Research HighlightsGaining Control of Glyphosate-Resistant Palmer Amaranth in Nebraska
By Carol Brown
Palmer amaranth is making its presence known across the Midwest, and in fields with glyphosate-resistant crops, it also has become resistant to this commonly used herbicide. Scientists across the country are studying how to get control over this prolific weed and it isn’t easy.
Amit Jhala, associate professor and weed management specialist at the University of Nebraska–Lincoln, has been studying herbicide programs to find the best combination that will control Palmer amaranth through research projects supported by the Nebraska Soybean Board.
“Palmer amaranth is widespread and present in almost every Nebraska county,” says Jhala. “According to a recent state survey, Palmer amaranth is the number one weed problem in the state’s crop fields.”
The two-year research project contained several objectives including the study of various glyphosate-resistant weeds through different pre- and post-emergent herbicide programs. And one objective focused mainly on controlling Palmer amaranth.
Jhala and his research team explored different herbicide programs in Roundup Ready® 2 Xtend and Enlist™ soybeans in combination with using narrow row spacing for their effectiveness in controlling or reducing seed production of Palmer amaranth populations. The study was conducted on a farmer’s no-till field near Carleton in Thayer County, who had been struggling with the weed for half a decade.
“We treated the Enlist soybeans with herbicides Sonic®, Zidua®, or Trivence®, applied pre- and post-emergence and found that they provided 84 to 97 percent control of Palmer amaranth early in the season over the two years,” Jhala says. “When these pre-emergence herbicides were followed by Enlist One, Liberty®, or their mixture and applied post-emergence, they provided 87 to 95 percent control.”
The researchers found that the areas not receiving a post-emergent herbicide treatment failed to maintain control of glyphosate-resistant Palmer amaranth compared to the areas treated with both pre- and post-emergence herbicides.
Weed Reduction Recommendations
There are things farmers can do to reduce the presence of glyphosate-resistant weeds including waterhemp and Palmer amaranth. Jhala and the research team have designed several recommendations based on the outcomes from this project.
“Growers need to understand the biology of Palmer amaranth as it is important to know when the weed emerges to gain effective control,” he says. “We recommend routinely scouting your fields to know when the weed emerges to ensure proper post-emergent herbicide application timing.”
Jhala also recommends using a diverse approach to weed management that is focused on reducing Palmer amaranth seed production and the number of its seeds in the soil seedbank. At harvest, if farmers can work toward preventing field-to-field movement of weed seeds, this can help to prevent seedbank build-up.
Other recommendations include:
- Use a multiple mode-of-action pre-emergence herbicide at soybean planting
- Use field management techniques such as narrow row spacing and cover crops, which will suppress Palmer amaranth and other weed growth through crop competitiveness
- Stay aware of crop rotations and types of herbicide-resistant cultivars to maintain weed control effectiveness. For example, Enlist corn should not be in rotation with Enlist soybean to avoid the reduction of herbicide effectiveness with post-emergent applications of Enlist One or Enlist Duo.
Palmer amaranth is a tough weed to manage but using multiple strategies including appropriate herbicide-resistant soybean varieties, herbicide treatments and agronomic management techniques, farmers can see positive results. Reducing weed pressure can help soybean productivity and ultimately improve yield.
Jhala and the research team have published several articles on this research:
University of Nebraska “CropWatch”:
“Weed Technology,” a peer-reviewed journal of Weed Science Society of America:
- Effect of row spacing and herbicide programs for control of glyphosate-resistant Palmer amaranth in soybean
- Management of glyphosate-resistant Palmer amaranth in Enlist soybean
This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.