An effective, multifaceted weed management program includes not only herbicides but also cultural and mechanical practices. With an end goal of reducing the soil seed bank, researchers are exploring non-herbicidal opportunities to help growers.

Palmer amaranth-infested soybean field

"Research shows the most effective way to manage resistant weeds is to apply multiple effective modes of action." - Dane Bowers

Cover crops differ by region, so growers should test options within the climate and soil conditions where they’ll be grown.

"In disaster fields with very high weed seed rates, we tested tilling one time with a moldboard plow, turning soil 8 to 10 inches and planting a cereal cover crop," said Jason Norsworthy, professor at the University of Arkansas.

Cereal rye reduces emergence of most small-seed weeds like Palmer amaranth and waterhemp by 60 percent to 65 percent. Norsworthy’s research shows a one-time moldboard plow followed by a cereal rye cover crop for two to three years will reduce Palmer amaranth by 95 percent.

Row spacing is another option to reduce weed populations.

"At 90 [percent] to 95 percent canopy, weed emergence decreases," he said. "Wide row spacing under dryland conditions takes 10 to 12 weeks for canopy to form, but a drill-seeded system with good moisture on 7½" row spacing [will] canopy five or six weeks after planting, taking four to five weeks of pressure off herbicides and reducing the herbicide input needed to maintain weed control."

On the research front, Norsworthy is collaborating with Australian colleagues on harvest weed seed control. His research in soybean fields shows 99 percent of waterhemp and Palmer amaranth end up in the combine during harvest; these captured weeds are windrowed and burned, killing 100 percent of the weed seed.

"Proactive resistance management is needed on every acre. Focus on reducing the soil seed bank and embarking upon a zero-tolerance program; that means scouting, managing weeds outside your fields and using multiple effective modes of action. Use all the tools available; that’s a proactive strategy," said Norsworthy.

Scout, Identify, Repeat
It’s important that growers know which weed species are in their fields, so weekly scouting should be routine.

"If new weeds, like waterhemp or Palmer amaranth, are spotted in fields, growers need to act very quickly to eliminate those weeds before they go to seed and become an even bigger problem," said Dane Bowers, Syngenta herbicide technical product lead.

Maintaining the viability of herbicide technologies is vital because no revolutionary tools are expected to come to market in the next several years.

"This speaks to the importance of applying multiple effective modes of action on the target weed. There are some premixes that contain multiple modes of action, but they may not necessarily be effective on the target weed," Bowers said. "Research shows the most effective way to manage resistant weeds is to apply multiple effective modes of action. This is even more effective than rotating modes of action."

Diversify Weed Management
Too often, a diverse weed management program only includes the herbicide program because growers are still depending almost entirely on herbicides for weed control.

"There are so many practices that help maintain the viability of our current herbicide tools," he said. "Crop rotation, cover crops, cultivation and harvest weed seed control are some of the ways to develop a truly diversified program, one that does not depend solely on herbicides. There may be additional investment up front, but the return on investment is strong over time if we can prevent resistant weeds from developing."