Prolific Weed Resistance Threatens U.S. Crop Yields

Talk to any row crop farmer or weed scientist in the United States, and you’ll realize that weed resistance to numerous chemistries is advancing at an alarming rate. For as long as farmers have battled weeds in their crops, weeds have fought back. Nature continues to be resilient and has proven, time and again, that there is no single solution to manage tough-to-control and resistant weeds.

Resistant Weed Evolution in Action

Herbicide-resistant weeds were first discovered in the United States in the late 1960s in a pine nursery, where triazine herbicides had been used repeatedly. Triazine resistance continued to spread and was the most widely recognized type of weed resistance for several decades.  During the early- to mid-1980’s, a new ALS mode-of-action chemistry was introduced into wheat and later other crops. Because of their low use rates and effectiveness against labeled weeds in wheat, farmers quickly adopted ALS herbicides, applying them year after year.

After four to five years of use, however, growers began to observe prickly lettuce and kochia escaping from ALS herbicide treatments. After four additional years, ALS-resistant weeds were widespread across the western United States.

A mature lambsquarter, one of the resistant weeds known to be aggressive.
A mature lambsquarter, one of the resistant weeds known to be aggressive.

Weed resistance has continued to expand over the years with resistant weed species now found in all major field crops, including corn, soybeans, cotton and wheat. The number of herbicide resistant weed species has continued to grow and some species exhibit resistance to PPO, glycine (glyphosate) and most recently HPPD mode-of-actions. In some cases, individual weed species have developed resistance to multiple modes-of-action.  Glycine resistance is particularly concerning due to the widespread utility of the product and the aggressive nature of some of the weeds that have developed resistance to glyphosate.

Resistant weeds that are known to be very aggressive, such as waterhemp, Palmer amaranth, marestail, lambsquarters and common and giant ragweed, are often referred to as ‘driver weeds’ since  they alone can make some fields unfit for growing corn, soybeans and cotton. In wheat, tough-to-manage and resistant weeds include kochia, Russian thistle, wild oat, green foxtail, prickly lettuce, Palmer amaranth and other common pigweed species.

Severity of Waterhemp and Palmer Amaranth Resistance

Farmers in the Midsouth and Southeast have recognized for several years that resistant Palmer amaranth can be devastating to fields. Resistant Palmer amaranth continues its spread into the Midwest, Southwest and even the Far West. Waterhemp is also in the firefight as the first broadleaf weed to become resistant to multiple herbicide classes: ALS inhibitors, PPO inhibitors, glyphosate and HPPD inhibitors. Alternative herbicide-tolerant seed traits and compatible herbicides will offer other options for corn, soybean and cotton growers.

The growing prevalence of weed resistance coincides with the fact that no new herbicide modes of action have been introduced in the past 30 years. Therefore, it is imperative to use best management practices to sustain the viability of existing chemistries and help protect crop yields.

Dr. Jason Norsworthy, University of Arkansas professor and weed scientist, points to his research and experience in following the development of Palmer amaranth and waterhemp weed resistance. “Growers need to take a year-round approach to weed control. Zero tolerance to prevent weeds from going to seed offers the best way to manage resistance and keep herbicides and fields sustainable,” he says.

“Growers need to use diversity in weed management practices to keep weeds confused and fight resistance,” Norsworthy says.

In a four-year research trial near Fayetteville, Ark., Norsworthy conducted a trial in which resistant Palmer amaranth weed seeds were introduced into four fields. In year one, only a streak of weeds escaped herbicide treatments. By year three, all four fields were overtaken by resistant weeds, resulting in complete crop failure.


Norsworthy also cites the example of one Arkansas farmer’s diversified weed control practices, which he began in 2006. The farmer was diligent in field scouting and using herbicide with multiple modes of action. Any weed escapes from emergence through harvest were removed by hand. The farmer was an early adopter of the zero-tolerance mindset, allowing him to keep herbicide costs in check, while his neighbors sometimes bore triple the herbicides costs to manage resistant weeds.

“Growers need to use diversity in weed management practices to keep weeds confused and fight resistance,” Norsworthy says. “One weed escape is too many. Rotate different classes of chemistry with different modes and sites of action, and use other cultural practices such as tillage, crop rotation and hand removal of weeds.”

Crop Science Solutions

Crop Science has a broad portfolio to combat tough-to-control and resistant weeds. A well-thought-out herbicide program, using multiple modes of action, should be implemented to sustainably manage weeds. 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. Not every product is suitable for every situation, and use of the correct application technique will ensure the best results.

The following Crop Science solutions are valuable tools to consider for your program.

Corn

When managing corn crops for high yields, remember it’s always best to start with a clean field. Start with an Autumn™ Super (2) application in the fall, and the following spring use Corvus® (2, 27), a pre-emergence residual herbicide which has overlapping modes of action to control early season problem weeds. Corvus pre-emergence herbicide from Crop Science is the only corn herbicide to offer burndown, residual and reactivation. Residual activity prevents new weeds, while reactivation controls late weeds.

The multiple modes of action in Corvus deliver consistent, broad-spectrum control of grasses and broadleaf weeds, including weeds resistant to glyphosate-, ALS-, PPO- and triazine-based herbicides.

A wide application window allows for application from pre-plant to early postemergence at V2, making it an effective, long-lasting first pass herbicide in a two-pass system. Depending on the weed spectrum in your field, such as fields without heavy Palmer amaranth and waterhemp pressure, Corvus may still be a great one-pass option.

A recommended two-pass program starts with a pre-emergence application of Corvus herbicide. The second pass should include a postemergence product such as Laudis® herbicide (27). If using Laudis following an application of Corvus, add another effective herbicide with a different mode of action, such as DiFlexx™ (4) herbicide, Liberty® (10) or atrazine, to ensure you are using multiple modes of action in your weed control.

Another pre-emergent herbicide Crop Science offers corn growers is Balance® Flexx (27). Follow a pre-emergent application of Balance Flexx with a postemergence herbicide to control multiple weed flushes. Balance Flexx has the unique power to reactivate with as little as a half-inch of rain to control late-emerging weeds. It controls glyphosate-, triazine-, PPO- and ALS-resistant weeds, including resistant marestail, common ragweed, waterhemp and Palmer amaranth. Balance Flexx even controls tough grasses, such as woolly cupgrass.

Balance Flexx fits into many cropping systems. Its superb weed control power combined with state-of-the-art crop safety innovation enables growers to readily rotate from corn to other key crops with little or no delay.

Capreno® herbicide (2, 27) is a postemergence herbicide option for corn from Bayer. It has the longest-lasting residual of any post product on the market. With multiple modes of action, Capreno controls more than 65 grasses and broadleaves, including those resistant to glyphosate, PPO, ALS, dicamba and triazines.

Liberty® is the preeminent weed management system with a unique chemistry and novel mode of action to offer superior control of a broad spectrum of resistant and tough-to-control weeds in LibertyLink® corn. It is THE non-selective post-emergence herbicide that still effectively handles grasses and broadleaf weeds including glyphosate resistant Palmer amaranthgiant ragweed, marestail, waterhemp and kochia.

For more information on products Bayer offers to help control weeds in corn as part of an integrated weed management program, visit our corn section.

Soybeans

Crop Science offers soybean grower season-long weed control options starting with the decision to purchase LibertyLink® soybeans.  LibertyLink soybeans are widely available in many soybean brands including Credenz®. This year’s soybean herbicide program allows grower to start and finish clean with the inclusion of a full line up of residual products from FMC combined with the powerful control of Liberty® (10) herbicide and finishing with Autumn™ Super (2) in the fall after harvest.            

Liberty herbicide is the only group 10 herbicide that offers superior control of a broad spectrum of resistant weeds and tough-to-control weeds including palmer amaranth, waterhemp, marestail, giant ragweed, and kochia. To learn more about how to S.T.O.P. weeds with Liberty, visit bayercropscience.us/stopweeds for proper application tips.

As Bayer continues to anticipate the needs of farmers in the future, we continue to invest in developing leading edge technologies like Balance™ GT soybeans and Balance® Bean herbicide*.

For more information on products Bayer offers to help control weeds in soybeans as part of an integrated weed management program, visit our soybean section.

*Balance Bean is not yet registered for sale or use in the United States. Balance GT is deregulated in the United States and authorized in some key importing countries. Additional international authorizations are pending.

Cotton

Crop Science offers cotton growers Liberty® (10), the preeminent weed management system with a unique chemistry and novel mode of action to offer superior control of a broad spectrum of resistant and tough to control weeds in LibertyLink® cotton. In fact, it is the ONLY non-selective post-emergence herbicide that still effectively handles grasses and broadleaf weeds including glyphosate resistant Palmer amaranth, giant ragweed, marestail, waterhemp and kochia. To learn more about how to S.T.O.P. weeds with Liberty, visit bayercropscience.us/stopweeds for proper application tips.

In addition to the LibertyLink herbicide trait in cotton, growers may choose cotton varieties with a GlyTol® trait, which allows tolerance to glyphosate. Growers who plant FiberMax® or Stoneville® cotton varieties with the GlyTol plus LibertyLink traits can spray glyphosate or Liberty herbicide to control a wide range of tough-to-manage and glyphosate-resistant weeds. While major problem weeds vary by geography, some of the more prominent weed pests controlled include Palmer amaranth (pigweed), marestail, morningglory, Russian thistle and grasses.

Growers should check labels; some older, commercially available FiberMax cotton varieties do not contain both the GlyTol and LibertyLink traits.

For more information on products Bayer offers to help control weeds in cotton as part of an integrated weed management program, visit our cotton section

Wheat

Huskie® herbicide (6, 27) is available to wheat growers in 40 states and includes a unique chemistry for use in cereals with multiple modes of action (MOA). Huskie controls many broadleaf weeds such as kochia, Russian thistle, prickly lettuce and wild buckwheat—including ALS- and glyphosate-resistant biotypes. Huskie® Complete herbicide (2, 27, 6) is available to wheat growers in Colorado, North Dakota, South Dakota, Minnesota, Montana, Wisconsin and Wyoming. This all-in-one wheat herbicide is a combination of Huskie plus thiencarbazone that provides grass control. Together, these modes of action provide good control of green and yellow foxtail along with 50 grass and broadleaf weeds.

Because Crop Science continuously provides growers with new solutions to weed management problems, the company recently introduced two new wheat herbicides.

The first is Wolverine® Advanced (1, 6, 27) which controls 69 grass and broadleaf weeds in wheat, thanks to three modes of action in a single product. Wolverine Advanced provides the same unique broadleaf control found in Huskie with the addition of ACCase grass chemistry to control green and yellow foxtail and barnyardgrass. This wheat herbicide is an excellent tool to consider as grass chemistry rotation partner to manage weed resistance.

Wheat growers also have the option of a second new herbicide, Varro™ (2) which controls grass weeds and is available in Colorado, North Dakota, South Dakota, Minnesota, Montana and Wyoming. Green and yellow foxtail as well as barnyardgrass are controlled by Varro. Just as important, Varro provides excellent rotational flexibility and allows a wide range of choices when it comes to broadleaf tankmix partners—while enhancing the performance of those partners.

Visit bayercropscience.us/crops/cereals for information on additional herbicide products Bayer offers as part of an integrated weed management program for wheat.

More Information

To learn more about using herbicides with effective modes of action, visit the Crop Science website at bayercropscience.us/products/weed-management/resources. You can also find information about Respect the Rotation™, the Crop Science resistance management program, at bayercropscience.us/learning-center/articles/herbicides-respect-the-rotation.

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