DD2005-50: Determination of the Safety of the BASF Canada Imidazolinone-Tolerant Clearfield™ Sunflower (Helianthus annuus L.) Hybrid X81359

Issued: 2005-08

This Decision Document has been prepared to explain the regulatory decision reached under Dir94-08 Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits and its companion document BIO2005-01 The Biology of Helianthus annuus L. (Sunflower) and Dir95-03 Guidelines for the Assessment of Novel Feeds: Plants Sources.

The Canadian Food Inspection Agency (CFIA), specifically the Plant Biosafety Office (PBO) of the Plant Health and Biosecurity Directorate and the Animal Feed Division of the Animal Health Directorate, have evaluated information submitted by BASF Canada regarding the imidazolinone tolerant sunflower hybrid X81359, which will be known commercially as Clearfield™ sunflower. The CFIA has determined that this plant with a novel trait does not present a greater risk to the environment, nor does it present livestock feed safety concerns when compared to currently commercialized sunflower varieties in Canada.

Unconfined release into the environment and livestock feed use of sunflower hybrid X81359 are therefore authorized as of August 12, 2005 and April 14, 2005 respectively. Hybrid X81359 and any other sunflower lines derived from it may be imported and/or released, provided (i) no inter-specific crosses are performed, (ii) the intended uses are similar, and (iii) it is known based on characterization, that these plants do not display any additional novel traits and are substantially equivalent to currently grown sunflower in Canada, in terms of their specific use and safety for the environment and for human and animal health.

Hybrid X81359 is subject to the same phytosanitary import requirements as its unmodified counterparts.

Table of Contents

I. Brief Identification of the Plant with a Novel Trait (PNT)

II. Background Information

III. Description of the Novel Traits

1. Development Method
2. Imidazolinone Tolerance
3. Stable Expression

IV. Criteria for the Environmental Safety Assessment

1. Potential of X81359 to become a Weed of Agriculture or to be Invasive of Natural Habitats
2. Potential for Gene Flow from X81359 to Relatives Whose Hybrid Offspring May Become More Weedy or More Invasive
3. Altered Plant Pest Potential of X81359
4. Potential Impact of X81359 on Non-Target Organisms
5. Potential Impact of X81359 on Biodiversity

V. Criteria for the Livestock Feed Assessment

1. Potential Impact of X81359 on Livestock Nutrition
2. Potential Impact of X81359 on Livestock and Workers/By-standers

VI. New Information Requirements

VII. Regulatory Decision

Appendix 1: Clearfield™ Sunflower Herbicide Tolerance Stewardship Plan

I. Brief Identification of the Plant with a Novel Trait (PNT)

Designation(s) of the PNT: Clearfield Sunflower Hybrid X81359

Applicant: BASF Canada

Plant Species: Sunflower (Helianthus annuus L.)

Novel Traits: Tolerance to imidazolinone herbicides

Trait Introduction Method: Naturally occurring mutation

Proposed Use of the PNT: Production of H. annuus for human food and livestock feed. This material will not be grown outside the normal production area for sunflower.

II. Background Information

BASF Canada has developed a sunflower hybrid tolerant to imidazolinone herbicides. This sunflower hybrid, designated X81359, exhibited no significant injury when treated with imidazolinone herbicides at normal field application rates. This will allow post-emergent use of imidazolinones in sunflower crops, thus providing an alternative means of weed control in sunflower production.

The imidazolinone tolerance trait originated from a wild population of Helianthus annuus and was introduced into domestic germplasm by conventional breeding. The herbicide tolerance trait is conferred by a single point mutation in the acetohydroxyacid synthase (AHAS) gene such that this enzyme, the target of imidazolinone herbicides, is no longer affected by imidazolinones.

Hybrid X81359 was field tested in the US and Argentina from 2001 to 2002.

BASF Canada has provided data on the identity of hybrid X81359, a detailed description of the modification method and breeding history, information on the modified gene, the resulting protein and its mode of action and the stability of trait expression.

Agronomic characteristics such as percent oleic acid content, seed yield, days to flower, plant height, and hybrid appearance were compared with those of unmodified H. annuus counterparts.

Nutritional components of X81359 such as proximates, amino acids and fatty acids were compared with an unmodified sunflower counterpart. Levels of anti-nutritional factors were also compared between X81359 and the unmodified counterpart.

The Plant Biosafety Office (PBO), Canadian Food Inspection Agency (CFIA) reviewed the above information, in accordance with the following assessment criteria for determining environmental safety of plants with novel traits (PNTs), as described in directive Dir94-08:

• potential of X81359 to become a weed of agriculture or to be invasive of natural habitats,
• potential for gene-flow from X81359 to wild relatives whose hybrid offspring may become more weedy or more invasive,
• potential for X81359 to become a plant pest,
• potential impact of X81359 or its gene products on non-target species, including humans,
• potential impact of X81359 on biodiversity

The Animal Feed Division, CFIA, has also reviewed the above information with respect to the assessment criteria for determining the safety and efficacy of novel livestock feed, as described in the regulatory directive Dir95-03:

• potential impact of X81359 on livestock nutrition; and
• potential impact of X81359 on livestock and workers/bystanders.

Additionally, CFIA has reviewed a method submitted by BASF Canada for the detection and identification of sunflowers containing this modified AHAS gene.

III. Description of the Novel Trait

1. Development Method

This imidazolinone tolerance trait originated from a wild population of H. annuus and was introduced into domestic germplasm by conventional breeding. The mutation was identified in a population of wild H. annuus in a field in Kansas. The field in which the spontaneous mutation occurred had been planted to soybean and treated for several previous seasons with the imidazolinone herbicide, imazethapyr. The herbicide tolerance trait is conferred by a single point mutation in the acetohydroxyacid synthase (AHAS) gene such that imidazolinone herbicides have reduced binding efficiency to the modified AHAS enzyme.

2. Imidazolinone Tolerance

Imidazolinone herbicides are active against the enzyme acetohydroxyacid synthase (AHAS), also known as acetolactate synthase (ALS).

AHAS is an enzyme found in bacteria, certain other micro-organisms and plants. This enzyme catalyses the first step in the biosynthesis of the essential branched chain amino acids isoleucine, leucine and valine. Herbicide-induced AHAS inhibition results in a lethal decrease in protein synthesis. Unmodified sunflowers are not tolerant to imidazolinone herbicides.

A single amino acid substitution in the AHAS gene, sufficient to alter the binding site such that imidazolinone herbicides no longer binds to the AHAS enzyme, resulted in the herbicide tolerant phenotype.

The novel imidazolinone tolerance is under the control of the native AHAS promoter and is believed to be constitutively expressed. Sequence information for the modified AHAS gene was submitted.

The tolerance to imidazolinone herbicides was demonstrated by comparison of the activity of the AHAS enzyme extracted from X81359 sunflower plants to that of imidazolinone-susceptible sunflower plants.

The levels of valine, leucine and isoleucine produced in sunflower are regulated by feedback inhibition of AHAS. BASF provided data to demonstrate that the modified AHAS shows similar feedback inhibition by valine and leucine as compared to unmodified AHAS. The modification of the AHAS does not affect feedback inhibition and hence, the regulation and levels of these amino acids.

Unlike known food allergens, AHAS is a minor protein in plant tissue, it is heat sensitive and trypsin susceptible. The AHAS protein from X81359 was shown to be heat sensitive, with no detectable activity of AHAS after 1 min of heating at 100°C. AHAS was completely degraded within 60 minutes of trypsin treatment. The AHAS activity in seed is not changed by the mutation, however the AHAS activity in imidazolinone-tolerant and imidazolinone-susceptible leaf tissue is significantly different. The unmodified form of the AHAS protein shows no amino acid similarity to known allergens. Since the amino acid sequence of mutated AHAS differs by one amino acid from that of unmodified sunflower, it would not be expected to be an allergen. HPLC data demonstrated similar protein banding patterns between the imidazolinone-tolerant and imidazolinone-susceptible sunflowers indicating that it is unlikely that secondary mutations causing unintended effects have occurred in the sunflower genome.

3. Stable Expression

The imidazolinone tolerance trait from the source material from which hybrid X81359 was derived was shown to segregate according to the manner expected for a single semi-dominant allele. Hybrid X81359, which is several generations removed from the original mutation selection, consistently shows imidazolinone tolerance.

IV. Assessment Criteria for Environmental Safety

1. Potential of X81359 to become a Weed of Agriculture or be Invasive of Natural Habitats

Sunflower production in Canada occurs in southern Manitoba, Saskatchewan and Alberta, with a very small amount of production in Ontario. Cultivated sunflower does not have a high potential for weediness. Sunflower plants can grow as volunteers in a cultivated field following a sunflower crop and are usually eliminated via cultivation or the use of herbicides. In the recent provincial weed surveys conducted in Manitoba (2002) and Saskatchewan (2003), volunteer sunflower was ranked the 66^th and 85^th most abundant weed, respectively.

According to the information provided by BASF Canada, no competitive advantage was conferred to X81359, other than that conferred by tolerance to imidazolinone herbicides. The mutation of the AHAS gene in X81359 has not affected the physiology of the plant, as supported by agronomic and compositional data. It is therefore not expected that X81359 would possess traits that would render it invasive of natural habitats since none of the reproductive or growth characteristics were modified.

Imidazolinone tolerance in itself will not cause X81359 to become more weedy or invasive in managed habitats than non-modified H. annuus. Imidazolinone-tolerant sunflower volunteers will not be controlled in subsequent crops if an imidazolinone herbicide is used as the sole weed control tool. However, control of imidazolinone tolerant sunflower as a volunteer weed in other crops or in fallow ground can readily be achieved by the use of classes of herbicides with other modes of action (i.e. non-Group 2 herbicides) or by mechanical means.

The above considerations have led the CFIA to conclude that sunflower X81359 has no ecological advantages when compared with currently commercialized sunflower varieties.

BASF Canada provided the CFIA with a stewardship plan that describes appropriate strategies that will allow the management of X81359 volunteers, as well as other approved sunflower lines expressing imidazolinone tolerance (see Appendix 1).

2. Potential for Gene Flow from X81359 to Relatives Whose Hybrid Offspring May Become More Weedy or More Invasive

Helianthus annuus L. is a native of North America. Its wild relatives and other Helianthus species are distributed widely across the Central Plains of Canada from north to south. The wild H. annuus is a common roadside weed in the southern parts of the prairies, particularly in Manitoba, extending into the central United States. The cultivated and wild H. annuus have many opportunities for hybridization as they often grow in close proximity in many locations. These species overlap in flowering time and are visited by the same pollinators. Genetic cultivar markers are readily found in wild populations of H. annuus indicating no strong barrier to the introgression of domesticated germplasm into wild populations. H. petiolaris, another annual species that occurs in pockets in Canada, has been known to hybridize with H. annuus.

Several perennial species occur in Canada. The most conspicuous is the H. maximiliani which flowers on the roadside in late summer and fall. Some H. giganteus occurs in pockets and the H. tuberosus (Jerusalem artichoke) is found primarily on riverbanks. This species has been cultivated to a small extent for its tubers. Hybridization with perennial species that are found in Canada occurs very rarely in nature. Artificial methods are required to cross H. annuus with these perennial species.

By far, the most likely introgression of genes from cultivated H. annuus would be into wild H. annuus. The CFIA has therefore determined that gene flow from X81359 to wild sunflower in Canada is very likely. However, the imidazolinone tolerance trait is already present at various levels in wild Canadian populations. In addition, gene flow from X81359 to wild sunflowers in Canada would not be expected to result in increased invasiveness of the offspring, as the imidazolinone tolerance trait is not associated with enhanced weediness or any other properties. The occurrence of imidazolinone tolerant wild sunflowers will not restrict weed management options as imidazolinone herbicides are not sold or used alone to control wild sunflowers in Canada and imidazolinone tolerant wild sunflowers will still be easily controlled with herbicides with other modes of action or cultivation.

BASF Canada provided the CFIA with a stewardship plan that describes appropriate strategies that will allow the deployment of cultivated sunflower lines expressing imidazolinone tolerance while minimizing outcrossing to wild sunflowers (see Appendix 1).

3. Altered Plant Pest Potential of X81359

H. Annuus is not a plant pest in Canada and the novel trait in sunflower X81359 is not expected to affect its plant pest potential. The mutation of the AHAS gene function in X81359 is not associated with disease or insect resistance and, therefore, is very unlikely to have altered its' plant pest potential. The CFIA has therefore determined that sunflower X81359 does not present a plant pest concern.

4. Potential Impact of X81359 on Non-Target Organisms

Single amino acid modification of the AHAS enzyme, which alters the herbicide binding site on the enzyme, is the molecular basis for imidazolinone tolerance in sunflower X81359. BASF Canada has submitted data and information indicating that the modified AHAS is substantially equivalent to the native AHAS enzyme. The mutation in the AHAS gene in X81359 has not significantly affected the biosynthesis of the branched-chain amino acids, valine, leucine and isoleucine, or the nutritional composition. The CFIA has therefore determined that the modified AHAS enzyme will not have altered impacts on interacting organisms, including humans, compared with the unmodified counterpart.

The AHAS enzyme is not a known toxin, does not confer resistance to agricultural pests and is commonly found in a wide variety of plants and micro-organisms with a history of safe use. No novel toxins were introduced into this variety. Therefore, no negative interactions with non-target symbiotic or consumer organisms are anticipated.

In addition, agronomic, morphological and compositional characteristics of X81359 are within the range of values displayed by currently commercialized sunflower varieties. The CFIA concluded that there were not likely to be significant unintended changes to sunflower X81359 that could have adverse impacts on non target organisms.

5. Potential Impact of X81359 on Biodiversity

Sunflower X81359 has no novel phenotypic characteristics which would extend its use beyond the current geographic range of sunflower production in Canada. In addition, X81359 is not different from conventional sunflower in terms of safety to non-target organisms, and X81359 sunflower does not present altered weediness or plant pest potential. The novel trait will not alter the ability of this line to persist in the Canadian environment. Cultivated sunflower outcrosses under natural conditions to wild relatives in Canada, and the transfer of novel trait to wild sunflower in is highly likely. However, the consequences of the transfer of the imidazolinone tolerance trait are minimal as various levels of imidazolinone tolerance have been detected in Canadian wild sunflower populations, imidazolinone herbicides are not used to control sunflowers in Canada without the addition of another herbicide with a different mode of action or cultural control, the trait does not confer any selective advantages in the absence of imidazolinone, and imidazolinone-tolerant wild sunflowers can be controlled by other herbicide modes of action and cultivation.

The CFIA has therefore concluded that the impact on biodiversity of sunflower X81359 is equivalent to that of currently commercialized sunflower varieties.

BASF Canada provided the CFIA with a stewardship plan that describes appropriate strategies that will allow the deployment of X81359, as well as other approved sunflower lines expressing imidazolinone tolerance, while managing the development of Group 2-herbicide resistant weeds and outcrossing with related plants (see Appendix 1). The stewardship plan submitted by BASF Canada is based on the biology of the sunflower plant and on associated agronomic practices.

The Clearfield™ Sunflower Herbicide Tolerance Stewardship Plan comprises the Best Management Practice Program for the Clearfield™ Sunflower Production System. As part of its stewardship plan, BASF Canada is responsible for communicating to Canadian sunflower producers the general recommendations of the Clearfield™ Sunflower Stewardship Guide. A number of strategies have been developed by BASF to communicate the best management strategies to growers adopting the technology and allow them to report any problems. In addition, BASF is required to monitor grower compliance to determine the effectiveness of the stewardship plan and make any changes to the plan as appropriate.

V. Criteria for the Livestock Feed Assessment

1. Potential Impact of X81359 on Livestock Nutrition

Nutritional composition

Nutritional composition data was obtained for whole sunflower grain obtained from X81359 and its comparator, 8377NS, grown in a trial in Wisconsin in 2001. Three replicates from each variety were analyzed for protein, fat, fibre, branched chain amino acids (Iso, Leu, Val), essential amino acids, (Cys, Met, Thr, Lys), minerals (P, Zn, Mg, Fe), and B vitamins (Thiamine, Niacin, Pantothenic Acid, Vitamin B6). Vitamin E analysis was conducted on the two varieties from a different trial (North Dakota), because of insufficient sample material from the Wisconsin trial. Overall there were statistically significant differences between X81359 and 8377NS in crude protein and Thr. Although total oil content was not different, differences in individual fatty acids were observed. The fatty acids were within the ranges published by the National Sunflower Association, and were within the normal ranges reported for NuSun hybrids.

Further analyses of whole grain from sunflower from a trial site of 12 varieties including X81359 and 8377NS, conducted in Kansas in 2003, further supported the nutritional equivalence of X81359 to conventional sunflower varieties. Analyses included: crude protein, crude fat, crude fibre, amino acids, B vitamins (pyridoxine B6, Niacin, Thiamine B1, pantothenic acid, Riboflavin B2, folic acid) Vit E, minerals, and fatty acids. In this trial, X81359 had slightly lower oil and stearic acid content. As well, one amino acid (tryptophan), Ca, and Mn were higher in X81359 than the conventional lines. These five observed differences were not consistent with the differences observed in the Wisconsin trial, and were determined to be unrelated to the herbicide tolerance trait.

Antinutritional factors

Phytic acid and trypsin inhibitor were analysed in the replicated Wisconsin trial discussed above. Trypsin inhibitor was not detected in either X81359 or 8377NS. Phytic acid was significantly lower in X81359.

These trials support the conclusion that the nutritional composition of X81359 is equivalent to conventional sunflowers.

2. Potential Impact of X81359 on Livestock and Workers/By-standers

The AHAS enzyme is found in a wide variety of plants and micro-organisms. AHAS is not a known toxin or allergen and a single amino-acid change would not be expected to change this. AHAS from sunflower X81359 is feedback inhibited by valine and leucine as is the unmodified AHAS, it is present in small amounts in the feed, it is heat labile and it is rapidly degraded under conditions in the gastrointestinal tract. The activity of AHAS in seed is not changed by the modification. The difference in levels of AHAS activity in imidazolinone-tolerant and imidazolinone-susceptible leaf tissue is not biologically significant as the AHAS enzyme in both still has the same physicochemical properties and functional activity. Based on the information provided by BASF, the modified AHAS is unlikely to be a novel toxin or allergen.

As this sunflower line was not subjected to mutagenesis, the potential for secondary mutations which might cause unintended effects is unlikely. Based on the detailed characterization provided (nutritional composition and agronomic data of the modified plant compared to the unmodified comparator, as well as similar protein banding patterns between the modified and unmodified sunflowers) it is unlikely that secondary mutations causing unintended effects have occurred in the sunflower genome.

Based on the predicted exposure levels and the results of the above tests, no significant risk to livestock and workers/by-standers is expected from exposure to the modified AHAS enzyme.

VI. New Information Requirements

If at any time, BASF Canada becomes aware of any information regarding risk to the environment, including risk to human or animal health, that could result from release of these materials in Canada or elsewhere, BASF Canada will immediately provide such information to the CFIA. On the basis of such new information, the CFIA will re-evaluate the potential impact of the proposed use and will re-evaluate its decision with respect to the livestock feed use and environmental release authorizations of this sunflower hybrid.

VII. Regulatory Decision

Based on the review of data and information submitted by BASF Canada, and through comparisons of hybrid X81359 with an unmodified sunflower counterpart, the Plant Biosafety Office, CFIA, has concluded that the modified gene and its corresponding novel trait do not confer to sunflower X81359 any characteristic that would result in an ecological advantage or plant pest risk following unconfined release and therefore poses minimal risk to the environment when compared to conventional sunflower varieties.

Based on the review of data and information submitted by BASF Canada, including comparisons of hybrid X81359 with an unmodified sunflower counterpart, the Animal Feed Division, CFIA, has concluded that the modified gene and its corresponding novel trait do not confer to sunflower X81359 any characteristic that would raise any concerns regarding its safety or nutritional composition. Sunflower and its byproducts are currently listed in Schedule iv of the Feeds Regulations and are, therefore approved for use in livestock feeds in Canada. Sunflower X81359 has been assessed and found to be as safe and as nutritious as traditional sunflower varieties. Sunflower X81359 and its products are considered to meet the present ingredient definitions and are approved for use as livestock feed ingredients in Canada

Unconfined release into the environment and livestock feed use of the sunflower hybrid X81359 are therefore authorized as of August 12, 2005 and April 14, 2005 respectively. Any other sunflower lines derived from it may be imported and/or released, provided (i) no inter-specific crosses are performed, (ii) the intended uses are similar, (iii) it is known, based on characterization, that these plants do not display any additional novel traits and are substantially equivalent to currently grown sunflower in Canada, in terms of their specific use and safety for the environment and for human and animal health.

Hybrid X81359 is subject to the same phytosanitary import requirements as its unmodified counterparts.

Please refer to Health Canada's Decisions on Novel Foods for a description of the food safety assessment of sunflower hybrid X81359.

Appendix 1: Clearfield™ Sunflower Herbicide Tolerance Stewardship Plan

1. Best Management Practice Program for the Clearfield™ Sunflower Production System

Introduction

The Clearfield™ Production System for Sunflower is an innovative cropping system that offers enhanced weed control. It creates a number of new opportunities to western Canadian producers:

• Superior, one pass, broad-spectrum post-emergence grass and broadleaf weed control.
• An additional weed management tool for weed control and resistance management.
• Combines high yielding sunflower hybrids from leading seed companies with ODYSSEY herbicide for superior weed control in all tillage systems.

The Clearfield "Trait" in Sunflower

The Clearfield trait for sunflowers was discovered in 1997 by researchers at Kansas State University. The tolerance trait was first identified in an imidazolinone resistant population of common (wild) sunflower that had arisen in a commercial soybean field.

Dr. Jerry Miller, a USDA sunflower geneticist, crossed the resistant wild sunflower to USDA cultivated sunflower genetic stocks. The herbicide tolerant trait was maintained through several backcross generations. This naturally occurring gene was incorporated into cultivated germplasm via traditional plant breeding techniques.

Tolerance to the imidazolinone family of herbicides is conferred by a single semi-dominant gene. For commercial herbicide tolerance expression, the gene must be homozygous. For this reason, Clearfield sunflower hybrid production requires the conversion of both male and female parent lines. Clearfield sunflower hybrids are not commercially cross-tolerant to the sulfonylurea (SU) herbicides, which are also ALS inhibitors, and will be injured by these herbicides. However, acceptable control of volunteers or wild sunflowers may not be achieved with an SU or other group 2 herbicide.

Key Sustainability Issues

The users, developers and marketers of herbicide tolerant cropping systems are responsible for sustaining the production system and must address the key issues of:

• Herbicide resistance management using an integrated approach.
• Control of herbicide tolerant crop volunteers.
• Managing out-crossing to non-Clearfield crops and weeds.

These key sustainability issues form the basis for our stewardship plans for crops grown using the Clearfield Production System. The Clearfield sunflower Stewardship Plan, like those for other Clearfield crops, can be summarized by the following guiding principles:

Clearfield™ Production System – GUIDING PRINCIPLES

There are a number of GUIDELINES that must be understood and followed by Agronomists and Growers when using Clearfield™ Production Systems.

• DO NOT exceed a maximum of 2 exclusive Group 2 herbicides on any one field, in any 4 year period
• ALWAYS follow an Integrated Weed Management (IWM) program that includes herbicides, cultural practices and crop rotations in order to manage weed populations and minimize weed seed development
• ALWAYS control volunteers in the season following a Clearfield™ crop
• USE practices which minimize the likelihood of out-crossing to similar crops or related weeds
• FOLLOW the Best Management Practices outlined in the our Clearfield™ Sunflower Stewardship Guide

Resistance Management in the Clearfield Production System

Herbicide Resistance Management Using an Integrated Weed Management (IWM) Approach

The objectives of herbicide resistance management are to achieve weed control while preserving the value of each herbicide and each herbicide group for the longer term.

An integrated approach to weed control is the Best Management Practice to delay the onset of weed resistance to herbicides. An integrated approach involves the use of all methods available to the grower in order to provide effective weed control in crop. The use of herbicides is one of a number of useful tools available to growers.

Development of Resistance

Herbicide Resistance Action Committee (HRAC) is an industry initiative which fosters co-operation between plant protection manufacturers, government, researchers, advisors and farmers. The objective of the working group is to facilitate the effective management of herbicide resistance. HRAC has identified a number of factors to consider when evaluating herbicide resistance risk. The most important factors influencing a plant's potential to develop resistance are:

• Biology and genetic make up of the weed species in question: Points of consideration include the following . Weeds that are extremely susceptible to an herbicide, are prolific seed producers and have a large amount of genetic variation within the species may have a greater potential to become resistant to an herbicide. The initial frequency of naturally occurring resistant biotypes in a weed population influences a weed population's potential to develop resistance. Also, the relative fitness or vigor of resistant weed biotypes affects resistance development. Generally speaking, for any particular weed species, the greater the initial frequency of resistance and the greater the fitness of the resistant biotype, the greater the potential for herbicide resistance to develop.
• History of herbicide use: continuous use of the same mode of action herbicide for several consecutive years, without tank mixing or sequentially applying herbicides with other modes-of-action, may increase the risk for resistant populations to develop. The greater the selection pressure exerted by an herbicide, the greater the potential for resistance. Although a higher rate of application or sequential applications results in a high level of weed control, it also represents an increased potential for the development of resistance. Likewise, lower herbicide rates, which provide less effective weed control, exert less selection pressure. Label herbicide rates are a reflection of efficacy trials that indicated crop yield responses and best control of a number of different weeds.
• Crop management practices: weed control that relies solely on herbicide use and does not combine tillage or other cultural practices with herbicide applications may increase the potential for resistant populations to develop. This includes using crop rotation practices that allow for non-chemical options for weed control as well as influence the ability to rotate herbicide mode of action and frequency of use.
• Environmental conditions: environmental conditions that are not conducive to herbicide breakdown in the soil may increase the potential for resistant populations to develop. Continuous dry weather can slow the breakdown of many herbicides (e.g. imidazolinones). High soil pH inhibits the breakdown of some herbicides like SU's. The longer a herbicide persists, the longer it exerts selection pressure on a weed population, particularly if there are multiple weed flushes in one growing season.
• Weed seed bank / Seed soil dormancy: a high soil seed bank within an individual field increases the selection pressure, which in turn increases the likelihood of resistance developing. Seed soil dormancy will also impact on resistance development. Plants with longer soil dormancy will tend to exhibit slower resistance development since selection pressure is reduced. Seeds that can survive for years in the soil may slow the onset of resistance. Weeds with a long seed life may create a large seed bank in the soil. This seed bank serves as a buffer against genetic changes in the weed population, since the seeds do not normally all germinate within one year. Conversely, weed seed with a short seed life germinate within one or two years. This rapidly depletes the quantity of susceptible weed seed and gives any resistant seed a competitive advantage when a selection pressure is applied.

Many exceptions to these generalizations exist, and this makes it difficult to predict which species will develop a resistant population. The time required for a weed population to develop resistance will vary, and depends on many factors, including:

• Selection pressure exerted by the herbicide;
• Herbicide rotation patterns;
• Seed germination dynamics;
• Use of herbicide combinations with different modes of action;
• Initial frequency of naturally occurring resistant individuals in the weed population; and
• The relative vigor of resistant biotypes of weeds.

Based on these factors, models have been developed to predict the development of resistance in a weed population. Current models provide an indication of the development of resistance; these indications are an essential input to the development of resistance management strategies and practices.

Identifying Weed Resistance

It is important to avoid confusing herbicide failure caused by weed resistance with herbicide failure caused by other factors. All other possible reasons for poor herbicide performance should be ruled out before considering the possibility of resistance. These include application error and poor environmental conditions at the time of herbicide application. Shifts in weed populations from susceptible species to species that are less sensitive can also cause weed control problems. Herbicide resistance should be suspected under the following conditions:

• A weed species that is normally controlled by the herbicide now escapes treatment, while other weeds on the label are controlled.
• Other factors such as application error or adverse weather conditions are ruled out.
• Irregular shaped patches of a weed develop in the field and are not controlled with the particular herbicide.
• Weed control records for the field indicate repeated use of a particular herbicide, or herbicides from the same herbicide group.

Weed Resistance Management

Herbicides have been grouped based on their mode of action. Herbicides that are in Group 2 are those classed as ALS/AHAS inhibitors. BASF markets herbicides that are in the imidazolinone chemical family and they are members of Group 2. Herbicides such as ABSOLUTE, ODYSSEY and ADRENALIN used in the Clearfield Production System are examples of Group 2 herbicides. BASF is therefore a key stakeholder in resistance management of ALS resistant weeds. BASF is committed to maintaining the efficacy of all of its herbicides in order to provide growers with effective, high performance, environmentally sound products for many years.

The key to the performance of Clearfield Production Systems is effective weed resistance management. BASF Crop Protection is committed to delivering sustainable cropping systems that incorporate best practice principles. The Clearfield Production Systems provide alternative options for growers within a well-managed rotation. This BMP guide is designed to assist agronomists and growers who have chosen to use the Clearfield Production Systems, in making decisions which best manage herbicide resistance in weed populations.

Following is a discussion and proposed strategies for managing ALS herbicide resistance in weed populations under the Clearfield Production System. The guidelines for managing the development of weed resistance presented here are consistent with recommendations put forward in provincial crop protection guides and the Weed Resistance Education and Action Program (WREAP, http://www.wreap.ca).

General Recommendations to Minimize Development of Weed Resistance

Development of herbicide resistant weeds can be avoided or delayed through good management practices. The recommendations listed below take into consideration many of the points discussed so far and are designed to provide an integrated approach to weed management in order to prevent or delay the onset of weed resistance. Three key areas of integrated weed management are chemical control, cultural practices and crop management.

Chemical Control

• Know your herbicide groups.

  Reliance on one product, or on products within the same group may eventually lead to weed resistance. Understanding herbicide classification by product group is necessary to develop an effective weed management strategy.

• Keep records of herbicide application.

  Herbicide application records and field mapping are necessary to effectively rotate herbicide groups

• Always read and follow herbicide label recommendations.

  The rate of herbicide recommended provides the most effective control over a wide range of environmental conditions. This will help to ensure weed seed is not added to the seed bank, while minimizing selection pressure.

• Use tank mixes or sequential applications of herbicides that have different modes of action. 

  Tank mixes allow you to control weeds in more than one way by combining two or more modes of action on the same weed. In order to be effective, both active ingredients need to provide control of the target weed and have similar residual activities. This minimizes selection pressure and delays the development of weed resistance.

• Rotate among herbicide groups.

  Rotate among herbicide groups for both grass and broadleaf weed control. Alternating products used according to mode of action is one of the most effective means of delaying development of weed resistance. Use the minimum number of applications of any one herbicide or herbicide group per season.

• Utilize non-selective herbicides.

  Non-selective herbicides applied pre-emergence are an effective means of controlling early flushes of weeds and/or weed escapes. They should be used in conjunction with an in crop herbicide that has an alternative mode of action.

Cultural Control/Crop Management

Cultural (non-chemical) weed control practices do not exert any chemical selection pressure and can help to reduce the level of weeds in the soil seed bank. These practices are important components of an integrated weed control strategy.

• Use crop rotations, notably rotations from broadleaf crops to grass crops.

  Different crops can help to alter the weed spectrum. It also makes it easier to rotate between herbicide groups. A four-year crop rotation is recommended for sunflower, mostly because of its high susceptibility to sclerotinia ( Sclerotinia sclerotiorum). In the intervening years, other broadleaf crops such as beans, peas, and canola should be avoided as they act as hosts for the disease.

• Plant competitive crops.

  Crops vary in their ability to compete with weeds. Crops such as barley that establish a heavy stand are much more competitive with weeds then crops such as lentils or flax. A competitive crop can reduce weed pressure.

• Delay planting

  Delayed planting will allow for the initial flush of weeds to germinate. This flush can be controlled with cultivation where practical, or through the use of a non-selective herbicide.

• Plant quality seed/Increase your seeding rat

  Certified seed offers a number of benefits and helps to ensure a uniform stand that will compete more effectively with weeds. A more dense plant canopy offers more weed competition.

• Combine tillage and/or timely cultivation with herbicide treatments, if practical.

  If your crop production system includes spring cultivation, seed as soon as possible after working the ground to give the crop a head start over the weeds.

An effective weed management strategy is comprised of multiple weed control options. Herbicide tolerant cropping systems provide yet another mechanism for effective weed control and should be considered as one of the tools for managing the development of weed resistance.

Integrated Weed Management (IWM) in the Clearfield Sunflower Production System

Clearfield crops are tolerant to the imidazolinones, which are Group 2 herbicides. Group 2 herbicides work by inhibiting acetolactate synthase, an enzyme that is required for the production of the amino acids leucine, isoleucine and valine in plants. Group 2 herbicides are known as 'ALS inhibitors'.

Continuous use of Group 2 herbicides may result in the selection of weed biotypes with a resistance to this Group of herbicides. Preservation of the effectiveness of this Group of herbicides is vital for efficient and cost-effective agricultural production in Canada. Therefore effective management strategies for weed control that delay or avoid the potential for the development of resistant weeds is an important focus of the Clearfield production system.

In addition to the general recommendations outlined in the previous section, a number of specific management strategies, outlined below, should be followed when using the Clearfield Sunflower Production System. Growers and agronomists should give consideration to each of the following points, in order to develop an integrated weed management plan utilizing Clearfield sunflower within their crop rotation.

• Follow the recommended crop rotation practices for sunflower when growing Clearfield sunflower.

The production of other crops allows for the use of alternate mode of action herbicides and tillage, where appropriate, to delay the development of herbicide resistance. Proper crop rotation promotes good agronomics by reducing disease and insect pressure in sunflower.

• Use alternate (non-group 2) mode of action herbicides with activity on sunflower in the rotational crops. This includes herbicides that are classed as growth regulators or photosynthetic inhibitors. Examples are discussed in more detail under the volunteer management section.

This reduces the selection pressure from continuous dependence on ALS inhibiting imidazolinone herbicides. It provides an alternative mode of action to control volunteer sunflowers that may be present

• Apply no more than two (2) Group 2 herbicides in any four (4) year period on the same field. Make only one application of a Group 2 herbicide per season.

The use of no more than two applications in four years will slow the development of resistance. Where possible, use sequential applications of partner herbicides or tank mix herbicides with multiple modes of action on target weed species. Consideration should be given to less frequent use of Group 2 herbicides to delay the onset of resistance development. This includes the use of Group 2 herbicides both within the Clearfield Production System and in conventional crops.

• If a Group 2 herbicide has been applied as a pre-emergence application, DO NOT apply further Group 2 herbicides to that crop. Make any further post-emergence applications with herbicides from a different mode-of-action group.
• Where it is possible, care should be taken to avoid applications of Group 2 herbicides in consecutive years unless at least two years previous effective weed control has been achieved, with methods other than the use of Group 2 herbicides.
• Do not plant Clearfield sunflowers on cropland or near road ditches, field borders, fence rows etc. with a history of heavy infestations of wild sunflower.

This will help minimize the potential of cross-pollination of wild type sunflowers with Clearfield sunflower.

• When possible, control wild sunflower in areas around Clearfield sunflower fields (road ditches, field borders, fence rows) through the use of non-group 2 herbicides and/or mowing prior to seed set. If the area adjacent to the field has a heavy infestation of wild sunflower, or if you cannot control the wild sunflower population because it is growing in an adjacent area that does not belong to you, DO NOT plant Clearfield sunflower.

This will help minimize the potential of cross-pollination of wild type sunflowers with Clearfield sunflower. It also promotes good sanitation practices by eliminating vectors for insects and disease.

• Control emerged wild sunflower prior to planting Clearfield sunflowers. Utilize non-group 2 burndown herbicides in no-till or min-till situations, or tillage in conventional-till systems.
  Non-selective herbicides applied pre-emergence are an effective means of controlling early flushes of weeds. This will reduce the reliance on ALS herbicides for controlling wild sunflower.
• Where Group 2 resistance is suspected within a weed population, testing of the relevant weeds should be carried out prior to the use of crops in the Clearfield Production System. Please ask your local BASF sales representative for more information.
• Integrated Weed Management should be undertaken on a field-by-field basis. Specific field planning should take into consideration the history of the field well as the future use options.

Resistance management guidelines for other herbicide mode of action Groups should be taken into account when developing and planning rotations

Controlling Volunteers from Clearfield Sunflower

Objective: control of all volunteers from Clearfield crops before flowering.

Summary

• Best Management Practice is to control volunteer plants in the year following when a Clearfield crop has been grown.
• Do NOT rely on Group 2 herbicides to control volunteer sunflowers.
  Clean up of farm equipment during all stages of sowing, harvesting, storage and transport are important in the effective control of volunteers because good sanitary practices will reduce or eliminate the movement of seed.
• Best Management Practice is to make volunteer control part of weed, pest and disease management strategies for the farm.
• Clearfield sunflowers will be controlled by all herbicides currently registered for control of this plant, with the possible exception of sulfonylureas, where a low level of cross-tolerance could result in unacceptable weed control. Clearfield Sunflower volunteers will be controlled by glyphosate, and a range of herbicides registered for use in wheat, barley and other crops.

Important Reasons for Control of Volunteers

• Volunteer plants act as competitive weeds in following crops.
• Volunteers may be important in the build-up and spread of major diseases.
• Cross-pollination from volunteer plants to conventional plants of the same crop species or to wild relatives increases the risks of herbicide tolerance spreading.

Controlling Volunteers

A number of cultural and chemical control options are available to control volunteers including:

• Minimize seed losses at harvest
• Close attention to timeliness of harvesting.
• Correct adjustments in the header.
• Seal all holes and cracks in harvesting equipment which allow spillage, even of small quantities of seed, (especially in the table, front elevator and grain tank).
• Clean out harvest equipment before switching fields.
• Practice good hygiene at harvest and during transport of grain.
• Practice good stubble management after harvest in light of crop rotation decisions.
• Use pre-seed glyphosate application to control weeds and volunteers that emerge prior to seeding.
• Control ALL volunteers in following crops with proper selection of in-crop herbicides.

Management Actions

Cultivated sunflowers are produced on a relatively limited acreage in western Canada. Wild sunflowers are also not listed as a common weed of agricultural land in western Canada. In the recent provincial weed surveys conducted in Manitoba (2002) and Saskatchewan (2003), volunteer sunflower was ranked as the 66^th and 85^th most abundant weed, respectively. Consequently, wild sunflowers, or volunteer sunflowers are often not targeted by herbicide companies when label recommendations are developed. Normally a cereal crop is planted following sunflowers. This is sound agronomic practice from the point of view of not only weed management, but also to limit the spread of insects and diseases that occur in sunflowers to crops such as canola, beans or peas which may also be susceptible.

In addition to the general management recommendations listed below, a summary of the available herbicide options for volunteer sunflower control in cereal crops (wheat and barley) is discussed. The listing is based on the 2004 Guide to Crop Protection and includes all herbicides that were rated as providing fair, good or excellent control of volunteer sunflowers. Weed control claims for volunteer sunflower control are not listed on any imidazolinone product label in Canada. ASSERT (imazamethabenz) herbicide is in fact, registered for use on conventional sunflowers grown as a crop.

• Keep good field records of herbicides used in previous crops and herbicide-tolerant varieties in neighboring fields to develop effective plans for controlling volunteers.
• A crop of the same species should not follow a Clearfield crop, as controlling volunteers within the same crop species is difficult. Generally speaking, this would also not be a good agronomic practice for disease and weed management.
• Non selective burndown products containing glyphosate should be utilized for volunteer sunflower control before seeding. Commercial sunflowers usually lack long term seed dormancy and possess traits such as synchronous germination. Therefore, pre-seed weed control with a burndown product is a viable tool for volunteer sunflower management.
• The following list highlights options for volunteer sunflower control in cereal crops. The herbicides are listed by active ingredient and herbicide Group, not trade name. The list of commercial products available to control sunflower would actually be larger. As mentioned earlier, while Clearfield sunflowers are not commercially cross-resistant to the sulfonylurea (SU) herbicides, which are also Group 2 herbicides, they may not be completely controlled by an SU alone. Choose a non-Group 2 herbicide for control, or tank mix an SU with a Group 4 herbicide such as 2,4-D or MCPA. There is a full range of growth regulator (Group 4) and photosynthetic inhibitors (Group 6) available that provide volunteer sunflower control in cereal crops.

  Active Ingredient	Herbicide Group
  Bromoxynil + MCPA	6, 4
  Bromoxynil + 2,4-D	6, 4
  Clopyralid + MCPA	4
  Dicamba + MCPA/2,4-D	4
  Dicamba + MCPA K+	4
  Dicamba + 2,4-D amine + Mecoprop	4
  Dicamba + Mecoprop + MCPA	4
  Diclorprop + 2,4-D	4
  Fluroxypyr + Clopyralid + MCPA	4
  Tribenuron methyl + 2,4-D	2, 4
  Thifensulfuron methyl + MCPA	2, 4
  Thifensulfuron methyl + Tribenuron methyl + MCPA (tank mix)	2 + 4
  Thifensulfuron methyl + Tribenuron methyl + 2,4-D (tank mix)	2 + 4

Managing Out-crossing to Non-Clearfield Crops and Weeds

What is Out-crossing and Gene Flow?

Gene flow is the movement of gametes, zygotes (seeds), individuals, or groups of individuals from one place to another and their subsequent incorporation in the gene pool of the new locality (Slatkin, 1987). Gene flow is a natural biological process and in plants it primarily occurs via pollen or seed dispersal (Levin and Kerster, 1974). The relative importance of gene flow to population genetic structure depends on the distance between donor and recipient populations, population size, how long the process has been in effect, and whether the new gene confers any fitness advantage to the recipient population (Waines and Hegde, 2003).

Out-crossing (or cross-pollination) is a type of mating in plants in which a male gamete of one individual fertilizes a female gamete of another individual (Waines and Hegde, 2003). The term out-crossing generally refers to mating within a species and the term has been used synonymously with gene flow (Gleaves, 1973; Handel, 1983). However, as defined earlier, gene flow can occur through means other then cross-pollination. We will restrict this discussion to pollen-mediated gene flow (out-crossing).

A wide range of factors influence the ability of a given plant to out-cross with others, including timing of flowering, stigma receptivity, pollen production, pollen dispersal, pollen viability and environmental factors. The likelihood of out-crossing varies greatly from species to species and variety to variety. Successful out-crossing may result in the offspring displaying characteristics of both parent plants.

The risk of out-crossing can vary between crop and weed species. The focus for management must be on the control of volunteers and managing herbicide tolerant crops and related species.

Potential for Out-crossing in Clearfield Sunflower

Sunflower has been demonstrated to hybridize with many other Helianthus species (Burke et al., 2002). The potential environmental impact of Clearfield sunflower hybrids resulting from interspecific and intergeneric crosses between the crop and related species depends on a number of factors, including:

• The potential for the related species increasing in weediness.
• The potential of the related species to cause ecosystem disruption.

Potential for Out-crossing to Species Related to Sunflower

Because the cultivated H. annuus was originally a cross pollinating crop and rather self-incompatible, it crosses readily with other Helianthus species. The genus has about 67 species (Heiser, 1978). It includes annuals and polyploid perennials, but it is the annual species that cultivated sunflower is most likely to cross with. In western Canada, the two species of Helianthus that occur most commonly are the annual Helianthus annuus (wild sunflower) and, to a lesser extent, the perennial Helianthus tuberosus (Jerusalem artichoke).

The wild H. annuus is the most common species of Helianthus in western Canada. It is a common roadside weed in the southern parts of the prairies, particularly in Manitoba. It is a tall branching plant with an indeterminate growth habit, numerous seed heads and self-incompatible flowers (Rogers et al., 1982). Wild sunflower is also well adapted to a range of environmental conditions which contribute to its weediness (Sieler and Rieseberg, 1997).

Cultivated hybrid varieties of sunflower are self-compatible. However, the level of self-fertility expressed is influenced by the environment (Snow et al., 1998). Sunflower was originally a cross- pollinating crop. Wild sunflower populations and open pollinated varieties are highly self-incompatible and must be cross-pollinated (Rogers et al., 1982). Pollen transfer occurs primarily by insect pollinators, primarily bees. Crop specific genetic markers have been demonstrated to be carried by bees distances of up to 1000 meters (Snow et al., 1998). Hybridization between wild and cultivated sunflower was demonstrated to occur at a level of 2% when the species were up to distances of 1000 meters apart (Arias and Rieseberg, 1994). These same researchers demonstrated that up to 42% of progeny from wild plants growing near cultivated fields of sunflower were F1 hybrids.

Several perennial species occur in Canada. The most conspicuous is H. maximiliani which flowers on roadsides in late summer and fall. Some H. giganteus occurs in pockets and H. tuberous (Jerusalem artichoke) is found primarily on riverbanks. Jerusalem artichoke has been cultivated to a small extent for its tubers. Hybridization with perennial species that are found in Canada occurs very rarely in nature. Some crosses of these species with cultivated sunflower have been achieved using artificial methods.

Potential for Introgression from Clearfield Sunflowers into Relatives

Genetic cultivar markers are readily found in wild populations of H. annuu (Linder et al., 1998). The authors conclude that there is no strong barrier to introgression between cultivated and wild sunflower and suggest the only limitation to the introgression of transgenes into wild populations would be if there is a decrease in fitness that accompanies the trait.

Marshall et al., 2001 studied gene flow, growth and competitiveness of imazethapyr resistant wild sunflower. They observed no differences in competitiveness of resistant wild sunflower compared to susceptible biotypes. In this same study gene flow from resistant to susceptible biotypes occurred with movement up to 15.5 meters.

Under suitable conditions, the germination of F1 hybrids in their first year has been observed (Snow et al., 1998). This study also indicated that even after 3 years of burial in soil at depths of 10-20 cm, approximately 20% of the hybrid seed was viable. The dormancy rate of the F1 hybrid depends on whether the cultivated or wild sunflower acted as the pollen donor.

A number of factors discussed above indicate that gene flow between cultivated and wild sunflowers can potentially spread wild sunflower resistance to imidazolinone herbicides:

• Outcrossing between cultivated and wild sunflowers occurs.
• Outcrossing between resistant and susceptible biotypes is a high probability.
• No difference in fitness has been found between resistant and susceptible sunflower biotypes.
• Dormancy characteristics of F1 hybrids may allow a seed bank to develop and persist.

Outcrossed CL sunflowers may not exhibit commercial levels of herbicide tolerance, since the trait is only semi-dominant

Managing Out-crossing of Clearfield Sunflowers to Related Species

Many of the integrated weed management practices outlined earlier to delay the development of weed resistance and for controlling Clearfield sunflower volunteers will be effective in minimizing the impact of outcrossing. These include: 

• Control all volunteers in the season after growing Clearfield Sunflowers. (Refer to earlier section, Controlling Volunteers with Clearfield Technology).
• Follow the recommended crop rotation practices for sunflower when growing Clearfield sunflower.

  The production of other crops allows for the use of alternate mode of action herbicides and tillage, where appropriate, to delay the development of herbicide resistance. Proper crop rotation promotes good agronomics by reducing disease and insect pressure in sunflower.

• Use alternate (non-group 2) mode of action herbicides with activity on sunflower in the rotational crops. This includes herbicides that are classes as growth regulators or photosynthetic inhibitors. Examples are discussed in more detail under the volunteer management section.

  This reduces the selection pressure from continuous dependence on ALS inhibiting herbicides. It provides an alternative mode of action to control volunteer sunflowers and other ALS resistant weeds that may be present.

• Apply no more than two (2) Group 2 herbicides in any four (4) year period on the same field. Make only one application of a Group 2 herbicide per season.
• Do not plant Clearfield sunflowers on cropland or near road ditches, field borders, fence rows etc. with a history of heavy infestations of wild sunflower. 

  This will help minimize the potential of cross-pollination of wild type sunflowers with Clearfield sunflower.

• When possible, control wild sunflower in areas around Clearfield sunflower fields (road ditches, field borders, fence rows) through the use of non-group 2 herbicides and/or mowing prior to seed set. If the area adjacent to the field has a heavy infestation of wild sunflower, or if you cannot control the wild sunflower population because it is growing in an adjacent area that does not belong to you, DO NOT plant Clearfield sunflower.

  This will help minimize the potential of cross-pollination of wild type sunflowers with Clearfield sunflower. It also promotes good sanitation practices by eliminating vectors for insects and disease.

• Control emerged wild sunflower prior to planting Clearfield sunflowers. Utilize non-imidazolinone burndown herbicides in no-till or min-till situations, or tillage in conventional-till systems.

  Non-selective herbicides applied pre-emergence are an effective means of controlling early flushes of weeds. This will reduce the reliance on imidazolinone herbicides for controlling wild sunflower

• Integrated Weed Management should be undertaken on a field-by-field basis. Specific field planning should take into consideration the history of the field well as the future use options.

  Resistance management guidelines for other herbicide mode of action Groups should be taken into account when developing and planning rotations.

2. Communication and monitoring of the Best Management Practice Program for the Clearfield™ Sunflower Production System

BASF intends to communicate the stewardship plan (BMP) to producers through both broad-based and targeted approaches. Broad-based communications include: Technical bulletins, Business Representative presentations and the BASF AgSolutions web-site. Targeted communication will be achieved through the Clearfield Commitment, which is a user agreement established between BASF and the producer growing Clearfield seed. The Clearfield Commitment will allow specific stewardship messages to be directed to all growers of Clearfield sunflowers via mail, e-mail, dedicated web-sites and the Ag Solutions toll-free line and staff.

Monitoring that the BMP is understood and being followed will be through audits, specific surveys of grower production practice and the use of proprietary databases.

References

Arias, D.M. and L.H. Rieseberg. 1994. Gene flow between cultivated and wild sunflowers. Theor. Appl. Genet. 89:655-660.

Burke, J.M., K.A. Gardner and L.H. Rieseberg. 2002. The potential for gene flow between cultivated and wild sunflower ( Helianthus annuus) in the United States. Amer. J. Bot. 89(9): 1550-1552.

Gleaves, J.T. 1973. Gene flow mediated by wind-borne pollen. Heredity 31:355-366.

Handel, S.N. 1983. Pollination ecology, plant population structure, and gene flow. P. 163-211. In L. Real (ed.) Pollination biology. Academic Press, Orlando, FL.

Heiser, C. B., Jr. Taxonomy of Helianthus and Origin of Domesticated Sunflower. 1978 In: Sunflower Science and Technology. Agron. 19. pp. 31-53. Ed. Carter, J. F.

Levin, D.A., and H.W. Kerster. 1974. Gene flow in seed plants. Evol. Biol. 7:139-220.

Linder, C.R., I. Taha, G.J. Seiler, A.A. Snow, and L.H. Rieseberg. 1998. Long-term introgression of crop genes into wild sunflower populations. Theor. Appl. Genet. 96:339-347

Marshall, M.W., K. Al-Khatib, and T. Loughin. 2001. Gene flow, growth, and competitiveness of imazethapyr-resistant common sunflower. Weed Science 49:14-21.

Rogers, C.E., T.E. Thompson, and G.J. Seiler. 1982. Sunflower species of the United States. Bismarck, ND: National Sunflower Association. 75 p.

Sieler, G.J. and L.H. Rieseberg. 1997. Systematics, origin and germplasm resources of the wild and domesticated sunflower. Pages 21-65 in A.A. Schneiter, ed. Sunflower Technology and Production. Agron. Monogr. 35. Madison, WI: ASA, CSSA, and SSSA.

Slatkin, M. 1987. Gene flow and the geographic structure of natural populations. Science (Washington, DC) 236:787-792.

Snow, A.A., P. Moran-Palma. L.H. Rieseberg, A. Wszelaki and G.J. Seiler. 1998. Fecundity, phenology, and seed dormancy of F1 wild-crop hybrids in sunflower ( Helianthus annuus, Asteraceae). Amer. J. Bot. 85(6): 794-801.

Waines, J. G., and S.G. Hegde. 2003. Review and Interpretation. Intraspecific gene flow in bread wheat as affected by reproductive biology and pollination ecology of wheat flowers. Crop Sci. 43:451-463.

This bulletin is published by the Plant Health and Biosecurity Directorate and the Animal Health Directorate of the Canadian Food Inspection Agency. For further information, please contact the Plant Biosafety Office or the Animal Feed Division at: