Fighting Back against Prolific Weed Resistance

For as long as growers have battled weeds in their fields, 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.

Resistance is advancing at an alarming rate – just talk to any weed scientist in the United States. Resistant weed species are now found in all major field crops and, in some cases, individual weed species have developed resistance to multiple sites of action.

Dr. Peter Dotray, Professor and Extension Weed Specialist of Texas A&M, describes the industry’s overall approach to weed management over the last few decades.

“Once we transitioned into the opportunity of using glyphosate as a herbicide in crops, I think a lot of other strategies were put on the shelf and growers quickly found that glyphosate followed by glyphosate followed by glyphosate was extremely effective.”

But the industry learned a major lesson.

Resistant Weeds Have Evolved

Certain weed populations quickly evolved, no longer responding to the herbicide, and as a result, growers have lost that chemistry as an effective tool for certain weeds.

Take Palmer amaranth, for example. Often cited as the most concerning driver weed across the South, it’s not only resistant to glyphosate, but is now documented to have such stacked resistance to multiple chemistries.1 Resistant weeds that are known to be very aggressive in the Midwest include waterhemp, marestail, lambsquarters, and common and giant ragweed.

The industry now knows that over-reliance on a single chemistry is not a sustainable strategy. A well-thought-out herbicide program, using multiple effective sites of action, should be implemented to sustainably manage weeds.

Bayer has a broad portfolio to combat tough-to-control and resistant weeds in corn. Talk to a specialist about recommendations for your farm. Consider the history of weeds in your fields, and choose a chemistry with no resistance to those weeds.

Combine Herbicides and Cultural Practices to Manage Weeds

Cultural practices such as tillage, crop rotation and hand removal of weeds can supplement herbicide treatment, leading to a more sustainable outcome. Dotray cites the importance of using multiple management strategies.

“We’ve taken a step backwards, so to speak, and are visiting now all the current strategies available to develop a more diversified weed management program,” he said. “It’s thinking about herbicides, rates, and incorporation. It’s also thinking about tillage, crop rotation, and the use of biological control where possible.”

Eric Prostko, a weed scientist at the University of Georgia, agrees that having multiple weed management strategies is best. He recommends starting clean by using a combination of tillage, cover crops and herbicides to remove pigweeds from the field.

“Make sure that when you get to the field to plant, there are no pigweeds in the field, because if you allow that to happen, you will lose,” he said.

Use Residual and Postemergence Herbicides

“It’s going to be almost impossible for growers to manage resistance and typical weed problems without the use of residual herbicides,” Prostko explains. “And when they are activated by moisture, whether through rainfall or irrigation, they work great.”

Finally, he advises being timely with post-emergence herbicides. Waiting until weeds are taller will make them more difficult to manage. But growers who begin soon after crop emergence, when weeds are smaller, can achieve better results.

“Whatever herbicide you choose … you want to spray those pigweed plants before the biggest pigweed in the field is 3 inches tall.”

Combine multiple, effective herbicide sites of action, plus cultural practices such as crop rotation and cultural approaches, to manage or delay resistance. Know your fields and know your weeds. Talk to a specialist about herbicide recommendations for your farm.

Bayer Solutions for Corn Growers

When managing corn crops for high yields, remember it’s always best to start with a clean field. The following program can help you manage weeds in your cornfields:

• Start with an Autumn™ Super (2) application in the fall.
• The following spring use Corvus^® (2, 27), a pre-emergence residual herbicide that has overlapping sites of action to control early-season problem weeds.* A wide application window allows for application from pre-plant to early postemergence at V2, making Corvus an effective, long-lasting first pass herbicide in a two-pass system. The recommended tankmix partner is atrazine (5).
• Another potential residual option with two modes of action for control of both large seeded and small seeded grasses and broadleaf species would be Harness Max (15,27). Harness Max can be used both as a pre-emergence residual herbicide and/or a post applied residual application up to 11 inch corn allowing for more flexibility of application timing. The recommended tankmix partner is atrazine (5).
• If heavier grass and broadleaf weed pressure is expected, consider stepping up your program with Trivolt (2,27,15).Similar to Corvus but with an added group 15 for longer residual control of grasses and small seeded broadleaf species, Trivolt’s application window allows for application from pre-plant to early postemergence at V2, making Trivolt an even stronger, long-lasting first pass herbicide in a two-pass system . The recommended tankmix partner is atrazine (5).
• The second pass should include a postemergence product, such as DiFlexx^® DUO (4, 27). The recommended tankmix partner is glyphosate (9).

Another pre-emergent herbicide Bayer 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 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^® (2, 27) postemergence herbicide delivers season-long control of more than 65 grasses and broadleaves, including those resistant to glyphosate, PPO, ALS, dicamba and triazines.

^{*Corvus and Balance Flexx are restricted use pesticides.}

Work Cited

1. Evans, J. A., Tranel, P. J., Hager, A. G., Schutte, B., Wu, C., Chatham, L. A. and Davis, A. S. “Managing the evolution of herbicide resistance.” 2016. onlinelibrary.wiley.com/doi/10.1002/ps.4009/full