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Research Highlights

Research Highlights
Changing weed management behaviors

By Daniel Lemke, photos by NDSU Extension

Two years ago, a bus tour program to Nebraska in order to learn more about Palmer amaranth left North Dakota State University (NDSU) Extension Agronomist Tom Peters fearful about the future of crop farming if the weed ever came to the state.

 “I was scared to death about our prospects if Palmer amaranth ever got established in North Dakota,” Peters recalls.

Since that 2017 program, Palmer amaranth was discovered in five North Dakota counties, although all known stands were small and were controlled. A follow-up program to Nebraska in August 2019 left Peters a bit more hopeful about a future with Palmer amaranth.

 “This time, I felt we could tackle the weed if an integrated plan could be implemented,” Peters says. “There’s hope.”

Creating Awareness

The North Dakota Soybean Council awarded NDSU Extension a grant for travel to the University of Nebraska West Central Research and Extension Center at North Platte to learn how Palmer amaranth is changing the way corn and soybeans are grown. The mission included NDSU Extension staff, members of the North Dakota Weed Control Board, farmers and ag retailers.

“Behaviors in Nebraska have changed since 2017,” Peters says. “Farmers there saw Palmer amaranth control as strategically important. They have zero tolerance for weed escapes.”

Controlling every Palmer amaranth plant is important because each weed can produce a prolific amount of seed.

Peters says that the delegation visited an area of Nebraska that primarily used a no-till approach which took tillage from the equation as a management tool. Farmers bulked up on their cultural practices and didn’t just rely on chemicals. Growers incorporated narrower row spacing, higher plant density, crop rotation, cover crops and field-edge control into their weed management plan.

“The farmers who understood the situation and changed their behavior used a continuous weed-management plan. Their practices were implemented in fields over the course of years,” Peters says.  “Farming practices in 2019 that are coupled with practices in 2020 will, ultimately, pay dividends in 2021.”

Farmers can quickly lose fields infested with Palmer amaranth if not managed and weeds controlled when they’re small.

Peters says that he saw areas where strategic management plans were working well. Those efforts included starting with clean fields using postemergence burndown herbicides. Growers then applied soil residual herbicides at multiple time points beginning with a pre-emergence application and followed by early post-emergence application. Finally, growers finished by using a post-emergence herbicide. Farmers applied a dicamba product or Liberty as the closer, typically just before row closure. If needed, weed escapes were pulled by hand. Those chemical programs were complimented with cover crops seeded after wheat, corn or soybean harvest to discourage later-germinating Palmer amaranth.

 “That’s the recipe we’re going to have to follow,” Peters explains. “We can’t allow Palmer amaranth to get established.”

Peters says that Palmer amaranth can be expensive to control. Given the current farm economics, farmers likely can’t spend an exorbitant amount on weed control. In Nebraska, if Palmer amaranth is established, Peters says that farmers may need to spend as much as $100 per acre on weed control.

Stutsman County NDSU Extension Agent Alicia Harstad took part in the Nebraska program to learn about research being done by the University of Nebraska and to see how farmers were coping with Palmer amaranth.

Palmer amaranth control is going to require an aggressive, multi-pronged approach to keep fields clean, according to NDSU researchers.

“We saw Palmer that was growing taller than corn,” Harstad says. “In that same cornfield, there were also 4- to 6-inch-tall Palmer plants growing under the canopy.”

Like Peters, Harstad knows that North Dakota farmers need to be proactive about weed control. She says that farmers are already struggling with herbicide-resistant kochia and that waterhemp is becoming increasingly difficult to control.

“If we are proactive about weed control, we can prevent complete field disasters. If we are reactive about weed control, it will be a losing battle,” Harstad says.

Long-Term Solution

 Agronomists and weed experts know that successful weed management will involve long-term plans, not just choosing which herbicides to spray that year.

“It could be a 3- or 4-year plan that includes crop rotations and cover crops, then think strategically about herbicides,” Peters says. “It’s a long-term view, rather than a year-by-year view.”

“We also need to think about rotating herbicides within the whole crop rotation, not only within the same growing season,” Harstad says. “For example, take a look and see what herbicide modes of action you are using in your soybeans this year, and try to avoid using the same ones in your corn the following year.”

Harstad says that farmers need to think beyond just herbicides and con-sider other factors that affect weed control, such as spray quality, using the right adjuvant, getting the crop  to canopy faster and other production practices.

“There isn’t an ‘easy’ button when it comes to weed control,” Harstad adds.

 While Palmer amaranth has a reputation as a formidable weed, Harstad encourages farmers to apply the same approach to managing a much more common pest.

“I am glad people are looking for and are worried about Palmer,” Harstad says, “but we need to make sure to not forget about waterhemp.”

 Whether for waterhemp or Palmer amaranth, commitment to diversified management practices will be a determining factor in a farmer’s ability to control these troublesome weeds.

A Noxious Guest

Palmer amaranth was added to the list of North Dakota’s noxious weeds in early 2019, becoming the 13th plant listed. Landowners and operators are required to do whatever they can to control the spread of noxious plants because those plants can be injurious to the land, people and livestock.

 Stan Wolf, Cass County weed officer, is one of the people responsible for enforcing the state’s noxious-weed laws. Weed officers are the first line of defense on all grasslands and rangelands in North Dakota. Wolf credits NDSU Extension with raising awareness about Palmer amaranth.

“Farmer owners and operators know about Palmer,” Wolf says. “They may not know exactly what it looks like, but they’re doing a good job of looking. The main thing is, if they see something that’s not normal, get it identified. Work with your agronomist, Extension or a weed officer, some-one who can identify what the plant is.”

 Wolf is concerned about Palmer amaranth not only on cropland, but also in other areas such as fencerows, pastures and shelter belts. He says that, if those areas aren’t controlled, weed seed is free to move around via water or birds. Of particular concern to Wolf are hunting food plots. He says that these areas are typically in hidden spots, out of sight from most people, which make them attractive to wildlife. Those areas could harbor noxious weeds such as Palmer amaranth, especially if the plot was seeded with products purchased from other states. Two of Minnesota’s Palmer amaranth infestations originated from conservation planting seed and contaminated cover-crop seed.

 “Who knows where that seed came from?” Wolf asks. “Any seed source is suspect if not produced locally in areas known to be Palmer amaranth-free.”

Noxious weeds listed in North Dakota include:

  • Absinth Wormwood
  • Diffuse Knapweed
  • Musk Thistle
  • Russian Knapweed
  • Yellow Toadflax
  • Canada Thistle
  • Houndstongue
  • Palmer amaranth
  • Saltcedar
  • Dalmatian Toadflax
  • Leafy Spurge
  • Purple Loosestrife
  • Spotted Knapweed