Research Article |
Corresponding author: Claire E. Wilson ( claire.wilson@inspection.gc.ca ) Academic editor: Curtis Daehler
© 2016 Claire E. Wilson, Karen L. Castro, Graham B. Thurston, Andrea Sissons.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Wilson CE, Castro KL, Thurston GB, Sissons A (2016) Pathway risk analysis of weed seeds in imported grain: A Canadian perspective. In: Daehler CC, van Kleunen M, Pyšek P, Richardson DM (Eds) Proceedings of 13th International EMAPi conference, Waikoloa, Hawaii. NeoBiota 30: 49–74. https://doi.org/10.3897/neobiota.30.7502
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The risk of introducing weeds to new areas through grain (cereals, oilseeds and pulses) intended for processing or consumption is typically considered less than that from seed or plants for planting. However, within the range of end uses for grain, weed risk varies significantly and should not be ignored. In this paper, we discuss pathway risk analysis as a framework to examine the association of weed seeds with grain commodities throughout the production process from field to final end use, and present inspection sampling data for grain crops commonly imported to Canada. In the field, weed seed contamination of grain crops is affected by factors such as country of origin, climate, biogeography and production and harvesting practices. As it moves toward export, grain is typically cleaned at a series of elevators and the effectiveness and degree of cleaning are influenced by grain size, shape and density as well as by grade requirements. In cases where different grain lots are blended, uncertainty may be introduced with respect to the species and numbers of weed seed contaminants. During transport and storage, accidental spills and cross-contamination among conveyances may occur. At the point of import to Canada, inspection sampling data show that grain shipments contain a variety of contaminants including seeds of regulated weeds and species that represent new introductions. However, grain cleaning and processing methods tailored to end use at destination also affect the presence and viability of weed seeds. For example, grains that are milled or crushed for human use present a lower risk of introducing weed seeds to new environments than grains that undergo minimal or no processing for livestock feed, or screenings that are produced as a by-product of grain cleaning. Pathway risk analysis allows each of these stages to be evaluated in order to characterize the overall risk of introducing weeds with particular commodities, and guide regulatory decisions about trade and plant health.
Pathway risk analysis, pest risk assessment, weed seeds, contaminants, grain, imports, screenings, Canada
Internationally traded grain commodities are recognized as a pathway for the introduction of weed seeds into new areas (
Regulating the spread of weeds via this pathway is the responsibility of individual countries under the guidelines of the International Plant Protection Convention (IPPC), and many countries have legislation and import requirements that mitigate the risk of introducing new weed species to some degree. However, according to the principles of the IPPC, regulations must be based on risk analysis and characterizing the risk associated with complex pathways such as this one remains a challenge. International standards for pest risk analysis are well developed for addressing individual species in terms of the likelihood they will enter, establish and spread in a new area, and the impacts they may have (
More recently, a pathways approach to pest risk analysis has been proposed (
In this paper we discuss the association of weed seeds with imported grain from point of origin to end use at destination, and provide a qualitative description of the pathway that can be used as a framework for pathway risk analysis. We identify six points, or events, along the pathway that have relevance for weed risk, namely: crop-weed associations at the point of origin; farming practices; grain handling practices; transport and storage; import requirements; and end use of grain in the country of destination (Canada) (Figure
Conceptual diagram of imported grain as a pathway for the introduction of weed seeds. Six points, or events, along the pathway that have relevance for weed risk are illustrated from left to right along a timeline from point of origin to end use at destination. Factors that increase the risk of introducing new weed species to Canada are shown in red boxes, while factors that decrease the risk are shown in green boxes.
Canadian grain imports 2010–2015. Average annual imports in MT/yr for ten grain crops or crop groupings most commonly imported to Canada, and top five countries of origin for each crop given as % total imports over the period 2010–2015. Note that import data includes seed for planting in addition to grain but this typically makes up a very small proportion (<5%) of the total for each crop (data from:
Crop | Scientific name | MT/yr | Top five countries of origin (% total imports) | ||||
---|---|---|---|---|---|---|---|
Corn | Zea mays subsp. mays | 1,065,056 | United States (97.5%) | India (0.8%) | Romania (0.5%) | Russian Fed. (0.4%) | Chile (0.2%) |
Rice | Oryza sativa L. | 385,065 | United States (60.7%) | Thailand (22.0%) | India (9.6%) | Pakistan (2.5%) | Italy (1.2%) |
Soybean | Glycine max (L.) Merr. | 282,962 | United States (85.0%) | India (10.4%) | China (1.9%) | Ukraine (0.7%) | Turkey (0.7%) |
Cereals Wheat Oats Barley |
Triticum aestivum L. Avena sativa L. Hordeum vulgare subsp. vulgare |
142,315 | United States (76.6%) | Denmark (11.7%) | Romania (4.6%) | Sweden (1.4%) | Rep. of Ireland (1.3%) |
Pulses Beans Peas Chickpeas Lentils |
Phaseolus spp., Vigna spp., Vicia spp. Pisum sativum L. Cicer arietinum L. Lens culinaris Medik. |
125,691 | United States (75.6%) | Australia (4.8%) | China (4.1%) | Thailand (2.8%) | India (2.2%) |
Canola | Brassica napus L., B. rapa L., B. juncea (L.) Czern. | 116,781 | United States (94.7%) | Chile (5.0%) | Australia (0.1%) | Ukraine (0.1%) | Uruguay (0.01%) |
Sunflower | Helianthus annuus L. | 28,495 | United States (86.6%) | Bulgaria (5.5%) | China (3.4%) | Argentina (2.9%) | Italy (0.5%) |
Flax | Linum usitatissimum L. | 10,575 | United States (74.7%) | Argentina (7.1%) | Russian Fed. (5.5%) | Kazakhstan (2.8%) | India (2.7%) |
Millet Proso millet Foxtail millet Japanese millet Pearl millet |
Panicum miliaceum subsp. miliaceum Setaria italica subsp. italica Echinochloa frumentacea Link, E. esculenta (A. Braun) H. Scholz Pennisetum glaucum (L.) R. Br. |
10,144 | United States (82.7%) | Ukraine (6.3%) | China (3.1%) | Russian Fed. (2.3%) | India (1.8%) |
Sorghum | Sorghum bicolor subsp. bicolor | 5,114 | United States (93.8%) | India (2.5%) | Argentina (1.5%) | Ethiopia (1.3%) | China (0.5%) |
Total (all crops) | 2,172,198 |
The pathway for weed seed dispersal in grain begins in the field where the crop is grown in the country of origin. The majority of Canadian grain is imported from the U.S., although significant amounts are also brought in from other countries, and trade patterns frequently shift to meet market demands (
The risk of introducing new weed species to Canada depends not only on the number of weed seeds contaminating imported grain, but on the particular species assemblages present, and the likelihood they will end up in suitable environments for establishment and spread. Many contaminants moving in the international grain trade may be common weeds already present in Canada, and thus do not present a risk of new species introductions. Others may be weeds from tropical climates unlikely to survive through Canadian winters, or weeds associated with crops not widely grown in Canada (e.g., rice). At a broad scale, information about the point of origin allows for generalizations about risk. For example, the risk of new species introductions is generally considered lower from countries with similar weed floras (i.e., fewer new species) or different climates (i.e., species less likely to survive), and higher from countries with different weed floras and similar climates. At this stage there is also the opportunity to determine whether particular weed species of concern (e.g., regulated species) occur in the area of origin. The level of risk will vary for each crop/country combination proposed for importation, and the more detailed the information about point of origin (e.g., state, county), the more specific the analysis can be. However, it should be noted that the value of a very detailed analysis at this stage may be compromised by industry practices further along the pathway, for example blending of grain lots from different origins (see Grain handling, below).
At smaller scales, crop production practices can also impact the diversity and prevalence of weeds at the field level and at harvest.
Crop production: Prior to planting, factors such as previous land use, crop rotation, pre-planting tillage, herbicide application, seed bank composition and crop seed purity can play a role in characterizing a field’s weed flora for a particular year (
Some crops and crop cultivars are inherently more competitive than others. Crop competitive ability varies from region to region, but a general ranking puts cereals first, followed by canola and then pulses (
Chemical weed control options also vary by crop. In general, broadleaved weeds are easier to control in cereals and other monocot crops, while grass weeds are easier to control in broadleaved crops. For some crops, such as flax and pulses, herbicide options tend to be more limited than those for others, such as cereal grains or corn (
In the case of organically grown crops, a variety of non-chemical weed control options, such as mechanical and thermal methods, mulching and intercropping, may be employed to keep weeds in check (
Harvest: At harvest, critical factors contributing to weed contamination levels include timing, weather conditions, crop vs. weed height, weed maturity and combine settings (
The action of the conventional combine includes reaping, threshing (separating the grain from the husks) and winnowing (blowing off fines and other foreign material). Weed seeds that have a pappus are easily dislodged and dispersed at harvest time and are more readily eliminated during the cleaning process (
Overall, knowledge of crop production and harvesting practices can be helpful for considering their effect on grain contamination at source. Although weed levels and species complexes vary from farm to farm, with different agronomic, harvesting and cleaning practices, generalizations can be made based on the information available and applied to the evaluation of risk. For example, crops that are typically more competitive, treated with herbicides, harvested at a greater height or have large seeds might be expected to harbour less weed seed contaminants (lower risk) than crops that are less competitive, grown organically, harvested close to the ground, or that have small seeds that are difficult to separate from weed seeds (higher risk). This information can be combined with that collected about point of origin to develop a more refined picture of the species and levels of weed contamination that might be expected with a particular grain crop after harvest.
From the farm, harvested grain typically moves through a series of elevators on its way to export, where it is cleaned and graded to determine its market value.
Cleaning: Cleaning removes dockage, which is material that can readily be removed from grain prior to grading, such as stones, straw, chaff, broken grains, contaminant seeds, dust and hulls (
Grading: The extent to which grain is cleaned is typically determined by grade requirements to meet government regulations, export standards or contract conditions. Numerical grades are a measure of grain quality and cleanliness and help determine the value of grain on the market (
The percentage of FM allowed in a grade can be an indicator of the level of contamination with weed seeds. For example, U.S. No. 1 grade soybeans must contain no more than 1% FM by weight, U.S. No. 2 grade no more than 2%, U.S. No. 3 grade no more than 3%, and so on (
Blending: In commercial trading, the quality of grain in demand fluctuates with changing markets and intended uses. Producers, handlers and exporters must balance the costs of cleaning grain against the value it will have on the market. In some cases there may be an incentive for producers or exporters to clean grain to the highest grade or value; however, in many cases there may be market demand for lower quality grain and the incentive is to clean only to the targeted level of the grade or contract (
Overall, the variation in composition of FM and the practice of blending are significant sources of uncertainty with respect to the potential numbers and species of weed seeds found in grain. Blending of grain lots from different origins with distinct weed floras has the potential to greatly increase the number of weed species in the resultant lot. Unfortunately, information on whether or not a particular grain lot has been blended and the origins of the original grain lots is very difficult, often impossible, to obtain.
Transport and storage of grain at every stage along the pathway introduces the possibility of cross-contamination and spills. The pathway may be simple or complex in terms of the number of transfers and conveyances prior to arrival at destination. From the point of origin, grain may be moved by truck, rail car and/or ship as it moves towards export and final destination, and may be unloaded and reloaded at a series of intermediate elevators and storage facilities along the way. Each step contributes to uncertainty with respect to the potential for cross-contamination and the risk of spillage post-import.
Cross-contamination: Ideally, good sanitation requires the thorough cleanout of all grain harvesting, transporting, and handling equipment between loads (
Accidental spills: Accidental spills are also an unfortunate reality of the grain handling system, as evidenced by the weed and volunteer grain flora along railway tracks, roadsides, ports and around mills and other grain processing facilities (
An example of a grain spill on a grand scale is that of a Malaysian cargo ship that went aground in Alaska in 2004, spilling most of a shipment of over 60,000 tons of U.S. No. 2 grade yellow soybeans produced in North Dakota and destined for processing and human consumption in China (
As with grain cleaning and blending, the possibility of cross-contamination of conveyances and spills during the transport and storage of grain illustrates the complexity of the pathway and introduces a significant element of uncertainty with respect to the species of weed seeds that might be found in imported grain.
Import requirements are an important means by which countries can reduce the risk of introducing new pests and protect their domestic industries and environments. Currently, all grain imported to Canada is expected to arrive free of soil and regulated pests, and a range of different requirements (e.g., import permits, phytosanitary certificates, treatment certificates) exist for particular crops and countries of origin (
Plants currently regulated as quarantine (i.e., prohibited) pests under Canada’s Plant Protection Act (
Scientific name | Common name |
---|---|
Aegilops cylindrica Host | Jointed goatgrass |
Alopecurus myosuroides Huds. | Slender foxtail |
Centaurea iberica Trevir. ex. Spreng. | Iberian starthistle |
Centaurea solstitialis L. | Yellow starthistle |
Crupina vulgaris Cass. | Common crupina |
Cuscuta spp.(except native species) | Dodder |
Dioscorea polystachya Turcz. | Chinese yam |
Echium plantagineum L. | Paterson’s curse |
Eriochloa villosa (Thunb.) Kunth | Woolly cup grass |
Microstegium vimineum (Trin.) A. Camus | Japanese stiltgrass |
Nassella trichotoma (Nees) Hack. ex. Arechav. | Serrated tussock |
Orobanche spp. and Phelipanche spp. (except native species) | Broomrape |
Paspalum dilatatum Poir. | Dallis grass |
Persicaria perfoliata (L.) H. Gross | Devil’s-tail tearthumb |
Pueraria montana (Lour.) Merr. | Kudzu |
Senecio inaequidens DC. | South African ragwort |
Senecio madagascariensis Poir. | Madagascar ragwort |
Solanum elaeagnifolium Cav. | Silverleaf nightshade |
Striga spp. | Witchweeds |
Zygophyllum fabago L. | Syrian bean-caper |
Inspection sampling data: Compliance with import requirements is monitored through inspection and sampling at the point of import. During the period 2007–2015 an import sampling program focussed on weed seeds in grain was initiated to monitor for regulated species and to gather information about contaminants moving in imported grain. In total, 947 samples were taken from imported shipments of the 10 grain commodities most commonly imported to Canada (see Introduction), and analyzed for presence of weed seeds (Table
Data from a Canadian sampling program showing weed seed contaminant species reported in imported grain 2007–2015. Crop species are provided in Table
Imported grain | Samples | Range of contaminant species reported per sample | Total number of unique contaminant species reported in all samples | ||||
---|---|---|---|---|---|---|---|
Size (kg) | n | (#) | Other Crops (#) | Common Weeds (#) | New species (#) | Total (#) | |
Corn | 1.0 | 198 | 0–22 | 29 | 74 | 7 | 110 |
Rice | 0.5 | 11 | 0–12 | 5 | 18 | 4 | 27 |
Soybean | 1.0 | 70 | 0–36 | 35 | 99 | 30 | 164 |
Cereals | 1.0 | 223 | 0–35 | 55 | 188 | 24 | 267 |
Pulses | 1.0 | 251 | 0–36 | 36 | 120 | 4 | 160 |
Canola | 0.5 | 52 | 0–18 | 18 | 57 | 3 | 78 |
Sunflower | 1.0 | 42 | 0–24 | 22 | 45 | 0 | 67 |
Flax | 0.5 | 7 | 0–13 | 5 | 21 | 3 | 29 |
Millet | 0.5 | 69 | 0–18 | 17 | 42 | 3 | 62 |
Sorghum | 0.5 | 24 | 0–16 | 12 | 21 | 1 | 34 |
Total | 947 | 0–36 | 84 | 288 | 66 | 438 |
Overall, 438 different contaminant taxa were reported in the samples analyzed, including 84 crops present as volunteer weeds or commodity handling contaminants, 288 common weeds already present in Canada, and 66 species which are absent from Canada or very locally introduced (i.e., less than 5 individual locations reported in less than 3 provinces), representing possible new introductions. A number of contaminants were only identified to genus and a few to family; for convenience they are referred to as ‘species’ from here on. The complete list of contaminants cross-referenced to the crops they were found in is included in Suppl. material
The number of contaminant species per sample ranged from 0 for all crops to between 12 (rice) and 36 (soybean and pulses) (Table
The 20 most frequently reported contaminant species for all crops combined are shown in Table
Top 20 most frequently reported contaminant species in imported grain crops examined in a Canadian sampling program 2007–2015. #Reports (%) indicates the number of samples a species was reported in of a possible 947 with percentages in parentheses, and #Crops indicates the number of crops it was reported in, of a possible 10.
Name of Contaminant | Common name | # Reports (%) | # Crops |
---|---|---|---|
Chenopodium album L. | Lamb’s-quarters | 356 (38%) | 10 |
Fallopia convolvulus (L.) Á. Löve | Wild buckwheat | 306 (32 %) | 9 |
Amaranthus retroflexus L. | Redroot pigweed | 287 (30%) | 9 |
Setaria italica subsp. viridis (L.) Thell. | Green foxtail | 262 (28 %) | 9 |
Avena fatua L. | Wild oat | 241 (25 %) | 9 |
Triticum aestivum L. | Wheat | 229 (24 %) | 9 |
Bassia scoparia (L.) A. J. Scott | Kochia | 222 (23 %) | 9 |
Thlaspi arvense L. | Stinkweed | 198 (21 %) | 8 |
Brassica napus subsp. napus | Canola or rapeseed | 190 (20%) | 8 |
Echinochloa crus-galli (L.) P. Beauv. | Barnyard grass | 177 (19%) | 10 |
Sinapis arvensis L. | Wild mustard | 143 (15 %) | 8 |
Setaria pumila subsp. pumila | Yellow foxtail | 127 (13 %) | 9 |
Bromus tectorum L. | Downy brome | 122 (13 %) | 4 |
Hordeum vulgare subsp. vulgare | Barley | 111 (12 %) | 7 |
Descurainia sophia (L.) Webb ex Prantl | Flixweed | 103 (11 %) | 6 |
Helianthus annuus L. | Sunflower | 103 (11 %) | 8 |
Persicaria lapathifolia (L.) Delarbre | Pale smartweed | 90 (10%) | 10 |
Salsola tragus L. | Russian thistle | 83 (9 %) | 8 |
Cirsium arvense (L.) Scop. | Canada thistle | 82 (9 %) | 5 |
Avena sativa L. | Oats | 79 (8 %) | 8 |
The 66 species that represent potential new weed introductions to Canada are shown in Table
Contaminants that represent potential new weed species introductions to Canada, reported in imported grain crops examined in a Canadian sampling program 2007–2015. #Reports indicates the number of samples a species was reported in of a possible 947, and #Crops indicates the number of crops it was reported in, of a possible 10.
Name of contaminant | #Reports | # Crops | Name of contaminant | #Reports | # Crops |
---|---|---|---|---|---|
Aegilops cylindrica Host | 54 | 2 | Anchusa azurea Mill. | 1 | 1 |
Rumex maritimus L. | 22 | 3 | Anoda spp. | 1 | 1 |
Cuscuta spp. | 10 | 4 | Blainvillea acmella (L.) Philipson | 1 | 1 |
Commelina benghalensis L. | 7 | 1 | Bromus sterilis L. | 1 | 1 |
Digera muricata (L.) Mart. | 5 | 1 | Codonopsis spp. | 1 | 1 |
Phaseolus spp. (except crops) | 5 | 1 | Crambe spp. | 1 | 1 |
Rapistrum rugosum (L.) All. | 5 | 1 | Cyanotis axillaris (L.) D. Don | 1 | 1 |
Euphorbia heterophylla L. | 4 | 1 | Cynodon dactylon (L.) Pers. | 1 | 1 |
Apera spica-venti (L.) P. Beauv. | 3 | 1 | Dactyloctenium aegyptium (L.) Willd. | 1 | 1 |
Consolida regalis Gray | 3 | 1 | Gaillardia megapotamica (Spreng.) Baker | 1 | 1 |
Digitaria ciliaris (Retz.) Koeler | 3 | 2 | Galium tricornutum Dandy | 1 | 1 |
Dinebra retroflexa (Vahl) Panz. | 3 | 1 | Ipomoea hederacea Jacq. | 1 | 1 |
Eleusine indica (L.) Gaertn. | 3 | 1 | Ipomoea lacunosa L. | 1 | 1 |
Hirschfeldia incana (L.) Lagr.-Foss. | 3 | 1 | Lepyrodiclis holosteoides (C. A. Mey.) Fenzl ex Fisch. & C. A. Mey. | 1 | 1 |
Alisma plantago-aquatica L. | 2 | 2 | Pedaliaceae spp. | 1 | 1 |
Bromus arvensis L. | 2 | 1 | Pennisetum spp. | 1 | 1 |
Bromus catharticus var. catharticus | 2 | 1 | Perilla frutescens (L.) Britton | 1 | 1 |
Celosia argentea L. | 2 | 2 | Persicaria nepalensis (Meisn.) H. Gross | 1 | 1 |
Corchorus olitorius L. | 2 | 1 | Phyllanthus spp. | 1 | 1 |
Cucumis spp. (except crops) | 2 | 1 | Rapistrum perenne (L.) All. | 1 | 1 |
Euphorbia davidii Subils | 2 | 1 | Rapistrum spp. | 1 | 1 |
Glaucium corniculatum (L.) Rudolph | 2 | 1 | Reseda odorata L. | 1 | 1 |
Nicandra physalodes (L.) Gaertn. | 2 | 2 | Rorippa islandica (Oeder) Borbás | 1 | 1 |
Panicum psilopodium Trin. | 2 | 2 | Salvia hispanica L. | 1 | |
Phyllanthus urinaria L. | 2 | 1 | Sesbania exaltata (Raf.) Rydb. | 1 | 1 |
Rottboellia cochinchinensis (Lour.) Clayton | 2 | 1 | Setaria pumila subsp. subtesselata (Büse) B. K. Simon | 1 | 1 |
Salvia columbariae Benth. | 2 | 2 | Sida spinosa L. | 1 | 1 |
Schoenoplectiella mucronata (L.) J. Jung & H. K. Choi | 2 | 1 | Sisymbrium orientale L. | 1 | 1 |
Sida rhombifolia L. | 2 | 2 | Spermacoce spp. | 1 | 1 |
Urochloa fusca (Sw.) B. F. Hansen & Wunderlin | 2 | 2 | Stachys annua (L.) L. | 1 | 1 |
Achyranthes aspera L. | 1 | 1 | Trifolium reflexum L. | 1 | 1 |
Alternanthera ficoidea (L.) P. Beauv. | 1 | 1 | Verbena officinalis L. | 1 | 1 |
Amaranthus caudatus L. | 1 | 1 | Veronica hederifolia L. | 1 | 1 |
Overall these results are similar to other studies which have reported large numbers of contaminant weed species in imported grain (
Grain commodities imported to Canada are used for human and animal food as well as industrial products. Wheat, rice, pulses, soybean, canola, sunflower and flax grain are primarily used for human food products in Canada, while corn, barley, oats and sorghum grain are mainly used for livestock feed, and millet grain for bird feed (
Human and industrial uses: Grain for human consumption or industrial uses is typically cleaned to a very high standard. Beyond the cleaning undertaken to meet grade or contract specifications prior to export, imported grain for human food or industrial end uses typically undergoes further cleaning in order to ensure quality and consistency of the resultant products (
In Canada, many imported grain commodities are used as livestock feed (
Livestock feed that is processed can undergo a number of transformative processes including particle size reduction by grinding or rolling with a hammer or roller mill, conditioning, pelleting and extrusion (
End use processing can clearly mitigate the risk of weed seed introduction in many cases, and is an important consideration in a pathway risk analysis for imported grain. Grain subject to cleaning and processing for human consumption and industrial uses presents a low risk of introducing weeds into new environments, as weed seeds are either removed during cleaning or devitalized during processing. In contrast, livestock and bird feeds subject to minimal processing represent a higher risk for the transmission of viable weed seeds. It is expected that the greater the degree of processing, the less likely the feed will contain viable weed seeds.
Screenings as a by-product of grain cleaning: Grain screenings represent a high risk relative to the grain they originate from, because they represent a concentration of the non-grain fraction that includes weed seeds and other material that remains after the grain has been cleaned. In Canada, grain screenings are most frequently used as components in livestock feed. The raw screenings are often processed by grinding and pelleting to reduce problems with feeding and handling. One study in Saskatchewan indicated that weed seed viability was almost completely destroyed in grain screenings that had been ground and steam pelleted and/or treated with ammonia (
However, screenings that are unprocessed or ground but not further processed present a potential risk for the introduction of weed seeds to farm properties and elsewhere. Studies have shown that sheep and steers fed unprocessed grain screenings had viable weed seeds in their manure (
Of all the end uses of grains, unprocessed or minimally processed screenings present the highest risk for containing viable weed seeds, and potentially large numbers of them. The weeds seeds in screenings can be unintentionally spilled in a variety of environments conducive to germination, including areas around mills, bins and farm properties, or be fed to livestock and dispersed into pastures. To address the risk posed by imported, unprocessed screenings and grain for cleaning (which generates screenings), import requirements have been established in Canada (
In summary, imported grains represent a very complex pathway for the possible introduction of new weed species to Canada. Weed-crop associations at the point of origin, along with crop production and harvesting practices, can be researched to develop predictions of what weed species might be associated with which imports; however, subsequent steps along the pathway such as grain cleaning, blending, and the potential for cross-contamination in transport and storage mean the weeds found in import sampling programs are not always the ones that might be expected. Import interception data presented here shows that all imported grain commodities sampled were a source of associated weed contaminants, however information about end use indicates that grain destined for human food or industrial purposes in Canada likely presents a negligible risk of introducing new weeds into the environment, due to extensive cleaning and processing at destination. Further research on the effects of specific processes on weed seed viability would be useful to confirm this. However, the greater risk lies with imported grain that is direct-fed or minimally processed for livestock feed, and the fate of dockage or screenings that are removed from grain during the cleaning process.
The pathway risk analysis approach provides a useful framework for characterizing the nature of a pathway, identifying events that affect pest risk, and highlighting possibilities for risk reduction or mitigation. In this case, a qualitative description of the pathway from point of origin to end use at destination provides a better understanding of the multiple interacting factors that may affect weed seed contamination in grain imports, and this may help to focus plant protection efforts in future. For example, future risk analyses on specific grain commodities may call for less focus on the analysis of crop-weed associations at the point of origin and production and harvesting practices and more focus on end use. Likewise, risk mitigation efforts might be most usefully focused on grain used for livestock feed and management of screenings, as compared to grain for human consumption or industrial purposes which present little risk of introducing new weeds to the environment.
We would like to acknowledge our retired colleague Ken Allison who co-authored a number of earlier risk assessments which informed the discussion presented here. We are grateful to the analysts in the Canadian Food Inspection Agency’s Saskatoon Laboratory Seeds Science and Technology Section who provided test results and species identification, and to Steve Jones, Ruojing Wang, Leanne Duncan, and others for help extracting and interpreting the inspection sampling data. We thank John Pauch and Chris Beckman at Agriculture and Agri-Food Canada for providing import data. We also thank a number of colleagues who provided valuable input and comments on drafts of the manuscript including Wendy Asbil, Stephen Darbyshire, Sarah Davis, Robert Favrin, Steve Jones, Michael Mander, Jason Murphy, Kristina Pauk and Ruojing Wang.
Weed seed contaminant species reported in imported grain in a Canadian sampling program 2007–2015
Data type: Species list and tabular occurrence data
Explanation note: Complete list of weed seed contaminant species reported in 947 samples of 10 imported grain crops in a Canadian sampling program 2007–2015, cross-listed to number of times reported and crops reported in.
Frequency distributions showing percentage samples with number of contaminant species reported per sample for 10 imported grain crops examined in a Canadian sampling program 2007–2015
Data type: Frequency distribution graphs
Explanation note: Ten frequency distribution graphs (one per crop) shown in a multi-panel.