F₁ populations to both mesotrione and atrazine were intermediate when compared with parental populations. In the case of atrazine, BC_S and F₂ populations segregated 1 : 1 and 1 : 3, respectively, for susceptibility (S) : resistance (R), at a dose that controlled the sensitive parent but not the F₁ or resistant parent. For mesotrione, variability was observed within the F₁ populations, suggesting that mesotrione resistance is multigenic and the resistant parents used in the cross were not homozygous at the resistance loci. Furthermore, at low mesotrione doses, more F₂ plants survived than expected on the basis of a single-gene trait, whereas at high doses, fewer F₂ plants survived than expected. Dry weight data confirmed the conclusions obtained from survival data. Specifically, atrazine responses segregated into two discrete classes (R and S) in both the F₂ and BC_S populations, whereas mesotrione responses showed continuous distributions of phenotypes in F₂ and BC_S populations. We conclude that metabolism-based atrazine resistance in MCR is conferred by a single major gene, whereas inheritance of mesotrione resistance in this population is complex.

Nomenclature: Atrazine; mesotrione; common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU.

Acetolactate synthase (ALS) –inhibitor resistance has been recently documented in a yellow nutsedge biotype in Arkansas rice production, with a target-site mutation resulting in an amino acid substitution from Trp574 to Leu. Preliminary observations have indicated that the resistant biotype showed distinct phenotypic characteristics. Two greenhouse experiments were conducted on the resistant biotype in comparison with three susceptible standards (1) to understand differential growth habit and spatial distribution, and (2) to characterize shoot emergence pattern and seedling vigor. The resistant biotype exhibited a drastically different growth habit with secondary and tertiary basal bulbs emerging away from the parent shoot, resulting in a wider spatial distribution and ground coverage compared to the very compact growth habit of susceptible biotypes. Unlike the susceptible biotypes, the rhizomes developing into tubers were not often connected to the primary basal bulb, but were originating randomly from daughter shoots. The resistant biotype produced an extensive subterranean network of rhizomes and basal bulbs, with wider root spread and distribution compared to the susceptible biotypes. The growth habit of the resistant biotype appeared to be intermediate between yellow and purple nutsedges. Further, the resistant biotype showed a considerably delayed emergence pattern with relatively high levels of tuber dormancy. Although the resistant plants exhibited low early-growth seedling vigor and biomass production compared to the susceptible biotypes (perhaps because of smaller tubers), final aboveground biomass production was greater than that of susceptible biotypes. The overall growth habit and phenotype of the resistant biotype may provide a competitive advantage over adjacent species through the ability to occupy niches and gain improved access to critical resources. The distinct growth pattern may also mean that tillage should not be relied upon for control because it can assist further spread by disconnecting and displacing the chains of rhizomes.

Nomenclature: Yellow nutsedge, Cyperus esculentus L.; purple nutsedge, Cyperus rotundus L.; rice, Oryza sativa L.

Yellow nutsedge is one of the most problematic weedy sedges in rice–soybean systems of the Mississippi Delta region. An acetolactate synthase (ALS)-inhibiting, herbicide-resistant (Res) yellow nutsedge biotype was recently documented in eastern Arkansas, which showed intermediary growth habit between yellow nutsedge and purple nutsedge and also exhibited differential photoperiodic sensitivity to flowering. The objectives of this study were to: (a) determine variation in reproductive characteristics of the Res biotype and three susceptible (Sus) yellow nutsedge biotypes, (b) understand the influence of photoperiod on growth and reproduction, (c) understand the potential role of seeds in population establishment, and (d) elucidate the phylogenetic relationships between the Res yellow nutsedge biotype and purple nutsedge. Tuber production per plant and tuber weight of the Res biotype were less than that of the Sus biotypes. Differences in quantitative traits, such as shoot and tuber production existed between the Res and Sus biotypes for photoperiods ranging from 12 to 16 h. Generally, photoperiods greater than 12 h increased shoot development in all yellow nutsedge biotypes, with differential responses among the biotypes. Number of tubers reached the maximum for the Res biotype at a 14-h photoperiod. Over a 90-d period, inflorescence formation was only observed in the Res biotype with maximum flowering and seed production in the 14-h photoperiod. Subsequent tests revealed up to 18% seed germination, suggesting that seed could also play a role (in addition to tubers) in the persistence and spread of the Res yellow nutsedge. Phylogenetic analysis based on ribosomal DNA internal transcribed spacer (ITS) regions and mitochondrial nad4 gene intergenic spacer sequences indicated that the Res biotype was more closely associated with Sus yellow nutsedge biotypes. Nevertheless, 100% similarity for the nad4 gene sequences between the Res yellow nutsedge biotype and a reference purple nutsedge suggests that the Res biotype is likely a result of hybridization between yellow and purple nutsedges, which perhaps explains the intermediary growth characteristics observed in the Res biotype.

Nomenclature: Purple nutsedge, Cyperus rotundus L.; yellow nutsedge, Cyperus esculentus L.; rice, Oryza sativa L.; soybean, Glycine max (L.) Merr.

Field experiments were conducted near Boone, IA, to quantify postdispersal seed predation of common lambsquarters and common waterhemp in corn (2003) and soybean (2004) managed with conventional, reduced, and zero-tillage systems. Seed predation in each tillage regime was quantified using selective exclusion treatments during July through September 2003 and June through October 2004. In addition, the activity density of ground-dwelling invertebrates was estimated with pitfall traps. Choice and no-choice feeding trials were conducted in the laboratory using the most abundant weed seed predators found in the field to determine seed preferences of the potential predator organisms. The greatest seed loss occurred during July and August. In 2003, seed predation was lower in zero tillage than in conventional and reduced tillages, but no differences in seed predation between tillage regimes were observed in 2004. Maximum seed predation for common lambsquarters was 53% in 2003 and 64% in 2004. Common waterhemp seed predation reached 80% in 2003 and 85% in 2004. The majority of seed predation was by invertebrate organisms. The most common invertebrate species captured with pitfall traps were field crickets (Gryllus pennsylvanicus De Geer [Gryllidae, Orthoptera]) and ground beetles (Harpalus pensylvanicus Burmeister [Coleoptera, Carabidae]). In 2003, field crickets were relatively more abundant in conventional and reduced tillage than in zero-tillage plots. In 2004, field crickets were more abundant in the reduced tillage than in the other two tillage regimes. No differences were detected for ground beetles among tillage regimes (P  =  0.57). Choice and no-choice feeding experiments confirmed the preferences of field crickets and ground beetles for common lambsquarters and common waterhemp seeds over the larger seeds of giant foxtail and velvetleaf. Under field conditions, the activity density of field crickets was a significant predictor of common lambsquarters (r²  =  0.47) and common waterhemp (r²  =  0.53) seed predation. Positive relationships were also detected between the activity density of ground beetles and common lambsquarters (r²  =  0.30) and common waterhemp (r²  =  0.30) seed predation. This research demonstrated that weed seed predation is an important component affecting weed seedbanks and that crop management practices that favor the activity of predators such as field crickets or ground beetles could influence weed populations. Also, the results suggested that tillage is more important in determining the number of weed seeds available on the soil surface to predators than directly affecting predator activity density.

Nomenclature: Common lambsquarters, Chenopodium album L.; common waterhemp, Amaranthus tuberculatus (Moq.) Sauer.; giant foxtail, Setaria faberi Herrm.; velvetleaf, Abutilon theophrasti Medik.; corn, Zea mays L.; soybean, Glycine max (L.) Merr.

Molecular assays are often implemented by weed scientists for detection of herbicide-resistant individuals; however, the utility of these assays can be limited if multiple mechanisms of evolved resistance exist. Waterhemp resistant to protoporphyrinogen oxidase (PPO)– inhibiting herbicides is conferred by a target-site mutation in PPX2L (a gene coding for PPO), resulting in the loss of a glycine at position 210 (ΔG210). This ΔG210 mutation of PPX2L is the only known mechanism responsible for PPO-inhibitor resistance (PPO-R) in waterhemp from five states (Illinois, Indiana, Iowa, Kansas, and Missouri); however, a limited number of populations have been tested, especially in Illinois. To verify the ubiquity of the ΔG210 in PPO-R waterhemp populations in Illinois, a previously published allele-specific PCR (asPCR) was used for the detection of the ΔG210 mutation to associate this mutation with phenotypic resistance in 94 Illinois waterhemp populations. The ΔG210 mutation was detected in all populations displaying phenotypic resistance to lactofen (220 g ai ha⁻¹), indicating the deletion is likely the only mechanism of resistance. With evidence that the ΔG210 mutation dominates PPO-R waterhemp biotypes, molecular detection techniques have considerable utility. Unfortunately, the previously published asPCR is time consuming, very sensitive to PCR conditions, and requires additional steps to eliminate the possibility of false negatives. To overcome these limitations, a streamlined molecular method using the TaqMan® technique was developed, utilizing allele-specific, fluorescent probes for high-throughput, robust discrimination of each allele (resistant and susceptible) at the 210th amino acid position of PPX2L.

Nomenclature: Lactofen; waterhemp, Amaranthus tuberculatus (Moq.) Sauer (syn. rudis) AMATA.

Field studies were conducted to determine the effect of emergence timing on the fitness of the next generation as represented by seed mass, maturation, and afterripening of common waterhemp cohorts. Five natural cohorts were documented both in 2009 and 2010. Different maternal environments resulting from varied cohort emergence timings did not influence seed maturation time and seed mass, but had an inconsistent effect on seed afterripening. Here are our major findings. (1) Waterhemp cohorts needed similar amounts of time to generate viable seeds (20 to 27 d after flower initiation) and the seeds produced were of similar size (2.0 to 2.35 g), and (2) waterhemp has strong primary dormancy that may be released within 4 mo during the afterripening process, depending on the dormancy level. Seeds produced by later cohorts were more sensitive to the afterripening period, suggesting more flexibility in life strategy. Seeds from the 2009 cohorts had similar afterripening patterns; newly harvested seeds had strong primary dormancy (<10% germination), which was gradually released during dry storage and reached the maximum germination (>80%) rate 4 mo after harvest (MAH). However, germination then dropped to 40% 6 and 8 MAH, suggesting the induction of secondary seed dormancy. Strong primary dormancy at harvest for 2010 seeds was sustained in dry afterripening, perhaps because of higher dormancy level, which was the result of less-favorable parental environments brought by 10 to 30 times higher population densities and 2.5 to 5 times higher accumulative precipitation than in 2009 (see Wu and Owen 2014). We also tested the soil seed-bank seed population densities for each waterhemp cohort and found that early cohorts greatly influenced the seed population densities at the soil surface level and the turnover rate of the soil seed bank. Results from this research will provide insights into better management of waterhemp, targeting a better understanding of the seed bank.

Nomenclature: Common waterhemp, Amaranthus tuberculatus (Moq.) Sauer.

Imazamox-resistant wheat varieties carry the Imi1 allele, which confers resistance to the imidazolinone (IMI) herbicide imazamox. This resistance trait allows the selective control of jointed goatgrass, a difficult-to-control winter annual grass weed. Allele movement between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. Hybrids (F₁) of IMI-resistant wheat and jointed goatgrass were identified in 2008 in a commercial wheat field in Eastern Oregon. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of the Imi1 allele in wheat × jointed goatgrass hybrids. Using polymerase chain reaction assays we detected the presence of the Imi1 allele. A total of 128 sites were surveyed over the 2 yr. Of 1,548 plants sampled, 1,100 were positive for the Imi1 alelle and of those, 1,087 were heterozygous and 13 were homozygous for the allele. We assessed hybrid yield components and how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and the area or management practice in the commercial fields was determined. Nonagricultural sites or production of IMI-resistant wheat in consecutive years were two factors associated with a greater proportion of IMI-resistant hybrids. Our results demonstrate that the Imi1 allele is moving from IMI-resistant wheat to jointed goatgrass, producing resistant hybrids and backcross plants. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Therefore, it is important to implement field management practices that reduce IMI-resistant hybrid production and to effectively manage nonagricultural areas with jointed goatgrass infestations to prevent introgression of the IMI-resistance allele.

Nomenclature: Imazamox; jointed goatgrass, Aegilops cylindrica Host.; winter wheat, Triticum aestivum L.

Weeds represent a major cause of agricultural losses worldwide. Most weeds share a common set of life history characteristics that predispose them to weediness, two of which are self-compatibility, which allows for ease of colonization through reproductive assurance, and high trait plasticity, which allows for tolerance of a wide variety of environments and abiotic conditions. However, self-fertilization typically comes at the cost of inbreeding depression. This study investigates the role of inbreeding depression and trait plasticity under abiotic stress in the tall morningglory, a widespread self-compatible agricultural weed in the southeastern United States. Results show very little inbreeding depression in this species, likely due to purging of deleterious alleles through repeated founder events in agricultural landscapes. In contrast, abiotic stress induced substantial plasticity in ecophysiological traits, reproductive traits, and biomass allocation. In terms of performance, drought sharply impacted reproduction but not vegetative growth, and nitrogen limitation sharply impacted both. These findings are applicable to the control of weedy morningglory and underscore the usefulness of evolutionary ecology to weed management.

Nomenclature: Tall morningglory, Ipomoea purpurea (L.) Roth.

Crop rotation promotes productivity, nutrient cycling, and effective pest management. However, in row-crop systems, rotation is frequently limited to two crops. Adding a third crop, especially a perennial crop, might increase crop-rotation benefits, but concerns about disruption of agricultural and ecological processes preclude grower adoption of a three-crop rotation. The objective of the present research was to determine whether weed seed banks differ between a sod-based rotation (bahiagrass–bahiagrass–peanut–cotton) and a conventional peanut–cotton rotation (peanut–cotton–cotton) and the importance of crop phase in weed seed-bank dynamics in a long-term experiment initiated in 1999 in Florida. Extractable (ESB) and germinable (GSB) seed banks were evaluated at the end of each crop phase in 2012 and 2013, and total weed seed or seedling number, Shannon-Weiner’s diversity (H′), richness, and evenness were determined. ESB increased in H′ (36%), richness (29%), and total number of weed seeds (40%) for sod-based compared with conventional rotation, whereas GSB increased 32% in H′, 27% in richness, and 177% in total number of weed seedlings. Crop phase was a determinant factor in the differences between crop rotations. The first year of bahiagrass (B1) exhibited increases in weed seed and seedling number, H′, and richness and had the highest values observed in the sod-based rotation. These increases were transient, and in the second year of bahiagrass (B2), weed numbers and H′ decreased and reached levels equivalent to those in the conventional peanut–cotton rotation. The B1 phase increased the germinable fraction of the seed bank, compared with the other crop phases, but not the total number of weed seeds as determined by ESB. The increases in H′ and richness in bahiagrass phases were mainly due to grass weed species. However, these grass weed species were not associated with peanut and cotton phases of the sod-based rotation. The results of the present study demonstrated that including bahiagrass as a third crop in a peanut–cotton rotation could increase weed community diversity, mainly by favoring increases in richness and diversity, but the structure and characteristics of the rotation would prevent continuous increases in the weed seed bank that could affect the peanut and cotton phases.

Nomenclature: Bahiagrass, Paspalum notatum Fluegg; cotton, Gossypium hirsutum L.; peanut, Arachis hypogaea L.

Crop and weed competition studies rarely determine how plant-to-plant interactions alter the structure and physiology of crop roots. Soybean has the ability to detect neighboring weeds and to alter growth patterns including the allocation of resources to root growth. In this study, we hypothesized that low red : far red light ratio (R : FR) reflected from aboveground vegetative tissue of neighboring weeds would alter soybean root morphology and reduce root biomass and nodule number. All experiments were conducted under controlled conditions in which resources of light, water, and nutrients were nonlimiting. Low R : FR reflected from aboveground neighboring weeds reduced soybean seedling root length, surface area, and volume, including the number of nodules per plant. An accumulation of H₂O₂, an increase in malondialdehyde (MDA) content, a reduction in flavonoid content, and a decrease in 1,1-diphenyl-2-picrylhydrazyl (DPPH)–radicle scavenging activity were observed. The reduction in flavonoid content was accompanied by a decrease in the transcription of GmIFS and GmN93 and an increase in transcript levels of several antioxidant genes. These molecular and physiological changes may have a physiological cost to the soybean plant, which may limit the plant’s ability to respond to subsequent abiotic and biotic stresses that will occur under field conditions.

Nomenclature: Soybean [Glycine max (L.) Merr.].

Goss's bacterial wilt and leaf blight of corn is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis (Cmn). This disease has recently re-emerged as an important disease in the Midwestern United States (US) and continues to spread. Cultural practices are currently the only methods available for controlling the disease. Weedy species in the genera Echinochloa, Setaria, and Sorghum have previously been described as alternative hosts of Cmn. The objective of this research was to use an isolate of Cmn from the eastern Midwest to examine the host status of previously confirmed hosts, as well as test whether additional weedy or cover crop species are alternative hosts of the bacterium. Plants were inoculated with a suspension of 1 × 10⁸ colony-forming units of Cmn per milliliter in a greenhouse experiment. Leaves were observed for typical symptoms of Goss's wilt 7 d after inoculation. Pathogen presence was determined by observing bacterial streaming microscopically, and isolating Cmn from symptomatic plants. Putative colonies of Cmn were confirmed with the use of morphological and molecular methods. Koch's Postulates were completed on populations of new plant species that showed symptoms. Results revealed three new hosts of Cmn: annual ryegrass, johnsongrass, and large crabgrass. In contradiction to previous reports, barnyardgrass was not a host of Cmn in this study. Results also confirm that giant foxtail, green foxtail, shattercane, and yellow foxtail are hosts of Cmn. These results redefine the known host range of Cmn and are important in identifying additional sources of inoculum to improve our understanding of the epidemiology of Goss's wilt.

Nomenclature: Annual ryegrass, Lolium multiflorum Lam.; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; giant foxtail, Setaria faberi Herm.; green foxtail, Setaria viridis (L.) Beauv.; johnsongrass, Sorghum halepense (L.) Pers.; large crabgrass, Digitaria sanguinalis (L.) Scop.; shattercane, Sorghum bicolor (L.) Moench ssp. arundinaceum (Desv.) de Wet & Harlan; yellow foxtail, Setaria pumila (Poir.) Roemer & J.A. Schultes; corn, Zea mays L.

Ground beetles are postdispersal weed seed predators, yet their role in consuming buried seeds is not well studied. We conducted greenhouse experiments to investigate how seed burial affects consumption of weed seeds (volunteer canola) by adult ground beetles (Coleoptera: Carabidae). Seed burial depth influenced seed consumption rates as demonstrated by a significant interaction between seed burial depth, carabid species, and gender of the carabid tested. We observed higher seed consumption by females of all species, and greater consumption of seeds scattered on the soil surface compared with seeds buried at any depth. However, there was evidence of seed consumption at all depths. Adults of Pterostichus melanarius (Illiger) and Harpalus affinis (Schrank) consumed more buried seeds than did those of Amara littoralis Mannerheim. Agricultural practices, such as tillage, bury seeds at different depths and based on the results of this study, these practices may reduce seed consumption by carabids. Soil conservation practices that reduce tillage (conservation or zero tillage) will favor greater weed seed predation due, in part, to the high availability of seeds at the soil surface or at shallow soil depths.

Nomenclature: Volunteer canola, Brassica napus L.

Postdispersal weed seed predation is a significant source of weed mortality in agroecosystems. The magnitude of seed predation, however, is variable. Understanding the relative importance of factors driving variability in seed predation rates will increase the potential utility of seed predation to farmers. We conducted landscape-scale field experiments to quantify and compare the effects of space, time of sampling, and habitat on weed seed predation. Seed predation assays, with and without vertebrate exclosures, measured seed predation rates at spatially explicit sample sites across 8.5 ha of crop and noncrop habitats on a diversified organic vegetable farm in Maine. Total and invertebrate seed predation averaged 8% and 3% d⁻¹, respectively. Vertebrate seed predators detected by motion-sensing cameras included small mammals and birds. A ground beetle, Harpalus rufipes, was highly dominant in pitfall traps, comprising 66% of invertebrate seed predators captured within crop fields. Seed predation was randomly distributed in space. However, time of sampling and habitat were highly significant predictors of seed predation. Variance partitioning indicated that habitat factors explained more variation than did time of sampling. Total seed predation was greater in crop and riparian forest habitats than in mowed grass, meadow, or softwood forest. Generally, invertebrate seed predation was greatest at sites with an intermediate degree of vegetative cover, whereas habitat type was the chief biotic determinant of vertebrate seed predation rates. These results suggest cover cropping and wetland conservation as practices that may bolster seed predation rates.

Nomenclature: Harpalus rufipes DeGeer.

Palmer amaranth is a troublesome weed in cotton production. Yield losses of 65% have been reported from season-long Palmer amaranth competition with cotton. To determine whether water is a factor in this system, experiments were conduced in 2011, 2012, and 2013 in Citra, FL, and in Tifton, GA. In 2011, infrequent rainfall lead to drought stress. The presence of Palmer amaranth resulted in decreased soil relative water content up to 1 m in depth. Cotton stomatal conductance (g_s) was reduced up to 1.8 m from a Palmer amaranth plant. In 2012 and 2013 higher than average rainfall resulted in excess water throughout the growing season. In this situation, no differences were found in soil relative water content or cotton g_s as a function of proximity to Palmer amaranth. A positive linear trend was found in cotton photosynthesis and yield; each parameter increased as distance from Palmer amaranth increased. Even in these well-watered conditions, daily water use of Palmer amaranth was considerably higher than that of cotton, at 1.2 and 0.49 g H₂0 cm⁻² d⁻¹, respectively. Although Palmer amaranth removed more water from the soil profile, rainfall was adequate to replenish the profile in 2 of the 3 yr of this study. However, yield loss due to Palmer amaranth was still observed despite no change in g_s, indicating other factors, such as competition for light or response to neighboring plants during development, are driving yield loss.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats.; cotton, Gossypium hirsutum L.

The overall objective of this study was to identify common patterns in the spatial distribution of the major weed species present in the corn-growing region of central Spain, exploring the scale dependence of these patterns and the possible associations or dissociations between individual species. Weed density was assessed in 16 commercial fields using digital images acquired in a 9-m by 9-m sampling grid. A set of six species was found in all the fields: black nightshade, common cocklebur, fierce thornapple, johnsongrass, purple nutsedge, and velvetleaf. Spatial analysis by distance indices and inverse distance weighting interpolation methods were performed to create weed distribution maps. The results showed aggregated spatial distribution patterns for all individual species regardless their life cycle, annual or perennial. Some associations and dissociations among species were found in the analysis of interactions. Nevertheless, the spatial patterns of co-occurrence of weed species were field-specific and therefore cannot be considered general patterns of weed co-occurrence. In order to explore the scale dependence of these results, an additional study was conducted in an experimental field located in the same area using a 1.0-m by 0.75-m sampling grid. Although this resolution allowed for a better definition of the positions of the weed patches and weed-free gaps, the results obtained revealed similar patterns to those observed with a coarser sampling resolution.

Nomenclature: Black nightshade, Solanum nigrum L. SOLNI; common cocklebur, Xanthium strumarium L. XANST; fierce thornapple, Datura ferox L. DATFE; johnsongrass, Sorghum halepense (L.) Pers. SORHA; purple nutsedge, Cyperus rotundus L. CYPRO; velvetleaf, Abutilon theophrasti Medik. ABUTH; corn, Zea mays L.

Clethodim resistance was identified in 12 rigid ryegrass populations from winter cropping regions in four different states of Australia. Clethodim had failed to provide effective control of these populations in the field and resistance was suspected. Dose–response experiments confirmed resistance to clethodim and butroxydim in all populations. During 2012, the LD₅₀ of resistant populations ranged from 10.2 to 89.3 g ha⁻¹, making them 3 to 34–fold more resistant to clethodim than the susceptible population. Similarly, GR₅₀ of resistant population varied from 8 to 37.1 g ha⁻¹, which is 3 to 13.9–fold higher than the susceptible population. In 2013, clethodim-resistant populations were 7.8 to 35.3–fold more resistant to clethodim than the susceptible population. The higher resistance factor in 2013, especially in moderately resistant populations, could have been associated with lower ambient temperatures during the winter of 2013. These resistant populations had also evolved cross-resistance to butroxydim. The resistant populations required 1.3 to 6.6–fold higher butroxydim dose to achieve 50% mortality and 3 to 27–fold more butroxydim for 50% biomass reduction compared to the standard susceptible population. Sequencing of the target-site ACCase gene identified five known ACCase substitutions (isoleucine-1781-leucine, isoleucine-2041-asparagine, aspartate-2078-glycine, and cysteine-2088-arginine, and glycine-2096-alanine) in these populations. In nine populations, multiple ACCase mutations were present in different individuals. Furthermore, two alleles with different mutations were present in a single plant of rigid ryegrass in two populations.

Nomenclature: Clethodim; rigid ryegrass; Lolium rigidum Gaudin.

Edamame, a specialty food-grade soybean popular among health-conscious consumers, is growing in popularity worldwide. Despite a well-developed soybean industry, most edamame consumed in the United States is imported from Asia. Considerable interest exists in growing edamame domestically; however, weed interference is a major problem, and until recently, only a single herbicide was registered for use on the crop. The objectives of this work were (1) to compare effectiveness of weed management treatments that utilize herbicides currently registered for use on edamame or that may be registered in the near future, (2) to determine the significance of edamame cultivar on performance of these treatments, and (3) to identify potential relationships between the crop and weed. Ten different weed management treatments were tested in three edamame cultivars over a 3-yr period. All weed management treatments increased marketable pod yield relative to the nontreated control, but only treatments with saflufenacil or S-metolachlor combinations were comparable to the hand-weeded weed-free treatment. Of the treatments studied, S-metolachlor followed by imazamox was among the greatest yielding, had the least weed density and biomass, and did not reduce crop population density. Also, cultivars differed in their weed-suppressive ability. Path analysis indicated certain relationships were consistent across cultivars, such as weed population density having a direct negative association with crop biomass; however, other edamame–weed interactions were not identical across cultivars. Although more improvements are needed, the vegetable industry is beginning to have nascent weed management options in edamame, which will likely reduce reliance on hand weeding and result in crop-production costs that are more competitive in the global market.

Nomenclature: Imazamox; S-metolachlor; saflufenacil; edamame, Glycine max (L.) Merr.

Nitrous oxide (N₂O) is a potent greenhouse gas with implication for climate change. Agriculture accounts for 10% of all greenhouse gas emissions in the United States, but 75% of the country's N₂O emissions. In the absence of PRE herbicides, weeds compete with soybean for available soil moisture and inorganic N, and may reduce N₂O emissions relative to a weed-free environment. However, after weeds are killed with a POST herbicide, the dead weed residues may stimulate N₂O emissions by increasing soil moisture and supplying carbon and nitrogen to microbial denitrifiers. Wider soybean rows often have more weed biomass, and as a result, row width may further impact how weeds influence N₂O emissions. To determine this relationship, field studies were conducted in 2013 and 2014 in Arlington, WI. A two-by-two factorial treatment structure of weed management (PRE POST vs. POST-only) and row width (38 or 76 cm) was arranged in a randomized complete block design with four replications. N₂O fluxes were measured from static gas sampling chambers at least weekly starting 2 wk after planting until mid-September, and were compared for the periods before and after weed termination using a repeated measures analysis. N₂O fluxes were not influenced by the weed by width interaction or width before termination, after termination, or for the full duration of the study at P ≤ 0.05. Interestingly, we observed that POST-only treatments had lower fluxes on the sampling day immediately prior to POST application (P  =  0.0002), but this was the only incidence where weed influenced N₂O fluxes, and overall, average fluxes from PRE POST and POST-only treatments were not different for any period of the study. Soybean yield was not influenced by width (P  =  0.6018) or weed by width (P  =  0.5825), but yield was 650 kg ha⁻¹ higher in the PRE POST than POST-only treatments (P  =  0.0007). These results indicate that herbicide management strategy does not influence N₂O emissions from soybean, and the use of a PRE herbicide prevents soybean yield loss.

Nomenclature: Soybean; Glycine max (L.) Merr.