Canola Interference for Weed Control
The increased incidence of herbicide-resistant weed species, and the related biological repercussions, poses a major threat to sustainable crop production. Integrated weed management, which involves greater reliance on non-chemical weed management tactics such as crop interference, needs to be included in canola production systems. Crop interference comprises both competition and allelopathy which favour the growth of the crop. This review examines canola plant traits associated with competitiveness and allelopathy. Competitive ability is evaluated by the ability of plant morphological traits to improve access to scarce light, nutrients and water in a limited space. Allelopathy refers to the harmful or beneficial effect of crop biochemicals on neighbouring weed species. Allelochemicals are a subset of secondary metabolites produced from intact living roots and crop residues that differ between cultivars and have specific defensive functions in the rhizosphere. Elite allelopathic cultivars can be identified by screening canola germplasm. The identification of the allelochemicals involved and their effects in the field also need to be explored. The impact of genetic variation, the mechanisms of allelopathic action, the source and fate of allelochemicals and associated biota in the rhizosphere all need to be considered in new cultivar development. The breeding of weed-suppressive allelopathic canola cultivars needs to be in the context of good agronomic performance. Although allelopathic canola cultivars are unlikely to eliminate all weed pressures in the field, the extent to which they contribute in weed management is worthy of exploration. It remains to be known whether combined competitive and allelopathic cultivars can be developed to maximise overall interference. The integration of agronomic practises with canola interference also needs to be developed.
Canola (Brassica napus L.) is a member of Brassicaceae family with low glucosinolates and erucic acid content relative to traditional rapeseed (B. napus L.). It is a major oilseed crop, ranked as the second most important global source of vegetable oil . Canola is also a potential source of specific protein and industrial raw materials including biopolymers, surfactants, adhesives and, more recently, biodiesel . The annual worldwide increase in canola production has been substantial and it is predicted to exceed 15 million tonnes by 2015 [28, 29]. Australia is the world’s second largest exporter of canola seed after Canada and canola is Australia’s third largest broad-acre crop after wheat and barley. The Australian Oilseed Federation  predicts that prospects for the Australian canola industry are excellent due to good commodity prices, market demand and its value in the farming system. Canola is, therefore, an attractive alternative crop for grain growers. The rapidly growing demand for canola worldwide implies both greater yield and greater area of production, utilising better management practises and improved cultivars.
Weeds commonly occur in canola crops  and their infestation is a major yield-reducing factor . Weeds interfere with crop plants, causing serious impacts as a result of competition for either above or below-ground resources . Canola is exposed to severe competition from weeds which are often considered as the most yield limiting factor in Canada . In India, Gill et al.  reported that the magnitude of loss from weeds ranged from 30 to 50 %, depending on the growth and persistence of the weed population in the standing crop. Grass weeds, such as annual ryegrass (Lolium rigidum), vulpia (Vulpia myuros) and wild oat (Avena fatua) were most abundant in canola crop of south-eastern Australia . Interference may be through severe soil nutrient depletion , water and shading.
Weed competition also reduces grain yield and quality and market value of the canola seed. In Canada, Rose and Bell  showed that presence of seeds of wild mustard (Sinapis arvensis) and stinkweed (Thlaspi arvensis) in canola seeds mixtures reduced the seed quality of canola by increasing the level of erucic acid in the extracted oil and the glucosinolate content of the remaining meal. In Australia, heavy infestations of wild radish (Raphanus raphanistrum) have reduced canola yields by up to 90 %  and such infestations greatly reduced the quality of canola meal both through crop stress and direct seed contamination of harvested product [33, 101].
The use of herbicides and herbicide tolerant canola cultivars has increased rapidly in Australia and worldwide. However, the over reliance on herbicides can reduce their effectiveness and lead to the evolution of herbicide-resistant weeds . High population densities of some weed species necessitate the input of more herbicides but the high use of herbicides exacerbates the development of the resistance problem [77, 126]. The widespread use of triazine-tolerant (TT) canola cultivars has increased the use of triazine herbicides and has led to increased triazine-resistant populations of wild radish in Australia . The escalating problem of herbicide-resistant weeds is a challenge to farmers as is the need to manage agrochemicals to minimise soil herbicide residues that can negatively impact on succeeding crops.
Integrated weed management systems have the potential to reduce herbicide use and their associated costs where there is greater reliance on non-chemical control tactics including enhancing crop interferences. It has been shown that the reliability of herbicide performance can be improved when combined with crop species or varieties of superior competitiveness [37, 91].
Interference is the term used to describe an induced effect by an individual plant on a neighbour through changes in the immediate environment . It comprises competition and allelopathy. Zimdahl  reported that it is the total adverse effect that both plants exert on each other when growing in a common ecosystem. Competition is the negative interaction between two or more plant species for existence and superiority within a limited space . Competition is greatest when available resources for both crop and weed are below the combined demand . The phenomenon occurs between individuals of the same species (intra-species) and between individuals of different species (inter-species). Allelopathy is distinct from other negative plant interference in that the detrimental effect is through release of chemicals by a donor plant . Molisch  indicated that this chemical interference can be both harmful and beneficial. At high concentrations allelopathic chemicals can act as inhibitors while at low concentrations they can sometimes stimulate neighbouring plant growth . Weed responses to crop allelopathy have become well documented in recent decades [129, 135]. However, the impact varies depending on the plant species, cultivar, growth stage and various stress factors. In this review, we examine both forms of canola interference, competition and allelopathy, and discuss possible ways to maximise this beneficial attribute for improved weed control.
Crops and weeds compete for various resources. The competitive ability of a particular plant is a major factor in suppressing the competitor. An increase in the biomass and/or population density of one species is the most likely route to increase competition for resources and thus influence the growth and survival of the affected species . Competition for resources between species occurs through both above and below-ground interaction. The competitive ability of a plant is an integrated response over time, with contributions from a range of traits.
Above-Ground Competition for Light and Related Canola Traits
Light is an essential determinant of the energy balance of the soil and plant, and it drives water and nutrient transport . Competition for light occurs in most cropping situations soon after seedling emergence [48, 131]. Plants intercept light using different light attributing characters and a successful plant is not necessarily the plant with more foliage but the plant with foliage in an advantageous position for light interception relative to that of its competitors . Leaves are the principal source of assimilate production during the vegetative phase. In rapeseed (B. napus), the lower leaves have been shown to export assimilates basipetally, while the upper leaves exported assimilates almost exclusively acropetally . They translocated and re-translocated the mobile nutrients in the plant system before they senesced . Leaves of rapeseed exerted and developed source-sink capacity during the early growth stage, the expansion rate of leaves being positively correlated with seed yield [38, 153]. Thus, during early development, light interception by the rapeseed plant influences growth rate that determines competitiveness with neighbours. Plant height, leaf size, number and leaf area index are directly related to the interception of radiation by leaves. In Canada, Beckie et al.  indentified from field observations that canola height was as an important criterion of plant competitiveness for resources. Daugovish et al.  confirmed that the greater competitive ability of wild oat or yellow mustard over canola was attributed to greater plant growth rate and plant height.
Other plant morphological traits such as stem elongation, upward leaf movement [21, 61, 110, 124] and leaf layer density  all contribute to competitiveness for light. These plant components usually relate to shade avoidance, allowing plants to photosynthesise and grow to become more competitive [11, 21, 124]. Further, the variation in morphological sensitivity of plants to light signals is known to vary among cultivars . Thus, choice of a suitable shade-avoidance cultivar, combined with agronomic tactics (e.g. crop density and row arrangement), also helps to manipulate crop plant photomorphogenesis. In Australia, vigorous hybrid canolas have generally been shown to compete more successfully with, for example, annual ryegrass than did TT canola varieties . The plant biomass measures of both cultivar types were negatively correlated with weed plant biomass . The study was consistent with Canadian results that suitable vigorous hybrid genotypes provide more competition against weeds [68, 69, 171]. Vigorous hybrids produce tall plants with much foliage, thereby reducing light penetration to the weed canopy. Choice of vigorous cultivars can be an effective crop interference tactic for weed management especially in the early establishment phase of a canola crop.
Below-Ground Competition for Nutrients and Water and Associated Canola Traits
Competition for below-ground resources constitutes an important aspect of crop-weed interaction. This below-ground interference has been reported to reduce plant performance more than do above-ground relations . Below-ground competition usually occurs for space, soil nutrients and water. Plants take up soil nutrients mainly by diffusion and mass flow mechanisms from the depletion zone (the concentration gradient surrounding the roots) . The competitive ability of a crop plant is likely determined by its capacity to make use of nutrients from this zone  and plants usually invest relatively more resources into roots compared with shoots for below-ground competition . Efficient nutrient acquisition by roots becomes an important key for plant competitive ability. Characteristics related to nutrient and water uptake include plant root size and depth, relative growth rate, biomass, root density and total surface area [1, 2, 31, 55]. The canola plant has an extensive root system  with abundant root hairs  to give it high root surface area and large potential to extract nutrients from the soil . Strong and Soper  reported that roots of Brassica plants proliferate in areas of high nutrient concentration, although differences exist among genotypes in their ability for nutrient acquisition. Nitrogen uptake by canola, for example, has been linked to total root biomass rather than higher uptake per unit of length . However, Laine et al.  demonstrated that if one half of the canola root system was starved of nitrogen, the other half was still able to supply the shoot with sufficient nitrogen through increased uptake per unit of root length. The optimisation of canola root traits for nutrient acquisition may link with its competitive ability against different weed species.
The conversion of soil resources to plant biomass (referred to as nutrient use efficiency) differs between species and cultivars . A typical canola plant usually has a higher demand for phosphorus and potassium than does a wheat plant [29, 137, 152]. These demands may influence success in gaining a greater share of the other nutrients to establish dominance over a less successful weed species. Duan et al.  reported that the rate of root biomass accumulation in canola was positively correlated with increased lateral root length  while, in another study, canola biomass was negatively correlated with weed biomass . The biomass of canola was regulated by the reduced pH in the rhizosphere resulting from the release of organic acids by its roots . In soil, insoluble phosphorus usually becomes more readily available to canola roots through the acidification of soil near the rhizosphere [2, 73, 138]. Understanding the process involved in the acquisition of soil resources, and the associated mechanisms by which canola competes, may help improve the below-ground competitive ability of canola for nutrient acquisition in the presence of weeds.
Plants provide a pathway for water movement between the soil and the atmosphere. This path begins in the soil with water uptake and is influenced by numerous biotic and abiotic factors . Plants experience competition for water when the moisture supply in the soil environment is reduced (e.g. uptake by neighbour) or is exceeded by the evaporative demand . Donald  asserted that the success of any cereal plant for water competition depends on the rate and completeness with which it can make use of the soil water supply. This capacity for water uptake by crops is determined by several attributes within the environment such as transpiration rate and stomatal resistance capacity [31, 78] and the efficiency of water use by plant roots and leaves . Poor stomatal control, for example, results in relatively high plant water use and this may increase competitiveness if the plant neighbours are water conservers . In canola, it is assumed that hybrid cultivars with early vigour use available soil water more quickly, thereby making it relatively unavailable for use by neighbouring weeds. The competitive ability of a cultivar may increase in a specific location due to the environmental influence on evapotranspiration. Although the mechanisms were not clear, it has been suggested that in cool environments hybrid canola induces non-favorable conditions for weed growth by reducing soil resources . Essential nutrients, once inside the canola plant, can be relocated to support growth and, advantageously, they are, therefore, not available to neighbouring weed species.
Plant avoidance and tolerance mechanisms to soil water stress are related to its root morphology and distribution. Pavlychenko and Harrington  found that the considerable depth of the root systems of wheat provided good adaption for drought tolerance and weed competition. Likewise in canola, a deep root system is likely a key trait of the plant’s ability to access sufficient water. Canola roots have been shown to extract water from a depth of 150 cm although up to 95 % of the total seasonal uptake was removed from the top 105 cm of the soil profile . Thus, cultivars with a deep root system trait may become more competitive by being able to adjust their avoidance or tolerance of soil water stress. Roots of canola and other Brassica species, however, are poorly adapted to dry regions and so agronomic adjustment of these early-seeding or early-maturing cultivars may be needed to improve tolerance to competition through better water use efficiency during the seed filling stage. In Western Australia, the early sowing and early flowering cultivars of B. napus produced the greatest total dry weight and seed yield due to efficient water use compared with a late sowing . Early flowering cultivars also showed better competitive ability in Canada because they proliferated their root systems as soon as they sensed a water source, enabling them to fully utilise those resources . These data demonstrate key aspects of canola roots in competitive interference: tolerance of water stress without changes in physiological adaption; and canola root architecture and cellular mechanisms.
Selecting a Competitive Canola Ideotype
The crop ideotype consists of morphological and physiological traits which contribute to enhanced yield relative to currently prevalent crop cultivars. Such a plant will make minimum demand on resources per unit of dry matter production . The design of crop ideotypes, however, may likely involve modifications related to the environment. An evaluation of the competitive ability of different cereal crops, such as rice, wheat and barley, clearly showed that no one ideotype was appropriate for every environment . Different combinations of plant traits could confer the best competitive advantage depending on growing season, climatic conditions and competitiveness with weed species as well as the timing of the competition . Olofsdotter et al.  reported that the best competitive plants also have good biotic and abiotic stress resistance. Little consideration has been given to the inclusion of specific plant traits for strong competiveness with weeds to enhance yield stability. Understanding which traits are most strongly associated with competitive advantage of canola is important for developing new cultivars and should include allelopathy in the development of a canola ideotype.
The term allelopathy originated from the Greek word “allelon” meaning each other and “pathos” meaning suffering and was first introduced by Austrian plant physiologist Molisch . The word “pathos” also means “feeling” or “sensitive” and could, therefore, be used to describe both positive (sympathetic) and negative (pathetic) interactions . The concept of allelopathy received further attention by Rice . He defined allelopathy as an important mechanism of plant interference mediated by the addition of plant-produced secondary products into the rhizosphere . The organic secondary products involved in inhibitory or stimulatory effects are referred to as allelochemicals and these can be released through volatilisation, leaching from plant leaves, residue decomposition and active root exudation [36, 133]. Chemicals with allelopathic potential are present in nearly all plants and their respective tissues . Under the appropriate environmental conditions, these phytotoxic compounds may be released into the environment in sufficient quantities to affect the growth of neighbouring plants . Allelopathy is a significant component of crop/weed interference and, therefore, a potential weed management tool [15, 82, 117]. Allelopathy includes the use of phytotoxic chemicals released from crop residues as well as from intact roots of living plants [163, 164].
Allelopathy of Canola Residues
Crop allelopathy evidence initially came from studies of the use of organic mulches and cover crops to suppress weed emergence. The presence of growth inhibiting substances in plant residues was reported by Collinson . The decomposition products of residues can exert effects on weed germination and establishment [14, 107, 127, 128, 163] either taken up by the recipient singly, additively or synergistically , adsorbed onto soil colloids , modified or reduced or biochemically modified (including non-toxic chemicals into toxic chemicals) by soil organisms [58, 147]. These inhibitory allelopathic effects of residues of both native and cultivated Brassica spp and their relatives have been reported for weed suppression. Boydston and Hang  reported that residues of soil-incorporated foliage of canola suppressed plant populations of common lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), barnyard grass (Echinochloa. crus–galli), hairy nightshade (Solanum sarrachoides) and longspine sandbur (Cenchrus longispinus [Hack.] Fern.) . In Australia, Jones et al.  reported that residues of barley, wheat and canola showed adverse effects on the survivability, growth and dry matter production of paradox grass (Phalaris paradoxa), wild oat (A. fatua) and turnip weed (Rapistrum rugosum). Several subsequent weed suppression studies showed that Brassica cover crops, such as rapeseed and mustard, have high potential to be used in an alternative weed management system. The researchers concluded that an allelopathic mechanism was involved [3, 24, 25, 85, 158]. Tissue damage and then hydrolysis of the Brassica plants released glucosinolate breakdown products, including isothiocyanates, oxazolidinethiones, ionic thiocyanate (SCN−) and organic cyanides [25, 67]. Most breakdown products of glucosinolates are volatile, whereas hydroxamic acids are water-soluble. In the soil, hydroxamic acids can be transformed into more toxic compounds by neighbouring weed species [59, 62]. Although the specific modes of action of these compounds on target weed species have not been thoroughly investigated, most compounds showed inhibitory effects on other species through reduced and delayed germination or inhibition of seedling emergence [115, 116]. The level and the time course of allelochemical release and of other residue-mediated alterations in the soil are largely dependent on the amount and decomposability of the residue, on soil biological, chemical and physical characteristics [34, 96] or on residue management practises . It is unclear whether canola living roots release these compounds in exudation or whether release occurs only during decomposition, and it is worthy of further investigation.
Biofumigation is defined as the use of biocidal compounds, primarily isothiocyanates, used as commercial fumigants, or released by Brassicaceous plants used as green manure or rotation crops, for suppression of soil-borne pests and pathogen [4, 83]. Such compounds have relatively high vapour pressure and are thoroughly dispersed throughout the surrounding soil where they may affect soil-borne fungi, pathogen, insects and nematodes . This finding has led to an increased interest in the development of biofumigation strategies, where naturally formed isothiocyanates could be used as a control measures. Incorporation of Brassicaceous plants in order to control pathogens and nematodes has proven to be effective in several studies [108, 112]. The use of canola as a break crop to help control take-all fungus (Gaeumannomyces graminis) in cereal rotations in Australia is also an example of this biofumigation effect. However, the inconsistent results in biofumigation studies (reviewed by Matthiessen and Kirkegaard)  implied that other factors were involved. The profile of isothiocyanate production varies between Brassica species [84, 142, 143, 149, 159], between individuals of the same genotype [53, 84], and even within different plant tissues of a single individual [57, 104]. Furthermore, it needs to be considered that there are beneficial organisms including biocontrol agents, that are also affected by glucosinolate breakdown products and their presence may have consequences for pest control in an integrated pest management (IPM) agro-ecosystem. The existence of the biofumigation capability, however, is demonstrative of the potential of root exudation for crop management. Their role for weed control remains to be evaluated fully.
Canola Allelopathy by Intact Roots of Living Plants
Weed suppression via living plant exudation is considered a promising approach to exploit allelopathy in annual crops [7, 51, 52, 134]. Belz [15, 16] claimed that weed suppression by crop plant root exudation is a valuable mechanism if this trait can be exploited in much the same way as defence mechanisms against insects or pathogens. The approach has already been reviewed for major grain crops including rice [45, 145], wheat [17, 169] and barley [17, 18, 97]. Those reviews showed that the allelopathic ability of a crop plant to defend itself against weeds was possible and there was considerable genetic variability to exploit such mechanism among cultivars. The family Brassicaceae is often reported as having allelopathic properties that can affect establishment and growth of other species by root exudation [14, 98, 156]. An intercropping study between wild mustard and broccoli demonstrated that broccoli yield was reduced by 50 % due to the phytotoxic effect of plant exudation from wild mustard . In the USA, cultivated or naturally occurring mustards often formed relatively pure stands when well established and, in the wild, they can be successful invaders of native grasslands [14, 163]. In Turkey, Uremis et al.  found that root exudates of the rapeseed cultivar Westar influenced the root growth of redroot pigweed (A. retroflexus L.), black nightshade (Solanum nigrum L.), common purslane (Portulaca oleracea L.), cutleaf ground cherry (Physalis angulata L.) and jungle rice (Echinochloa colonum) more than shoot growth, whereas other cultivars Jumbuck, Tobin, Lisoune and Galant showed less allelopathic activity through their root-secreted chemicals. This suggests that canola plants are also likely to show allelopathy through root exudation and raises the prospects of creating elite allelopathic canola genotypes with improved weed-suppressive capability.
Canola Root Exudates and Phyto-Chemistry
Plant living root hairs and actively growing primary and secondary roots typically release large quantities of secondary metabolites (known as root exudates) . This phenomenon has long been regarded as a passive process of secreted photosynthetically fixed carbon into the soil . Root exudates or secondary metabolites represent one of the largest direct inputs of plant chemicals into the rhizosphere and almost certainly root exudates comprise the major sources of allelochemicals . Stressed plants secrete particular secondary metabolites for their defensive activity [6, 162]. For example, Arabidopsis thaliana (Brassicaceae) secretes a large number of defence metabolites when grown alone . However, once a plant neighbour is identified, the repertoire of metabolites is reduced but overall their secretion increases significantly . In addition to the role in plant defence, some metabolites have physiological functions by serving as mobile and toxic nitrogen transport and storage compounds [166, 167]. However, these multiple functions do not compromise the main role of secondary metabolites as chemical defence and signalling compounds .
Exuded compounds are highly species-specific. They move safely into the environment through a variety of plant sequestration (e.g. sub-cellular vesicles) and transport mechanisms (e.g. protein embedded) [9, 19, 162]. Allelopathy in Brassica spp. appears to be associated with the presence of several groups of exudated metabolites such as phenylpropanoid, flavonoids, isothiocyanates and glucosinolates [26, 85, 90, 115, 116, 119, 121, 166]. These sulphur-containing compounds are indole derivatives at the C-3 position of the indole ring [44, 123]. Phenylpropanoids have a wide variety of functions including defence against microbial attack and other sources of injury . Glucosinolates provide pathogenic organism defence  and can accumulate and modify to yield a variety of products including isothiocyanates, thiocyanates and nitriles but this depends on the nature of glucosinolates and the stress imposed [35, 105]. Choesin and Boerner  measured the direct release of isothiocyanate from growing the root of B. napus but they did not evaluate its effect on weed species.
Research is needed to clarify the type of chemicals released by intact canola roots and their role in weed inhibition. Such findings would facilitate investigation of the biochemistry and metabolomic pathways of these chemicals in plants in respect of canola allelopathy. It would also provide opportunities for new weed controlling cultivars.
Root Exudates and Rhizosphere Communication
The rhizosphere is the narrow region of soil directly influenced by root secretions and associated soil biota [19, 157]. In this zone, plant root-secreted chemicals can influence several processes such as resources (e.g. soil nutrients) and non-resource plant-mediated interaction , microbial communities and their populations  and neighbouring plant species . These influences may play an important role in communication between other plants in the rhizosphere .
Survival of a plant species in a particular rhizosphere depends on the mechanisms of adaption to interaction with biotic and abiotic components. The root rhizosphere is considered the place that provides habitat for plant roots and microorganisms and is inhabited by a wide range of microorganisms, including bacteria, fungi, algae, viruses and protozoa. These microbes may have a profound effect on allelopathic activity by altering and/or transforming the amount of allelochemicals . On the other hand, allelochemicals may also influence the microbial community [71, 87] and these mechanisms can involve both stimulation (by providing nutrient sources) and inhibition (by interfering with nutrients) . Various soil-borne organisms are highly sensitive to the Brassica plant-secreted 2-phenylethyl isothiocyanate, with bacteria being more tolerant than eukaryotic organisms . In contrast, growth of ectomycorrhizal fungi was found to be stimulated by root exudates of various Brassica spp. . Rumberger and Marschner [140, 141] reported that canola roots released sufficient amounts of 2-phenylethyl isothiocyanate into the soil rhizosphere to have a selective effect on the bacterial community. Bacterial community composition was significantly correlated with phenylethyl isothiocyanate concentration and moreover changed with plant growth stage [140, 141]. However, despite high residence time of this chemical, Choesin and Boerner  found in their study that this root secretion did not have an inhibitory effect on Medicago sativa L. This suggests that specific signals might be exchanged between Brassica plants and microorganisms, although this is not yet clear. It would be also interesting to know whether the possible allelopathic chemicals described for canola roots act directly against the neighbouring plants or indirectly through modifications by soil microorganisms.
Crop plant roots are continually interacting with roots of neighbouring plant species; and are capable of detecting and responding in multiple ways [20, 42, 56, 99, 148]. Roots may communicate with other roots with the help of various secondary metabolites, which are secreted into the rhizosphere in response to biotic and abiotic stresses.
Several research studies suggest that such a response of roots to their neighbours is not only explained by nutrients but also involves non-nutrient causes [8, 9]. Cahill and McNickle  divided these apparent non-nutrient root responses into three classes: (a) segregation (root growth away from neighbours), (b) neutral (no specific directionality of root growth) and (c) aggregation (root growth towards neighbouring roots). The actual sensing of the neighbour presence might be based on either physical touching of roots  or without physical touching via chemicals signals released by roots [6, 76]. For chemical signals, secondary metabolites have been largely credited with being involved in plant–plant interaction on the assumption that these compounds tend to be phytotoxic and persist in the soil . Of course, such compounds could be hormonal or pseudo-hormonal in their influence on non-same neighbours. Pierik et al.  reported that the high specificity of root exudates has the potential to transport such specific signals into the rhizosphere. It has been reported that the proteins in the root exudates are secreted differently depending upon the presence and identity of the neighbouring root [6, 43]. Such canola-neighbour root interactions have not been elucidated. Establishing these interactive mechanisms by canola exudates will elucidate the true complexity of the competitive arena (Fig. 1).
Canola-weed below-ground interactions (resources and non-resources based) involve various signals, such as variations in nutrient concentrations, soluble root exudates and the activities of soil microbes
Weed control remains one of the most important mechanisms of yield loss in Western Canada. The widespread use of herbicide-resistant canola initially provides a significant improvement to weed control, but environmental challenges, below optimal plant populations, herbicide-resistant weeds and the continued persistence of hard to control weeds (such as cleavers) remain challenges for growers.
Best tips for weed management:
- Early weed control. A combination of preseed weed control and one in-crop application before the four-leaf stage of the crop is often enough. Canola that gets off to a good start with weed competition reduced early in the season rarely sees an economic benefit to a second in-crop application of herbicide.
- Tank mix Combining two or more crop protection products (that are compatible) in one application. partners are added to preseed glyphosate to improve weed control and reduce selection of herbicide resistant weeds.
- Use weed control options in rotation crops to target weeds that are harder to manage (such as cleavers and volunteer canola, for example) in wheat.
- Use principles of integrated weed management to reduce the development and build-up of herbicide-resistant weeds, and improve overall results from herbicides.
Weed management is essential for profitable canola production. Of the important requirements for high yield potential, weed management is more important than fertilizer, more important than genetics 1 . Weeds are highly competitive, and can use up resources — moisture, nutrients, access to sunlight — that would otherwise be available to the crop. Yield loss from weed competition can be significant.
Weeds can also result in:
- increased insect and disease damage through multiplication on weed hosts
- spread of more weed seeds
- increased cultivation and/or chemical control practices (as a management strategy)
- reduced seedbed soil moisture and structure as a result of increased tillage to kill weeds prior to seeding
- delayed swathing and combining from swaths that are hard to pick up, resulting in plugging equipment and higher seed moisture content (from increased weed seeds in with harvested grain)
- increased heating and spoilage in storage due to green weed seeds in with harvested grain
- increased dockage with higher cleaning and transportation costs
- contamination resulting in reduced grades and quality from similar inseparable size and shape weed seeds (e.g. cleavers)
Weed competition is generally more damaging when:
- low canola plant population lacks vigour Seed properties that determine the potential for rapid uniform emergence and development of normal seedlings under a wide range of field conditions.
- weed density is high and the weeds are vigorous
- moisture and nutrient supply favours weed growth more than crop growth
- weeds significantly reduce moisture supply to the crop
- weed growth is ahead of the crop
- environmental conditions have resulted in a shallow rooted crop 2
Timing of control
Consider the weeds present and their growth stages when making the spray timing decision. Cleavers, stork’s bill and spiny annual sow thistle have rapid growth rates, and even under cool conditions advance through their leaf stages while other species are delayed. Timing of control is even more critical when these species are present.
Fields with a large population of weeds, especially advancing winter annuals, should get a preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. . University of Saskatchewan research shows that where weed competition is significant, an early preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. application can have a greater influence on crop yield than an early seeding date. Even if the crop is not seeded until later, an early burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. is still preferred. The study found that a late burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. treatment yielded significantly lower than early treatment, regardless of when the plots were eventually seeded 3 . Table 1 provides the preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. herbicide options prior to planting canola 4 .
Table 1. Preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. herbicide options prior to planting canola
Preseed spraying with phenoxy herbicides is not recommended as serious damage to the emerging canola seedlings can occur. Preseed ahead of cereals offers more herbicide options, including phenoxy herbicides.
Winter annual weeds like flixweed, shepherd’s purse, narrow-leaved hawk’s-beard and stinkweed can remove a lot of soil moisture in the spring as they begin to grow very early and are very inefficient water users. Early preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. stops this uptake, preserving moisture and nutrients for the crop.
How long to wait between spraying and seeding: For annuals and winter annuals, glyphosate needs only 24 hours to get to the growing point and set the control process in motion. (It may be quicker than that for some specialty glyphosates in good growing conditions.) After a day, the crop can be seeded. For perennial weeds, the recommended delay ranges from three to five days depending on weather conditions. If sunny and warm, translocation will take place fairly quickly so three days should be enough. If weather is cloudy and/or cool when applied, delaying five days is recommended before seeding. When deciding how long to wait to seed, remember that the conditions on the day the glyphosate is applied are the most important to efficacy, making them relatively more important than conditions in the days that follow.
Post-seeding/pre-emergence spray window
Preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. also reduces risk if weather causes a delay in post-emergence application. Weeds present when the crop emerges will have a substantially greater negative impact on the yield of the crop if the pre-emergent application is missed altogether.
The cleaner you have the field as canola emerges from the ground, the better. If preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. is missed, one option is to apply glyphosate or (glyphosate with a Tank mix Combining two or more crop protection products (that are compatible) in one application. partner) post seeding and pre-emergence. This can be a narrow window. Shallow seeded canola can emerge within five days under warm and generally moist soil conditions, and since growers need to leave weeds for a couple days to start growing again after the seeding operation, the window for post-seed/pre-emergence spraying is very narrow. The time lag between seeding and spraying is necessary to allow weeds buried by the seeding operation to re-emerge and to allow weeds uprooted to overcome the stress placed on them. However, spraying and missing a few of these weeds is more beneficial than waiting to spray in-crop. Ensure to check for crop emergence before applying products that need to be applied pre-emergence.
Benefits of early weed removal, from the one- to four-leaf stages of the canola crop, are supported by numerous research studies across Western Canada. This early control allows the crop to get established, and controls early emerging weeds before they take up excessive nutrients and moisture. Also, weeds that emerge after the crop has reached the four-leaf stage are usually so far behind the crop that they will have little impact on yield, and contribute very little to the weed seedbank.
Waiting a few extra days means that more late-germinating weeds may be up and controllable, but these studies show that this does not translate into yield.
Once the canopy has closed, the crop should outcompete the weeds all on its own. Second in-crop applications at this time do not usually provide an economic benefit as the plants are competitive enough on their own in comparison to the weeds, but can in some cases reduce weed seeds entering the soil seed bank.
For perennial weeds that have escaped preseed and first post-emergence applications but are delayed relative to the crop, pre- or post-harvest glyphosate applications may be a better alternative than a second post-emergence spray, especially if the weeds are delayed enough to avoid seed set prior to swathing. Although this can be a balancing act as well.
The following figure depicts herbicide timing and canola staging information, which was sourced from the 2021 Saskatchewan Guide to Crop Protection.
The two tables above summarize two studies from Western Canada showing the yield benefit to early weed control. The second table, the larger of the two, summarizes trials by the Canola Council of Canada at Crop Production Centres across western Canada. Large plots were sprayed for weeds at the one- to two-leaf stage of the crop (early), the three- to five-leaf stage (mid) and at the six-plus leaf stage (late).
In some years, the mid timing was missed or not included in the trials. In some years, two broader timings were used as opposed to three narrower timings. In 24 seeding date/locations out of 27, the earliest timing gave the best yield. The early applications showed a sixteen per cent yield advantage compared to the latest timing.
Table 2 depicts the in-crop weed control options for canola 4 .
Table 2. In-crop weed control options for canola
Always follow labels when applying herbicides. Applications made after the label window can lead to reduced weed control due to advanced weed staging and reduced herbicide contact with the weeds through the increased canola canopy closure. Late applications can also cause canola buds to abort, increasing the potential for permanent yield loss. Finally, late applications and higher than recommended rates of registered herbicides can increase the risk of elevated residues in the marketable grain. Foreign buyers are testing for pesticide residues that exceed registered maximum residue limits The greatest amount of a particular residue (ex. pesticide residue) that is allowed to be found in a sample (ex. of a canola shipment). (MRLs) or are not registered at all.
Yield loss from late herbicide applications vary on a field by field basis depending on herbicide rate, overall health of the field, and growing conditions during and after the application. Canola under any type of existing stress will likely have greater losses from a late herbicide application. It simply won’t be able to recover as quickly. If canola is healthy at the time of spraying but conditions are hot and dry soon afterward, canola may not be able to compensate by adding more flowers later in the flowering period. Losses cannot be predicted, but they could be up to 30 per cent, and even more in overlap areas.
Aerial options for weed control
If weeds and crop are advancing toward the end of the application window and the ground is too soft and wet to support a sprayer, then aerial spraying may be the best economic response. Here are the options for aerial herbicide application in canola:
- Roundup WeatherMax is the only glyphosate registered for aerial application at this crop stage, but use is subject to certain conditions which are outlined in detail on the label. (Many others are registered for pre-harvest.) In season sometimes other glyphosate products will get emergency use aerial registry so speak to your aerial applicator about your options.
- Liberty (glufosinate) is registered for aerial application at this stage. Remember, Liberty works best at higher water volumes. Clethodim is also registered for aerial application.
- None of the Clearfield herbicide system products are registered for aerial application on Clearfield canola. However you can still use the clethodim products on the Clearfield System for a grassy weed control option.
- Draft does not have an aerial label
- Assure II, Equinox and Poast have aerial labels.
- Quinclorac does not have an aerial label.
For more on these requirements, read product labels and the provincial guides to crop protection.
Benefits of aerial application: If weeds are plentiful, at the same stage or ahead of the crop, and canola canopy closure is unlikely to effectively limit their growth, then it will probably pay to control these weeds by air if you have no other option. Aerial spraying also avoids wear and tear on sprayers and on the field when conditions are wet. Under these very wet conditions, ground sprayers can leave deeps ruts to contend with in subsequent spray applications and at harvest, while destroying the crop in those tracks. Getting a sprayer unstuck can be a long, messy job, and dealing with the ruts at harvest can be time consuming and annoying.
Downsides to aerial application: Buffer zone requirements for aerial application are substantially further from sensitive habitats than buffers for ground application. For Liberty, for example, the buffer zone for aerial application is 30 metres from non-target plants and animals compared to a one metre buffer for ground application. Aerial application also represents another cost, although there is not that much additional cost now between aerial and ground application when you are hiring a custom applicator. Before applying by air, know what crops are surrounding the field to the N, NE, E, SE, S, SW, W, NW. Note what herbicide system is used in neighbouring canola crops as well. The aerial applicator will need all of this information.
Fall is a good time to clean up weeds, especially winter annuals and perennials. Cold temperatures help trigger winter annuals and perennials to start moving food reserves down into below ground tissues, so waiting until after cooler weather may improve results. When you spray at this time the glyphosate gets translocated down into the root system and so you get better control of these hard to kill weeds. Refer to your provincial guide to crop protection to determine which rate of glyphosate will work best for you.
When applying in the fall:
- Make sure the plants are actively growing with new supple leaf area to target. Six inches of growth is a rough guide. Weeds cut off at harvest need time to accumulate new leaf tissue. This is essential for herbicide uptake and efficacy.
- Frost damaged leaves may look green but they’re not healthy and will not take up herbicide. If frost has occurred, avoid application until leaf condition of the target weeds can be evaluated. Leaf tissues that are blackened, browned or dark green yet brittle are all symptoms of cold temperature damage. Leaves should be vibrant green, shiny and pliable. Look for new growth. That means the weed is actively growing again and will take in the herbicide.
- Spray in the afternoons when temperatures are warmer and heavy dew is off the plant. Ideally, you want to apply when temperatures are above 10 degrees Celsius and rising on days with predicted highs above 15 degrees Celsius, but preferably higher. If applying on days where temperatures are at the minimum, bright sunshine is critical. Bright sunny conditions are ideal for moving herbicides to the roots where they will have the most impact next year. Check product labels for specific recommendations as temperature requirements may vary.
- Use products registered for use before canola in fields that are planned for canola the following spring.
- Know whether the weeds present in the fall will last the winter. Canola volunteers, for example, that emerge in the fall are unlikely to last the winter and do not need to be sprayed. In fields where crops were harvested prior to mid-August, annual weeds growing back from below the cut may have a good chance of producing significant additional seed prior to a killing frost. These annual weeds may benefit from mowing or spraying.
Ideally, growers will adopt an integrated weed management program to reduce the reliance on herbicides for weed control.
Integrated weed management
Integrated weed management (IWM) combines different agronomic practices to reduce the reliance on any one weed control technique. Controlling weeds with only one or two techniques gives weeds the chance to adapt to those practices. We have many examples of herbicide resistant weeds on the Prairies, including cleavers with Group 2 resistance, kochia with Group 2 and Group 9 resistance, and wild oats and green foxtail with Group 1 resistance.
For information on managing herbicide resistance in canola and other crops, see CropLife Canada’s Manage Resistance Now.
IWM employs a range of control practices to keep weeds “off balance” and keep weed populations at manageable levels. Weeds are less able to adapt to a constantly changing system that uses many different management practices. Practices that limit the introduction and spread of weeds include:
Crop rotations or crop sequences that include oilseeds, spring cereals, winter cereals, pulses and forage crops allow the use of different herbicides and cultural practices that make it difficult for weeds to adapt and build up. Crop rotations must take advantage of crop life cycle and weaknesses in the life cycle of the existing weed problem.
Certain crop varieties are more competitive than others. Varieties that cover the ground more rapidly and close their crop canopy earlier can help shade out weeds. More vigorous growing canola hybrids sown at a higher plant population are very competitive and result in reduced weed growth and lower dockage.
Higher seeding rates
Higher plant populations allow the crop to shade weeds and make it more difficult for weeds to access nutrients and water. Canola crops that close the canopy quickly may only require one in-crop herbicide spray compared to less dense (thin) crops, which may require two applications. In most cases, seeding rates can be moderately increased to obtain weed management benefits without significant negative consequences.
Varied seeding dates
While most crops in a rotation benefit from early seeding it can be beneficial on individual fields to vary the seeding date from year-to-year. Early-seeded canola can result in the crop emerging before the weeds. Some weeds like green foxtail, lamb’s-quarters, kochia, redroot pigweed and Russian thistle require more growing degree days to germinate and emerge. An early-seeded canola crop could emerge far enough ahead of these weeds that herbicides may not be required. However, cool season weeds like wild oats, quackgrass, wild mustard and stinkweed may present a problem with early seeding.
In certain situations, later seeding may be used to reduce the number of weeds in the crop. Shallow till the soil early in the spring to aerate it and promote weed seed germination. Delay seeding until weed growth has reached a maximum. This growth must be destroyed either by a preseed herbicide application or a tillage operation prior to seeding. Disadvantages of this approach are that tillage can cause the loss of surface soil moisture resulting in poor, uneven germination and emergence, and that delayed seeding may reduce crop yield.
Shallow seeding depth
Seeding at one to 2.5 centimetres (one inch), provided adequate moisture is present, means quicker seedling emergence which helps the crop get ahead of or keep up with the weeds. Deep seeding delays crop emergence and weakens seedlings and they are less competitive.
Narrow row spacing
Narrow row spacing allows plants to be more competitive. Research at the Agriculture and Agri-Food Canada Agriculture and Agri-Food Canada is a department of the Government of Canada. ( AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. ) research centre in Scott, Saskatchewan showed that weed biomass increased and canola yields decreased with wider row spacing 5 .
The trend is to use wider row spacings, which has advantages for residue clearance and lower equipment cost per foot. Herbicide tolerant varieties also make weed control more effective with wider rows. However, narrower rows canopy faster to improve crop competition, which can reduce the risk of weeds adapting to these systems.
Use of pedigreed seed
Choose varieties that are very competitive in growth habit. Choose a variety A variety is a variant of a species that evolved in nature without the intervention of humans, e.g. Brassica oleracea variety (in short form, var.) botrytis (cauliflower), var. capitata (cabbage), var. italica (broccoli), etc. with a herbicide resistance that will be effective on the weeds/volunteers in your field.
Tillage or direct seeding A seeding method where the crop is sown into the field without any previous spring tillage operations.
Seedbed preparation can affect crop and weed growth. Spring tillage to control weeds can also loosen the seedbed and, in moist conditions, create clods — two conditions that reduce seedbed quality and hurt canola emergence.
Research at the AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. research centre in Indian Head, Saskatchewan found that soil disturbances from tillage increased weed densities. The research also found that weed densities early in the season were lower than at the end of the seeding season. direct seeding A seeding method where the crop is sown into the field without any previous spring tillage operations. using a burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. treatment may be all that is required to control weeds, especially at later seeding dates. This is possible because a tillage operation that encourages a new flush of weeds within the crop is eliminated.
direct seeding A seeding method where the crop is sown into the field without any previous spring tillage operations. systems leave crop residue between rows, which shades the soil and keeps it cool. For this reason, weed emergence can be slower in direct seeding A seeding method where the crop is sown into the field without any previous spring tillage operations. . A second flush of weeds is less likely to occur because of higher stubble residue between crop rows. Green foxtail is an example of one weed species that is reduced in a direct seeded soil.
For growers who use tillage for weed control, fall may be the better time ahead of canola crops. This timing reduces the stubble stand that traps snow and reduces soil erosion. However fall timing provides mechanical weed control while giving the soil time to resettle, allowing for improved seed placement and seed to soil contact the following spring. Some fall tillage considerations are listed below:
- Shallow cultivation in the fall, resulting in a light soil A soil that is high in sand content. covering of weed seeds, is the most effective means of stimulating wild oats germination. Harrowing is relatively ineffective as it does not provide a sufficient soil cover to favour germination.
- Deeper tillage penetration may be required to destroy roots of perennial weeds such as Canada thistle and quackgrass. Deeper tillage may also be required to prepare the soil for soil-incorporated herbicides or fertilizers, and to bury some of the straw.
- Deep fall tillage buries weed seeds deeper and induces dormancy in wild oats and other smaller weed seeds. Small weed seeds with a long dormancy period (wild mustard and stinkweed) most readily lose dormancy when left on the soil surface. Deep fall tillage can also induce dormancy in volunteer canola.
- Fall spraying is more effective than tillage for winter annuals like stinkweed, flixweed, tansy mustard, groundsel, and winter annual cleavers, which do not germinate until late September or October. These weeds germinate, establish a root system and form a rosette that is not killed by winter frosts. The plants overwinter and continue to grow as soon as snow disappears in the spring. By the time fields can be worked in the spring, these weeds have often become well-established and hard to kill, requiring extra tillage that dries out the seedbed. These weeds can be more effectively controlled by late fall spraying than with tillage. Tillage often transplants the weeds, allowing them to grow.
Banding nitrogen fertilizer gives seedlings an advantage over the weeds in accessing the nitrogen in the early stages. Most weeds germinate near the soil surface, therefore, banded fertilizer is not as accessible to the weeds, reducing the number of weeds germinating and their seedling vigour Seed properties that determine the potential for rapid uniform emergence and development of normal seedlings under a wide range of field conditions. . Broadcast nitrogen, especially if it’s incorporated, can stimulate weed seed germination (especially wild oats) and is more readily available to weed seedlings. Phosphorous fertilizer with the seed, especially when early seeding, is essential for establishing vigorous seedlings. Apply safe levels of nitrogen, phosphorus and potassium with the seed to avoid killing or injuring seedlings, making the stand less competitive.
Insect and disease protection
Seed treatments combined with shallow seeding into a firm, moist seedbed may reduce seedling diseases, allowing plants to be more competitive with weeds. Protection from insects such as the flea beetle will enhance seedling vigour Seed properties that determine the potential for rapid uniform emergence and development of normal seedlings under a wide range of field conditions. and growth.
- Ensure that farm equipment is clean so that weeds are not spread from field-to-field or from one farm to another.
- Cover screenings and grain during transport.
- Restrict domestic animal movement from weedy fields to clean fields.
- Do not allow weeds to set seed.
- Control weeds in ditches, field edges and around sloughs.
- Disrupt or destroy weeds by machine, hand mowing, hand pulling of small patches, early harvest, tillage, grazing, ensiling, burning weeds prior to maturity and mulching small patches with clean straw, manure or plastic.
- Control the spread of weeds through manure by proper rotting or composting of the manure and livestock bedding.
- Control or restrict wind and water movement from weedy to weed-free areas.
Chemical control measures
From an IWM standpoint, use herbicides in combination with good agronomic practices. Focus on combinations of optimal agronomic factors that enhance crop health and competitiveness, favour viable weed management systems, and support integrated crop management practices. AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. research found that combining the best variety A variety is a variant of a species that evolved in nature without the intervention of humans, e.g. Brassica oleracea variety (in short form, var.) botrytis (cauliflower), var. capitata (cabbage), var. italica (broccoli), etc. , the highest seeding rate and the earliest weed removal treatment increased yield 41per cent compared to the combination of a less vigorous variety A variety is a variant of a species that evolved in nature without the intervention of humans, e.g. Brassica oleracea variety (in short form, var.) botrytis (cauliflower), var. capitata (cabbage), var. italica (broccoli), etc. , the lowest seeding rate and the latest time of weed removal 6 .
Choose herbicides based on the type of weeds and the tolerance of the crop to various herbicides. Apply the right herbicide in the proper way, at the optimum time and in the recommended concentration. This information can be obtained from the label on the herbicide container, and from public or private crop specialists. Since registrations change from year to year, obtain provincial herbicide guidelines annually. Check the annual provincial publication for current recommendations and always refer to product labels.
Harvest weed seed control
Two options for integrated weed management at harvest are: seed crushers added to the back end of combines and modifications to chaff-spread patterns. These practices can help manage herbicide-resistant weeds and protect herbicides that are currently used from losing their effectiveness.
Herbicide-tolerant canola systems
The herbicide-tolerant (HT) systems in canola are:
- Clearfield – canola resistant to imidazolinone chemistry (Odyssey, Absolute, Solo, Ares, Tensile)
- Sulfonylurea-tolerant canola (Draft)
- LibertyLink – canola resistant to glufosinate ammonium (Liberty)
- Roundup Ready – canola resistant to glyphosate (Roundup)
There are also some varieties that offer a stacked trait with herbicide tolerance to both glyphosate and glufosinate ammonium.
The main features of each weed control system are summarized below.
- Clearfield products and registered tank mixes can be applied to Clearfield varieties.
- Controls various annual grassy and broadleaf weeds using post-emergent applications.
- Extended control (residual) herbicides offer control of some second flushes of certain shallow germinating weeds.
- Systemic mode of action with affected weeds showing stunting, yellowing/reddening within three to 10 days of spraying.
- Products with the same mode of action (Group 2) will not control volunteer Clearfield canola in subsequent years. Products may be co-packaged with a Group 1 or Group 4 product.
- Allows weed resistance management by switching from a Group 1 to a Group 2 herbicide or using a co-packaged product Group 2 and 4 or Group 2 and 1. Broadleaf resistance is a concern, especially on fields with previous Group 2 use.
- Some restrictions for recropping in the year following application.
- None of the Clearfield herbicide system products registered for aerial application on Clearfield canola.
- Draft and registered tank mixes can be applied to Sulfonylurea-tolerant varieties.
- Controls various annual grassy (when tank mixed with a group 1) and broadleaf weeds using post-emergent applications.
- Extended control (residual) herbicides offer control of some second flushes of certain shallow germinating weeds.
- Systemic mode of action with affected weeds showing stunting, yellowing/reddening within three to 10 days of spraying.
- Products with the same mode of action (Group 2) will not control volunteer canola in subsequent years. Products may be co-packaged with a Group 1 or Group 4 product.
- Broadleaf resistance is a concern, especially on fields with previous Group 2 use.
- Some restrictions for recropping in the year following application.
- Draft is not registered for aerial application on Clearfield canola.
- Liberty herbicide and registered mixes can be applied to LibertyLink varieties.
- Controls various annual grassy and broadleaf weeds, and suppresses some perennial weed species.
- Liberty works best when applied with water volumes a minimum of 10 gallons per acre.
- Applied post-emergent. Activity is through contact with green tissue, with affected plants show burning/yellowing within two to five days after application. Plants die one to two weeks after treatment.
- Unique mode of action (Group 10) and can be tank mixed with Group 1 clethodim, quizalofop and Group 4 quinclorac. Good option for grassy and broadleaf weed resistance management.
- No restrictions on recropping in the year following application.
- Liberty works best on warm sunny days. On cloudy days, increase water volumes and consider spraying later in the day when temperatures are at their highest.
- Liberty is registered for aerial application. Centurion and Select are also registered for aerial application.
- Avoid spraying Liberty when temperatures are above 28 degrees Celsius as canola injury can occur.
- Glyphosate and registered mixes can be applied to Roundup Ready varieties.
- Registered glyphosate rates and the application window are different for TruFlex or OPTIMUM GLY varieties.
- Controls various annual grassy and broadleaf weeds, and controls or suppresses some perennial weed species.
- Applied post-emergent. Activity is systemic, with affected weeds showing yellowing at the terminal buds within seven to 10 days after application.
- Glyphosate will not control volunteer Roundup Ready canola in subsequent years. An additive must be included with glyphosate to control the volunteers.
- Unique mode of action (Group 9), good option for grassy and broadleaf weed resistance management for Groups 1, 2, 3 and 8. Can be tank mixed with quinclorac.
- No recropping restrictions in the year after application.
- Roundup WeatherMax is the only glyphosate registered for aerial application at the post-emergence crop stage, but use is subject to certain conditions which are outlined in detail on the label. (Many others are registered for pre-harvest.)
How to choose an HT system
Consider management goals when picking an HT canola. Growers may find one HT system the best choice for some fields and another HT system the best choice for other fields.
Use weed spectrum to narrow the choice. Choose the product that matches the most important weeds in a particular field. Consider herbicide cost, too. Ordering early or in a company program may reduce costs.
List the other non-weed factors. These include variety A variety is a variant of a species that evolved in nature without the intervention of humans, e.g. Brassica oleracea variety (in short form, var.) botrytis (cauliflower), var. capitata (cabbage), var. italica (broccoli), etc. performance, harvestability, and disease resistance. However, these factors must not take priority away from the weed control issue. Do not put a great performing variety A variety is a variant of a species that evolved in nature without the intervention of humans, e.g. Brassica oleracea variety (in short form, var.) botrytis (cauliflower), var. capitata (cabbage), var. italica (broccoli), etc. into a field with a weed infestation that the available herbicide for the system will not control.
How to manage volunteer canola
Volunteer canola is a weed and competes with the crop for nutrients and water and sunlight. Volunteers in a canola crop do not make a positive contribution to yield. Volunteers do not have seed treatment, so they can introduce seedling diseases and increase flea beetle pressure. Also, volunteers in non-canola years provide a host for blackleg and clubroot, reducing the effectiveness of crop rotation for managing these diseases. Finally, volunteers can have single HT traits, but with outcrossing more volunteers also have stacked traits — Roundup Ready, Liberty and/or Clearfield traits.
Effective management steps for volunteers with single-trait or stacked herbicide tolerance:
The vast majority of seed in the canola seed bank is eliminated in the first two years after canola is grown, so growing at least one and preferably two or three crops between each canola crop can greatly reduce the volunteer population. Cereals provide a broad range of preseed and in-crop herbicide options for any type of canola volunteer. Stacked volunteers, even if they happen to have all three traits, are still susceptible to common herbicides, including 2,4-D, MCPA and bromoxynil, and various other products. Group-6 Basagran can control all canola volunteers in peas, flax, beans and other broadleaf crops.
Scout rotation crops
When scouting, also check for volunteer canola that are not controlled by herbicide application. Early detection allows time for control before seed set and clubroot gall formation.
Control ahead of canola
If some volunteers germinate on a field planned for canola, carfentrazone, bromoxynil, pyraflufen, halauxifen are registered for preseed application before canola, providing tank mixes for straight glyphosate.
Keep detailed notes
In addition to regular growing season records, keep notes on herbicides and herbicide-tolerant systems used on each field.
Leave canola seeds on the soil surface after harvest
When using fall tillage, delay it until a few weeks after harvest. That gives time for seeds to germinate in the fall (most of which will die through the winter) and for birds and insects to eat seeds. Also, dormancy of canola seed does tend to be reduced if canola remains near the soil surface. Seeds of B. napus Also referred to as Argentine canola, it is the species of canola currently commonly grown in Canada. varieties that are incorporated into the soil develop induced secondary dormancy and can persist for up to four years in the soil.
Reduce harvest losses
The fewer seeds shelled out, the lower the volunteer seedbank. The minimum harvest loss is typically about half a bushel per acre, which is five times the typical seeding rate. On many fields, harvest losses are 20 times the seeding rate, or more.
Avoid tillage in the spring
This will allow for maximum volunteer emergence. However, if using a convention tillage system, growers can maximize weed control benefits by shortening the interval between tillage and seeding operations.
Rotate canola with cereal, pea and forage crops
Diversifying a rotation permits use of a wider selection of herbicides. Lengthening the rotation depletes the volunteers from the soil weed seed bank. Include fall seeded crop, growing silage and green manuring to control volunteers.
Apply herbicides early
Spraying volunteer canola at the 2-4 leaf stage is much more effective than at the 5-6 leaf stage.
Choose herbicides that work
Growers have good options to control single trait and stacked trait volunteers ahead of cereal rotation crops.
Products available for use ahead of other crops are shown in Table 3 7 .
Table 3. Herbicides to control emerged weeds before seeding or after seeding but prior to crop emergence
Major weeds of canola
An important step in weed management is to identify the problem weeds present in each field. Every year, evaluate individual fields for weed types, populations and locations. Keep an up-to-date list of weeds found in each field. Local crop specialists can help identify the weeds and many good weed identification books are available.
Volunteer canola is a weed and competes with the crop for nutrients and water and sunlight. Volunteers in a canola crop do not make a positive contribution to yield. Volunteer canola left untreated can introduce seedling diseases and increase flea beetle pressure. In non-canola years, volunteer canola provides a host for blackleg and clubroot, reducing the effectiveness of crop rotation for managing these diseases.
Cleavers can be very competitive with canola because it clings to plants when growing toward the light and reduces crop development. The clinging bristles can result in swathing difficulties and bunching of the swath. More important, cleavers seeds are similar in size and shape to canola, making them a serious contaminant that will downgrade the crop. Canola that is graded No.1 Canada must have less than one per cent other seeds that are conspicuous and not readily separable from canola. This includes cleavers. The cut off is 1.5 per cent for No.2 and 2.0 per cent for No.3.
Preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. is the most effective way to control cleavers. Fields with increasing populations of cleavers will need a higher rate of preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. . The 180 grams (half-litre) rate of active per acre of glyphosate is not likely enough. A rate of 360 grams (one litre) per acre will control these weeds more effectively. If cleavers are larger, a 540 gram (1.5-litre) rate may be required. A selection of Tank mix Combining two or more crop protection products (that are compatible) in one application. partners can be added to pre-burn applications of glyphosate to improve cleavers control. If cleavers are a problem in a field, a fall application of glyphosate the season before following by a spring preseed burnoff Refers to a pre-seed glyphosate (herbicide) application that stunts the growth of the perenial weeds in a field (since glyphosate only affects top growth and isn’t taken up by the roots in the spring) in order to give the crop (to be seeding shortly after) a better chance at outcompeting the weeds for sunlight, water and nutrients. can go a long way to reduce populations. Overwintering cleavers are very hard to control and get big fast in the spring, so control them the fall before whenever possible.
Dig up a few cleaver plants to determine staging. Those with large established roots are probably winter annuals and will need a higher rate. Cleavers that emerge in the fall can survive a cold winter if they’re under a nice blanket of snow. Early spring control is essential; otherwise winter annuals will quickly grow to a size where herbicide control is much less effective. If cleavers are small and look like they germinated in the spring, a lower recommended rate may be enough. In-crop cleavers efficacy, regardless of herbicide system can be improved with the addition of quinclorac.
Glyphosate-resistant kochia was first confirmed in Southern Alberta in 2011, making it the first glyphosate-resistant weed confirmed in Western Canada. Interestingly, a 2010 predictive model by Hugh Beckie, research scientist with AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. in Saskatoon, ranked kochia first among weeds at risk of developing glyphosate resistance on the Prairies. The prediction was accurate.
Glyphosate-resistant kochia is now commonly found in all three Prairie Provinces. Kochia produces 15,000 to 25,000 seeds per plant, and mature weeds can spread these seeds over a fairly wide area, once they break off from their stems and start to tumble with the wind. Farmers will want to test “escapes” (kochia plants that aren’t killed by the herbicide applied to target them) for glyphosate resistance, so the weeds can be targeted for intense management before they start to shed their seeds. A few hours of hand rouging may be required, but this can be time well spent to stop a small patch from spreading.
The preseed window is a critical time to control kochia as 80 per cent of kochia seedlings emerge before the crop. It’s important to note that all kochia plants in Western Canada are considered Group 2 resistant. For good long-term management and prevention of glyphosate-tolerant kochia escapes, use a rotation of broadleaf control products with good activity on kochia and with mixes that include actives from Groups other than 2 and 9. Tank mixes and pre-packs that include bromoxynil, dicamba or Group 14 active ingredients are the choices available. Among those products, only CleanStart (with Group 14) is approved for use ahead of canola and must be applied at the high rate to control glyphosate-resistant kochia. Preseed kochia management ahead of cereals will allow more product choices. Taking measures to manage glyphosate resistant kochia before it is visible in the field is critical to managing it successfully.
Canada thistle is three to four times more competitive than wild oats. The graph below shows estimated yield losses of canola caused by Canada thistle, based on the number of thistle plants per square metre of canola crop.
Researchers at the AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. in Lacombe, Alberta developed an equation to predict yield loss through competition from Canada thistle. The equation is:
Per cent yield loss = -3.83 + 1.4X (x is the number of Canada thistle shoots per square metre)
The more samples taken, the greater the accuracy in estimating yield loss. A minimum of 10 samples is recommended per quarter section. For example, if a canola field has an average of 15 Canada thistle shoots per square metre, the estimated yield loss would be:
Per cent yield loss = -3.83 + 1.4 x 15 = 17
Since no control measure provides 100 per cent weed kill, and since weeds are not killed immediately on emergence, all of this loss can never be recovered. The recovery of at least 75 per cent of the yield loss by spraying with a herbicide would result in an increase of 213 kilograms per hectare (3.8 bushels per acre). Decisions must be made on whether the use of a herbicide is economical given current herbicide costs and canola prices.
Sow thistle does not usually occur uniformly throughout fields, but where it does, the crop can suffer high yield loss. Soil, rainfall and farming practices influence the competition between canola and sow thistle.
Researchers at the AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. in Regina, Saskatchewan developed an equation for determining yield loss in canola:
Per cent Yield Loss = -3.81 + 13.76 x (square root of X)
(Where X = the number of sow thistle rosettes and shoots per quarter of a square metre)
In a field with 20 sow thistle rosettes in a square metre (or five per quarter of a square metre) the yield loss would be:
Per cent yield loss = -3.81 + 13.76 x (square root of five)
= -3.81 + 13.76 x 2.24
In general, thistle populations increase in seasons of normal to above normal soil moisture. This can be important if the weeds are not controlled in the year before the canola crop.
Volunteer wheat and barley
Volunteer cereals are strong competitors at the early growth stages. Volunteer wheat and barley at seven to eight plants per square metre can reduce canola yield by 10-13 per cent, as shown in studies at the University of Manitoba.
Equal numbers of volunteer cereals were found to be 1.5 times as competitive as wild oats. At higher densities, barley was much more competitive than wheat.
Canola yield losses are more severe when volunteer wheat or barley emerges before the canola crop.
Wild oats can significantly reduce canola yields. Estimates of yield losses due to wild oat weed density have been developed for healthy, well-fertilized canola crops with good stand establishment.
Researchers at the AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. in Lacombe, Alberta developed an equation to predict canola yield losses from wild oats:
Per cent yield loss = 3.22 x (square root of X), Where X = the number of wild oat plants per square metre
An average infestation of wild oats would range from 60 to 100 plants/m 2 . A heavy infestation would range from 300 to 500 plants per square metre. If a field had 100 wild oat shoots per square metre on average, the per cent yield loss would be: Per cent yield Loss = 3.22 x (square root of 100)
= 32.2 per cent
Research by the Alberta Research Council (now Alberta Innovates) in Vegreville, Alberta has shown canola yield losses caused by green foxtail. The graph below summarizes this work.
Green foxtail can result in significant yield losses if it emerges ahead of or with the canola crop. If green foxtail density is less than 100 plants per square metre and the majority of the weeds have emerged more than one week after the crop, yield loss is expected to be insignificant. Green foxtail is a poor competitor in cooler regions unless in dense stands. Canola is an efficient competitor against green foxtail. Under low light intensity green foxtail plants grow very poorly and remain small.
Research by the Alberta Research Centre (now Alberta Innovates) in Vegreville estimated yield losses in canola due to quackgrass density. The graph below summarizes this work.
The economic threshold An economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. in canola is about 20 to 25 quackgrass shootspersquare metre, but varies with the price of canola. Quackgrass usually occurs at high densities in localized patches within a field, but can spread rapidly by its underground rhizomes. Pre harvest, post-harvest and preseed burnoffs are essential in the strategy to eliminate quackgrass populations in Western Canada.
Incorporation of soil-applied herbicides
The recommended procedures for mixing or incorporating pre-emergent herbicides into the soil are found on the herbicide label. Such procedures will usually give the desired results. Exceptions occur due to the wide range of soil and climatic conditions. Tillage and incorporation procedures that are necessary for these herbicide applications are not favourable to soil moisture conservation and soil erosion prevention.
Factors to consider when incorporating herbicides
The depth of incorporation for maximum effectiveness varies with the herbicide and the weed to be controlled. Course texture soils and soils with low organic matter may require different herbicide rates than other soils.
Uniform application and proper incorporation is essential to receive full benefit from a soil-incorporated herbicide. Loose, mellow soils are ideal for mixing the herbicide uniformly to the desired depth. Hard and lumpy soils, and those that are wet and sticky make it difficult to get uniform and proper mixing.
Dry soils will, in some cases, limit herbicide activity. An incorporation procedure that dries out the soil just before or even after seeding can result in poor emergence of the crop. Timing and method of incorporation are important where weather conditions will dry out the soil. A loose or lumpy soil surface will lose moisture in fall and spring. Harrow and pack cultivated soils that are likely to dry out after every tillage operation. Granules placed on hard, lumpy soil before incorporation can fall between the lumps and go too deep, whereas liquid attaches to the soil. Therefore, if granules are to be used, it may be necessary to pre-work the soil to get it into a condition where the granules may be mixed within the desired soil layer. Lumpy or sticky soils may require several tillage operations to get them into a good condition for proper incorporation.
Where soil erosion is not a problem, fall application is usually preferable to conserve spring seedbed moisture. On some soils, the recommended incorporation procedure for either fall or spring may leave the soil so loose, soft and fine that it will be subject to both wind and water erosion. On these soils, consider a minimum incorporation if alternate methods of weed control are available.
Harrows are very useful for shallow incorporation. Tine harrows behind cultivators contribute to better mixing, due to the lively action of the tines. Drag harrows on stubble fields may not work well for incorporation if they are constantly loaded with a lot of trash.
Tandem disc harrows, discers and any disc-type implement will provide deeper incorporation, more soil breakdown and faster and more uniform mixing to the depth of tillage than most other implements. Use these where deep incorporation is needed, where the soil is lumpy or sticky and where soil erosion is not a problem.
Tilling before applying a herbicide will produce a loose but smooth soil surface. This increases the depth and uniformity of herbicide incorporation.
Cultivators tend to give shallow incorporation and uneven mixing. Deep tillage of 10 centimetres (four inches) or more will give less soil disturbance. For good mixing, a higher travel speed of 10 to 13 kilometres per hour (eight miles per hour) is necessary. This higher speed throws soil upward and sideways to give more even incorporation. Higher speeds also help to break up lumps. Any brand of cultivator can be used, but shank spacing, speed, depth and kind of sweeps must be adjusted.
In stubble, the most effective incorporation of liquid herbicide is obtained if the field is first worked until about two-thirds of the surface is black. Granules can be applied directly to stubble before tillage, but spread the straw bunches. Weed or volunteer crop growth on fields will also absorb liquid herbicide.
Choose the incorporation procedure after a careful assessment of the field, climatic conditions and herbicide requirements.
Herbicide-resistant weeds are becoming more common across the Prairies. Kochia and cleavers are widely resistant to Group 2 herbicides. Kochia with resistance to Group 9 herbicide was detected on the Prairies in 2011. Group 1 resistant wild oats are in many fields. There are other examples. Table 4 depicts the herbicide-resistant weeds in Western Canada 4 .
Table 4. Herbicide-resistant weeds in Western Canada
|Weed||Herbicide Group||Locations Confirmed|
|Barnyard grass||Group 2||MB|
|Canada Fleabane||Group 9||Occurs in several US states|
|Canada Fleabane||Multiple Resistant: Groups 2 & 9||Occurs in Ontario|
|Cleavers||Group 2||AB, MB, SK|
|Cleavers||Multiple combinations of: Groups 2 & 4||AB|
|Chickweed||Group 2||AB, MB, SK|
|Cow Cockle||Group 2||AB|
|Downy Brome||Group 2||Occurs in Montana|
|Foxtail, green||Group 1||AB, MB, SK|
|Foxtail, green||Group 2||MB, SK|
|Foxtail, green||Group 3||AB, MB, SK|
|Foxtail, green||Multiple combinations of: Groups 1 & 3||MB, SK|
|Foxtail, yellow||Group 1 + 2||MB|
|Hemp-nettle||Group 2||AB, MB|
|Hemp-nettle||Multiple combinations of: Groups 2 & 4||AB|
|Kochia||Group 2||AB, MB, SK (overwhelming majority)|
|Kochia||Group 4 (dicamba and fluroxypyr)||SK|
|Kochia||Group 5||Occurs in North Dakota and Montana|
|Kochia||Group 9 (glyphosate)||AB, MB, SK|
|Kochia||Multiple Resistant: Groups 2 & 9||AB, MB, SK|
|Kochia||Multiple Resistant: Groups 2 & 4||SK|
|Kochia||Multiple Resistant: Groups 2, 4 (dicamba) & 9||AB|
|Kochia||Multiple Resistant: Groups 2, 4 & 9||AB|
|Lamb’s-quarters||Group 5||Occurs in Ontario|
|Marshelder (false ragweed)||Group 2||Occurs in North Dakota|
|Mustard, Ball||Group 2||AB|
|Mustard, Wild||Group 2||AB, MB, SK|
|Mustard, Wild||Group 4||MB|
|Mustard, Wild||Group 5||MB|
|Narrow-leaved hawk’s-beard||Group 2||AB|
|Persian Darnel||Group 1||AB, SK|
|Ragweed, giant||Group 2 & 9||Occurs in Ontario, Minnesota|
|Redroot pigweed||Group 2||MB, SK|
|Redroot pigweed||Group 5||Occurs in Ontario|
|Russian thistle||Group 2||AB, SK|
|Russian thistle||Group 9||Occurs in Montana|
|Shepherd’s-purse||Group 2||AB, MB, SK|
|Smartweed, pale||Group 2||MB|
|Spiny Annual Sow-thistle||Group 2||AB, MB|
|Stinkweed||Group 2||AB, MB, SK|
|Waterhemp||Group 2 & 9||Occurs in North Dakota, MB|
|Waterhemp||Group 2, 5, 9 & 14||Ontario|
|Wild buckwheat||Group 2||AB|
|Wild oat||Group 1||AB, MB, SK|
|Wild oat||Group 2||AB, MB, SK|
|Wild oat||Group 8||AB, MB, SK|
|Wild oat||Multiple combinations of: Groups 1 & 2,||AB, MB, SK|
|Wild oat||Groups 1 & 8,||AB, MB, SK|
|Wild oat||Groups 2 & 8,||AB, MB, SK|
|Wild oat||Groups 1, 2 & 8,||AB, MB, SK|
|Wild oat||Groups 1, 2, 8 & 25,||AB, MB, SK|
To reduce the risk of selecting for resistant weeds on your farm:
- Control weeds early. Herbicides are more effective on small weeds.
- Rotate herbicide groups.
- Rotate with crops that allow for a wide range of herbicide groups, and that have different seeding and harvest timing (ex. winter cereals.)
- Use tank mixes. Hitting weeds with two modes of action reduces the risk of herbicide resistant weeds escaping and setting seed.
- Use the right herbicide at the right rate and apply at the right time. Cutting rates, for example, may reduce herbicide efficacy and increase weed seed return to the soil seed bank.
- Control weeds throughout the season to reduce the weed seed bank.
- Employ other integrated weed management practices so herbicides are not the only method of weed management used on the farm.
Weather effects on herbicide performance
Herbicides in general tend to work best in warm sunny conditions when weeds are actively growing and cycling nutrients into their growing points. In these conditions, weeds will take in herbicides most efficiently.
However, ideal conditions can vary by product. For example, cloudy days don’t provide the photosynthetic activity required for many herbicides, including Group 10 glufosinate. And nights near freezing followed by days with highs that barely reach 10 C will not provide high metabolic activity required for best results from Group 9 glyphosate.
Performance is on a sliding scale. Cloudy days with highs of 10 degrees Celsius, after a night near zero degrees Celsius will tend to result in herbicide performance at the low end. Sunny days with highs of 15 degrees Celsius after a night of three degrees Celsius will provide improved control. Sunny days with highs of 20 to 25 degrees Celsius after a night of 10 degrees Celsius will provide optimum control.
Cool humid conditions are also prime conditions for herbicide injury to the crop. The leaf cuticle (waxy layer) is thinner allowing more rapid uptake of herbicide into the plant and cool conditions reduce the speed at which the herbicide is inactivated in the crop. This can lead to a flash of injury in the crop. This injury is temporary in most cases and once good growing conditions return, the crop recovers and yield loss is rare.
Talk to your local product rep to see how they will support the use of their product when applied in cool temperatures. Set expectations according to weather conditions.
This Canola Watch article has suggestions on how to manage herbicide applications in different weather conditions.
Spray drift – management
Growers have two key considerations when it comes to spraying — good coverage and drift management. A fine mist spray may provide better coverage of the target area, but this mist is also more prone to drift. Drift means a loss of efficacy, a waste of product, and perhaps most worrisome, damage to non-target areas — including neighbor’s crops, shelterbelts and yard sites.
Here are tips to reduce spray drift:
Choose a herbicide that can handle large droplets. With canola, growers often do not have much choice when it comes to herbicide. They use the product to match the herbicide tolerance system. Where possible, consider the following when making a herbicide choice:
- Glyphosate is suited to low drift (coarse droplet) sprays, but remember that at the low water volumes that favor glyphosate, coarse sprays may not provide enough droplets per square inch. A combination of coarse spray and low (but not ultra-low) water volume is best to make sure you get droplets on even the smallest weeds.
- Liberty can work with a coarse spray nozzle but it needs at least 10 gallons per acre to maintain efficacy.
- Group 2 products for the Clearfield system in general perform well with coarse sprays.
Use a low drift nozzle to produce a coarse to very coarse spray. Ideally, find a nozzle that can achieve a coarse spray at a broad range of pressures. But too coarse and coverage can be inadequate, especially at low water volumes. The image below uses water-sensitive paper to demonstrate coverage based on droplet size and water volume.
Keep the boom low. A boom at 50-60 centimetres (20-24 inches) above the target will have half the drift of a boom 90-100 centimetres (36-40 inches) above the target. Auto-boom height controllers make it possible to lower the boom and maintain the desired travel speed without having to worry about the boom tips hitting the ground.
One challenge with a lower boom is to achieve 100 per cent spray pattern overlap at the same time. Wolf prefers to see 100 per cent overlap, not the 30 per cent overlap in many recommendations, because that provides a uniform number of droplets across the whole field. Spray from one nozzle should reach to the middle of the spray pattern of the adjacent nozzle. Pick a fan angle and pressure that achieves this at a low boom height.
Aim for 100 per cent overlap nozzle to nozzle. This provides equal coverage across the whole boom width.
Know what’s down wind and what harms it. Canola is sensitive to drift. Consider your neighbor’s Roundup Ready canola when you’re spraying Liberty on InVigor canola, for example.
Let the weather help. Calm early mornings can actually increase spray drift damage. The spray can hang in the air, making it impossible to predict when and where it will settle. This dense cloud can do a lot of damage to a non-target field or yard site. Bright sunny days with some wind are ideal times to spray to minimize drift damage. In these conditions, thermal turbulence tends to move any spray that does hang in the air up and down instead of laterally along the ground. Anything that goes up is rapidly diluted in the atmosphere.
Do headlands and around yard sites first. Get these areas done first so that if the wind picks up you have a buffer area already created. When following unusual patterns, use GPS to track what areas have been sprayed.
Talk to people who might be affected. When spraying on days that are not ideal, talk to neighbors who might be affected by spray drift. Damaging a sensitive crop or a relationship is probably not worth the risk.
Better herbicide performance – tips
Here are ways to get the most from a herbicide:
Spray when weeds are actively growing. Herbicide activity is usually reduced when applied to weeds subjected to cold and/or moisture stress.
Follow label instructions. Specifically, apply the herbicide at the growth stage most susceptible to target weeds. Apply the recommended rate of herbicide. Use the water volume stated on the label. Use the wetting agent included or recommended. Do not add extra wetting agent unless indicated on the product label. Apply at weather conditions best suited to individual herbicides. (For more on this, read the section “Weather effect on herbicides performance”.)
Timing is more important than the method of application. AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. research from Scott, Saskatchewan showed that spraying 7 days after crop emergence generated higher yields than spraying 17 days after emergence, no matter the droplet size. Finer spray may be more efficacious than coarse spray, but finding a suitable day to apply a fine spray without significant drift can be difficult. When it comes to crop yield, the research found that using a low drift nozzle early to get rid of weed competition is better than waiting longer for a relatively calm day to use a finer spray. Hire a plane if necessary to achieve the earlier spray timing.
If the coarse nozzle means you can spray sooner because the risk of drift is too high for a fine spray, then the coarse spray has a clear advantage.
Do not spray when wind velocity exceeds 15 kilometres per hour (nine miles per hour), or use a wind shroud on the sprayer.
Choose a nozzle that produces a coarse spray in the middle of its pressure range. A nozzle that produces a coarse spray through the middle of its pressure range provides the most flexibility to change spray pattern and droplet size in response to changing ground speed and pressures. For example, a nozzle that produces coarse spray at 40 to 80 psi offers more flexibility to slow down or speed up and still achieve a coarse spray. The charts shown below provide examples. The table below shows coverage based on nozzle spray type and pressure in gallons per acre (gpa).
Know the nozzle’s pressure range and choose the middle of it. This expands on the previous tip. When travel speed increases, spray pressure must increase considerably more to maintain the spray rate per acre. For example, to increase speed by 10 per cent, pressure may have to increase by 25 per cent. Droplet size shrinks and drift potential increases with higher pressure. Know what speed range a nozzle provides to achieve coarse sprays. This range may be fairly narrow for high pressure nozzles. For low pressure nozzles, the speed range tends to be a lot wider.
Higher ground speed is not the only way to increase productivity. Speeding up increases the amount of acres covered in a day, but it also causes greater pressure fluctuations to maintain spray rates over a range of speeds. Travel slower to reduce the amount of fine droplets, and make it safer to drive with the boom lower. The pressure gauge can be used as a speedometer.
Consider other methods besides higher speeds to increase productivity.
- Larger tanks to cover more acres between fills. Keep in mind that larger tanks add weight. Heavier units cannot always get through a field in soft conditions where a lighter more nimble unit can. If a heavier higher horsepower unit can get through a soft field, it often leaves ruts.
- Use a wider boom. With automatic boom height control, wider booms are easier to handle. Going from a 90 to 120 foot boom increases productivity significantly. Make sure the pump can achieve consistent pressure across the whole width.
- Choose a sprayer with a more convenient loading area and with a high speed pump. This will reduce fill time.
Select water volume and spray quality based on target weeds and herbicide characteristics.
- Grassy weeds are the most challenging target because they have small and vertical leaves that are difficult to wet. Avoid very coarse sprays on grassy weeds. Grassy weeds also tend to require higher water volumes.
- Broadleaf weeds will tolerate larger droplets.
- Contact products need better leaf coverage than systemic products.
- Test the wettability of weeds. Pour some water on the weed. A weed that gets wet and stays wet is unlikely to present a droplet size issue. Coarse sprays will usually provide good efficacy. But if the water beads and runs off, use higher water volume and do not use a very coarse spray. Cleavers, lamb’s-quarters and kochia are weeds that are hard to wet.
- Canopy density affects performance. If a spray needs to get through a dense canopy to hit low targets, use a higher water volume and slower travel speed.
- Avoid the combination of extra coarse droplet size and low water volumes. This will not provide the droplets per square inch required to hit every weed.
Use high quality water. Water containing large amounts of organic matter, clay, and other particles will result in nozzle plugging and reduced weed control.
Double nozzles (with forward and backward facing jets) may be slightly oversold for weed control. Double nozzles are more suited to fungicide application on a vertical target, such as a wheat head.
But in studying double nozzles, AAFC Agriculture and Agri-Food Canada is a department of the Government of Canada. researchers found that angled tips tend to provide better coverage, especially for low drift nozzles at low boom heights. In that case, a tip that angles spray forward can improve spray efficacy. With a high boom, angling does not provide the same benefit.
Not all weeds can be controlled with large droplets. A good test to see how well weeds will retain a herbicide spray is to shower them with water and see how wet they get. If the water sticks, they should retain a coarse spray fairly easily. If the water rolls off and the plant stays dry, these weeds may be harder to control with larger droplets. Cleavers, kochia, lamb’s-quarters, and volunteer canola are examples of hard-to-wet weeds. As they get larger and their waxy cuticle prevents herbicide uptake.
For these weeds:
- Do not use very coarse droplets. Dial down to coarse or even medium droplets if it’s warm, sunny and not too windy.
- Use higher water volumes. With Liberty, use at least 10 gallons per acre. Glyphosate tends to work better with lower volumes, but don’t have the volume so low that there aren’t enough droplets to hit all small weeds.
- Contact products need better leaf coverage than systemic products. Higher water volume and smaller droplets are important for contact products.
Cleaning the sprayer tank – tips
Properly cleaning a sprayer tank and boom protects a sensitive crop. It protects people working with the sprayer. It protects the sprayer and its components. Here are practical tips:
Be prompt and thorough. Remove pesticide from mixing and spray equipment immediately after spraying – it makes the job easier. The main areas of concern are the tank wall, sump, plumbing, and filters. First, spray the tank completely empty while still in the field. It’s okay to cover previously sprayed areas – all herbicides must be crop-safe at twice the label rate to be registered by the PMRA. Reduce the rate to be certain. Second, add 10x the sump’s remnant of clean water, circulate, and spray it out in the field as well. Repeat. These two rinsing steps will take care of the majority of the cleaning and won’t take very long. Having a clean water tank on the sprayer and a wash-down nozzle makes this job easier.
Do a visual inspection. Herbicide residue may precipitate out of solution in some parts of the sprayer or plumbing. A thorough visual inspection can identify these problem areas and ensure that they are cleaned properly.
Clean the tank wall. Removal of residues from tank walls is best accomplished with a direct, pressurized spray. Make sure all parts of the wall have been in contact with clean water. Use a wash-down nozzle if it provides complete and vigorous coverage of the interior tank surface.
Empty the sump as completely as possible by spraying it out. Any spray liquid or herbicide concentrate remaining in the sump area will be re-circulated in the sprayer. In this case, the only way to remove the remaining herbicide is through dilution by repeatedly adding water and each time draining the sump as much as possible.
Plumbing can be a significant reservoir of herbicide residue. To remove residue from plumbing, pump clean water through the boom while ensuring that all return and agitation lines also receive clean water and all residue is flushed out. This may require opening and closing various valves several times, and repeating the process with new batches of clean water.
Use several small washes. The most effective use of a volume of rinse water is to divide it equally across several repeat washes. For example, a single 600 gal wash is as effective as two washes with 70 gallons each, and three with 30 gal each, assuming a 10 gallon sump remainder. More wash cycles allow for less water in total.
Clean filters. Nozzle screens and in-line filters can be a significant reservoir for undiluted or undissolved herbicide and are one of the most overlooked parts of sprayer decontamination. Remove all filters and nozzle screens and thoroughly clean these with fresh water. Run clean water through plumbing leading to the screens.
Clean all nozzles in a nozzle body. When cleaning a spray boom, rotate through all nozzles in a multiple-nozzle body to ensure clean water reaches all parts of these assemblies. Remove screens that may have been used with herbicide, even if just for a short while.
Use tank cleaning adjuvants. Adjuvants such as ammonia can assist the tank decontamination process. Ammonia does not neutralize herbicides, but it does raise the pH of the cleaning solution which helps sulfonylurea herbicides dissolve. When decontaminating after use of an oily (EC) formulation, the use of a wetting agent such as AgSurf will assist in removing oily residue that may trap SU herbicide on tank and hose material. Commercial cleaners are available.
Dispose rinsate A liquid mixture of pesticides diluted by water and cleaning agents produced from cleaning pesticides application equipment or pesticides containers after use. carefully. Ideally, finish with an empty tank, having sprayed it all on the field. Do not drain the tank while stationary unless you are certain it is free of pesticide and that you are away from sensitive areas and waterways.
A new area of research that addresses the issue of rinsate A liquid mixture of pesticides diluted by water and cleaning agents produced from cleaning pesticides application equipment or pesticides containers after use. disposal is biobeds. Biobeds contain a mixture of soil, compost, and straw to host the microbes that degrade pesticides Pesticides (herbicides, insecticides or fungicides) used to protect against or reduce the amount of damage caused by weeds, pest insects or plant diseases. . Initial results show that placing dilute pesticide waste into biobeds breaks them down more quickly and prevents them from reaching ground waters more effectively than placing them onto soil.
For more on sprayer tank clean out, check out the Sprayer tank clean out tips Canola Watch article and see the Introduction section of the Guide to crop protection or Guide to field crop protection.