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Hordeum spp. (barley grass) and Bromus spp. (brome grass) are annual weed species that achieve long‐distance seed dispersal via attachment to sheep, but also lead to sheep carcass damage due to the penetration of seeds into animal tissues. Range expansion of invasive weed populations predicted as a consequence of climate change may be associated with an increase in seed contamination incidence globally. An examination of Australian abattoir data (2006–2012), along with recent weed distribution patterns, indicates carcass contamination was associated with rainfall and abundance of key annual grass weeds, primarily Bromus spp. and Hordeum spp., although other factors may play a contributing role. Significant differences in seed contamination in slaughtered sheep were noted between states and regions, with widespread contamination occurring throughout all agro‐ecological zones. Abattoir, sheep age and sex significantly influenced incidence, with contamination highest in adult males. Incidence increased with mean monthly rainfall, but declined with mean elevation, and significant interactions were noted between mean monthly temperature and state, and between mean elevation and year. Results show the need for vigilant recording of seed injuries in sheep to enhance weed surveillance efforts and effective integrated weed management of contributing annual grass weeds.

Background and objectives: Integrated weed management in commercial wheat production is urgently needed due to increasing herbicide resistance and production costs. Benzoxazinoids (BXs), which include benzoxazinones and benzoxazolinones, are unique bioactive metabolites produced by certain members of the Poaceae including maize, wheat, rye and some dicots. BXs play important roles in plant defence and are causal agents of allelopathic interference. We investigated the role of genetics, environment and crop growth stage on BX abundance in the roots and rhizoplane of selected commercial wheat cultivars, and quantified their microbial transformation products (aminophenoxazinones) in roots and rhizosphere soils. Methods: Cultivar trials of competitive wheat (Triticum aestivum L.) genotypes were conducted in two moderate to low rainfall (449–572 mm) locations in southeastern Australia in 2015 and 2016. Replicated shoot, root, rhizoplane, and rhizosphere soil samples were collected for metabolic profiling at selected crop phenological stages, extracted and further analysed for known benzoxazinoid metabolites by liquid chromatography coupled with high resolution mass spectrometry. Results: Fifteen BXs and related microbially derived aminophenoxazinones were detected in wheat shoots, roots, rhizoplanes and rhizosphere soils in both years and locations. MBOA, HMBOA and HMBOA-Glc were the three most abundant BX metabolites in wheat tissues, with the heritage cultivar Federation producing the highest levels of MBOA. The phytotoxic aminophenoxazinones APO and AMPO were the most abundant BX microbial transformation products and were detected in wheat roots, rhizoplanes and rhizospheres. Abundance varied with cultivar, growth stage, location and year. Conclusions: Microbially-produced aminophenoxazinones generated from both heritage and modern wheat root exudates were detected and quantified in rhizosphere soils, with abundance dependent on cultivar, growth stage, and season. Concentrations of microbial metabolites APO, AMPO, and AAPO were higher in the rhizosphere of young wheat seedlings in contrast to that of mature plants suggesting that phenoxazinone production was upregulated early in the season. Our findings demonstrate that BX metabolites at all life stages of wheat potentially undergo rapid biotransformation to aminophenoxazinones under field conditions, resulting in ecologically relevant concentrations sufficient for weed suppression by certain wheat cultivars.

Barley grass (Hordeum spp.) is a short-lived annual weed which competes with preferred crop and pasture species and frequently contaminates wool and carcasses, and irritates the ears, eyes and noses of sheep. Barley grass, annual ryegrass (Lolium rigidum L.) and wild radish (Raphanus raphanistrum L.) are annual winter crop weeds which reduce grain yield through competition. In three consecutive years (2015 to 2017) cereal, legume and cereal/legume forage crops were grown and harvested in early October, late October or early November consistent with an early silage harvest (ES), late silage harvest (LS) or hay cut (H). A spring wheat cultivar was sown over each site in the following year (2016 to 2018). Weed density was recorded during the forage crop and wheat phases. Forage crop weed populations varied between years. Late paddock preparation for sowing in 2015 effectively eradicated barley grass from all forage crops, however, the competitiveness of legume and cereal/legume crops against annual ryegrass was reduced. In contrast, legume and cereal/legume mixtures tended to have higher barley grass densities than cereal crops in 2016 and 2017, when paddock preparation was earlier. Cutting in October 2015 reduced annual ryegrass and wild radish populations in 2016 wheat by 92.0% and 86.7%, respectively. In 2017 and 2018, regrowth and subsequent seed set following cutting appeared to negate cutting time effects for all crop and crop/legume combinations. Late paddock preparation, an early October forage harvest and effective regrowth control provided the best opportunity for barley grass, annual ryegrass and wild radish control in a single year.

Weeds are an increasingly significant issue inhibiting agricultural production worldwide. Forage conservation could form part of an integrated weed management program if ensiling killed weed seeds. In Experiment 1, seeds of five grass (Hordeum spp., Bromus diandrus, Bromus hordeaceum, Lolium rigidum and Vulpia spp.) and two broad-leaved temperate weed species (Echium spp. and Raphanus raphanistrum), that were either untreated, ensiled in pasture (Trifolium subterranean/Lolium rigidum mixture) forage for a minimum of three months, underwent 48 h in sacco digestion in steers or ensiled prior to digestion were tested for germinability and viability. In Experiment 2, seeds of eight tropical weed species (Cenchrus ciliaris, Rumex spp., Bidens pilosa, Sorghum halepense, Urochloa panicaoides, Paspalum dilatatum, Brachiara eruciformis and Choris truncata) were ensiled in Sorghum bicolor forage. In Experiment 3, L. rigidum and R. raphanistrum seeds were ensiled in either Medicago sativa forage wilted to 336.9, 506.5 or 610.7 g/kg dry matter, or in chaff to which water or water plus acid was added at rates to achieve 350, 450 or 550 g/kg dry matter content with lactic plus acetic acid added in the ratio of 3:2 at 80, 45 or 10 g/kg DM, respectively. In Experiment 4, L. rigidum and R. raphanistrum seeds were ensiled in cotton wool to which water or water plus acid was added at the same rates as in Experiment 3. Germinability of all seeds following ensiling was substantially reduced or nil. The extent of the reduction varied with species and experiment. In sacco digestion reduced germinability in Experiment 1, but to a lesser extent than ensiling, while ensiling plus digestion reduced germination rates to 0%.

Weed suppressive potential of annual pasture legumes has been previously described, the mechanism of interference with weeds has not been clearly elucidated. We, therefore, aimed to delineate the role of secondary metabolites synthesized and released by pasture legumes through a series of studies to: 1) characterize key metabolites present in plant tissues, residue and the rhizosphere and 2) correlate their presence with weed suppressive properties. Field and in vitro experimentations coupled with targeted and non-targeted metabolic profiling was performed to evaluate the abundance of key phytotoxic metabolites using UHPLC QTOF-MS. Further, those metabolites strongly correlated with weed suppression and phytotoxicity were predicted by chemometric analyses and their concentration evaluated in field soils collected from the same legume site. Field experimentation, methanolic extracts and dried residues of Biserrula pelecinus L. and Ornithopus compressus L. exhibited marked phytotoxicity in a series of laboratory experiments. Metabolic profiling revealed that both foliar tissues and rhizosphere soils of pasture legumes possessed a high abundance of flavonoids and their precursors. Chemometric analyses suggested the strong association of quercetin, isoquercetin, kaempferol, and kaempferol-7-O-glucoside with weed suppression under field conditions. Specifically, the abundance of quercetin and kaempferol was significantly higher in soils collected from established stands of biserrula and yellow serradella in contrast to arrowleaf, gland and subterranean clover. Current study provided an evidence for the role of certain annual legume-produced flavonoids in weed suppression in southern Australia and further insight into their localization and release in the soil rhizosphere.

Amaranthus retroflexus L., an introduced invasive weed in southern Australia, has been associated with acute renal failure and/or mortality in a number of livestock species. While its leaves, flowers and stems are generally reported to contain high levels of nitrogen, few studies have fully characterised the chemical composition of A. retroflexus foliage with respect to mammalian toxicity. We performed extensive metabolic profiling of stems, leaves, roots and inflorescence tissues of A. retroflexus collected from three spatially and/or temporally distinct toxicity outbreaks, and report on the 1) composition of primary and secondary metabolites in methanolic extracts of A. retroflexus tissues using HPLC and HPLC-MS QToF and 2) chemometric analysis of A. retroflexus extracts in relation to the associated toxin(s). All tissues of A. retroflexus possessed an abundance of N-containing metabolites, particularly quaternary ammonium compounds which were identified as betaines, two of which (valine betaine and isoleucine betaine) are rarely encountered in plants. Cytotoxicity to murine fibroblasts was highest in extracts of leaf tissue and was associated with a single, a small modified peptide with high similarity to N-acetyl-L-α-aspartyl-L-alanyl-L-α-aspartyl-L-α-glutamyl-O-(carboxymethyl)-L-tyrosyl-L-leucinamide, a synthetic phosphotyrosyl mimic involved in cell signaling processes. One possible mode of action leading to acute renal failure in grazing livestock by a modified peptide such as this is proposed.

Parthenium weed (Parthenium hysterophorus L.) is an invasive plant species in around 50 countries and a &lsquo, Weed of National Significance&rsquo, in Australia. This study investigated the relative toxicity of the leaf, shoot and root extracts of two geographically separate and morphologically distinct biotypes of parthenium weed in Queensland, Australia. Parthenium weed exhibited higher phytotoxic, cytotoxic and photocytotoxic activity in leaf tissue extracts in contrast to shoot and root. The germination and seedling growth of a dicot species (garden cress) were inhibited more than those of a monocot species (annual ryegrass) using a phytotoxicity bioassay. The cytotoxicity of leaf extracts was assessed in a mouse fibroblast cell suspension assay and increased under high ultraviolet A(UV-A) radiation. A major secondary metabolite, parthenin, was found in abundance in leaf extracts and was positively correlated with cytotoxicity but not with photocytotoxicity or phytotoxicity. Ambrosin and chlorogenic acid were also detected and were positively correlated with germination inhibition and the inhibition of radicle elongation, respectively. In addition, other currently unidentified compounds in the leaf extracts were positively correlated with phytotoxicity, cytotoxicity and photocytotoxicity with two to three molecules strongly correlated in each case. Both parthenium weed biotypes investigated did not differ with respect to their relative toxicity, despite their reported differences in invasive potential in the field. This suggests that secondary chemistry plays a limited role in their invasion success.

Herbicide resistance in weeds restricts control options, thereby escalating economic loss and threatening agricultural sustainability in cereal production. Field evaluation of the crop performance, competitive traits, and consequent weed suppressive potential of 13 commercial winter wheat (Triticum aestivum L.) cultivars was performed in central NSW Australia with a focus on the evaluation and modelling of above-ground interactions. In 2015 and 2016, replicated field trials were established with genetically diverse commercial wheat genotypes under moderate to low rainfall conditions in Wagga Wagga (572 mm) and Condobolin (437 mm) New South Wales, respectively. The heritage cultivar Federation and a commercial cultivar of winter cereal rye (Secale cereale L.) were included as known weed-suppressive controls. Crop and weed growth, as well as early vigour, leaf area index, and photosynthetically active radiation, were monitored at various crop phenological stages including early growth, vegetative, flowering, grain fill, and harvest. Significant differences between wheat cultivar and location were observed for crop biomass, early vigour, leaf area index, weed number, weed biomass, canopy architecture, and yield in both 2015 and 2016. Differences in weed establishment were largely impacted first by rainfall and season and secondly by crop architecture (i.e., height, size, canopy) and phenology (i.e., growth stages). Early vigour and early canopy closure were instrumental in suppressing weed establishment and growth. Cultivar performance and competition with weeds were also clearly influenced by both environmental factors and genotype, as evidenced by differences in early cultivar performance, yield, and weed suppression by season and location. Specifically, Federation, Condo, and Janz wheat cultivars were superior performers in terms of weed suppression in both locations and years, however, Federation produced up to 55% lower yield than recently introduced cultivars. Partial least squares (PLS) regression was performed to develop a predictive linear model for weed competition in commercial wheat cultivars based on weed dry biomass as the response variable and selected aboveground crop canopy traits as predictors. In 2015, the model differed in accordance with crop growth stage, but the impact of predictors on weed biomass at both locations was not significant. In 2016, under local above average rainfall conditions, the model showed a significant negative correlation (p &lt, 0.001) of most predictors on weed biomass (r2 = 0.51 at Condobolin, r2 = 0.62 at Wagga Wagga), suggesting the most influential factors in reducing weed numbers and establishment as crop vigour, biomass, and height. Our results indicate the establishment of competitive wheat cultivars in the absence of post-emergent herbicides resulted in a two to five-fold increased weed suppression over less suppressive genotypes, without significant yield penalties. Therefore, cultivar choice constitutes a cost-effective and sustainable weed management tool, particularly when weed pressure is significant.

Barley grass (Hordeum murinum subsp. glaucum.) is an annual weed associated with grain revenue loss and sheep carcass damage in southern Australia. Increasing herbicide resistance led to a recent investigation into effective integrated weed management strategies for barley grass in southern Australia. Field studies in Wagga Wagga, New South Wales (NSW) during 2016 and 2017 examined the effect of post-emergent herbicide applications and strategic defoliation by mowing on barley grass survival and seed production in a mixed legume pasture. Statistically significant differences between herbicide-only treatments in both years showed propaquizafop to be more than 98% effective in reducing barley grass survival and seed production. Paraquat was not effective in controlling barley grass (58% efficacy), but led to a 36% and 63.5% decrease in clover and other weed biomass, respectively, after 12 months and increased lucerne biomass by over three-fold after 24 months. A single repeated mowing treatment resulted in a 46% decline in barley grass seedling emergence after 12 months and, when integrated with herbicide applications, reduced other weed biomass after 24 months by 95%. Resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides observed in local barley grass populations led to additional and more focused investigation comparing the efficacy of other pre- and post-emergent herbicides for barley grass management in legume pastures. Haloxyfop-R + simazine or paraquat, applied at early tillering stage, were most efficacious in reducing barley grass survival and fecundity. Impact of defoliation timing and frequency on barley grass seedlings was also evaluated at various population densities, highlighting the efficacy of repeated post-inflorescence defoliations in reducing plant survival and seed production. Results highlight the importance of optimal environmental conditions and application timing in achieving efficacious control of barley grass and improving pasture growth and biomass accumulation.

Herbicide resistance in both grasses and broadleaf weeds is on the rise across Australia, with an increasing number of cropping weeds experiencing resistance to multiple herbicides. One contributing factor to this issue is the adoption of conservation agriculture (CA). CA is a system of residue management that avoids the use of cultivation for establishment of annual broadacre crops. Another contributing factor is poor management of herbicide mode of action strategies in broadacre farming. One key tool for integrated weed management (IWM) strategies is the use of competitive grain crop cultivars and post-harvest crop residues, which can effectively suppress or delay weed seedling emergence and provide an initial advantage for the crop in terms of early weed suppression. The ability of various dual-purpose grazing or non-grazing grain crops and their residues to suppress weeds until subsequent planting the following year was compared in two successive field experiments in the Riverina region of New South Wales (NSW), Australia. We evaluated 1) the impact of residues of several grain crops on winter and post-harvest summer annual weed establishment from 2012 to 2014 and 2) in-crop and post-harvest weed suppression in 2014–2015 using a genetically diverse set of canola cultivars, including those found to be highly weed-suppressive in the first trial. Replicated field trials were established in Wagga Wagga, in a moderate rainfall zone (mean 572 mm/year) from 2012 to 2015 using commercially available crop cultivars. Differences in in-crop weed infestation and in post-harvest crop fallows associated with grain crop cultivar and species were observed in each of three years. Significant weed suppression associated with grazing and non-grazing wheat residues was observed after harvest, with grazing wheat exhibiting significant suppression of fleabane and witchgrass up to 130 days post-harvest. Grazing and non-grazing canola provided strong and significant suppression of fleabane and witchgrass for up to 140 days following harvest. Grazing cereal cultivars were generally more suppressive of weeds than non-grazing cultivars. Early vigour and ability to intercept light and accumulate biomass resulted in suppression of in-crop weed growth in canola trials, with GT-50 the most weed suppressive canola cultivar. Weed biomass differed with cultivar in both years, and appeared to be inversely related to early crop vigour, suggesting the importance of crop biomass in regulating weed competition in the crop. Cultivars CB Taurus and GT-50 were consistently the most weed suppressive when residues remained in plots 150 days post-harvest. These results indicate that establishment of certain species and cultivars of grain crops may effectively suppress weed growth both in-crop and post-harvest, in the absence of post-emergent herbicides. In addition, the choice of canola cultivar for desired weed suppression impacts the subsequent ability of the crop and its residues to successfully interfere with weed growth.

Citrullus lanatus (camel melon) is an important summer weed of Australian fallows, and can rapidly develop monocultural stands in sandy soils receiving adequate soil moisture. As a general review on the biology of C. lanatus , this paper reviewed the current published literature (including our recent published studies) and also summarised extensive field and laboratory studies performed on its biology, phenology and management in the South-West Slopes of New South Wales. Recent population genetics studies conducted in Australia have shown that the species is monotypic, and was introduced as a single colonisation event in the mid 1800's. Our studies showed genetic diversity in C. lanatus to be non-existent across Australia and invasive ranges and highest in the native range in Africa. Further genetic analyses have shown the species in Australia is identical to Citrullus lanatus var. citroides , the citron melon, native to Africa and now naturalised across Africa, Asia and North America, where it is a weedy nuisance or occasionally a food source for livestock and humans. Although limited genotypic diversity may facilitate potential biocontrol strategies for Citrullus lanatus in Australia, biocontrol may be difficult due to its close genetic similarity to commercial watermelon, a major horticultural crop in Australia and more globally. In Australia, field germination was observed to occur during a two to three-month period between late spring and summer, when warm soil temperatures occurred and field establishment was typically observed after significant rainfall events and was associated with soil moisture availability. Controlled environment seed dormancy findings indicated that dormancy was significantly reduced by storage at ambient laboratory temperatures over eighteen months. Seed dormancy was transient and appeared to be both physical and physiological in nature, and was dependent on the period of after-ripening during the post-harvest period. Key reproductive attributes, including high seed production, self-compatibility and pollination facilitated by several non-specific pollinators have likely resulted in increased spread of this weed in Australia and more globally. Management of C. lanatus is achieved using IWM strategies including pre and post-emergent applications of herbicides as well as limiting fruit production by cultural practices including mowing, grazing and cultivation.

Cucumis myriocarpus is an annual cucurbitaceous summer weed infesting fallow fields and pastures. Infestation results in reduced moisture availability for winter cereal crops as well as reduced crop yields and pasture quality. The need to manage this weed is of paramount importance given its adverse effects on farming systems, biodiversity and grazing livestock and its ranking as the number one weed of importance in Australian summer fallows of grain crops. Land management practices, including movement of grazing animals and over-stocking, are potentially assisting the spread of Cucumis myriocarpus fruits and viable seed. The plant is characterized by the presence of small, ellipsoid to spherical melon fruits with spiny appendages. Each plant can produce up to 50 or more melons per plant, with each fruit containing up to 200 viable seeds. Seed is often dormant upon fruit maturity and our results under controlled environmental conditions suggest both physiological and physical factors influence dormancy. Under field conditions, seedlings can form large vines growing upto 3 m in length. Field pollination experiments suggest that this melon is mainly self-pollinated by insects, including bees, flies and wasps. Cucumis myriocarpus is generally managed by the use of various broadleaf phenoxy herbicides and systemic post-emergent products. It is found in this study that this weed established through multiple flushes of germination, hence multiple herbicidal applications coinciding with rainfall events one suggested for more efficacious management. However, rotation of infested pastures with cereal crops such as canola and wheat also results in improved control. Additional studies into the impact of soil with and physical properties, disturbance and grazing, are recommended for development of more efficacious control measures. This review discusses taxonomy, genetic variation, biology and ecology and management of this important summer annual weed.

Background Echium plantagineum, a native of Europe and Africa, is a noxious invasive weed in Australia forming monocultural stands in pastures and rangelands. It produces a complex mixture of bioactive secondary metabolites, including toxic pyrrolizidine alkaloids (PAs), that protect the plant from insect and livestock herbivory and naphthoquinones (NQs), which suppress competition from weeds, insects and pathogens, and also influence invasion success. However, the extent to which allelochemical production is impacted by environmental factors, thereby influencing plant defense against pests, remains unclear. Results Following plant stress induced by drought, herbivory and high temperature, extracts of E. plantagineum shoots and roots were subjected to metabolic profiling by UPLC-MS-DAD- QToF mass spectrometry. Abundance of NQs, especially deoxyshikonin, shikonin and dimethylacrylshikonin, rapidly increased in roots exposed to elevated temperatures. Water withholding initially increased NQ abundance, but prolonged drought resulted in reduced total PAs and NQs. Intraspecific competition elevated the production of NQs, whereas simulated herbivory had no initial effect on NQs. Following herbivory, the abundance of the PA 3'-O-acetylechimidine-N-oxide in seedling shoots was increased. Conclusions Differential accumulation of defense metabolites by E. plantagineum following exposure to various stressors suggested stress-dependent biosynthetic regulation, particularly with respect to NQ production, which was rapidly induced following drought, intraspecific competition and high temperature treatment, thereby positively impacting resistance or defense against herbivores, weeds and pathogens. We propose that trade-offs between above- and below-ground metabolism in E. plantagineum may facilitate allelochemical production in response to stress, rendering plants with an enhanced ability to defend against other neighboring plants, insects and microbes, with allelochemical production further facilitated by catabolic recycling following lengthier exposure to stress. © 2019 Society of Chemical Industry.

Conyza bonariensis (L.) Cronq. (syn. of Erigeron bonariensis L.) is a difficult-to-control summer weed species in the cropping belt of south-eastern Australia. Herbicide resistance may be affecting the ability to control C. bonariensis in the agricultural region of north-eastern Victoria; therefore, a survey was conducted to evaluate resistance to herbicides from several mode-of-action groups. Of the populations collected, 40% were resistant to glyphosate at 1080 g ha–1. Chlorsulfuron failed to control any of the populations collected. Further research identified multiple herbicide resistance to glyphosate, chlorsulfuron, metsulfuron-methyl and sulfometuron-methyl in five of nine populations fully characterised. Resistance was not found to 2,4-D, clopyralid or paraquat. There was no correlation between prevailing land use and the frequency of glyphosate-resistant populations, suggesting that resistance had been selected in multiple production systems. The high frequency of resistance could explain the difficulty experienced in controlling C. bonariensis across north-eastern Victoria and demonstrates the importance of integrated weed management to manage this weed.

International audience; Allelopathy is described as the interference to plant growth resulting from chemical interactions among plants and other organisms mediated through release of plant-produced bioactive secondary metabolites referred to as allelochemicals. A number of mechanisms have been studied for the release of allelochemicals from various plant tissues including volatilization or leaching from aerial parts, exudation from roots and decomposition of plant residues in soil. Despite differences in biological activity and mode of action, related compounds commonly share similar biosynthetic pathways while some classes of metabolites can be produced using diverse biosynthetic pathways. Recently considerable research has also been undertaken to critically understand the role of allelochemicals in plant succession and plant invasion in native and nonnative ecosystem. In addition, numerous studies have been performed on the selection and utilization of weed suppressive crops and their residues for weed management in sustainable agriculture systems. A better understanding of allelochemical production with respect to plant defence strategies, both physical and chemical, may also allow us to better protect and manage developing crops, limit the spread of invasive weeds, preserve native plant stands and create strategies for allelochemical development and application as novel pesticides. The use of sensitive analytical techniques associated with performance of metabolomics in concert with other omics technologies has led to new advances in the identification of unique allelochemicals, the biosynthetic pathways associated with their production, their complex role(s) in the soil rhizosphere and their production as impacted by a changing climate. Identification of novel plant metabolites, including allelochemicals, may result in a source of biologically based pesticides through the provision of complementary structures for future synthesis and as an aid in the development of new molecular target sites.

Mixed farming systems have traditionally incorporated subterranean clover (Trifolium subterraneum L.) and lucerne (Medicago sativa L.) as key components of the pasture phase across south-eastern Australia. However, poor adaptation of subterranean clover to acidic soils, insufficient and inconsistent rainfall, high input costs, soil acidification and the emergence of herbicide-resistant weeds have reduced efficacy of some traditional clover species in recent years. To overcome these challenges, numerous novel pasture species have been selectively improved and released for establishment in Australia. Despite their suitability to Australian climate and soils, limited knowledge exists regarding their weed-suppressive ability in relation to establishment and regeneration. Field trials were therefore conducted over 3 years in New South Wales to evaluate the suppressive potential of selected pasture legume species and cultivars as monocultures and in mixed stands against dominant annual pasture weeds. Pasture and weed biomass varied significantly between pasture species when sown as monocultures, but mixtures of several species did not differ with regard to establishment and subsequent weed infestation. Arrowleaf clover (T. vesiculosum Savi.) and biserrula (Biserrula pelecinus L.) cv. Casbah showed improved stand establishment, with higher biomass and reduced weed infestation compared with other pasture species. Generally, weed suppression was positively correlated with pasture biomass; however, yellow serradella (Ornithopus compressus L.) cv. Santorini exhibited greater weed suppression than other pasture legumes while producing lower biomass, thereby suggesting a mechanism other than competition for resources affecting weed-suppressive ability. Over the period 2015–17, arrowleaf clover and biserrula cv. Casbah were generally the most consistent annual pasture legumes with respect to yearly regeneration and suppression of annual pasture weed species.

Barley grass (Hordeum spp.) is a relatively short lived annual that provides high quality grazing early in the season, but its seed heads cause contamination of wool and carcasses, and may irritate the mouth, eyes and nose of sheep. Treatments were imposed on established subterranean clover (Trifolium subterraneum L.) annual pasture in the same plots for three consecutive years (2015 to 2017) to evaluate changes in barley grass content. Treatments included: grazing alone (G), herbicide followed by grazing (HG), or a forage conservation harvest in early October, late October or early November consistent with an early silage harvest (ES), late silage harvest (LS) or hay cut (H). Grazing plus herbicide markedly reduced (P < 0.05) barley grass numbers compared with all other treatments, but increased (P < 0.05) the growth of annual ryegrass (Lolium rigidum L.). ES reduced (P < 0.05) barley grass and increased (P < 0.05) subterranean clover compared with H, but broadleaf weed content benefitted by LS in contrast to either ES or H. Although herbicide application was the most effective method for barley grass control, forage harvest timing could be used to beneficially manipulate pasture composition.

Geographically distinct populations of Paterson’s curse (Echium plantagineum L., Boragineacea), found near roadsides across New South Wales and Victoria, Australia were surveyed along 3 distinct longitudinal transects in spring of 2011 for presence of pyrrolizidine alkaloids and naphthoquinones in sampled plants. Composite samples of shoots and roots were collected from each of 45 sites; shoot extracts were subjected to solid phase extraction and LC-ESI/MS for determination of pyrrolizidine alkaloids (PAs) and related N-oxides (PANOs), while root periderm extracts were analysed for naphthoquinone content spectrophotometrically and by LC-ESI/MS. Metabolic profiling of 12 possible PAs and PANOs showed their consistent appearance in all shoot extracts, with lepthamine N-oxide, echimidine-N oxide and echumine N-oxide predominant. The three major PANOs were significantly higher in northern sampling locations than those further south. Root extracts contained shikonin and several related naphthoquinones, as well as two of the major PANOs found in the leaves. Naphthoquinones were highest in the northwest corner of the sampled region. The patterns of abundance of secondary metabolites in E. plantagienum suggest that climate change might result in greater production of defensive compounds by E. plantagineum, making this weed increasingly toxic to livestock.

Weed management systems often seek biological solutions to minimize the environmental impacts related to the use of herbicides in agricultural systems. The suppressive effects of allelochemicals can sometimes be used effectively to provide biological pest and weed control. Jerusalem artichoke (Helianthus tuberosus L.) has been reported to be a highly invasive weed in European cropping systems, but this plant is also gaining interest as a cultivated crop in Italy. The aim of this study was to investigate the impact of Jerusalem artichoke and its extracts upon the germination and growth of seedling weeds and crops. HPLC coupled to MS was used for identification of the compounds associated with the observed growth inhibition. The weed suppressive activity of local biotypes of Jerusalem artichoke on weed growth and establishment was also evaluated in a field experiment. Shoot extracts of the cultivar Fuseau were consistently most inhibitory to germination and seedling growth of lettuce, particularly the diethyl ether extract. Solvent partitioning of the aqueous extracts of dried artichoke shoots resulted in greater inhibition in comparison to the aqueous extract itself. The diethyl ether extract provided 2-fold greater growth inhibition, in comparison to the aqueous extract, at concentrations of less than 0.5 mg extract per Petri dish. HPLC MS evaluation of the diethyl ether extract led to the identification of salicylic acid (o-hydroxybenzoic acid), and the closely related compound p-hydroxybenzaldehyde, as well as minor quantities of o-coumarinic acid and coumarin in the active fraction inhibiting lettuce seedling growth. Field observations provided further evidence for the allelopathic potential of H. tuberosus residues, as significant weed growth inhibition was observed in Jerusalem artichoke-infested plots with soil-incorporated residues in comparison to non-infested field sites, both in terms of weed seedling emergence and growth. The allelopathic potential of H. tuberosus may be of interest in the implementation of integrated weed management programmes by considering the suppressive ability of this plant and its residues for weed management in the field.

Jerusalem artichoke has been reported to colonize several ecological niches and agronomic crops in southern Europe. This plant is also of interest because of its high biomass production and its potential to produce ethanol for biofuel. Allelopathy may be an advantageous trait in Jerusalem artichoke under cultivation, as it potentially reduces weed interference with the crop, theoretically allowing a reduction of mechanical or chemical input required for weed management. However, this trait may also be unfavorable if other crops are cultivated in rotation with Jerusalem artichoke or in areas infested by this species. The aim of this study was to investigate the sensitivity of selected diverse crops (wheat, lettuce, corn, tomato, rice, and zucchini) and weeds (barnyardgrass, black nightshade, common lambsquarters, common purslane, large crabgrass, and pigweed) to the presence of Jerusalem artichoke dried leaf tissues in laboratory experiments performed under controlled conditions. The simulated soil incorporation of different Jerusalem artichoke residues (four cultivars and a weedy population) was carried out in a series of laboratory and greenhouse experiments. Jerusalem artichoke reduced the radicle growth of seedling lettuce (60%), tomato (30%), large crabgrass (70%), and barnyardgrass (30%), whereas total germination of these species was less affected. Sensitivity to Jerusalem artichoke residues was species dependent; germination and initial growth of corn were not affected, whereas winter wheat, lettuce, tomato, rice, and zucchini seedlings were more sensitive to residue presence. Our experiments show that both wild and cultivated decomposing Jerusalem artichoke residues, particularly leaves and stems, possess phytotoxic potential. Additional field experimentation remains to be conducted to determine if allelopathy in the field contributes to its invasibility.

Seed contamination significantly affects production capacity and animal welfare in Australian sheep flocks and causes considerable financial loss to producers and processors across sheepmeat value chains. Seven grass-weed species contribute to seed contamination in Australia, with barley grass (Hordeum spp.) identified as a key perpetrator. Herbicide resistance and variable dormancy emerging in southern Australian barley grass populations are thought to enhance its capacity for successful pasture invasion, further exacerbating the potential for seed contamination in sheep. The present article reviews the current literature regarding the impact and incidence of seed contamination on sheepmeat production, with particular reference to key grass-weed species prevalence across Australia. Data are presented on a recent incidence of carcass contamination across years, where incidence varied between 11% and 80% from 2009 to 2013, contracting to between 2% and 60% during 2014 and 2015. Key areas requiring future research are defined. Understanding the biology of key grass weeds, historical influences and economic consequences associated with seed contamination in sheep may assist in defining future risks to sheep production and improve weed management. Furthermore, examining more recent data describing the current status of seed contamination across Australia and the associations with causal weed species may aid the development of critical weed-management strategies in highly infested regions, subsequently limiting the extent of future seed contamination.

A series of field studies were conducted from 1999 to 2005 in Ithaca, NY, at the Cornell Turfgrass Research Center as part of the National Turfgrass Evaluation Program (NTEP) to evaluate a collection of 78 fine-leaf fescue cultivars (Festuca spp.) for turfgrass quality, seedling vigor, and ability to inhibit the establishment of common annual and perennial weeds. Using these criteria, we evaluated the overall suitability of the cultivars for use in turfgrass settings, as well as their potential weed suppressive or allelopathic ability. The ability of fine-leaf fescue to displace weeds was visually evaluated by density-wise comparison, and several cultivars of the 78 studied consistently established well and provided good to very good suppression (greater than 70%) of common turf weeds when established at the same planting density. Other cultivars provided moderate (between 35% and 70%) to (< 30%) little weed suppression. Greater weed suppressivity is likely associated with the differential ability of fescue cultivars to establish rapidly and to form a dense canopy, as well as potential allelopathic interference. This study was conducted in conjunction with laboratory experiments that revealed that certain fine-leaf fescue cultivars produced phytotoxic root exudates that were released into the rhizosphere over time. Additional field studies conducted in Ithaca showed that cultivars Intrigue, Columbra, and Sandpiper were consistently more weed suppressive than the other fine-leaf fescues evaluated. Although our understanding of the dynamics of production and degradation of fine-leaf fescue root exudates in the rhizosphere is limited, recent field studies also suggest that allelopathic interference as well as the ability to rapidly establish influence subsequent weed infestation in fine-leaf fescue stands. From a more practical standpoint, certain fine-leaf fescue cultivars, including Intrigue, Columbra, Sandpiper, and Reliant II, could be recommended for use in low-maintenance turf settings in the northeastern United States due to their aesthetic appeal and their limited weed infestation in circumstances where herbicides are not applied.

Terrestrial weeds continue to evolve in association with the rapid global changes in our land-use systems, due to their regenerative strategies, their adaptability to change, and their inherent diversity. Currently, invasive weeds are estimated to cost the United States’ economy up to $35 billion per year, and this total is rapidly increasing, as greater numbers of invasive species become naturalized. Several invasive exotic species have more recently established across New York State, creating difficulties for agricultural producers, roadside and natural areas managers, and homeowners and resulting in millions of dollars expended annually for their control. Three perennials that have become particularly problematic in New York State in recent years in both agricultural and roadside settings include Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.), mugwort (Artemisia vulgaris L.), and pale swallow-wort (Vincetoxicum rossicum (Kleop.) Barbar.). This review describes their history, biology, ecology, and potential for management and focuses on key characteristics contributing to their spread in New York State and adjacent regions.

Fifteen herbaceous perennials were evaluated in field experiments in two New York State locations to determine their utility in roadside and landscape areas as weed suppressive groundcovers. Four species, Alchemilla mollis, Nepeta x faassenii, Phlox subulata, and Solidago sphacelata were strongly weed suppressive in both managed (weeds removed) plots and unmanaged (weeds not removed) plots. Weed suppressivity of perennial groundcovers was significantly increased in year two in both locations when perennials were well established. The most suppressive perennials showed several similar characteristics likely associated with their successful establishment. Successful groundcovers possessed dense foliage which strongly reduced light transmittance at the soil surface and emerged relatively early in spring. Lamiastrum galeobdolon and Thymus praecox proved to be more successful over a 2-year period when managed by weed removal in early spring. Although Leymus arenarius, a relatively tall monocot, also inhibited weed growth, this species demonstrated invasive characteristics due to its spread outside plots by fast-growing rhizomes.

Summary 1. Establishment success of non-native invasive species is often attributable either to habitat invasibility or inherent species traits. In this study we explored the interplay between these two factors in the establishment, expansion and plasticity in growth of the clonally reproducing invasive weed mugwort Artemisia vulgaris in two contrasting habitats, as well as the potential management practice of monthly mowing. 2. We investigated resource allocation patterns and spatial distribution of ramets originating from two naturalized populations over a 3-year period. Ramets from these morphologically distinct populations were transplanted into each of two contrasting habitats to determine the invasive potential of these populations and the relative resistance of each habitat to invasion. 3. Total ramet production, average ramet height and spatial distribution patterns differed significantly between the two populations, but the degree of variation in the response was habitat dependent. There were no interpopulation differences in total biomass production. Plastic responses in resource allocation patterns, spatial distribution of ramets and relative growth rates were observed, demonstrating differences in invasive potential between the two mugwort populations. 4. The two habitats differed in invasibility. This could have been the result of differences in community structure, competition for available resources, disturbance and/or invader traits. In addition, monthly defoliation (mowing) reduced mugwort ramet production by as much as 90% and as little as 10%. 5. Synthesis and applications. This study demonstrates that variation exists in habitat invasibility, and that intraspecific variation in growth patterns occurs in mugwort. The interaction between habitat traits and species characteristics was found to be important when determining invasion success. We also demonstrated that monthly mowing following the introduction of mugwort can substantially decrease the rate of spread of this clonal species, which may provide an effective management opportunity both for this species and for other clonal invaders.

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization, and root exudation. Allelochemical structures and modes of action are diverse and may offer potential for the development of future herbicides. We have focused our review on a variety of weed and crop species that establish some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are ...

Seven sorghum accessions were evaluated quantitatively and qualitatively for the composition of their root exudates. Utilizing a unique capillary mat growing system, root exudates were collected from all sorghum accessions. Exudates were subjected to TLC and HPLC analysis to evaluate their chemical composition. Within each sorghum accession, variation existed in the amount of exudate produced and the chemical constituents of each exudate. Sorgoleone was the predominant constituent identified in each accession's exudate. Other closely related compounds, including 5-ethoxysorgoleone, 2,5-dimethoxysorgoleone, three other minor components (MW = 364, 388, and 402), and one unidentified component comprised the minor constituents of the root exudate. Our past work has shown that several of these compounds have potent phytotoxic activity as photosystem II inhibitors, thereby lending further support to the concept that Sorghum spp. are allelopathic and weed suppressive.

Research was conducted in 1996 and 1997 at the Horticulture Research Farm in Lexington, KY, to evaluate full season weed control and phytotoxicity for rates of sulfentrazone alone and in combination with Gallery (0.55 kg ai/ha) (0.49 lb ai/A), Treflan (2.18 kg ai/ha) (1.94 lb ai/A), and Pennant (3.27 kg ai/ha) (2.92 lb ai/A). In general, by 12 WAT, control provided by all treatments was only fair to moderate. The greatest reductions in weed cover were observed with the high rate of sulfentrazone (0.55 kg/ha) (0.49 lb ai/A), the combination of sulfentrazone (0.41 kg/ha) (0.37 lb ai/A) plus Pennant, and combinations of sulfentrazone plus Treflan. In all cases, weed control was improved with the use of a higher rate of sulfentrazone, and not necessarily associated with the presence of the partner herbicides. Phytotoxicity was mainly associated with contact of the herbicide with developing foliage, but sulfentrazone was particularly damaging to the herbaceous species Liriope and Hemerocallis, where damage was observed throughout the plants. Sulfentrazone was present in all cases where plants exhibited severe phytotoxicity in response to herbicide treatment. The most sensitive species evaluated in this study were: Abies, Liriope, and Hemerocallis. The most tolerant species included: Fraxinus, Crataegus, and Euonymus. Postemergence directed applications or shielded preemergence applications of sulfentrazone were recommended to possibly decrease phytotoxicity to sensitive species.

Biorational alternatives are gaining increased attention for weed control because of concerns related to pesticide usage and dwindling numbers of labeled products, particularly for minor-use crops. Allelopathy offers potential for biorational weed control through the production and release of allelochemics from leaves, flowers, seeds, stems, and roots of living or decomposing plant materials. Under appropriate conditions, allelochemics may be released in quantities suppressive to developing weed seedlings. Allelochemics often exhibit selectivity, similar to synthetic herbicides. Two main approaches have been investigated for allelopathic weed suppression. One is use of living rotational crops or mulches that interfere with the growth of surrounding weeds [e.g., tall red fescue, Festuca arundinacea Schreb.; creeping red fescue, F. rubra L. subsp. commutata; asparagus, Asparagus officinalis L. var. altilis); sorghum, Sorghum bicolor (L.) Moench; alfalfa, Medicago saliva L.; black mustard, Brassica nigra (L.) Koch; and oat, Avena saliva L.]. Attempts to select germplasm with enhanced suppressive ability have been limited. The second is use of cover crop residues or living mulches to suppress weed growth for variable lengths of time (e.g., winter rye, Secale cereale L.; winter wheat, Triticum aestivum L.; and sorghum). Cover crop residues may selectively provide weed suppression through their physical presence on the soil surface and by release of allelochemics or microbially altered allelochemics. The ability to understand the physiological basis for allelopathy in a crop plant may allow the weed scientist or ecologist to work closely with molecular biologists or traditional plant breeders to selectively enhance the traits responsible for weed suppression.

Bioassays using binary mixtures that included a cover crop with known allelopathic potential and a weed species were employed to determine the importance of allelopathy compared to resource competition as interference mechanisms. Responses of weed species germinated with cover crops in a petri dish were measured. Interference between weed and cover crop seedlings was determined in a greenhouse experiment using the additive design, which included partitions to reduce above- and below-ground competition and used capillary mat subirrigation to control moisture and fertilizer availability. Germinating sorghum reduced radicle length of weeds, whereas germinating rye tended to increase weed radicle length. Methods limited above-ground competition, so likely interference mechanisms were below-ground competition and allelopathy. Germination with a cover crop had little effect on germination and shoot length of weeds. Increased density of rye but not of sorghum reduced growth of barnyardgrass seedlings. Reduced number of barnyardgrass leaves in the presence of rye was likely due to allelopathy. Suppression of barnyardgrass dry weight attributed to allelopathic interference by rye was successfully separated and compared to the combined effects of competition and allelopathy.

Sweet corn and pumpkin were planted no-tillage (NT) into cover crop residue treatments of vetch, rye, crimson clover, and ladino clover controlled with glyphosate, and a bare ground conventional tillage (CT) control. Objectives included evaluation of crop growth, yield, and weed suppression in NT versus CT treatments. Herbicide application was also investigated, with a plus and minus herbicide treatment (alachlor plus cyanazine for sweet corn, or ethalfluralin for pumpkin) as the main factor in the factorial experiment, and cover crops the subfactors. Weed control 4 wk after planting was dependent upon cover crop. The fewest weed numbers and least biomass were found in the ladino clover plots, but clover regrowth and subsequent competition with the cash crop were severe. Herbicides also affected weed biomass at 4 wk after vegetable planting, with least biomass in herbicide-treated plots. Neither cover crop nor herbicide treatment significantly affected weed weight by 8 wk after planting or pumpkin fruit weight at harvest. Pumpkin yield was not influenced by herbicide application. The vetch cover, although harboring greatest weed biomass, produced the greatest total yield (ear weight) in sweet corn. When averaged over cover crop, sweet corn yields were higher in herbicide-treated plots than in untreated ones. Both sweet corn and pumpkin maturity were generally delayed in the absence of herbicide treatments or in the presence of cover crop residues, especially clover and rye residues.

Greenhouse experiments that used capillary mat subirrigation to maintain constant soil moisture and to sup- ply fertilizer continuously were conducted to evaluate the effects of sorghum or rye residue on early growth of barn- yardgrass and velvetleaf. The separate effects of root residue and of shoot residue were compared to the combined effects of root plus shoot residues and to an uncovered soil control. Residues included as nontoxic controls were leached shoot tissue and poplar excelsior. Shoot residue, leached shoot tissue, and poplar excelsior were surface-applied on an equal light transmittance basis such that mass of poplar excelsior > shoot residue > leached shoot tissue. The presence of rye root residue delayed emergence of barnyardgrass. Surface- applied residues tended to decrease barnyardgrass height, but velvetleaf stem length was greater in treatments with surface residue. Although cover crop shoot residues had little effect on weed growth after 18 d, weed growth decreased in the presence of cover crop root residues and poplar excel- sior. Nomenclature: Barnyardgrass, Echinochloa crus-galli L. #3 ECHCG; velvetleaf, Abutilon theophrasti Medicus # ABUTH; rye, Secale cereale L. 'Wheeler'; sorghum, Sor- ghum bicolor (L.) Moench 'Pioneer Hybrid #8333.' Additional index words. Capillary mat subirrigation, cover crop, alielopathy, physical effects.

The relative phytotoxicity of sorgoleone as measured by seed germination and seedling growth of selected crop and weed species and inhibition of photosynthetic oxygen evolution in atrazine-resistan...

Sorghum allelopathy has been reported in a series of field experiments following sorghum establishment. In recent years, sorghum phytotoxicity and allelopathic interference also have been well-described in greenhouse and laboratory settings. Observations of allelopathy have occurred in diverse locations and with various sorghum plant parts. Phytotoxicity has been reported when sorghum was incorporated into the soil as a green manure, when residues remained on the soil surface in reduced tillage settings, or when sorghum was cultivated as a crop in managed fields. Allelochemicals present in sorghum tissues have varied with plant part, age, and cultivar evaluated. A diverse group of sorghum allelochemicals, including numerous phenolics, a cyanogenic glycoside (dhurrin), and a hydrophobic p-benzoquinone (sorgoleone) have been isolated and identified in recent years from sorghum shoots, roots, and root exudates, as our capacity to analyze and identify complex secondary products in trace quantities in the plant and in the soil rhizosphere has improved. These allelochemicals, particularly sorgoleone, have been widely investigated in terms of their mode(s) of action, specific activity and selectivity, release into the rhizosphere, and uptake and translocation into sensitive indicator species. Both genetics and environment have been shown to influence sorgoleone production and expression of genes involved in sorgoleone biosynthesis. In the soil rhizosphere, sorgoleone is released continuously by living root hairs where it accumulates in significant concentrations around its roots. Further experimentation designed to study the regulation of sorgoleone production by living sorghum root hairs may result in increased capacity to utilize sorghum cover crops more effectively for suppression of germinating weed seedlings, in a manner similar to that of soil-applied preemergent herbicides like trifluralin.

Post-rice irrigated soils offer several potential advantages for the growth of subsequent crops, but Australian producers have often been reluctant to grow grain legumes immediately following a rice crop due to physico-chemical constraints. A field experiment was thus conducted to explore the potential for producing grain legumes following rice in comparison to those following a fallow during 2012 and 2013. Two grain legumes, field pea and faba bean, were sown 5, 7 and 12 weeks after rice harvest in 2013 at Yanco, NSW, and plant growth indicators and grain yield were compared. Early sowing of field pea following rice gave the best outcome, with plants flowering three weeks earlier and yielding 1330 kg·ha−1 more grain than after fallow. In contrast, faba bean yield was 35 kg·ha−1 less after rice than after fallow across the three sowing dates. Higher pea yield was consistent with the early emergence of seedlings, higher light interception and overall greater plant growth following rice. Post-rice crops also had 10-fold less weed infestation than crops in a similarly-established fallow treatment and, thus, required far less weed management. Legume crops sown at the later seeding date had significantly reduced (~50%–60%) yields compared to those of the first two sowings; this is most likely a reflection of reduced temperatures and day lengths experienced during vegetative and reproductive growth phases.

A reduced-till (RT) experiment determined the effect of rye seeding density and method of kill on rye biomass persistence, weed suppression, and tomato yield. ‘Wheeler’ rye was seeded at 56, 110, and 170 kg ha−1in the fall of 1988 and 1989 in Champaign, IL; Lexington, KY; and Lafayette, IN. One wk before transplanting tomatoes, rye was desiccated either by applying glyphosate at 1.1 kg ha−1and mowing (RT-glyphosate) or by mowing alone (RT-mowed). Both methods left rye residues on the soil surface as a mulch. Seeding density did not affect rye biomass, weed control, or tomato yield. Rye biomass differed depending on location and year, ranging from 320 to 1150 gm−2. Rye was completely killed by glyphosate, but regrew following mowing. RT-glyphosate suppressed weeds for 4 to 8 wk, which was similar to the suppression from conventional tillage (fall plowing with spring disking and harrowing) with trifluralin and metribuzin (CT-herbicide). Weed communities varied, but redroot pigweed and giant foxtail were present at all three locations. All treatments required supplemental weed management for commercially acceptable control. Except for IL in 1990, processing tomato yields (kg ha−1) in RT-glyphosate with hand-weeding were equal or better than yields in the CT treatments.

The recent emphasis on development of alternative, non-chemical weed strategies for landscape and roadside management has led to the study and utilization of well-adapted landscape groundcovers, including turfgrass and herbaceous ornamentals that can successfully withstand and suppress weed invasion. By selecting groundcovers which exhibit growth characteristics that result in consistent interference with weed establishment, one can successfully achieve effective long-term weed control in the landscape using reduced herbicide and labor inputs. In addition to allelopathic characteristics resulting in enhanced weed suppression due to the production and release of phytoinhibitors in the landscape, some groundcovers effectively suppress weedy invaders through competition for space, sunlight, moisture, nutrients and even direct alteration of the rhizosphere environment. Additional information related to weed biology and the impact of cultural practices used in the landscape on weed infestation are also critical when considering long-term and sustainable weed suppression for landscape and natural settings. Until recently, little information was available regarding the development of low maintenance landscapes for use in the Northeastern United States, with the exception of roadside trials documenting utilization of certain direct-seeded groundcovers in highway medians. Recent studies in both landscape and roadside settings have indicated that the selection and utilization of certain herbaceous perennial groundcovers and turfgrasses may not only result in lower labor inputs with respect to maintenance, they may further prevent the spread of noxious invasive weeds by severely limiting their ability to establish. When surveyed, stakeholders reported that weed suppressive groundcovers had much greater aesthetic appeal than those that were partially infested. Many of these groundcovers suppress weeds effectively by intense competition for resources, particularly by reduction in the amount of available light at the soil surface due to dense canopy cover. In addition, some groundcovers such as Nepeta x faasennii and Festuca rubra inhibit weed seedlings by their potential release of allelochemicals into the rhizosphere. Although still relatively novel, allelopathic groundcovers and turfgrasses offer interesting possibilities for future development of allelochemicals as bioherbicides and attractive, stress-tolerant and pest-resistant landscape plantings.

The establishment and management of nine cover crops in Kentucky production systems were evaluated in field experiments over a 2.yr period. 'Wheeler' rye, 'Barsoy' barley, and 'Tyler' wheat cereal grains produced greater biomass (180 to 260 g/m2) than the pasture species tall fescue, creeping red fescue, and white clover (55 to 110 g/m2). 'Kentucky 31' tall fescue, creeping red fescue, and white clover proved most difficult to control, and significant regrowth occurred regardless of herbicide or rate applied. HOE-39866 (1.7 kg ai/ha) was effective in rapidly controlling all cover crops except tall fescue by 30 days after application. Sethoxydim and fluazifop (0.4 and 0.3 kg ai/ha, respec- tively) effectively controlled the cereals and two ryegrass species. Glyphosate applied at 1.1 and 2.2 kg ai/ha was also effective, while 0.6 kg ai/ha controlled only cereal grain growth adequately. After chemical control, pasture grass plots contained fewest weeds/m2 with some reduc- tions likely due to density and regrowth of the sods. Cover crops were effective in suppressing weed growth at 45 days after chemical control. However, significant weed growth existed in all cover crop plots by 60 days after kill. Row crop establishment increased linearly with increasing glyphosate rate. Cereal grain covers provided the most compatible planting situations for greatest seedling estab- lishment, with rye and wheat providing greatest weed suppression. Generally, increased weed suppression pro- vided by a cover crop was accompanied by reduced row crop establishment, with greatest reductions observed in pasture grass plots. Cucumber was most easily established while snap pea was most difficult. Nomenclature: Fluazi- fop, (?)-2-(4-((5-(trifluoromethyl)-2-pyridinylloxy)phen- oxy)propanoic acid; HOE-39866, the ammonium salt of DL-homoalanin-4-yl-methyl phosphinic acid; glyphosate, N-(phosphonomethyl)glycine; sethoxydim, 2-(1-(ethoxyim- ino)butyl - 5 - (2-(ethylthio)propyl) - 3 - hydroxy - 2 - cyc- lohexen-l-one; tall fescue, Festuca arundinacea Schreb. 'Kentucky 31'; creeping red fescue, Festuca rubra L. ssp. commutata; Dutch white clover, Trifolium repens L.; cucumber, Cucumis sativus var. sativus 'Calypso'; snap pea, Pisum sativa L. 'Sparkle'; rye, Secale cereale L. 'Wheeler'; winter wheat, Triticum aestivum L. 'Tyler'. Additional index words. Minimum tillage, germination, glyphosate, sethoxydim, fluazifop, HOE-39866, row crop.

The weed suppressive activity of Sorghum species has been associated with phytotoxic compounds that are exuded from the roots, and contain primarily sorgoleone (1). The concentration of 1 ranges from about 40 to 800 pg/mgroot extract, based on quantitative analysis of seven genetically different Sorghum accessions. Minor constituents were isolated, including novel resorcinolic lipids. Analogues of 1 with the aliphatic side chain varying in number of carbon and double bonds have also been isolated by thin layer-argentation chromatography. The minor constituents that have been tested inhibit photosystem II electron transport, indicating that they contribute to weed suppressive activity of Sorghum. In studying the biosynthesis of 1, 2- 1 3 C-acetate was found to also get incorporated into 4,6-dimethoxy-2-[(8'Z,11'Z)-8',11',14'-pentadecatrienyl]-l,3-benzenediol, providing significant information on the possibility of altering the biosynthesis of 1 to a more phytotoxic and stable methylated resorcinolic lipid.

Sorghum [Sorghum bicolor (L.) Moench] roots exude a potent bioherbicide known as sorgoleone, which is produced in living root hairs and is phytotoxic to broadleaf and grass weeds at concentrations as low as 10 microM. Differential gene expression was studied in sorghum (S. bicolorxS. sudanense) cv. SX17 between roots with abundant root hairs and those without root hairs using a modified differential display approach. A differentially expressed gene, named SOR1, was cloned by using Rapid Amplification of the 5' ends of cDNA (5'-RACE). Real-time PCR analysis of multiple tissues of sorghum SX17 revealed that the SOR1 transcript level in root hairs was more than 1000 times higher than that of other tissues evaluated, including immature leaf, mature leaf, mature stem, panicle, and roots with hairs removed. Semi-quantitative RT-PCR revealed that SOR1 was expressed in the sorgoleone-producing roots of sorghum SX17, shattercane [S. bicolor (L.) Moench], and johnsongrass [S. halepense (L.) Pers.], but not in the shoots of sorghum or in the roots of sweet corn (Zea mays L.) 'Summer Flavor 64Y', in which sorgoleone production was not detected by HPLC analysis. Similarity searches indicated that SOR1 probably encodes a novel desaturase, which might be involved in the formation of a unique and specific double bonding pattern within the long hydrocarbon tail of sorgoleone.

Weeds consistently appear in less maintained landscape and turf settings, particularly in weak or stressed turf or unattended landscape beds. To achieve consistent control, it is necessary to better understand the reason why weeds are often persistent in these settings. Understanding the plant’s interaction with its environment, including rhizosphere ecology, and the impact of stress on plant growth are important considerations in landscape and turf weed management. In landscapes, the most critical time for effective weed management is before planting. Effective landscape planting and design will help prevent future weed problems.

Roots of many weed and crop species contribute biologically active chemicals into the environment known as root exudates. Root exudates are known to influence growth and establishment of crop and weed species, and these are released from living root systems. Many perennial woody and herbaceous plants have deep and extensive root/rhizome subterranean systems, which can produce prolific amounts of root exudates over long periods of time. Root exudates contribute many types of organic compounds to the rhizosphere. In addition to simple and complex sugars and growth regulators, root exudates contain different classes of primary and secondary compounds including amino acids, organic acids, phenolic acids, flavonoids, enzymes, fatty acids, nucleotides, tannins, steroids, terpenoids, alkaloids, polyacetylenes, and vitamins (Table 10.1; Rovira 1969; Schonwitz and Ziegler 1982; Rice 1984; Uren 2000). Uren (2000) suggested that the amount of root exudates produced varies with the plant species, cultivar, the age of the plant, and substrate and stress factors.

For the past 5 years, we have evaluated more than 100 herbaceous perennial groundcovers, including both grasses and grass mixtures, as well as ornamental broadleaf materials, for their ability to establish, suppress weeds, provide aesthetic appeal, and resist pests in various landscape and roadside settings across New York State. By working in cooperation with the NYSDOT, we have developed recommendations for materials that have performed well in difficult, potentially stressful, roadside and landscape settings. We have performed replicated research and demonstration trials that have clearly shown that certain species and cultivars provide effective weed suppression; great aesthetic appeal due to foliar texture, color, or flowering, resist pests and diseases; and require low maintenance over time. In addition, certain materials tolerate high levels of salt (NaCl), simulating roadside salt application exposure, in supplemental greenhouse studies. Materials generally suppressed weeds effectively by forming a dense canopy in a short period of time, and reducing light interception at the soil surface under this dense canopy. Certain groundcovers also appeared to exhibit strong potential allelopathic properties when grown either in field or laboratory settings. The selection of new plant materials for use in low-maintenance landscape settings offers potential to reduce time and maintenance inputs in difficult landscape or roadside settings, with the added benefit of reducing pesticide application in these settings for weed management. Additional studies are currently underway to develop further recommendations for use of warm- and cool-season turfgrasses in these settings.

Previous experiments showed that legumes grown in the presence of living or herbicidally treated quackgrass residues or extracts exhibited reduced seedling root and shoot growth and decreased nodulation and nitrogen fixation. Aqueous extracts of quackgrass shoots were most inhibitory to plant growth. Upon sequential partitioning of an aqueous extract of quackgrass shoots, the ether extract possessed the most activity and caused 50% reductions in radicle elongation of eight crop and weed species at concentrations of less than 240 μg/ml (small-seeded species) and 1000 μg/ml (large-seeded species). Snapbeans (Phaseolus vulgaris L. "Bush Blue Lake") grown aseptically in agar containing an ether extract at 100 and 200 μg/ml exhibited severe root browning, lack of root hair formation, and a two- to three-fold reduction in root and shoot dry weights. The ether extract of quackgrass shoots had no inhibitory effect on the growth of fourRhizobium species in Petri dishes or two species in broth culture. Inhibitors present in the ether extract may influence the legumeRhizobium symbiosis indirectly by reducing legume root growth and root hair formation. The ether extract of quackgrass shoots was separated using high-pressure liquid, thin-layer, and liquid column chromatography in an attempt to isolate and identify the inhibitors responsible for the inhibition of seedling growth. Two closely related flavonoid inhibitors were isolated from the ether extract. One was identified as 5,7,4'-trihydroxy-3',5'-dimethoxyflavone (tricin). Both flavonoids caused 50% inhibition of radicle elongation in cress (Lepidium sativum L. "Burpee curly") seeds at concentrations of less than 125 μg/ml. Both flavonoids were found in ether extracts of quackgrass shoots and rhizomes, but the largest amounts of both compounds occurred in quackgrass shoots collected from the field.

Experiments were conducted under controlled conditions to investigate the apparent allelopathic effects of sudex [Sorghum bicolor (L.) Moench ×Sorghum Sudanese (P.) Stapf, cv. FFR 201] on weed and vegetable species. Allelopathic potential, as measured by radicle elongation of herbaceous indicator species, decreased with increasing sudex age. Greatest potential allelopathic activity of sudex shoot tissue was observed when sudex was collected at 7 days of age. Small-seeded broadleaf species were more inhibited in the presence of sudex shoot tissue than were grass species. Two major phytoinhibitors were isolated from aqueous extracts of sudex shoot material by partitioning with diethyl ether, followed by thin-layer and liquid column chromatography. Phytoinhibitors were identified asp-hydroxybenzoic acid andp-hydroxybenzaldehyde, potentially the enzymatic breakdown products of the cyanogenic glycoside dhurrin. The I50 values of these compounds using a cress (Lepidium sativum L.) seed bioassay were 140 and 113 μg/ml for the acid and aldehyde, respectively. Sudex tissue collected at 7 days of age possessed a greater percentage of these phytoinhibitors on a per gram basis than did older sudex tissue. As sudex tissue age increased, the percentage ofp-hydroxybenzaldehyde in ether extracts of tissue also increased, while the percentage ofp-hydroxybenzoic acid decreased.