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  2003 Table of Contents

Weed Management Strategies for CLEARFIELD* Wheat Systems
Across PNW Precipitation Zones
D. A. Ball, C. Rainbolt, D. C. Thill, J. P. Yenish

Winter wheat ranks high in importance as an agricultural crop in the Pacific Northwest states of Washington, Idaho, and Oregon. These states rank 3rd, 7th, and 18th in U.S. winter wheat production with a total estimated value of production of over $633 million in 2000 (National Ag. Statistics Service). Winter wheat is a winter annual grass that is planted and emerges in the fall; overwinters as a small plant; grows fast and develops tillers in the spring; and is harvested in July and August. Winter annual grass weeds such as jointed goatgrass, downy brome, feral rye, and Italian ryegrass have the same growth cycle as winter wheat and are difficult to control in conventional wheat-fallow rotations. These weeds annually account for millions of dollars of lost wheat production and reduced quality – dockage. There has been moderate success in controlling winter annual grasses in wheat by utilizing multiple-year crop rotations with spring crops and fallow periods, and with chemical control of weeds before and after the wheat crop. However, before Beyondä herbicide in CLEARFIELD* wheat, there was no herbicide that could effectively control jointed goatgrass or feral rye in winter wheat.

The CLEARFIELD Production System for wheat is a unique system comprised of herbicide-resistant wheat varieties, Beyond herbicide to manage problematic weed species, and a stewardship agreement with growers to ensure the use of best management practices for system sustainability. The first publicly-developed CLEARFIELD winter wheat varieties to be released in the United States, ‘Above’ (from Colorado State University), 'AP502 CL' and 'AP401 CL' (marketed by AgriPro Wheat) are resistant to Beyond herbicide for use in the CLEARFIELD Production System for wheat. These two hard-red varieties and one hard white variety are best adapted to areas of the central Great Plains from western Nebraska through eastern Colorado and western Kansas and into the Texas panhandle. In the Pacific Northwest, General Mills has licensed two new CLEARFIELD winter wheat lines developed by Northwest Plant Breeders, Inc. One of these varieties has been released under the name 'CLEARFIRST'. Public and private plant breeders in the Pacific Northwest are actively developing other CLEARFIELD winter wheat varieties for Pacific Northwest production regions.

How was the CLEARFIELD Production System for wheat developed?
BASF Corporation developed CLEARFIELD crop systems for corn, rice, canola, and sunflower. In the late 1980s, BASF scientists used a chemical mutagen for wheat to induce a mutation using the French wheat cultivar, ‘Fidel’, and found a herbicide-resistant plant. Herbicide-resistant Fidel was not commercially acceptable in the U.S.A. so BASF (then American Cyanamid) cooperated with breeders from the Texas Agricultural Experiment Station to incorporate the herbicide resistance into a commercially acceptable line. Crosses to transfer the herbicide resistance to adapted wheat varieties were completed at Texas A&M University in 1996. Populations segregating for herbicide resistance and other traits were obtained by several universities from BASF for further testing and development. At the same time varieties were being developed, weed scientists at Oregon State University, University of Idaho, Washington State University, and BASF were studying how to best use Beyond™ herbicide under Pacific Northwest conditions.

CLEARFIELD* wheat variety development in the Pacific Northwest
Currently, the one variety released for commercial use in the PNW, CLEARFIRST is a tan-chaffed, soft white winter wheat with a Madsen background developed by Northwest Plant Breeders, Inc. and Dr. Calvin Konzak. It is adapted to PNW wheat growing regions. Marketing will be done by General Mills based out of Idaho Falls, Idaho. CLEARFIRST wheat was placed in pilot demonstrations for the 2002-03 production season. The pilot program consists of 28 locations managed by producers throughout Washington, Oregon, Idaho, and Utah ranging in size from 10 acres to over 30 acres. Weeds species targeted in the pilot demonstrations include wild oat, Italian ryegrass, feral rye, jointed goatgrass, downy brome and volunteer cereals (including winter and spring wheat and barley). CLEARFIRST was planted in conventional, minimum-till, and zero-tillage systems ranging from 8-inch precipitation to over 30-inches of annual precipitation. Certified CLEARFIRST seed is being raised on 2000+ acres in Washington and Idaho for the 2003-04 season with anticipation of 150,000 +/- acres of certified seed available for growers across the Pacific Northwest in the fall of 2003. Public and private plant breeders in the Pacific Northwest are actively developing other CLEARFIELD winter wheat varieties for Pacific Northwest production regions. Release of advanced lines from Oregon State University and the University of Idaho are expected in 2003.

Are the CLEARFIELD wheat varieties genetically modified organisms (GMOs)?
No foreign, non-wheat, DNA was introduced or inserted into CLEARFIELD wheat varieties at any time during the development. CLEARFIELD wheat varieties are thus classified as “non-GMO” in the U.S.A. and are not subject to restrictions in either domestic or overseas markets. The induced mutagenesis process described above is a traditional plant breeding technique and has been used for several decades to create crop varieties (including wheat) that are grown on large acreages in the U.S.A. and around the world.

How does the herbicide system work in susceptible and resistant plants?
Beyond, and similar herbicides, inhibit the activity of the enzyme, acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS), which is necessary for the biosynthesis of the branched chain amino acids valine, leucine, and isoleucine. In response to a Beyond herbicide application, ALS is inhibited in susceptible plants depriving them of the previously mentioned, essential amino acids. This causes the eventual death of treated plants. The ALS enzyme is unique to bacterial and plant species and is not found in the animal kingdom. Mutation gave rise to an altered form of the ALS enzyme that is not affected by the herbicide at normal application rates. Beyond herbicide received EPA Federal registration for use in CLEARFIELD wheat in December 2001.

What weed species does Beyond control?
Beyond is a broad-spectrum herbicide (grass and broadleaf weeds) that provides post-emergence and some in-season residual weed control. Grasses controlled by Beyond include jointed goatgrass, feral rye, downy brome, Italian ryegrass, wild oat, and volunteer cereals. In field studies with Beyondä, feral rye and Italian ryegrass were more difficult to control than jointed goatgrass, indicating that a properly timed application at full rate is necessary to optimize control of these two weeds. Beyond also controls several winter annual broadleaf weeds including flixweed, henbit, chickweed, shepherd's purse, field pennycress, and other mustard species. Spring applications of Beyond will control or suppress summer annual broadleaf weeds such as common lambsquarters, pigweed, and wild buckwheat.

CLEARFIELD Stewardship
Stewardship of the CLEARFIELD* production system is important to prolong its longevity for future generations of Pacific Northwest wheat producers. Sustaining the life of this technology is also in the public interest. With the Pacific Northwest wheat crop worth an estimated $633 million annually, the loss of production due to winter annual grasses and dockage could account for 10 percent, or $63 million dollars, in a single year. The biggest threat to prolonging the life of CLEARFIELD technology is developing herbicide-resistant weed biotypes. This could happen by either selecting for resistance among naturally occurring weed biotypes or, in the case of jointed goatgrass, direct transfer of the CLEARFIELD resistance trait through natural outcrossing. Because wheat and jointed goatgrass are genetically related, and the herbicide tolerance gene is found on a set of chromosomes common to both wheat and jointed goatgrass, outcrossing from wheat to jointed goatgrass can occur and has been documented under Pacific Northwest conditions (Zemetra et al. 1998). Unsuccessful stewardship might lead to the emergence of a wheat-jointed goatgrass hybrid that (when backcrossed naturally to jointed goatgrass) would give rise to a jointed goatgrass population resistant to Beyond and related herbicides. The following information outlines the stewardship program developed by BASF, and our comments to expand on the BASF program. Please note that mention of the CLEARFIELD program, Beyond herbicide, or the discussion of these products do not constitute our endorsement or recommendation for use.

BASF Stewardship Requirements
According to BASF, growers must purchase certified CLEARFIELD wheat seed every year from a CLEARFIELD seed retailer. This means that saving seed to plant next year’s crop will not be allowed (NO “brown-bagging” or “bin-running”). Seed increase fields (Foundation, Registered, and Certified) are grown following strict guidelines that ensure that fields are free of noxious weeds and “off-type” wheat. The use of registered or certified seed ensures proper herbicide tolerance to Beyond and prevents contamination from a non-CLEARFIELD variety. The penalty for planting CLEARFIELD wheat saved seed could be $100/acre or more. Proof-of-purchase records for CLEARFIELD wheat seed and Beyond herbicide must be provided to BASF prior to servicing of any claim. The stewardship requirements and recommendations (common sense good stewardship practices) were developed to prolong the life of CLEARFIELD technology in the field. Northwest wheat producers need to study these carefully before signing a CLEARFIELD stewardship grower agreement with BASF prior to, or at the time of seed purchase. Growers who use Beyond herbicide agree to use it in accordance with the product label, including stated label rates and timing.

BASF Stewardship Recommendations
Avoid continuous use of CLEARFIELD wheat on the same land. This reduces the reliance on Beyond in selection of herbicide resistant weed biotypes and promotes crop rotation and use of alternate control methods. Utilize crop rotation . Rotation of CLEARFIELD* winter wheat with spring crops breaks the cycle of winter annual weeds and allows the use of alternate mode-of-action herbicides. Use herbicides with different modes-of-action. Limit the sole reliance of ALS-inhibiting herbicides no more than 2 out of 4 years unless other control practices are implemented on target weeds (this recommendation will be discussed, further). Where needed to achieve broad spectrum control of multiple weed species, use sequential applications or tank mix herbicides with multiple modes-of-action on target weed species. This strategy should span across crops and years to provide sound weed resistance management. Properly manage weeds in wheat-fallow-wheat rotations. In the fallow year, control weeds (especially winter annuals) before they set seed with burndown (non-ALS) herbicides or tillage. Specific recommendations for jointed goatgrass include treating the entire CLEARFIELD wheat field with a labeled rate of Beyond™ herbicide. The labeled rate of Beyond will provide a high level of jointed goatgrass control and reduce the chance of outcrossing. Control jointed goatgrass in fence rows, road ditches, and pastures around CLEARFIELD wheat fields before jointed goatgrass seed set to further reduce the chance of outcrossing.

Additional Points About Stewardship
The development of herbicide resistant grass weeds such as jointed goatgrass, downy brome, and others is a very real possibility with over-reliance on CLEARFIELD wheat and Beyond herbicide. As previously mentioned, Beyond herbicide belongs to a herbicide class known as ALS inhibitors. Use of this class of herbicide has produced a dramatic array of herbicide resistant weeds since about 1982 when the first ALS resistant weeds were identified (Figure 1). There are currently 73 weed species identified, world-wide, that are resistant to ALS inhibitor herbicides (Heap 2002), including such common Pacific Northwest weeds as kochia, Russian thistle, prickly lettuce, and downy brome.

To determine how rapidly herbicide resistance might occur with Beyond herbicide in CLEARFIELD wheat, historical observations from other weed species can be used as an indication. As an example, continuous use of GleanÒ herbicide (an ALS inhibitor) in wheat resulted in the development of herbicide-resistant kochia and prickly lettuce 5 years after the introduction its commercial introduction (Thompson et al. 1994). Continuous use of BeaconÒ (another ALS inhibitor) in Kentucky bluegrass seed production resulted in herbicide-resistant downy brome after 4 years of continuous use (Mallory-Smith et al. 1999, Ball and Mallory-Smith 2000).

To estimate the impact of cropping practices and Beyond herbicide use on ALS inhibitor resistance development in jointed goatgrass, we independently constructed two separate simulation models to predict the rate of Beyond resistance development in jointed goatgrass. Both predictive models are based on known information about crop management impacts on jointed goatgrass populations, information on the effectiveness of Beyond herbicide for control of jointed goatgrass, and knowledge of natural mortality factors in the field (Hanson et al. 2002, Rainbolt and Thill, unpublished data). Estimated changes in jointed goatgrass seed numbers in the soil over a 10 to 15 year period were calculated by the simulation models based on scenarios where BeyondÔ herbicide were used every year, or once in two, three, or four years.

Figure 1. World-wide occurrence of herbicide resistant weed biotypes by herbicide class.

In the model developed at Oregon State University (Hanson et al. 2002), numbers of Beyond resistant jointed goatgrass individuals increased and surpassed susceptible jointed goatgrass numbers after 4 years when used each year in a continuous, CLEARFIELD* winter wheat system (Figure 2A). In a winter wheat – fallow crop rotation, resistant jointed goatgrass numbers surpassed susceptible numbers after 9 years if CLEARFIELD wheat and Beyond herbicide were used every crop year (Figure 2B). This scenario is within the BASF guidelines that allow CLEARFIELD wheat to be used every 2 out of 4 years. In a simulation of a winter wheat – fallow crop rotation with CLEARFIELD wheat grown every other crop (once in four years), a Beyond resistant population did not surpass the susceptible population during the 10 year simulation (data not shown). These results are similar to those obtained from a life-table model constructed by University of Idaho researchers (Rainbolt and Thill, personal communication). In their predictive model, developed independently from the O.S.U. model, a Beyond resistant jointed goatgrass population surpassed the susceptible jointed goatgrass population in 4 years, if CLEARFIELD wheat was grown and sprayed every year. In simulations of CLEARFIELD use 1 year out of 2, resistant jointed goatgrass populations surpassed the susceptible population after 10 years. In simulations of CLEARFIELD use 1 year out of 3, the resistant population surpassed the susceptible after 14 years, and it did not develop if CLEARFIELD wheat was grown and sprayed 1 year out of 4 (data not shown). Based on these simulation results, and from knowledge of prior cases of ALS inhibitor resistance, we recommend that the use of CLEARFIELD wheat should be limited to at least one year out of 3 to prevent the development of Beyond resistant weeds including jointed goatgrass.

In the intermediate, transitional rainfall areas of eastern Washington, a very common crop rotation consists of winter wheat – spring barley (or spring wheat) – fallow. In higher rainfall areas of the Pacific Northwest, a more intensive crop rotation of winter wheat – spring barley (or spring wheat) – legume (or Brassica) is common. With these types of crop rotations, we are less concerned about developing an ALS inhibitor resistant jointed goatgrass population, because winter wheat is grown only once every third year. Our major concern for the development of Beyond resistant weeds is in dry areas where winter wheat – fallow is the standard cropping practice. In these systems, it is desirable to attempt diversification in crop rotations, and/or limit use of CLEARFIELD* wheat to once every other crop (1 year out of 4).

Figure 2. Simulated response of BeyondÔ susceptible and resistant jointed goatgrass populations in A) continuous CLEARFIELD winter wheat, and B) winter wheat – fallow crop rotation with CLEARFIELD wheat grown every crop year (2 years out of 4). (Hanson et al. 2002).

BASF's CLEARFIELD Stewardship Grower Agreement
Wheat producers who desire to plant CLEARFIELD wheat varieties are required to sign an agreement with BASF stating they will adhere to the stewardship requirements. Signing this agreement is coordinated by the seed dealer at the point of sale. The wheat producer acknowledges that CLEARFIELD wheat technologies are protected under U.S. patent law, CLEARFIELD wheat varieties are PVPA protected, and crops grown from protected CLEARFIELD varieties may only be sold in normal commercial channels for wheat and not saved or sold for use as seed. Seed producers will obtain the signed agreements and forward the seed purchase information to BASF who maintains a CLEARFIELD wheat technology database. The database matches grower seed and herbicide purchases.

What will be the label rate and cost of Beyond?
The label rate will be 4 to 6 oz/acre and 5 oz will cost approximately $20/acre. Growers should check with their local ag chemical retailer for specific Beyond pricing.

What will CLEARFIELD wheat seed cost?
Seed cost is not fixed for any varieties of certified seed so the cost of seed will depend on the demand for the seed plus possible royalties from the variety owner. A technology fee will not be levied on the price of CLEARFIELD wheat seed by BASF.

CLEARFIELD* Wheat Resources on the World Wide Web: www.clearfieldsystem.com

BASF Corporation Web site developed solely for CLEARFIELD crop production systems. www.jointedgoatgrass.org

Web site for the National Jointed Goatgrass Initiative covering jointed goatgrass biology, distribution, control, and genetics.

Beyond is a trademark and CLEARFIELD is a registered trademark of BASF Corporation.
©2002 All Rights Reserved.

Always read and follow label directions.

Some text used in this article was adapted from Colorado State University Crop Production Series no. 3.116, CLEARFIELD Wheat, by J. Johnson, S. Haley, and P. Westra. The authors acknowledge these individuals for the written material used to produce this manuscript. The authors would also like to thank BASF for providing information on their stewardship program, the grower agreement, and CLEARFIELD wheat varietal information.

Literature Cited:

Ball, D. A. and C. Mallory-Smith. 2000. Sulfonylurea herbicide resistance in downy brome. Proceedings West. Soc. Weed Sci. 53:41-42.

Hanson, D. E., D. A. Ball, and C. A. Mallory-Smith. 2002. Herbicide resistance in jointed goatgrass (Aegilops cylindrica): simulated responses to agronomic practices. Weed Technol. 16: 156-163.

Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. October 30, 2002. Available at: www.weedscience.com

Mallory-Smith, C., P. Hendrickson, and G. Mueller-Warrant. 1999. Cross-resistance of primisulfuron-resistant downy brome to sulfosulfuron. Weed Sci. 47:256-257.

Mallory-Smith, C., D. C. Thill, and M. J. Dial. 1990. Identification of sulfonylurea herbicide-resistant prickly lettuce. Weed Technol. 4:163-168.

Thompson, C. R., D. C. Thill, C. Mallory-Smith, and B. Shafii. 1994. Characterization of chlorsulfuron resistant and susceptible kochia. Weed Technol. 8:470-476.

Zemetra, R., J. Hansen, and C. A. Mallory-Smith. 1998. Potential for gene transfer between wheat and jointed goatgrass. Weed Sci. 46:313-317.

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