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Decision Document DD2004-48
Determination of the Safety of BASF Canada's  Imidazolinone-Tolerant (CLEARFIELD™) Wheat Teal 11A

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Issued: 2004-06

Appendix 1: CLEARFIELD™ Wheat Herbicide Tolerance Stewardship Plan

1. Best Management Practice Program for the CLEARFIELD™ Wheat Production System


The CLEARFIELD™ Production System for Wheat is an innovative cropping system that offers a new way to grow wheat and enhanced weed control. It creates a number of new opportunities to western Canadian producers:

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:

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


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

FOLLOW the Best Management Practices outlined in the our CLEARFIELD™   Wheat 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 manufactures, 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 best control and crop yield responses.

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 influence the ability to rotate herbicide type 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:

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:

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 better method practices 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 WREAP.

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

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.

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™ Production System

CLEARFIELD™ Production System 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 delay or avoid the potential for the development of resistant weeds is an important focus of the CLEARFIELD™ production system.

The CLEARFIELD™ Production System for wheat currently exclusively utilizes the herbicide ADRENALIN. This CLEARFIELD™ herbicide combines herbicides with two modes of action. The two active ingredients are imazamox (Group 2) and 2,4-D ester (Group 4). These two herbicides have both have activity on a number of the target weeds in wheat. As outlined earlier, combining two modes of action is an effective means of delaying the onset of herbicide resistance.

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™ Wheat Production System. Growers and agronomists should give consideration to each of the following points, in order to determine an integrated weed management plans utilizing CLEARFIELD™ wheat within their crop rotation.

Controlling Volunteers from CLEARFIELD™ wheat

Objective: control of all volunteers from CLEARFIELD™ crops before flowering.


Important reasons for control of volunteers

Controlling volunteers

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

Management actions

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.

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 spikelet formation, 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™ Wheat

While wheat is a predominantly self-pollinating crop, it is recognized that low levels of out-crossing can occur (Allan, 1980). Specific information is provided here in relation to possible out-crossing of resistant traits to other wheat varieties and related species.

Description of Triticum aestivum varieties with CLEARFIELD™ Technology

CLEARFIELD™ wheat varieties have tolerance to the BASF imidazolinone herbicide, ADRENALIN®. CLEARFIELD™ wheat, was derived from inbreeding and chemically induced modification. It was not derived from recombinant DNA technology, i.e. it is not genetically engineered (GE). This variety was developed via conventional backcrossing methods and is in the CWRS class.

Potential for out-crossing to species related to wheat

Close relatives of wheat in Canada are limited to diploid wheat T. monoccum and tetraploid wheat T. turgidum var. durum (durum wheat). There are only a few reports concerning natural cross hybridization with related species and genera. Hybridization within the genus Triticum has been shown to be generally less than 6%. Wheat is primarily self-pollinated and the potential for out-crossing with a related species is unlikely. There are no known wild Triticum species to exist in North America.

A well-known inter-generic combination involving wheat is triticale, derived from crossing between wheat and rye (Secale cereale). There have been no eports of triticale serving as a bridge for hybridization with other wild grass species.

The risk for gene flow between wheat and weedy relatives is extremely low. There are a number of weedy relatives of wheat present in Canada from the genera Aegilops, Agropyron, Secale, Haynaldia, Hordeum and Elymus. Complex (artificial) hybrids have been made between wheat and several weedy relatives. However, it has only been achieved through deliberate cross-pollination in controlled environment settings. There have been no known naturally occurring hybrids or derived species reported.

Potential for out-crossing to non-CLEARFIELD™ wheat varieties

Although wheat cultivars are classified as self-pollinators, out-crossing in spring wheat has been documented at very low incidences. In ten Canadian spring wheat cultivars, out-crossing rates ranged from 0% to 6.7% in adjacent rows (Hucl, 1996). In another study, Hucl and Matus-Cadiz (2001) examined gene flow in four wheat cultivars. In this study, maximum gene flow in adjacent rows (30 cm distance) ranged from 0.2% to 3.8%. Typically when describing average out-crossing rates in wheat an average out-crossing frequency of <1% is used. This percentage is based on the Hucl (1996) study where the average out-crossing rate of ten cultivars over two years was 0.88% at a 20 cm distance from the pollen source. Comparatively, canola has relatively high out-crossing rates with an average out-crossing frequency of 20%.

As indicated earlier, a wide range of factors influence the ability of a given plant to out-cross with others, including spikelet type, timing of flowering, stigma receptivity, pollen production, pollen dispersal, pollen viability and environmental factors. We have very briefly tried to describe how some of these factors can influence out-crossing in spring wheat.

In ten Canadian spring wheat cultivars, out-crossing rates ranged from 0% to 6.7% in adjacent rows (Hucl, 1996). In this study, the variety Oslo that has open spikelets produced one of the highest out-crossing rates, while cultivars such as CDC Makwa and Columbus, which have dense spikes, had the lowest out-crossing rates.

Glume opening, the extrusion of anthers and the duration of opening are all significant factors affecting out-crossing. The most important factor that influences cross-pollination and potential gene flow during anthesis is the extent to which the flower opens (Waines and Hegde, 2003). In the same study mentioned earlier, Hucl (1996) demonstrated that cultivars with high out-crossing rates tended to have greater floret opening at anthesis.

Stigma receptivity decreases gradually from the moment of flower opening and has been shown to range from 2 to 13 days after anthesis (De Vries, 1971). The wide range in receptivity has been attributed to environmental conditions and is longest under moderate temperature and humidity.

Waines and Hegde (2003) discussed the role environment plays in effecting anthesis and seed set. In summary, the elimination of plant stress during flowering and the early stages of seed development will diminish the success of gene flow, and will favor self-pollination.

Pollen production in self-pollinating plants like wheat has been demonstrated to be much less then open-pollinated crops like rye. Wheat plants were demonstrated to have only one tenth the pollen production of rye plants (D'Souza, 1970). Under optimum field environmental conditions, wheat pollen viability was approximately 0.5 hours (D'Souza, 1970; De Vries, 1971).

Wheat pollen is relatively heavy compared to other grass pollen (Lelley, 1966). In addition humidity, temperature and wind play a role. Humid weather makes pollen heavy and will reduce dispersal distances from the plant. Dry hot weather causes desiccation and reduces pollen viability. Wheat pollen travels relatively short distances. Jensen (1968) reported that 90% of wheat pollen remains within 6 meters of its source. Hucl and Matus-Cadiz (2001) examined gene flow of 4 wheat cultivars over distance and also examined the impact of wind direction. For the four cultivars, the maximum gene flow rates were recorded at the shortest distance of 30 cm from the source (adjacent rows) and were found to range from 0.2% to 3.8%. Gene flow rapidly decreases as distance from the source increased. At a 27-meter distance, gene flow was observed in only 2 of 32 samples (0.09%). These types of studies have formed the basis for the 30-meter reproductive isolation distances from all wheat species and wheat volunteers when conducting for confined field trials of wheat.

CLEARFIELD™ wheat is a CWRS wheat developed from the same germplasm pool reflected in the Hucl (1996) study. Out-crossing rates for CLEARFIELD™ wheat lines should be within the range reported in this study.

There have been no reports showing that Triticum aestivum exhibits characteristics of a pest or is a weed in Canada. Centuries of breeding of wheat varieties has selected for a number of traits, which give our modern wheat varieties a poor ability to survive in the wild. Traits such as heads that did not shatter were favored due to easier harvest but this trait placed the wheat plants at a disadvantage to plants of other species, which could more efficiently distribute seed. In addition, hull-less type plants were easier to thresh but exposed the developing seed to environmental extremes.

Despite these disadvantages, plants of modern wheat cultivars are occasionally found in uncultivated fields and roadsides. These occurrences are usually associated with grain dropped during harvest or transport. Plants growing in these environments do not persist and are usually eliminated by mowing, cultivation, and/or herbicide application.

Wheat plants can also grow as volunteers in a cultivated field following a wheat crop. Volunteer wheat plants that germinate in a crop or summer fallow following the use of CLEARFIELD™ crops should be controlled before flowering, as outlined in the Controlling Volunteers section of these Stewardship Guidelines. After almost 200 years of cultivation in Canada and throughout the world, there have been no reports of wheat becoming an invasive pest.

The preceding discussion demonstrates that out-crossing to other wheat varieties is possible, however the likelihood of this occurring can be minimized if good hygiene and rotation practices are followed and if volunteers are controlled prior to flowering.

Managing out-crossing of CLEARFIELD™ wheat to related species

As stated, the likelihood of out-crossing to related species is low for wheat. However the following practices should be followed to help in minimize the potential for out-crosses occurring.

2. Communication to growers

BASF will utilize a number of vehicles to communicate the general recommendations of the CLEARFIELD™ Wheat Stewardship Plan to growers:

Communication with our end use customer is extremely important to maintaining a high level of customer satisfaction. Currently there are a variety of options available to growers to contact the company to address questions, request information, or to report problems. These options have been demonstrated to be effective, and have allowed BASF to adopt a policy committing we will respond to Customer Care Inquiries (product performance inquiries) within 48 hours of receiving a call. Growers will use the same means of communication to report problems associated with our stewardship recommendations. Growers communicate with BASF in a variety of ways, including:

3. Monitoring activities


Allan, R.E. 1980. Wheat. P. 709-720. In W.R. Fehr and H.H. Hadley (ed.) Hybridization of crop plants. ASA and CSSA, Madison, WI.

De Vries, A. Ph. 1971. Flowering biology of wheat, particularly in view of hybrid seed production-A review. Euphytica 20:152-170.

D'Souza, V. L. 1970. Investigations concerning the suitability of wheat as pollen-donor for cross-pollination by wind compared to rye, Triticale, and Secalotricum. Z. Pflanzenzuecht. 63:246-269.

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.

Hucl, P. 1996. Out-crossing rates for 10 Canadian spring wheat cultivars. Can. J. Plant Sci. 76:423-427.

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

Lelley, J. 1966. Observation on the biology of fertilization with regard to seed production in hybrid wheat. Der Zuchter 36:314-317.

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

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

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.

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