Research Highlights

Research Highlights
Exploring Genetic Basis of 2,4-D Herbicide Resistance in Palmer Amaranth

Palmer amaranth in soybeans. Photo: United Soybean Board

By Carol Brown

Over the years, scientists have successfully developed crops resistant to herbicides so weeds can be killed without disrupting crop growth. And, over time, several species of weeds have become resistant to some herbicides. Weed specialist Mithila Jugulam is approaching weed management from another direction by looking at the weeds themselves to find out why they become herbicide resistant.

The Kansas State University agronomy professor is leading a project, supported by Kansas Soybean Commission, to determine Palmer amaranth resistance to 2,4-D, a widely used herbicide. Her research results will help shape weed management strategies in Kansas and beyond.

“Palmer amaranth is a top-priority weed in Kansas and most of the United States,” says Jugulam. “It has been found that a Palmer amaranth population in Kansas developed resistance to six different herbicides including 2,4-D. We found that out of the six herbicides, resistance to at least five is mediated by metabolism, which is a unique aspect of this Palmer amaranth population. Development of metabolic resistance can have serious consequences in terms of weed management.”

Metabolic resistance is the ability of a weed to break down, or degrade, a herbicide. Enzymes within a plant help protect it against foreign substances and can degrade the herbicides, says Jugulam.  Each plant’s makeup of the enzymes involved in degradation is different and enables it to be more tolerant to certain chemicals than others. In crops such as corn and sorghum these enzymes provide a natural tolerance to 2,4-D by rapidly degrading the herbicide. She and her research team found Palmer amaranth has a similar mechanism as corn in 2,4-D degradation.

Figure 1. Experiments of 2,4-D dose response confirmed that the AHR (auxinic herbicide-resistant) Palmer amaranth is 8- to 10-fold more resistant compared to two known AHS (auxinic herbicide-susceptible) Palmer amaranth populations.

By studying the genetics, Jugulam is exploring how quickly Palmer amaranth with metabolic resistance can spread, which could have implications at the field-level, the state and beyond. In growth chambers and greenhouses, she is crossing Palmer amaranth herbicide-resistant plants with susceptible plants to generate progeny that can provide clues to how the trait is carried on.

“The F1 (first generation) progeny may tell us if herbicide resistance is nuclear- or gene-controlled, or neither,” says Jugulam. “We also have developed the next generation, or F2, to determine how many genes are involved in controlling 2,4-D resistance. We are studying these to see whether resistance is a single, dominant gene, which can spread fast within the population. On the other hand, if there are multiple resistant genes involved, then resistance could move slowly instead.”

Weed management for now and the future

Farmers and their crop consultants need to be cognizant of their weed management strategies to help herbicides do their job. History has shown that using the same herbicide such as glyphosate in the same fields year after year has enabled weeds to develop resistance to this effective herbicide.

“If Palmer amaranth is showing to be resistant to 2,4-D herbicide, growers need to be prudent in their herbicide applications to help ensure resistance,” remarks Jugulam. “My colleagues at Kansas State are testing multiple tactics to reduce weed populations in crop fields that involve both field management and chemical-based weed control. Still, the most effective way to control weeds is the use of herbicides, so we don’t want weeds to evolve their resistance to them.”

Weed management strategies including cover crops, row spacing, crop rotation, shallow tillage and herbicide rotation working together can go a long way to reduce weed populations in farmers’ fields. Jugulam and her research team continue to explore how weeds adapt to survive herbicide application resulting in resistance. Their results will help crop growers in Kansas and beyond improve their weed management strategies for greater crop productivity.

Published: Sep 20, 2021

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.