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Herbicide Resistance Management Issues In Plants With Novel Traits (PNT’s)

A Discussion Paper Presented to
The Canadian Food Inspection Agency, Plant Biosafety Office
(26/02/2002)

Table of Contents

Introduction

This paper presents a discussion to the Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biotechnology Office on issues related to the use of herbicides on herbicide resistant plants with novel traits (PNT’s). The paper discusses strategies and ideas that might be employed to reduce the occurrence of herbicide resistant volunteers, reduce the occurrence of multiple resistant volunteers and, reduce the occurrence of herbicide-resistant weeds that may emerge from the repeated use of a single type of herbicide. The paper reviews the current state of knowledge of herbicide resistant crops and their subsequent volunteers and presents a discussion of potential herbicide resistance management (HRM) strategies that could be used to reduce their emergence and impact in cropping systems. In addition, some suggestions received from weed scientists, plant breeders and agronomists are included.

This document is not intended to review and summarize the current status of herbicide- resistant (HR) weed biotypes in Canada. The excellent review presented by Beckie et al. (2001b) identified herbicide-use patterns and cropping systems that influence the selection of resistance, and outlined strategies for managing herbicide resistance in weeds in Canada.

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Regulatory suggestions for CFIA and other regulators

1. Define area of production zones.

2. Limit the number of same-type resistant crops in an area.

3. Restrict introduction of HT crops/cultivars with high out-cross potential. For example, significant differences in out-crossing between wheat varieties have been observed. When/if HT wheat is released the HT factor should only appear in varieties with low out-crossing potential.

4. Ensure HT seed sources have rapid, accurate genetic ID procedures in place prior to release.

5. Future approval for crops with high out-crossing potential could be restricted to those with no or short term (single season) dormancy.

6. Crops with the HT factor with the exception of Brassica napus canola have not contributed to HR volunteers or multiple volunteers. Flax, wheat and Brassica rapa canola have not been grown commercially and soybean and corn volunteers are not considered problem weeds. B. napus conventional, HT and multiple HT canola volunteers are identified as weedy concerns in field surveys. In recommended rotations, when a cereal follows canola, numerous herbicide control options are labelled. In pre-seeding applications in zero or reduced-tillage systems, glyphosate has been the main herbicide applied. This application provides effective control of conventional, glufosinate HT and imidazolinone HT volunteers. There is a need to develop broadleaf combinations or mixtures to control glyphosate and multiple HT volunteers pre-seed. For example, 2,4-D, MCPA, bromoxynil and a low residual sulfonylurea like tribenuron methyl in combination with glyphosate would provide broad-spectrum pre-seed weed control including volunteer glyphosate HT canola. In some situations amitrole and paraquat could provide effective pre-seed control and resistance management for volunteer HT canola.

7. Encourage herbicide rotation. Regulatory recommendation only.

8. Herbicide mixtures to manage volunteers and reduce selection pressure could be a requirement for future HT crop releases.

9. Sequential application between herbicide Groups for control of HT volunteers would delay resistance development and in some cases assist in controlling late emerging plants that often escape treatment and set seed (non-regulatory?).

10. Reduce herbicide rate for residual products. Based on European experience, lower rates of imidazolinone and sulfonylurea herbicides could delays resistance development. The pending introduction of imidazolinone HT wheat to cropping systems could lead to excessive use of imidazolinone herbicides. A common 4-year, prairie rotation of canola-wheat-peas-wheat where imidazolinone could be applied continuously would fast track Group 2 resistance. Applying lower rates successfully would require that more sophisticated information be provided to growers to tailor label rates according to environmental conditions and herbicide sensitivity of the species present in their fields.

11. Labelling or classification of herbicide mode of action by risk (high, moderate, and low) for selection for herbicide resistance in specific species of HT volunteers and weeds.

12. There is a need to develop a ‘degree of risk’ formula for combinations and the various herbicide management strategies discussed above.

13. Ensure future production concerns regarding volunteer management and resistance reduction strategies are available prior to release of HT crops (see # 14).

14. A complete HRM strategy package should be available with each new HT crop/cultivar introduction. Volunteer HT crops appearing in the rotation are weeds and must be managed as such. In some cases, an efficacious herbicide(s) may be available but, in most instances the management package must include a complete HRM strategy. Those non-herbicide strategies discussed in this paper will be an essential component of HRM and should be available to growers when the HT or PNT crop or cultivar is released.

Several of the items listed above can be regulated by CFIA and/or PMRA. However, successful management of HT crop volunteers and the prevention of weed resistance will require both regulation and information. The requirements of registration for a PNT could include the requirement for a scientifically reviewed information package on management of volunteers and herbicide resistance in succeeding years of the rotation.

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Summary

This discussion paper raises issues related to the management of plants with novel traits (PNT’s). The points raised result from a review of the current literature and the comments of thirty research scientists throughout Canada. The scientific literature and the interviews indicated three general strategy areas; preventing establishment; managing crop development and seed sources to minimize HR volunteers; and, practicing integrated management to reduce the impact of HR volunteers as weeds and minimize the selection pressure for HR weeds. It is recognized that some of these strategies are not within the CIFA mandate, however, they represent opportunities for the industry as a whole to successfully manage PNT’s and existing HR crops.

1. Prevent establishment with restricted introductions

• Abstinence - no plant, no problems
• Increase stakeholder involvement. Growers could be consulted on new introductions (i.e. HT wheat)
• Limit area of production to zones
• Restrict GM crops with high outcross potential from the production system

2. Minimize the establishment of significant populations of HR volunteers

• Introduce crop cultivars with low levels of out-crossing (i.e. wheat)
• Separate PNT’s and GMO crops at registration (are they different?)
• Limit the number of same type resistant crops in an area
• Ensure conventional seed is PNT and weed free
• Ensure genetic seed lot ID available for specific traits before release
• Increase in pedigreed seed production isolation distances
• Select cultivars with low or no secondary dormancy
• Develop crops with short term dormancy (single season)
• Develop strategies to eliminate out crossing (non-out crossing transgenic)
• Require a gene for shatter resistance in B. napus canola
• Terminator gene type technology
• Tissue specific promoters (expression at certain growth stages)
• Ensure industrial and pharmaceutical crops can never enter the food chain (underground production)
• Gene factor expressed in chloroplast only
• Modify plants using their own DNA

3. Reducing the impact of HR volunteers in the cropping system.

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Licensing and Release of Plants with Novel Traits (PNT’s)

When a crop with a novel trait for herbicide resistance has been introduced into an area that trait will likely require future herbicide resistance management (HRM).

There are several approaches to managing HR crops that can be discussed under the following; (1) preventing establishment by restricted introductions or plant breeding strategies; (2) avoiding the possibility of significant population levels being established; and, (3) reducing the impact of HR volunteers in the cropping system by using integrated management strategies.

(1) Preventing establishment with restricted introductions.

The use of the abstinence approach in the introduction of PNT’s would prevent introduction. If there are no plants, then there will be no problems. However, t his approach would not have been acceptable to growers in the case of Brassica napus canola. HT canola has been on the market since 1995 and has increased to >75 % of the canola acreage (Harker et al., 2001b, Légère et al., 2001).

The pending registration and introduction of Roundup Ready wheat into Canada is controversial. While the technology will simplify, and in some cases improve in-crop weed control in wheat there are a number of concerns surrounding the release of this crop (Van Acker and Entz. 2001). The benefits could include improved control of herbicide-resistant weeds, improved management of perennial weeds, a reduced risk of herbicide injury to wheat and the removal of off-type wheat within a crop. The risks could include the loss of farmer saved seed options for wheat, the contamination of wheat segregated for sale as non-GMO, the risk of developing glyphosate resistant weeds in direct-seeding systems (Beckie et al. 2001b, c), a high frequency of volunteer HR wheat in the following crop (Thomas et al., 1999), the likelihood of higher cost herbicide options for volunteer control (Rainbolt et al., 2001), the potential loss of glyphosate as a burnoff treatment in direct-seeding conservation systems (Derksen et al., 1996), the persistence of volunteer wheat in the rotation as reported from survey results (Thomas et al., 1999), and the ability of wheat to outcross to a significant extent in some cultivars (Hucl and Matus-Cádiz, 2001) are all marketing and management problems for Canada’s growers.

The production of imidazolinone tolerant wheat will not present the same challenges in volunteer management. However, the application of imidazolinone herbicides to wheat in addition to use in canola and peas, combined with the highly residual nature of the imidazolinone herbicides could produce high selection pressure for Group 2-HR broadleaf and grass weeds.

It has been suggested that the production of industrial and pharmaceutical crops be regulated to ensure that those traits can never enter the food chain.

Faced with these concerns and several years experience with HR Brassica napus canola (Bing et al. 1996, Downey. 1999, Hall et al. 2000, Harker et al. 2001b, c, Warwick et al. 1999) perhaps growers should have a greater voice in the decision on the release of PNT crops (particularly those GM crops with a high outcross potential) and a role in defining the areas or zones of introduction.

(2) Avoiding the possibility of significant population levels of HR volunteers establishing.

Volunteer crops are common weeds and weediness depends upon species, management practices, seed shatter prior to harvest and disbursement of seed at harvest. Measured after herbicide application, volunteer canola (B. rapa or B. napus) occurred in 8, 13, and 11% of fields in Alberta (Thomas et al. 1998a), Saskatchewan (Thomas et al. 1996), and Manitoba (Thomas et al. 1998b), respectively. Densities in direct-seeded fields in Manitoba were double that in conventional-tillage fields (Thomas et al. 1994).

Volunteer wheat is a significant weed in 8, 9, and 10% of fields in Alberta (Thomas et al. 1998a), Saskatchewan (Thomas et al. 1996), and Manitoba (Thomas et al. 1998b), respectively, and can persist for at least 5 yr in the seed bank (A. G. Thomas, unpublished data). Volunteer wheat is controlled by glyphosate, applied pre-seeding or in glyphosate-HR canola crops. Alternatively it can be controlled by Group 1 herbicides in canola or other broadleaf crops or by glufosinate in canola. It is poorly controlled in barley (Hordeum vulgare L.). Thus, imidazolinone-HR wheat is unlikely to be more difficult to control than conventional cultivars. Should glyphosate-HR wheat become registered, its control will be limited both pre-seeding and in-crop and Group 1 product use might increase for control of glyphosate-HR volunteers. There is a need for a new non-selective herbicide with a different mode of action (Dunan and Westra 2000).

The movement of HR genes impacts the general public’s perception of biotechnology safety. Consequently, maintaining populations of HR volunteers of all crops at low levels is desirable for both the cropping system and the general public’s perception. In the absence of herbicide selection, it is unlikely HR crops are more competitive than conventional crops species, suggesting they will not invade disturbed or natural areas (Warwick et al. 1999).

Measures to assist in the maintenance of HR volunteers at low levels could include the use of an end or termination type of gene technology. This would ensure that volunteer plants expressing the HR trait do not appear in the rotation. The risk would include the loss of farmer saved seed options for wheat. Similarly, development of HR crops with the HR gene factor expressed in the chloroplast only would eliminate many of the concerns of out-crossing to other crops of the same species or wild relatives. Other possible approaches might include modification of plants utilizing their own DNA and the development of tissue specific promoters where the HR factor is only expressed at certain growth stages.

Recent research has shown large differences in out-crossing levels between wheat cultivars are occurring (Hucl and Matus-Cádiz 2001). Based on this information, planned releases of HR wheat should be restricted to those cultivars demonstrating low-level out-crossing. Increasing the isolation distances for production of pedigreed wheat seed could also be considered. In canola, Downey and Beckie concluded from their studies (unpublished data) that the present CSGA recommendation standards for isolation of Breeder, Foundation and Certified canola seed production are adequate. They also concluded that when isolation requirements are met, contamination with transgenes in non-herbicide tolerant varieties is primarily caused by mechanical mixing (seeding, harvesting, cleaning, storage, etc.) rather than by cross-pollination between fields of herbicide tolerant and susceptible varieties. The regulatory system should ensure that genetic seed ID for all seed lots is available for specific traits prior to registration. At present, fast and accurate tests are not available for the imidazolinone and bromoxynil tolerant genes. Reducing the probability of HR volunteers appearing in certified seed is identified as an important component of HR management strategies (Thomas P. 2001)

If more than one HR trait has been developed for a species and the species is partially or completely allogamous, pollen flow could create multiple-HR volunteers. Multiple-HR volunteers have been reported in B. napus (Champolivier et al. 1999, Hall et al. 2000, Simpson et al. 1999). In 1997, a field in Alberta was planted with both imidazolinone-HR and glufosinate-HR B. napus, adjacent to a field of glyphosate-HR B. napus. Volunteers were selected with glyphosate in 1998. These volunteers flowered and produced seed. Seed contained individuals resistant to glyphosate and glufosinate; glyphosate and imazethapyr; and glyphosate, imazethapyr, and glufosinate (Hall et al. 2000).

Under field conditions, pollen flow from one field to another generally results in less than 1% out-crossing in the first 100 m (Downey 1999). However, assuming a 0.2% out-crossing rate in a field yielding 1400 kg ha^-1 with a harvest loss of 5%, Downey (1999) estimated some 35,000-hybrid seeds (3.5 seeds m-2) would remain in the recipient field although most would be killed by spring frost or cultivation. Because of the large acreage of HR canola in western Canada, it is predicted that many fields contain multiple-HR volunteers. In reduced tillage cropping systems, there is a need for pre-seed glyphosate/broadleaf herbicide mixtures to manage HR canola volunteers resistant to glyphosate.

Based on the B. napus canola experience, it may be desirable in future releases to limit the number of same type resistant crops in an area or zone.

Plant breeding strategies to develop shatter resistance in crops with high out-crossing rates are currently being investigated (P. A. O’Sullivan, personal communication). A high level of shatter resistance combined with low or no secondary dormancy would have a major impact on HR management strategies (Gulden et al. 2000). HR Crops with short term (single season dormancy) should also be considered.

It has also been suggested that it may be possible to develop plants types containing a tolerance mechanism turned on when exposed to the desired herbicide and turned off when a second product is included in the spray mix. Development of a repressor system where both products must be present if tolerance is to be expressed should also be investigated.

(3) Reducing the impact of HR volunteers in the cropping system.

a) HRM strategies with herbicide rotation.

Not all herbicides have the same probability of selecting for resistance in weeds (Beckie et al. 2001b). It is generally accepted that Group 1 and 2 herbicides pose a high risk for selecting for HR biotypes relative to herbicides from other groups (Dellow et al. 1997, Gressel 1997, Heap 1999, Lebaron and McFarland 1990).

In Canada, six HR crops species - Argentine canola (Brassica napus L.), Polish canola (Brassica rapa L.), flax (Linum usitatissimum L.), corn (Zea mays), soybean (Glycine max L), and wheat (Triticum aestivum L.) are currently registered or soon to be registered (Anonymous 1994a, b, 1996, 1999a, b, c, 2000a, b). HR crops can play a role in slowing the selection of HR weeds by increasing crop and herbicide rotational options Beckie et al. 2001b). HR crops could increase profitability and the use of herbicides with little or no impact on the environment (Burnside 1992). Beckie et al (2001b) suggest the impact of HR is largely dependent on the herbicide group and cropping area. Weeds resistant to Group 6 (benzonitriles), Group 9 (glyphosate) and Group 10 (glufosinate) herbicides are very rare (Heap 1999, 2000). Increasing the use of these products will slow selection of weed resistance to herbicides of other modes of action including Groups 1 and 2. Frequent application of herbicides from Groups 6, 9, or 10 would, however, increase the selection pressure for very rare resistance genes. Beckie et al (2001a, b, c) also note that the existence of glyphosate-HR weed biotypes of three species suggests that other weeds may also be selected for resistance. Given the importance of glyphosate in reduced tillage cropping systems, repeated use of glyphosate should be discouraged. The greater the risk of a herbicide mode of action of selecting for resistance, the less frequently herbicides from that group should be applied. The increasing production of imidazolinone-HR crops will increase the selection for Group 2-HR broadleaf and grass weeds.

To reduce the impact of HR volunteers in the cropping system it is essential that growers maintain accurate field cropping histories, isolate fields with different HR systems, document crop types and varieties on neighbouring fields for future reference, monitor population changes (Beckie et al. 1999b, c, 2002), practice site specific management and increase their specific level of resistance awareness (Bourgeois and Morrison 1997a, b, Bourgeois et al 1997c, Friesen et al. 2000, Goodwin 1994, Morrison and Devine 1994, Thomas et al. 1999). Thomas et al. (1999) noted that the follow-up questionnaire to the 1997 Manitoba Weed Survey indicated that about one-half (53%) of growers were aware of herbicide resistance in the area but only 18% were aware of or suspected that they had resistance on their farms, suggesting some level of denial by the producers. Recent surveys (Beckie et al. 1998) suggest more than 50% of fields in Manitoba contain herbicide resistant wild oats.

A better understanding of environmental conditions and the herbicide sensitivity of the species present in their fields would allow growers to reduce requirement in some situations and possibly slow resistance development (Holm et al. 2000, Stevenson et al. 2000, O’Donovan et al.2002, Zhang et al. 2000). For example, reduced but effective ALS inhibitor resistance rates used to control common chickweed in Europe compared to those used in western Canada doubled the time for resistance evolution by reducing the time that herbicides remain active in the soil (Kudsk et al. 1995).

Growers who include mixtures of herbicides with different modes of action coupled with crop rotation and other cultural practices were less likely to select HR weed populations (Shaner et al 1997). Little ALS resistance is reported in Europe or Japan where Group 2 herbicides are used in rotation or mixture with other herbicides (Heap 1999, Itoh et al. 1999). Beckie et al. (2000) suggest that testing suspected populations to determine resistance patterns will identify remaining herbicide options for growers. Guidelines for rotating and mixing herbicides need to be developed (Beckie et al. 1999, Bourgeios et al. 1997b, Goodwin 1994, Wrubel and Gressel 1994). Guidelines could include the labelling of herbicides indicating their site of action (Beckie et al. 1999a).

b) HRM strategies at harvest and late fall.

Legere et al. (2000) reported that cultural practices used by growers, with the exception of frequent fallow, did little to decrease resistance development in wild oat. However, in the case of HR volunteer crops, a combination of cultural practices could be an effective management technology. Reduced losses at harvest from both the cutter bar and combine would impact significantly on future HR volunteer and weed populations. Swathing at the correct stage would reduce shatter losses in crops such as B. napus canola (Thomas P. 2001). A chaff collection system behind the combine prevents distribution of both crop and weed seeds and reduces future populations (Shirtliffe et al. 1998). Another option would be to place weed and crop seeds lost through the combine in narrow rows to facilitate future management with herbicides or controlled burn situations. Seeding winter crops (wheat, rye, triticale) that are highly competitive with spring germinating annual weeds and volunteer crops can be utilized in developing HR management strategy (Beck 2001, LeBaron and McFarland 1990, Powles 1997, Powles et al. 1997). Fall-seeded or dormant-seeded canola has provided a new seeding date option for prairie canola growers. Benefits of fall or early-spring seeded canola include earlier maturity (flowering 2-3 weeks earlier which avoids hot, dry periods during flowering and seed set), 38% higher yields, higher oil content and reduced plant height (Kirkland and Johnson 2000). Additional benefits of fall or early-spring seeded crops may be considerable in HRM and IWM cropping systems. Weeds and HR crops that have adaptation to invade crops seeded on conventional spring dates may be disadvantaged when required to compete with crops that emerge, mature and are harvested earlier in the season.

c) HRM strategies at seeding and in-crop.

Selection of extended, diversified crop rotations is essential to avoid the mass invasion of weeds such as wild oat that thrive in short crop rotation sequences (Harker et al. 2001). Spring-seeded crops that are seeded, sprayed and harvested at constant dates provide a niche for weeds and volunteer HR crops to survive. “A good rotation has diversity in plant types, planting dates and harvest periods” (Beck 2001).

A healthy, vigorous crop will reduce the amount of biomass and seed produced by both weeds and volunteers. An important step in HRM is the selection of a competitive crop and variety, and a seeding rate that optimizes crop yield and competitiveness with invading weeds and volunteers. Barley has been shown to be more competitive than other field crops, including wheat and canola. However, within each crop there are differences in competitive ability between types and varieties. Kirkland and Hunter (1991) reported that semi-dwarf wheat cultivars were less competitive with wild oat than conventional height cultivars. In barley, O’Donovan et al. (2000b) reported that wild oat seed production and crop yield loss were highest in the semi-dwarf types Falcon and CDC Earl. Small increases in crop seeding rates can increase crop competitiveness, reduce weed biomass and possibly reduce the herbicide requirements (Blackshaw et al. 2000a, Kirkland 1993, Kirkland et al. 2000, O’Donovan 2000a, 2002).

Rapid emergence from shallow depth when soil moisture is sufficient promotes a vigorous, competitive crop. At Fort Vermilion, AB, barley emergence from wet soil was reduced 30% at 2.5-inch compared to 1.0-inch seeding depth (O’Donovan et al. 2002). Shallow seeding promotes early crop emergence and crops that emerge ahead of weeds and volunteers will have a major competitive advantage. O’ Donovan et al. (2002) reported that wild oats (20/m2) emerging 5 days ahead of barley will result in 17% yield loss compared to only 3% yield loss when the barley emerged 5 days ahead of the wild oats. An increase in canola seeding rate can also reduce the impact of weeds (O’Donovan and Newman 1996), but this strategy is much more effective when combined with a competitive canola variety and shallow seeding.

The contribution of nitrogen (N) fertilizer placement to management of annual weeds has been demonstrated in both conventional and zero-tillage systems. Several studies have shown that increasing the N rate can enhance crop competitiveness over weeds provided that the N is placed to favour the crop. In one study, weed densities, weed biomass and N uptake measured early in the growing season averaged 20 to 40% less, and grain yield of wheat at maturity averaged 12% higher where fertilizer N was side-banded compared to broadcast (Kirkland and Beckie 1998). In a four-year study with continuous barley under zero tillage, green foxtail populations decreased from over 100 plants/m2 where no N was applied to only 3 plants/m2 where N was banded at 120 hg/ha (O’Donovan et al. 2002). It is also significant that populations were generally lower under zero compared to conventional tillage. In conventional tillage systems, tillage just prior to seeding can control emerged volunteer HR canola (Thomas P. 2001). However, this practice can also induce seeds into a secondary dormancy that can contribute to persistence in the seed-bank (Gulden et al. 2000).

The inclusion of forages in crop rotations can have a suppressive effect on weed populations and allow crop producers to reduce herbicide-input costs. A survey of over 100 fields in Manitoba showed that populations of species such as Canada thistle, wild oat and wild mustard were lower in cereal fields that had previously contained alfalfa compared to cereal fields that followed a cereal crop (Ominski et al. 1999). At Lethbridge, field experiments were conducted to determine weed suppression attained with yellow sweet clover grown as a green manure fallow replacement crop. Sweet clover was under sown in field pea, flax, or Indian mustard and then killed in June of the following fallow year (Blackshaw et al. 2001). Sweet clover strongly suppressed a number of weed species during the first fall and spring of fallow. Residues remaining after termination of growth continued to provide excellent weed suppression. Weed densities in April before planting the succeeding wheat crop were 75 to 97% lower in sweet clover than in untreated fallow treatments. Weed suppression was similar whether yellow sweet clover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that a portion of the weed suppression effect may be due to allelopathic compounds being released from decomposing yellow sweet clover. Several other clover species including alsike clover were also shown to suppress weeds (Ross et al. 2001). Schoofs and Entz (2000) reported that several annual, winter annual and biennial forage cropping systems were at least as effective as the sprayed wheat control in suppressing wild oat, however, effects on other weeds, especially broadleaf weeds were variable. The largest difference in weed communities attributable to farming system was between the systems with annual cropping histories and those that included perennials in the cropping history (Leeson et al. 2000). Studies conducted at Lacombe and Melfort indicated that in the absence of herbicides, cutting barley for silage was very effective in reducing wild oat populations, especially when the crop was cut at an early growth stage (Harker and Kirkland 2001). It required two years of early-cut silage to reduce the wild oat seed-bank but after that early-cut silage kept wild oat populations at low levels. This study indicates that early-cut silage can be used as an effective weed management tool for wild oat, especially in situations where herbicide options may be limited due to the presence of HR weeds or volunteer crops.

Other management strategies to assist in reducing the impact of HR volunteers include, determination of threshold levels, most appropriate timing for herbicide application (pre, in-crop or post-harvest), early application to reduce competition versus preventing seed set of late emerging HR volunteers. And, elimination by mechanical means like inter-row tillage, pre-emergent tillage and clipping taller weeds in shorter growing crops (Johnson and Nielsen 2000). A successful HRM strategy program will be based on many of the strategies presently identified and discussed above as essential ingredients in Integrated Weed Management (Blackshaw et al. 2000b, O’Donovan 2002, Powles 1997, 2000, Shrestha et al. 2001).

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Literature Cited

Anonymous 1994a. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biotechnology Office. Regulatory Directive Dir94-10: The biology of Linum usitatissimum L. (flax). A companion document to the Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits. [Online] http://www.inspection.gc.ca/english/plaveg/pbo/dir/dir9410e.shtml. [accessed January 16, 2002].

Anonymous 1994b. (CFIA) Canadian Food Inspection Agency, Plant Health and Production  Division, Plant Biotechnology Office. Regulatory Directive Dir94-11: The biology of Zea mays L. (corn/maize). A companion document to the Assessment Criteria for Determining  Environmental Safety of Plants with Novel Traits. [Online]  http://www.inspection.gc.ca/english/plaveg/pbo/dir/dir9411e.shtml. [accessed January 16,2002].

Anonymous 1996. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biotechnology Office. Regulatory Directive T-1-10-96: The biology of Glycine max (L.) Merr. (soybean). A companion document to the Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits. [Online] http://www.inspection.gc.ca/english/plaveg/pbo/dir/t11096e.shtml. [accessed January 16, 2002].

Anonymous 1999a. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biotechnology Office. Regulatory Directive Dir1999-01: The biology of Triticum aestivum L. (wheat). A companion document to the Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits. [Online] http://www.inspection.gc.ca/english/plaveg/pbo/dir/dir9901e.shtml. [accessed January 16,2002].

Anonymous 1999b. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biotechnology Office. Regulatory Directive Dir 1999-02: The biology of Brassica rapa L. (canola/rapeseed). A companion document to the Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits. [Online] http://www.inspection.gc.ca/english/plaveg/pbo/dir9902e.shtml. [accessed January 16, 2002].

Anonymous. 1999c. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biotechnology Office. Regulatory Directive Dir94-09: The biology of Brassica napus L. (canola/rapeseed). A companion document to the Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits. [Online] http://www.inspection.gc.ca/english/plaveg/pbo/dir9409e.shtml. [accessed January 16, 2002].

Anonymous 2000a. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biosafety Office. Assessment Criteria for Determining Environmental Safety    of Plants with Novel Traits. [Online]  http://active.inspection.gc.ca/eng/plaveg/bio/pntvcne.asp. [accessed January 16, 2002].

Anonymous 2000b. (CFIA) Canadian Food Inspection Agency, Plant Health and Production Division, Plant Biosafety Office. Decision Documents, Determination of Environmental Safety. [Online] http://www.inspection.gc.ca/english/plaveg/pbo/dde.shtml. [accessed January 16, 2002].

Beck, D. 2001. No-till guidelines for the semi-arid prairies. [Online] Available http://www.dakotalakes.com/publications.htm [accessed February 07, 2002].

Beckie, H. J., D. J. Kelner, R. C. Van Acker , and A. G. Thomas. 1998. Manitoba weed survey of herbicide-resistant wild oat in 1997. Weed Survey Series Publ. 98-5, Agriculture and Agri-Food Canada, Saskatoon, Saskatchewan. 36 pp.

Beckie, H. J., F. Chang, and F. C. Stevenson. 1999a. The effect of labelling herbicides with their site of action: a Canadian perspective. Weed Technol. 13:655-661.

Beckie, H. J., L. M. Hall and F. J. Tardiff. 2001a. Impact and management of herbicide-resistant weeds in Canada. Proc. Brighton Crop Prot. Conf. – Weeds. British Crop Protection Council, Farnham, Surrey, UK. pp.747-754.

Beckie, H. J., L. M. Hall and F. J. Tardiff. 2001b. Herbicide resistance in Canada-where are we today? In R. E. Blackshaw and L. M. Hall, eds. Integrated Weed Management: Explore the Potential. Expert Committee on Weeds, Saint-Anne-de-Bellevue, Qc. pp. 1-36.

Beckie, H. J., L. M. Hall and S. I. Warwick. 2001c. Impact of herbicide-resistant crops as weeds in Canada. Proc. Brighton Crop Prot. Conf. – Weeds. British Crop Protection Council, Farnham, Surrey, UK. pp.135-142.

Beckie, H. J., I. M. Heap, R. J. Smeda, and L. M. Hall. 2000. Screening for herbicide resistance in weeds. Weed Technol. 14:428-445.

Beckie, H. J. and F. A. Holm. 2002. Response of wild oat (Avena fatua) to residual and non-residual herbicides in canola (Brassica napus). Can. J. Plant Sci. (in press).

Beckie, H. J. and S. Jana. 2000. Selecting for triallate resistance in wild oat. Can. J. Plant Sci. 80:665-667.

Beckie, H. J., A. G. Thomas, and A. Légère. 1999b. Nature, occurrence, and cost of herbicide-resistant green foxtail (Setaria viridis) across Saskatchewan ecoregions. Weed Technol. 13:626-631.

Beckie, H. J., A. G. Thomas, A. Légère, D. J. Kelner, R. C. Van Acker, and S. Meers. 1999c. Nature, occurrence, and cost of herbicide-resistant wild oat (Avena fatua) in small-grain production areas. Weed Technol. 13:612-625.

Beckie, H. J., A. G. Thomas, and F. C. Stevenson. 2002. Survey of herbicide-resistant wild oat (Avena fatua) in two townships in Saskatchewan. Can. J. Plant Sci. (in press).

Bing, D. J., R. K. Downey, and G.F.W. Rakow. 1996. Hybridizations among Brassica napus, B. rapa and B. juncea and their two weedy relatives B. nigra and Sinapis arvensis under open pollination conditions in the field. Plant Breed. 115:470-473.

Blackshaw, R. E., J. R. Moyer, R. C. Doram, and A. L. Boswell. 2001. Yellow sweetclover, green manure, and its residues effectively suppress weeds during fallow. Weed Sci. 49: 406-413.

Blackshaw, R. E., G. P. Semach, and J. T. O’Donovan. 2000a. Utilization of wheat seed rate to manage redstem filaree (Erodium cicutarium) in a zero-tillage cropping system. Weed Technol.  14: 389-396.

Blackshaw, R. E., L. J. Molnar, H. H. Muendel, G. Saindon, and X. Li. 2000b. Integration of cropping practices and herbicide improves weed management in dry bean (Phaseolus vulgaris). Weed Technol. 14: 327-336.

Bing, D. J., R. K. Downey, and G.F.W. Rakow. 1996. Hybridizations among Brassica napus, B. rapa and B. juncea and their two weedy relatives B. nigra and Sinapis arvensis under open pollination conditions in the field. Plant Breeding 115:470-473

Bourgeois, L. and I. N. Morrison. 1997a. Mapping risk areas for resistance to ACCase inhibitor herbicides in Manitoba. Can. J. Plant Sci. 77:173-179.

Bourgeois, L. and I. N. Morrison. 1997b. A survey of ACCase inhibitor resistant wild oat in a high risk township in Manitoba. Can. J. Plant Sci. 77:703-708.

Bourgeois, L., I. N. Morrison, and D. Kelner. 1997c. Field and grower survey of ACCase resistant wild oat in Manitoba. Can. J. Plant Sci. 77:709-715.

Burnside, O. C. 1992. Rational for developing herbicide resistant crops. Weed Technol. 6:621 625.

Burnside, O. C. 1992. Rationale for developing herbicide-resistant crops. Weed Technol. 6:621 625.

Champolivier, J., J. Gasquez, A. Messéan, and M. Richard-Molard. 1999. Management of transgenic crops within the cropping system. In Gene Flow and Agriculture: Relevance for Transgenic Crops. Farnham, Surrey, UK: British Crop Protection Council. pp. 233-240.

Dellow, J. J., M. Incerti, R. Britton, and A. Bishop. 1997. Herbicide resistance extension strategy for the south eastern wheat belt of New South Wales, Australia. Proc. Second International Weed Control Congress, Copenhagen, Denmark. Flakkebjerg, Slagelse, Denmark: Dept. Weed Control and Pesticide Ecology.  pp. 487-492.

Derksen, D. A., R. E. Blackshaw and S. M. Boyetchko. 1996. Sustainability, conservation tillage and weeds in Canada. Can. J. Plant Sci. 76:651-659

Downey, R. K. 1999. Risk assessment of outcrossing of transgenic Brassica, with focus on B. rapa and B. napus. Proceedings 10^th International Rapeseed Congress, Canberra, Australia.

Dunan, C. and P. Westra. 2000. What risks do herbicide resistant weeds present to chemical companies? Abstr. Third International Weed Science Congress, Foz do Iguassu, Brazil. Corvallis, OR: International Weed Science Society. p. 155-156.

Friesen, L. J. S., G. M. Ferguson, and J. C. Hall. 2000. Management strategies for attenuating herbicide resistance: untoward consequences of their promotion. Crop Protection 19: 891-895.

Friesen, S. and J. C. Hall. 2000. Confirmation of resistance to fenoxaprop-ethyl, imazamethabenz-methyl, and flamprop-methyl by two wild oat populations. Weed Sci. Soc. Am. Abstr. 40:61.

Goodwin, M. 1994. An extension program for ACCase inhibitor resistance in Manitoba: a case study. Phytoprotection 75(Suppl.):97-102.

Gressel, J. 1997. Burgeoning resistance requires new strategies. In Weed and Crop Resistance to Herbicides. R. De Prado, J. Jorrín, and L. García-Torres, eds. London, UK: Kluwer Academic Publishers. pp. 3-14.

Gulden, R. H., S. J. Shirtliffe, and A. G. Thomas. 2000. Secondary dormancy in volunteer canola (Brassica napus L.). Proc. Can. Soc. Weed Sci., Saint-Anne-de-Bellevue, Qc. Pp. 62-67.

Hall, L., K. Topinka, J. Huffman, L. Davis, and A. Good. 2000. Pollen flow between herbicide-resistant Brassica napus (Argentine canola) is the cause of multiple resistant B. napus volunteers. Weed Sci. 48:688-694.

Harker, K. N., R. E. Blackshaw, and G. W. Clayton. 2001a. Timing weed removal in field pea (Pisum sativum). Weed Technol. 15:277-283.

Harker, K. N. and K. J. Kirkland. 2000. Early- and normal-cut barley silaging effects on wild oat (Avena fatua) populations. Weed Sci. Soc. Am. Abstr. 40:70.

Harker, K. N., G. W. Clayton, and R. K. Downey. 2001b. The Canadian GMO canola experience. Proc. GCIRC Bulletin, Number 18, (in press). Poznan, Poland.

Harker, K. N., G. W. Clayton, T. K. Turkington, J. T. O’Donovan, R. E. Blackshaw, and P. Thomas. 2001c. Implementing integrated weed management in canola. In R. E. Blackshaw and L. M. Hall, eds. Integrated Weed Management: Explore the Potential. Expert Committee on Weeds, Saint-Anne-de-Bellevue, Qc. pp. 91-96.

Heap, I. M. 1999. International survey of herbicide-resistant weeds: lessons and limitations. Proc. Brighton Crop Prot. Conf. - Weeds. Farnham, UK: British Crop Protection Council. pp.769-776.

Heap, I. M. 2000a. International Survey of Herbicide-Resistant Weeds. [Online] http://www.weedscience.com [accessed November 2, 2000].

Heap, I. M. 2000b. 1999 international survey of herbicide-resistant weeds. Weed Sci. Soc. Am. Abstr. 40:43.

Holm, F. A., K. J. Kirkland and F. C. Stevenson. 2000. Defining optimum herbicide rates and timing for wild oat (Avena fatua) control in spring wheat (Triticum aestivum). Weed Technol. 14:167-175.

Hucl, P. and M. Matus-Cadiz. 2001. Isolation distances for minimizing out-crossing in spring wheat. Crop Sci.41:1348-1351.

Itoh, K., G. X. Wang, and S. Ohba. 1999. Sulfonylurea resistance in Lindernia micrantha, an annual paddy weed in Japan. Weed Res. 39:413-423.

Johnson, E. N. and M. E. Nielsen. 2000. Pre-emergent tillage in field pea effective, but timing critical. (abstract) Proc. Expert Committee on Weeds, Saint-Anne-de-Bellevue, Qc. Pp. 106.

 Kirkland, K. J. 1993. Weed management in the absence of herbicides. Journal Sustainable Agric.3:95-104.

Kirkland, K. J. and H. J. Beckie. 1998. Contribution of nitrogen fertilizer placement to weed management in spring wheat (Triticum aestivum L.). Weed Technol. 12:507-514.

Kirkland, K. J., F. A. Holm, and F. C. Stevenson. 2000. Appropriate crop seeding rate when herbicide rate is reduced. Weed Technol. 14:692-698.

Kirkland, K. J. and J. H. Hunter. 1991. Competitiveness of Canada prairie spring wheats with wild oat (Avena fatua L.) Can. J. Plant Sci. 71:1089-1092.

Kirkland, K. J. and E. N. Johnson. 2000. Alternative seeding dates (fall and April) affect Brassica napus canola yield and quality. Can. J. Plant Sci. 80: 713-719.

Kudsk, P., S. K. Mathiassen, and J. C. Cotterman. 1995. Sulfonylurea resistance in Stellaria media [L.] Vill. Weed Res. 35:19-24.

LeBaron, H. M. 1985. Principles, problems, and potential of plant resistance. In Agricultural Chemicals of the Future. J. L. Hilton, ed. Beltsville, New Jersey: Rowman and Allanheld Publishers. pp. 223-236.

LeBaron, H. M. and J. McFarland. 1990. Herbicide resistance in weeds and crops. In M. B. Green, H. M. LeBaron, and W. K. Moberg, eds. Managing Resistance to Agrochemicals: From Fundamental Research to Practical Strategies. Washington, D.C.: American Chemical Society. pp. 336-352.

Légère, A., H. J. Beckie, F. C. Stevenson, and A. G. Thomas 2000. Survey of management practices affecting the occurrence of wild oat (Avena fatua) resistance to acetyl-CoA carboxylase inhibitors. Weed Technol. 14:366-376.

Légère, A., M. J. Simard, A. G. Thomas, D. Pageau, J. Lajeunesse, S. I. Warwick, and D. A. Derksen. 2001. Presence and persistence of volunteer canola in Canadian Cropping systems. Brighton Crop Prot. Conf. – Weeds. British Crop Protection Council, Farnham, Surrey, UK. pp.143-148.

Leeson, J. Y., J. W. Sheard, and A. G. Thomas. 2000. Weed Communities associated with arable Saskatchewan farm management systems. Can. J. Plant Sci. 80:177-185.

Morrison, I. N. and M. D. Devine. 1994. Herbicide resistance in the Canadian prairie provinces: five years after the fact. Phytoprotection 75(Suppl.):5-16.

Morrison, I. N., B. G. Todd, and K. M. Nawolsky. 1989. Confirmation of trifluralin-resistant green foxtail (Setaria viridis) in Manitoba. Weed Technol. 3:544-551.

O’Donovan, J. T., K. N. Harker, G. W. Clayton, and L. M. Hall. 2000a. Wild oat (Avena fatua) interference in barley (Hordeum vulgare) is influenced by barley variety and seeding rate. Weed Technol. 14:624-629.

O’Donovan, J. T., K. N. Harker, G. W. Clayton, and T. K. Turkington. 2002. Integrated weed management: reducing herbicide costs and managing resistance. Proc.  Agri-Future Farm Tech. Expo. Alberta Tillage Conservation Soc., Red Deer, AB. (in press).

O’Donovan, J. T. and D. W. McAndrew. 2000b. Effect of tillage on weed populations in continuous barley (Hordeum vulgare). Weed Technol. 14:726-733.

O’Donovan, J. T. and J. C. Newman. 1996. Manipulation of canola (Brassica rapa) plant density and herbicide rate for economical and sustainable weed management. Proc. 2^nd. Int.. Weed Contr. Conf., Copenhagen, Denmark. P.969-974.

Ominski, P. D., M. H. Entz, and Kenkel. 1999. Weed suppression by Medicago sativa in subsequent cereal crops: a comparative survey. Weed Sci. 47:282-290

Powles, S. B. 1997. Success from adversity: herbicide resistance can drive changes to sustainable weed management systems. Proc. Brighton Crop Prot. Conf. - Weeds. Farnham, UK: British Crop Protection Council. pp.1119-1126.

Powles, S. B., D. F. Lorraine-Colwill, J. J. Dellow, and C. Preston. 1998. Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia. Weed Sci. 46:604-607.

Powles, S. B., M. Monjardino, R. Llewellyn, and D. Pannell. 2000. Proactive versus reactive herbicide resistance management: understanding the economic sense of herbicide conservation versus exploitation. Abstr. Third International Weed Science Congress, Foz do Iguassu, Brazil. Corvallis, OR: International Weed Science Society. p. 148-149.

Powles, S. B., C. Preston, I. B. Bryan, and A. R. Jutsum. 1997. Herbicide resistance: impact and management. Adv. Agron. 58:57-93.

Rainbolt, C. R., C. Thill, A. Ball, J. P. Yennish, and F. L. Young. 2001. Managing volunteers following herbicide resistant crops. Proc. West. Weed  Sci. Soc., pp. 90.

Ross, S. M., J. R. King, R. C. Izaurralde, and J. T. O’Donovan. 2001. Weed suppression by seven clover species. Agron. Journal. 93:820-827.

Schoofs, A. and M. H. Entz. 2000. Influence of annual forages on weed dynamics in a cropping system. Can. J. Plant Sci. 80:187-198.

Seefeldt, S. S., D. R. Gealy, B. D. Brewster, and E. P. Fuerst. 1994. Cross-resistance of several diclofop-resistant wild oat (Avena fatua) biotypes from the Willamette Valley of Oregon. Weed Sci. 42:430-437.

Shaner, D. L., D. A. Feist, and E. J. Retzinger. 1997. SAMOA: one company’s approach to herbicide-resistant weed management. Pestic. Sci. 51:367-370.

Shirtliffe, S. J., M. H. Entz, and B. D. Maxwell. 1998. The effect of chaff collection and harvest timing on the seed spread of wild oat (Avena fatua). Weed Sci. Soc. Am. Abstr. 38:39.

Shrestha, A., I Rajcan, K. Chandler, and C. J. Swanton. 2001. An integrated weed management strategy for glufosinate-resistant corn (Zea mays). Weed Technol. 15:517-522.

Simpson, E. C., C. E. Norris, J. R. Law, J. E. Thomas, and J. B. Sweet. 1999. Gene flow in genetically modified herbicide tolerant oilseed rape (Brassica napus) in the UK. In Gene Flow and Agriculture: Relevance for Transgenic Crops. Farnham, Surrey, UK: British Crop Protection Council. pp. 233-240.

Thomas, A. G., B. L. Frick, and L. M. Hall. 1998a. Alberta Weed Survey: Cereal and Oilseed Crops 1997. Weed Survey Ser. Publ. 98-2. Saskatoon, SK: Agriculture and Agri-Food Canada. 242 p., excl. maps.

Thomas, A. G., B. L. Frick, R. C. Van Acker, S. Z. Knezevic, and D. Joosse. 1998b. Manitoba Weed Survey: Cereal and Oilseed Crops 1997. Weed Survey Ser. Publ. 98-1. Saskatoon, SK: Agriculture and Agri-Food Canada. 192 p., excl. maps.

Thomas, A. G., D. Kelner, R. F. Wise, and B. L. Frick. 1994. Manitoba Weed Survey - Comparing Zero and Conventional Tillage Crop Production Systems. Weed Sur. Ser. 97-1. Manitoba Agriculture, Carmen, MB. 130 p.

Thomas, A. G., J. Y. Leeson, and R. C. Van Acker. 1999. Manitoba Weed Survey – Farm Management Practices in Manitoba 1997 Weed Survey Questionnaire Results. 1997. Weed Survey Ser. Publ. 99-3. Saskatoon, SK: Agriculture and Agri-Food Canada. 297 p.

Thomas, A. G., R. F. Wise, B. L. Frick, and L. T. Juras. 1996. Saskatchewan Weed Survey: Cereal, Oilseed and Pulse Crops 1995. Weed Survey Ser. Publ. 96-1. Saskatoon, SK: Agriculture and Agri-Food Canada.  419 p.

Thomas, P. 2001. How to manage volunteer canola. Alberta Canola Growers Commission, Edmonton, AB. pp. 10.

Van Acker, R. and M. Entz. 2001. Agronomic benefits and risks of using Roundup Ready wheat in western Canada. Proc. Manitoba Agronomy Conf., Manitoba Agriculture and Food, Crops and Soils Branch, Carmen, MB. (in press).

Warwick, S. I., H. J. Beckie, and E. Small. 1999. Transgenic crops: new weed problems for Canada? Phytoprotection 80:71-84.

Wrubel, R. P. and J. Gressel. 1994. Are herbicide mixtures useful for delaying the rapid evolution of resistance? A case study. Weed Technol. 8:635-648.

Zhang, J., S. E. Weaver, and A. S. Hamill. 2000. Risks and Reliability of using herbicides at below-labeled rates. Weed Technol. 14:106-115.