Research Highlights

Developing Strategies to Manage Herbicide-Resistant Palmer Amaranth

By Laura Temple

Palmer amaranth has become the primary broadleaf weed problem in soybean fields throughout Tennessee and many other states. A prolific type of pigweed, it is difficult to manage.

“Palmer amaranth can grow from a seed the size of a period at the end of a sentence to 12-inches tall in just 11 days,” says Dr. Larry Steckel, row crop weed management professor with the Department of Plant Sciences, University of Tennessee Institute of Agriculture. “It competes aggressively with soybeans and other row crops, and it continues to develop resistance to herbicides.”

To help farmers manage herbicide-resistant weeds, especially Palmer amaranth, the Tennessee Soybean Promotion Board invests soy checkoff funds in ongoing research and Extension support. Steckel oversees this research and education, which also receives support from the national soy checkoff.

“Our small-plot research is now focusing on providing early warning on new types of herbicide resistance in Palmer amaranth,” Steckel explains. “Resistance to auxin herbicides like 2,4-D and dicamba has been documented in Tennessee and nearby states, and we are monitoring its movement while identifying strategies to effectively control it.”

Herbicide Tank Mixes and Timing

In soybeans, a pre-emergence herbicide that controls Palmer amaranth is a necessity, according to Steckel. This allows the crop to get established and in-season control to be more manageable.

Herbicide-tolerant soybean systems provide more options for post-emergence control. However, populations of Palmer amaranth are developing resistance to multiple herbicides, including glyphosate, fomesafen, 2-4,D and dicamba.

“Tank mixes of glyphosate and an auxin herbicide, either 2,4-D or dicamba, depending on the technology system, are providing less control of Palmer amaranth than they did just four years ago,” Steckel says. “Timing is critical. Applying the tank mix when the weeds are small helps. Then, we recommend following up with a glufosinate application seven to 14 days later to clean up escapes. This combination has been consistently effective when applied properly.”

He notes that glufosinate, or Liberty, is temperamental, requiring ideal conditions to be fully effective.

“A follow-up of glufosinate is most effective when applied mid-day in hot, humid conditions,” he explains. “Inconsistent control can be caused by a host of environmental reasons including application near sundown, cool weather right around application or rain within four hours of spraying.”

However, Steckel and his team are already looking at the next concern.

“Glufosinate-resistant Palmer amaranth has been confirmed in Arkansas, just across the Mississippi River from Tennessee,” he says. “We are monitoring the potential spread of this population, but that reinforces that pre-emergence herbicides need to be the primary control method for this weed, with post-emergence herbicide as supplemental control.”

Evaluating Cultural Management Practices

Because new herbicide technologies are not likely in the short-term, the research Steckel leads includes ongoing evaluation of other methods to help control Palmer amaranth.

“In soybeans, planting in narrow rows and higher populations shade the ground more quickly, reducing some germination of Palmer amaranth,” he says.

His team is also investigating how cover crops can be tailored to support Palmer amaranth control. Their work has found that a cover crop blend of a grass like wheat or rye with a legume like a type of clover or vetch improve weed control, likely in part because of the biomass they produce.

“We’ve seen that terminating cover crops just before planting, or planting green into cover crops, can reduce Palmer amaranth emergence by 50 percent,” Steckel explains. “This also delays their emergence and growth to 3- to 4-inches by about 40 days, giving the crop more time to get established and providing farmers with more flexibility for post-emergence herbicide control timing.”

He believes farmers can learn to dial in their cover crop mix for both the following cash crop and improved weed control. His team is also learning about the allelopathic effects of cover crops on weed germination, or the release of biochemicals that can suppress weed germination and development.

According to Steckel, crop rotation, especially with corn or grain sorghum, helps farmers manage Palmer amaranth. However, he notes that some fields flood regularly in the spring, limiting crop options so those fields end up in continuous soybeans despite the best of intentions.

In some of those flat fields, Steckel does say that deep tillage that buries Palmer amaranth seed deep can help. However, he clarifies that tillage isn’t really an option on most of Tennessee’s highly erodible land.

Published: Nov 29, 2021