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Weeds are a major constraint to canola (Brassica napus L.) production, reducing grain yield and quality. The repeated use of pre- and post- emergent herbicides to control several grasses and broadleaf weeds has escalated the problem of herbicide resistance in weeds. The development of herbicide-tolerant canola cultivars has increased the flexibility of weed management but has also increased the risks of outcrossing with wild relatives and weed shifts to resistance. Herbicide-resistant weed species, and the related biological repercussions, pose a major threat to sustainable weed management. These developing risks have led researchers to examine integrated weed management (IWM) techniques for sustainable weed control. Weed control strategies using non-chemical tactics have valid roles for managing weeds. However, in broad-acre commercial fields, the effectiveness of several non-chemical selections are less proven than commercial chemical herbicides. Canola competition and allelopathy for weed suppression are potential components for integrated weed management in canola. This review examines current chemical and non-chemical options available for developing IWM strategies for profitable canola production, as well as future research directions. [ABSTRACT FROM AUTHOR]

The rapid spread of herbicide-resistant weeds coupled with the decline in the number of new herbicide modes-of-action under development poses a serious threat to the future productivity of canola ( Brassica napus L.). Crop competition is now an essential component of integrated weed management to reduce weed impacts and retard the spread of resistance. Canola plays an important role in any cereal-dominated crop rotation by providing diversification to broaden options for weed management, as well as other agronomic benefits for disease, pest and nutrition management. This review examines the opportunities to manipulate canola agronomy and increase the competitive ability by choice of cultivar, seeding rate, row spacing and orientation, and fertilizer use. The integration of such options for weed management prolongs the effectiveness and availability of herbicides. [ABSTRACT FROM AUTHOR]

Solanum elaeagnifolium Cav. is considered as one of the worst weeds of crop and pasture systems in temperate Australia. Effective long-term control is difficult due to the extensive root system. Field experiments were conducted at two locations in south-eastern Australia between 2006 and 2008 to examine a range of herbicides for control of S. elaeagnifolium on seed production and root regrowth. Herbicide performance was affected by herbicide, weed growth stage and environmental factors. Pyridine herbicides, such as pre-packed mixtures of aminopyralid + fluroxypyr and triclopyr + picloram + aminopyralid were the most effective and consistently reduced within-season aerial growth by 60–90% in both seasons. Overall control using glyphosate-based treatments was generally reduced due to emergence of new stems following herbicide application. Three picloram-based treatments provided the best and most consistent long-term control on root regrowth after two seasons, reducing stem emergence by 45–88%, especially with a late application of herbicides. The efficacy of residual herbicides such as atrazine or imazapic + imazapyr depends on rainfall conditions. Seedset control was best achieved with herbicides applied at the start of flowering stage, with no viable seed produced following treatments of 2,4-D amine + picloram and triclopyr + picloram + aminopyralid. These two treatments also significantly reduced viable seed production when applied at the early berry stage. The results indicate that an application at early flowering followed by a late application in autumn is necessary to effectively control the seedset (seedbank) and the root regrowth (rootbank) of S. elaeagnifolium . [ABSTRACT FROM AUTHOR]

Canola (Brassica napus L.) is an important rotational crop in the temperate cropping zone of southern Australia. Herbicide-resistant weeds are rapidly spreading and reducing canola grain yield and quality. Crop competition is a useful tool for reducing weed costs and dependence on herbicides, and retarding the spread of herbicide resistance. The potential interaction of canola seeding rate and cultivar for weed management has not been quantified in Australia. A field experiment was conducted in three environments to examine the impact of two contrasting canola cultivars (a low vigour type and a high vigour hybrid) at four seeding rates (10-100 plants/m2) on volunteer wheat (~50 plants/m2). Significant but variable effects of crop seeding rate, cultivar and weed were detected on canola density and grain yield, and on the suppression of volunteer wheat. The canola hybrids suppressed volunteer wheat more than the less vigorous cultivars in all the experiments. There was no benefit of increasing canola seeding rate above the normally recommended rate of 40 plants/m2 for weed suppression. The seed production of volunteer wheat on average doubled when canola density dropped from 40 to 10 plants/m2. Treatment effects on canola grain yield losses from weeds were less than those on weedsuppression. The grain yield of both cultivars was reduced between 30% and 40% with weeds at a canola density of 40 plants/m2 and plateaued above this density in weedy conditions. Maintaining canola plant establishment and using competitive cultivars is critical to avoiding weed seedbank replenishment, and reducing canola yield losses from weed competition. [ABSTRACT FROM AUTHOR]

Allelopathy is one crop attribute that could be incorporated in an integrated weed management system as a supplement to synthetic herbicides. However, the underlying principles of crop allelopathy and secondary metabolite production are still poorly understood including in canola. In this study, an allelopathic bioassay and a metabolomic analysis were conducted to compare three non-allelopathic and three allelopathic canola genotypes. Results from the laboratory bioassay showed that there were significant differences among canola genotypes in their ability to inhibit root and shoot growth of the receiver annual ryegrass; impacts ranged from 14% (cv. Atr-409) to 76% (cv. Pak85388-502) and 0% (cv. Atr-409) to 45% (cv. Pak85388-502) inhibition respectively. The root length of canola also differed significantly between genotypes, there being a non-significant negative interaction (r = -0.71; y = 0.303x + 21.33) between the root length of donor canola and of receiver annual ryegrass. Variation in chemical composition was detected between organs (root extracts, shoot extracts) and root exudates and also between canola genotypes. Root extracts contained more secondary metabolites than shoot extracts while fewer compounds were recorded in the root exudates. Individual compound assessments identified a total of 14 secondary metabolites which were identified from the six tested genotypes. However, only Pak85388-502 and Av-opal exuded sinapyl alcohol, p-hydroxybenzoic acid and 3,5,6,7,8-pentahydroxy flavones in agar growth medium, suggesting that the synergistic effect of these compounds playing a role for canola allelopathy against annual ryegrass in vitro. [ABSTRACT FROM AUTHOR]

Crop competition and allelopathy are two cultural control options for possible inclusion in cropping systems. This research aimed to identify superior allelopathic canola genotypes through a two-year field study. First year screening results of 312 diverseBrassicagenotypes showed genotypes differed significantly in their ability to suppress weed infestations. Crop plant height was correlated with the competitive ability of several genotypes, while other genotypes showed good weed-suppressive ability despite being short. Thirty-six of the genotypes grown in the field had been previously assessed for their allelopathic ability to inhibit the growth of annual ryegrass (Lolium rigidum) seedlings using anin vitrotechnique. The highly allelopathic genotypes: Av-opal, Sardi603, Rivette and Atr-beacon performed well against annual ryegrass in the laboratory and also against other species, includingCapsella bursa-pastoris,Sisymbrium orientaleandHordeum leporinumin the field. The weakly allelopathic Barossa and X-06-6-3725 genotypes performed poorly both in the laboratory studies and in the field. The following year, field testing of selected genotypes at two sowing dates further suggested that the most allelopathic genotypes in the laboratory bioassay were generally those that suppressed weed numbers and their biomass in the field. The late sowing time increased the natural weed pressure leading to a decrease in both canola grain yield and quality. Many of the highly allelopathic canola genotypes, which caused low weed populations in the field, had relatively low grain yield. This suggests that the allelopathic trait is independent of local adaptation and yields potential under weed-free conditions. Ideally, cultivars with both high allelopathy and high competitive ability would be most useful to help farmers maximise yield and control weeds. Selection for allelopathy in canola shows potential as a future non-chemical weed control option and requires further investigation. [ABSTRACT FROM PUBLISHER]

Silverleaf nightshade is one of the worst agricultural weeds on a worldwide basis. Improved understanding of its life cycle will be useful for weed management. This research showed that the growth and reproduction of silverleaf nightshade were affected significantly by the time of emergence. Plants (both root- and seed-generated) that emerged early in the growing season, such as in September or November (spring), were significantly taller and produced more biomass and fruit than plants that emerged in January (summer) and March (autumn). Delayed emergence resulted in a shorter vegetative phase and less fruit production. As a result, the silverleaf nightshade plants that emerged in September produced a large amount of seeds, while the plants that emerged later in the season did not flower. Thus, control of early-emerging plants is important. The dynamics of fruit production indicated that silverleaf nightshade fruit formed in December and peaked in March, which suggests that a control action is required before December to control the soil seed bank. [ABSTRACT FROM AUTHOR]

Silverleaf nightshade (Solanum elaeagnifolium Cav.) is a problematic summer-growing perennial weed in Australia. The genetic diversity of silverleaf nightshade is poorly understood. Nine silverleaf nightshade specific and 10 cross-species simple sequence repeat (SSR) primer pairs were utilised to investigate the genetic variations among 94 silverleaf night-shade populations collected in Australia. High genetic diversity was found within silverleaf nightshade populations, with an average genetic similarity of 0.43. The Unweighted Pair Group Method with Arithmetic mean based dendrogram indicated the presence of genetically diverse silverleaf nightshade populations in Australia. However, no well supported genetic structure was found. The Mantel test indicated that there is no significant correlation between genetic variation and geographic distance. These results suggested a lack of geographic structure in genetic diversity, which is probably due to the long distance spread of seeds of silverleaf nightshade. The high genetic diversity of silverleaf nightshade could contribute to the inconsistency in control efficacy between populations. [ABSTRACT FROM AUTHOR]

Wheat varietal autotoxicity and varietal allelopathy were assessed based on plant extract and root exudate bioassays under laboratory conditions. Aqueous extract of wheat differed in varietal autotoxicity and varietal allelopathy, inhibiting wheat germination by 2–21%, radicle growth by 15–30%, and coleoptile growth by 5–20%, depending on the combination of the receiver and donor. Extracts of cv Triller or cv Currawong were more allelopathic to other wheat varieties than cv Batavia and cv Federation. Triller extract was more autotoxic than Federation. Assessment of root exudates by the equal-compartment-agar-method further identified the significant differences in varietal autotoxicity and varietal allelopathy of root exudates between wheat varieties, with root exudates of Triller or Batavia showing stronger autotoxic or allelopathic effects than Currawong or Federation. The varietal autotoxicity and allelopathy of root exudates also showed a characteristic radial inhibitory pattern in the agar growth medium. These results suggest that careful selection of suitable wheat varieties is necessary in a continuous cropping system in order to minimize the negative impacts of varietal allelopathy and varietal autotoxicity. Factors affecting autotoxicity in the field and strategies in autotoxicity management are discussed. [ABSTRACT FROM AUTHOR]

Laboratory experiments were conducted under controlled conditions to examine the avoidance responses of seedling roots of annual ryegrass ( Lolium rigidum ) to the presence of seedling roots of a highly-allelopathic canola (rapeseed) genotype ( Brassica napus cv. Av-opal). The postulated concentration gradient of the canola (donor) allelochemicals was altered by changing the seedling density and/or the proximity in the nutrient-free agar medium. In addition, activated carbon was tested for its ability to nullify the effect of the allelochemicals. The ryegrass (receiver) roots initially grew towards the canola roots and then grew away from them, showing bent or curved growth, with generally shorter and thicker roots. This effect was greatest at high canola density and at the closer proximity, but was negated by the addition of activated carbon. These findings add considerable weight to the conclusion that certain genotypes of canola exuded allelochemicals which are detected by the annual ryegrass roots causing an active response that modifies and inhibits the growth of this weed. [ABSTRACT FROM AUTHOR]

The chemical basis for wheat seedling allelopathy on the growth of annual ryegrass was investigated by the identification and quantification of multiple allelochemicals from wheat seedlings. Results indicated that 58 wheat accessions differed significantly in seedling allelopathy and inhibited the root growth of ryegrass from 10 to 91%, depending on accession. Analysis of allelochemicals by GC/MS/MS indicated that allelopathy was significantly correlated with the levels of measured allelochemicals in the shoots and roots of young wheat seedlings. Ryegrass root growth was also negatively correlated with the levels of p-hydroxybenzoic, vanillic, and trans-ferulic acids in root exudates. Wheat allelopathic potential was negatively correlated with the levels of the eight known allelochemicals quantified in the shoots, roots, and water-agar medium, with multiple regression coefficients (r) of -0.61, -0.71, and -0.71, respectively. In comparison with weakly allelopathic accessions, strongly allelopathic accessions produced significantly higher amounts of allelochemicals in the shoots and roots of the wheat seedlings and also exuded larger quantities of allelochemicals into the growth medium. Wheat accessions with strong seedling allelopathy might be useful for management of weeds during the establishment stage, thereby reducing the need for commercial herbicides in early-season application.