Wheat growers need help in assessing resistant weeds

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Setting the stage for a quality wheat crop and a profitable growing season hinges on the ability to effectively control competitive weeds. For more than two decades, weed resistance to individual families of chemistry has become commonplace, and this grueling challenge is not diminishing. With each growing season, previously successful herbicides with modes of action, like ALS and ACCase inhibitors, are becoming less effective against troublesome cereal crop weeds such as Italian ryegrass, green foxtail, kochia and wild oat. Aiming to keep this ever-growing issue top of mind, Syngenta visited with industry experts to give wheat growers topline tips to consider as they come face-to-face with weed resistance today.

Sales agronomists and crop consultants should consider the points explained here when talking to their wheat- and barley-growing clients.

Watch for Weeds

Herbicide resistance is the inherited ability of weeds to survive and reproduce following a herbicide application that would normally kill them. “If you overuse the same herbicide mode of action, you will face herbicide resistant weed species, and the severity will depend on the cropping system,” said Donn Thill, professor of weed science, University of Idaho. Fortunately, a proactive and diversified weed management strategy can help cereal growers stay ahead of resistant populations.

In the Pacific Northwest, winter annual grasses and some broadleaf weeds are top of mind for wheat farmers. “Growers fear them most as they’re the most difficult to control in wheat,” said Thill. “Such weeds include downy brome and prickly lettuce, as well as jointed goatgrass, for which we rely on Clearfield herbicide tolerant wheat to help control,” he added.

To help address some of the most stubborn weeds and promote vigorous growth, Syngenta has also introduced its own line of Clearfield wheat varieties, such as AP604 CL, AP700 CL and SY605 CL, as a component of an integrated management solution. “Clearfield wheat technology allows the use of a Group 2 herbicide to be used in rotation with a Group 1 herbicide for resistance management in cereals,” said Don Porter, cereals technical brand asset lead, Syngenta. The Clearfield technology, combined with imazamox active ingredient herbicides is a good system to control stubborn weeds, such as jointed goatgrass and feral rye that are not effectively controlled by other selective cereal herbicides,” Porter suggested.

In the Northern Plains, recently noted resistant weeds include ACCase-resistant green foxtail and ALS-resistant kochia. “We’ve also been trying to manage and control ACCase-and ALS-resistant wild oat biotypes for several years now, and that’s the major concern today,” said Kirk Howatt, Ph.D., associate professor at North Dakota State University. “In terms of broadleaf weeds, the control we achieve is typically strong with products we currently have available for use in cereals, but we do have weeds like kochia that are becoming increasingly tolerant to various modes of action. We’re also seeing glyphosate resistance with ragweed and water hemp,” Howatt explained.

Howatt encourages keeping a close eye on weed species becoming increasingly tolerant to herbicides that once offered complete control, as well as evaluating a long-term strategy for greater return on investments over several years. “Preventive management solutions, or ‘the Cadillac treatment’ may cost more in the short-term,” he said, “but they help set the stage for a long-term gain and guard against potentially expensive problems before they occur.”

Start with a Competitive Crop

When problem weeds cannot be controlled by a previously effective herbicide, there is enhanced weed-crop competition for water, nutrients and light. “Weed scientists recommend growers use all the tools available to them in the most effective way possible,” Thill said. “The number one method of weed control is growing a competitive crop. If you begin with that, and you do all the right things to ensure the crop is aggressive and strong, weed control will be simpler to manage,” he added.

“Select the right varieties, plant at the right time and at the right depth, use the proper seeding rate and adjust for correct spacing; these agronomic practices can be followed by all growers to get the crop off to a healthy start,” he said. In addition, seed treatment insecticide and seed treatment fungicide help ensure optimum root health, stand establishment, increased vigor,  maximum yield potential, as well as offer protection against the trifecta of yield-robing diseases Rhizoctonia, Pythium and Fusarium in cereals.

Alternate, alternate, alternate

Over-reliance on the same herbicide mode of action is one of the key contributors to the growing problem. Experts say there is no substitute for alternating herbicides with different modes of action and chemical classes in the fight against resistance. “Yields can increase significantly with proper herbicide management,” said Howatt. “It’s unreliable and not recommended to cut labeled herbicide rates,” he added. For most effective management, be sure to use sequential applications and/or tank mixtures of herbicides with different modes of action; apply herbicides at the full label rate and at the proper growth stage; extend the range of available herbicides by employing a diverse crop/fallow rotation; and prevent weed escapes from producing seed by controlling weeds early on.

Scout with a Watchful Eye

There is also no substitute for careful, timely observation and scouting to keep resistant weeds from spreading. Be sure to clean tillage, seeding and harvest equipment when leaving fields with herbicide-resistant weeds to keep them from moving to other acres. “Be familiar enough with your fields to catch resistant biotypes early, and be aware of minor or major changes, like green streaks where seed has fallen,” Howatt said. Awareness of field history and yield impact, a watchful eye and a proactive management program will help growers save money and hassle in the long run.

Rotate Crops and Think Tillage

Cultural management options to help thwart resistance are tillage and crop rotation. Cereal crops rotated with crops like buckwheat, clovers and/or alfalfa can help keep weeds from adapting to a certain environment, making more control options available. These crops obstruct weeds, release natural weed-killing chemicals and simultaneously improve soil structure, prevent erosion and offer crop nutrients.

While no-till systems improve soil health and save farming costs, they can also enable perennial weeds to flourish. Howatt recommends delayed seeding to ensure adequate time to kill weeds with a burndown herbicide first.

To help meet the challenge, Syngenta has a diverse portfolio of cereal herbicides to meet the weed control needs of each field and help farmers grow more wheat. To help avoid additions to the seed bank, consider a burndown herbicide in addition to an in-season application of a postemergence herbicide. “Axial XL will typically provide the best control for wild oat, and is likely still controlling these resistant populations,” said Howatt. “It is still critical to rotate to other herbicides so that it remains effective,” he added.

Crop consultants and ag retailer agronomists need to check out the old and new chemistries for weed control because new ones have been coming onto the market, as Syngenta has noted about its line-up of products. Knowing the strengths and weaknesses of each and every one of them is important when talking to customers and requires investing some time by trained agronomists.

Syngenta has made it a standard practice to include management information and the mode of action group number on its product labels to help everyone remain ahead of resistance. As winter 2012 sets in and planning begins for the next growing season, remember that diversity is key in any resistance management program, Syngenta stresses. Refrain from heavy reliance on any one method of control to set the stage for strong, robust crops and profitable returns for years to come.


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