Research Highlights

Research Highlights
Research Survey Monitors Herbicide-Resistant Palmer Amaranth in South Carolina

Dicamba-based herbicide systems have become common in South Carolina soybeans because of the varieties available. This option still controls Palmer amaranth well, as shown by the response to dicamba post-emergence treatments. The control (0) is on the left, a full rate (1X) in the center and a double rate (2X) on the right. Photo: Mike Marshall

By Laura Temple

Years ago, timely glyphosate applications controlled Palmer amaranth. But this aggressive pigweed species seems to readily outsmart weed control programs and steal yield from soybeans and other crops.

“We started seeing Palmer amaranth develop resistance to glyphosate in Roundup Ready crop systems just six to eight years after the technology was introduced,” says Mike Marshall, assistant professor and weed scientist at Clemson University. “We surveyed Palmer amaranth weeds in 2010 to 2011 and found that most populations were resistant to at least twice the recommended rate of glyphosate.”

With the introduction of new weed control systems for soybeans in 2017 based on dicamba and 2,4-D herbicides, farmers received much-needed help controlling the weed.

“By 2021, these technologies had been available for five years,” he explains. “We thought that would be a good time to survey Palmer amaranth around the state again, to see if or how herbicide resistance is developing in current populations.”

Though glufosinate, the herbicide used in the Liberty Link system, has been available for years, it is still an effective option. Left to right shows how Palmer amaranth collected from around South Carolina responded to the control (0), the recommended rate (1X) and a double rate (2X). Photo: Mike Marshall

The South Carolina Soybean Board funded this research survey to ensure soybean producers have recent data to inform their herbicide program decisions. Marshall’s research looked at the activity of nine herbicides, including pre- and post-emergence options and residuals. He wants farmers to know which products control Palmer amaranth well and how they can preserve the effectiveness of those tools.

Marshall and his team collected mature seedheads from Palmer amaranth escapes in fields before harvest during the fall of 2021. They gathered 126 total samples, averaging four or five samples each from 24 counties with significant row crop production. 

“Finding weeds to sample was a challenge, which means farmers are currently doing a good job managing Palmer amaranth,” he says. “Most samples came from corners or edges of fields. However, we conducted a pretty comprehensive survey of the weed throughout the state.”

Palmer amaranth response to a post-emergence residual treatment of a PPO inhibitor, Reflex. Left to right shows the control (0), a single rate (1X) and a double rate (2X), reinforcing the importance of including a residual herbicide in every application. Photo: Mike Marshall

They planted one sample per county of the collected seeds in a greenhouse and treated each population with the different herbicides either pre-emergence or at the 2- to 4-leaf stage, when weeds were about 1 to 2 inches tall. They tested each herbicide at the recommended rate and twice that rate.

“Overall, results showed what we expected,” Marshall reports. “Most populations tested resisted control from glyphosate and ALS inhibitors. However, we observed reliable control with the remaining herbicides, including 2,4-D, dicamba, glufosinate and PPO inhibitors — our post-emergence options.” 

He notes that a few populations weren’t controlled effectively, but further investigation will determine if true resistance is starting to develop. 

When looking at pre-emergence herbicides, seven Palmer amaranth populations survived a single rate and one population survived the double rate of the group 15 herbicide, Dual Magnum, which is used on almost every crop grown in the region. Resistance to this mode of action has been confirmed in Georgia and North Carolina. Two populations survived the HPPD inhibitor isoxaflutole, which Marshall says is relatively new to the area because it isn’t used much in South Carolina corn. Just one population survived the atrazine treatment. Palmer amaranth resistant to group 5 herbicides like atrazine has also been confirmed in Georgia. 

“The survey reinforced the importance of not relying on a single herbicide mode of action to control Palmer amaranth,” he explains. “Farmers should keep using multiple modes of action and residual herbicides in every tank.”

He asks that farmers let their local extension agent know if they have Palmer amaranth escapes so they can investigate the issue. He also recommends pulling up lone plants and carrying them away, rather than dropping them in the field.

“We need a zero-tolerance policy,” Marshall says. “Palmer amaranth is so prolific that even with years of successful control, it only takes one year of slipping up or missing spray timing to replenish the seedbank to create years of future problems.”

He emphasizes that farmers are doing a good job of scouting and managing the weed. But because Palmer amaranth has proven its ability to adapt to control measures, he will continue to closely monitor the weed.

“We may repeat the survey again in five years to monitor any changes in our Palmer amaranth population,” he concludes. “At the same time, we will keep looking for other ways to control it.”

South Carolina Palmer Amaranth Survey Results
Palmer amaranth population response by county to nine herbicide treatments. The “++” indicates survivors at both the single and double application rate, “+-” represents survivors at only the single rate, and “X” indicates no weed germination from that population, even in the controls.  Source: Mike Marshall

Published: Sep 25, 2023

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.