ALS Resistance Affects Herbicide Decisions for Corn and Soybean Crops

ALS Resistance Affects Herbicide Decisions for Corn and Soybean Crops

ALS herbicides remain valuable tools for weed management in corn and soybeans, but the overuse of this herbicide group increases the risk for ALS-resistant biotypes in crops. Photo courtesy : Bayer

With all the talk about glyphosate resistant weeds, it’s important that growers keep in mind the lessons learned more than two decades ago when key problem weeds in wheat, including kochia and Russian thistle, developed widespread resistance to ALS herbicides.

Today, corn and soybean crops – as well as wheat and cotton – also fight ALS-resistant broadleaf weeds and grasses such as marestail, waterhemp, Palmer amaranth, cocklebur, sunflower, shattercane, giant foxtail, cheatgrass and others.

ALS products (Group 2) remain valuable weed management tools for growers because they still control a broad spectrum of weeds and grasses. However, overuse of ALS herbicides will allow selection for ALS-resistant biotypes to populate crops.

Those decades-ago lessons resulted in growers continuing to balance the use of effective herbicide chemistries with different sites of action, combined with additional best management practices, to create an integrated approach to crop protection.

To protect the effectiveness of ALS herbicides, here’s a quick checklist:

  • Know how the chemistry fits with the key problem weeds in your fields.
  • Integrate ALS herbicides with herbicides from other chemistry groups with different sites of action.
  • Take advantage of cultural practices such as crop rotation, tillage and cover crops to help manage weeds.
  • Never let weeds go to seed and enter the soil seedbank, where they can cause problems for years to come.

Dr. Curtis Thompson, Kansas State University weed scientist, explains that ALS resistance doesn’t have the same impact on corn and soybeans as it does in wheat because glyphosate-tolerant corn and soybeans came into the marketplace to help alleviate ALS resistance. Now that glyphosate resistance is widespread in corn and soybeans, growers are taking a closer look at ALS herbicides. Also, seed traits combined with herbicide tolerance to glufosinate offer an alternative to glyphosate systems in corn and soybeans as well as to ALS herbicide-tolerant soybeans.

“Although we have resistance to several chemistries today, including ALS, glyphosate, PPO, triazines, growth regulators, HPPD and ACCase, we still get a lot of use from these products,” Thompson says. “The key is to develop a weed control strategy using pre-emergence residual herbicides combined with postemergence herbicides, if needed, that effectively control problem weeds in your fields. Avoid the repeated use of a single chemistry that allows for selection of resistant biotypes.”

He adds that researchers, retailers and growers alike are discussing the best timing for herbicide applications using a combination of fall, winter, early spring, pre-emergence and/or postemergence treatments. Thompson advises checking with your local university Extension for current recommendations.

Resistance-management practices other than herbicides should also be part of an integrated weed management approach to controlling weeds in corn and soybeans. These include crop rotation, tillage and cover crops to aid weed control in row crops. In addition, tank-mixing multiple sites of action can control weeds and prevent further resistance development.

Bottom line, although cultural practices help, experts agree that the only acceptable goal for managing weed resistance to ALS and other herbicides is to have zero tolerance for the distribution of seeds by resistant weeds. The old way of thinking of weed control as an economic threshold is obsolete.

Bayer Weed Management Solutions

Bayer offers a broad portfolio of herbicides with multiple sites of action to manage tough-to-control and resistant weeds.*

In corn and soybeans, Bayer’s LibertyLink® system couples high-performing genetics with the superior weed control of Liberty® (10), the only working nonselective herbicide effective on tough-to-control grasses and broadleaves.** Liberty can be tank-mixed for multiple sites of action.

Corvus® (2, 27) is the #1 pre-emergence corn herbicide offering three sites of action when tank-mixed with atrazine, for consistent control on the toughest broadleaf weeds and grasses all season.¹ Capreno® (2, 27) postemergence corn herbicide offers long-lasting residual and two sites of action that both effectively control the toughest grass and broadleaf weeds.

To learn more about using herbicides with effective sites of action, see the Herbicide Resistance Management Guide. View Bayer’s product portfolio to learn more about weed control from Bayer or contact your local Bayer representative.

* Not every product is suitable for every situation, nor registered in every state, and proper application techniques will enhance results. Before applying any herbicide, please read the entire label for the best possible results and to confirm that the product is effective on the weeds you wish to control.

**The active ingredient in Liberty is a group 10 herbicide, which is the only broad-spectrum herbicide that effectively controls grasses and broadleaf weeds, and it has no known resistance in U.S. broadacre crops.

¹Corvus is a Restricted Use Pesticide. Corvus is not registered in all states.

Bayer, the Bayer Cross, Liberty, LibertyLink, Corvus and Capreno are registered trademarks of Bayer.

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