Multi-state researchers explore best practices for herbicide-resistant weeds

BY CAROL BROWN

Researchers across the country are working to help farmers improve weed management on their farms. Herbicides have proved effective, but several weed species are becoming resistant, and for some producers, it is not desirable to apply a lot of chemicals to their fields. The search continues to find the best management practices for control, especially for pesky weeds such as waterhemp and Palmer amaranth.

Weed management is a long-standing research topic that the United Soybean Board has supported for decades — but weeds keep evolving, necessitating the study of eradicating weeds to evolve as well. Bryan Young, a weed science professor at Purdue University, has worked in pursuit of reducing weed pressure in crop fields for years.

Young oversees a current USB multi-state project that explores best management practices for herbicide-resistant weeds in a soybean production system. The project contains several objectives, each pointing to the goal of sustainability through chemical and non-chemical weed management approaches, while also maintaining the utility of herbicides through improved herbicide stewardship.

“I define herbicide stewardship as a twofold management effort: herbicide resistance as well as herbicide off-target
movement,” Young explained. “We want to help mitigate the possible evolution or the spread of new herbicide-resistant weed species. It also encompasses making sure that once herbicide is applied, it stays where you intended to apply it.”

Young and his research team have divided the project into studies involving herbicide management and studies that explore non-chemical weed control methods including cover crops and mechanical devices. The project is divided into eight components across 16 states. The studies vary by geography to examine how location plays a part in the
results.

“We tried to make sure the geography is appropriate for certain practices or whether the geography would have
a large influence,” Young says. “We have projects in dry locations and in environments that have more moisture; others are in cooler environments. These geographies all have different effects on how weeds are managed.”

One component of the research focuses on weed seed control during harvest through mechanical seed destruction. Researchers in six states are testing the effectiveness of seed mills, which are attachments to the combine that grind up weed seeds before they are spread across the field with the crop chaff.

“Earlier, these harvest weed control instruments weren’t easy to find or buy. Now they are becoming a bit more mainstream,” commented Young. “But they are expensive. We are also studying the economics of their use to see what kind of return-on-investment farmers may have against other means of weed control.”

Another component of this research is using cover crop biomass for weed suppression. Rodrigo Werle,
assistant professor and weed specialist at the University of Wisconsin-Madison, and his team are leading this component. They are researching the efficacy of planting green into a cereal rye cover crop through a project being conducted in 12 states.

“From a weed suppression standpoint, it’s about cereal rye cover crop biomass accumulation in the spring to reduce weed emergence and growth,” Werle said. “One way to allow for more cover crop biomass accumulation is through planting green, which is planting soybeans directly into the living cover crop then terminating the cover crop afterward.”

From this research, Werle and his team identified some issues with this strategy that are important for farmers to
know. In some plots, although the weed suppression was good, they found some reduction in soybean stands and yield. Results also showed that the cereal rye cover crop didn’t completely eliminate the need for herbicide use. Farmers who are using cover crops for weed suppression may have some trade-offs to consider.

“Our overall results showed that using a cover crop can help the herbicide program become more robust,” Young
said. “Instead of just getting 90 percent suppression of waterhemp and Palmer amaranth, we could get a 97-98 percent control when a cover crop was used in combination with an effective herbicide program.”

Another component of this research project explores the effectiveness of using harvest chaff lines for weed control.

“Chaff lining is taking the chaff that comes out of the combine and funneling it into a narrow strip,” explained
Young. “This leaves a sort of windrowed line where most of the weed seeds should be. Hopefully, through the winter months there will be some degradation of those weed seeds from the mulching effect.”

Mulching of the chaff lines utilizes moisture, temperature and microorganisms to break down the chaff and weed
seeds. There will still be viable weed seeds come spring, but the bulk of them should be in the chaff lines, allowing herbicide spraying to be more targeted. This could reduce the amount of herbicide applied, which also reduces costs.

This component of the research project is being conducted in Illinois, Iowa, Louisiana, Kansas, Kentucky, and Wisconsin to test the idea in both warmer and colder geographies. Results are still being analyzed on this portion of the project.

The project also includes components that study inter-seeded winter wheat in soybeans, rainfall activation of
residual herbicides, soil residual activity of metribuzin, and herbicide off-target movement and monitoring surface
temperature inversions.

Results of all these studies will be finalized for public outreach. Some of these topics already have information
on the USB Take Action website, and more will be added as results are finalized.

Institutions involved in this USB project:
University of Arkansas
Southern Illinois University
University of Illinois
Iowa State University
Purdue University
Kansas State University
University of Kentucky
Louisiana State University
Michigan State University
Mississippi State University
University of Missouri
University of Nebraska
North Dakota State University
Ohio State University
Penn State University
University of Tennessee