Supercharged herbicide resistance creates control challenges

Weeds resistant to multiple types of herbicides continue to gain strength and complexity. There are two main groups of herbicide resistance: target-site resistance (TSR) and non-target-site resistance (NTSR).

Target sites in plants are enzymes (proteins) that are critical for plant growth.

Initially, TSR occurs when weeds adapt to change their target site/protein structure to prevent a specific herbicide from binding to a specific target site, allowing the weed to survive. An example is the evolution of weed resistance to ALS herbicides or glyphosate over time, resulting from repeated use.

More recently, NTSR and metabolic resistance are becoming increasingly common in weed populations such as waterhemp, Palmer amaranth, kochia, and marestail. Non-target-site resistance is a larger concern, as plant cells in weeds “learn” to chemically alter or sequester herbicides, rendering them immobile, nontoxic, or inactive.

Crafty weeds

Such metabolic resistance mechanisms extend beyond resisting one herbicide to encompass multiple herbicide groups, sometimes without any exposure to a herbicide. For example, in 2021, an Illinois waterhemp population developed metabolic resistance to dicamba (Group 4) without having been sprayed with dicamba.

Pat Tranel, weed scientist and associate head of Crop Science at the University of Illinois, said that farmers have selected these populations for decades, allowing weeds to accumulate resistance mechanisms.

“In 2011, we had six different mechanisms of resistance in waterhemp, and five of those were target site changes. In the last 12 years, we’ve identified nine new mechanisms of resistance, with eight of those being non-target site,” he said.

Rotating sites of action for decades has helped slow the selection of weeds with target-site resistance. However, all these changes can potentially select for weeds with non-target-site resistance.

“It’s a cumulative effect where weeds can have all the target-site mechanisms, and now they’re adding non-target-site herbicide resistance mechanisms on top of them to create complex resistant populations,” Tranel said.

He said he believes we’ve reached a tipping point regarding the continued use of chemistry, especially given the fact that researchers and farmers both know herbicide resistance will never leave a weed population.

“We know we can’t solve chemical resistance with more chemicals. We’re still using glyphosate, but we can’t rely on it like we once could. So, we must use other tactics to keep these resistant weeds from going to seed,” Tranel said. “Stopping additions to the weed seed bank should be an annual goal, not just saving yield.”

Hope for the future

Although the current herbicide group number chart helps growers rotate chemistries, Tranel envisions a future chart showing which herbicides from different groups metabolize similarly so they are not applied together.

“There is a lot of research ongoing to help make the unpredictable more predictable, but we’re not there yet,” he said.

Understanding metabolic herbicide resistance requires the application of genomic tools and resources to critical weed species. In April 2021, the International Weed Genomics Consortium (IWGC) was founded as a collaboration between public university and private company researchers to advance the sequencing of weed species’ genomes.

Like the early days of the fledgling crop biotech industry when genetic sequencing of corn and soybean genomes was underway, the IWGC is seeking the same with critical weeds — only with much better and faster tools. Once researchers understand weed genomes and the genetic basis for herbicide resistance, genetic markers can be used to help farmers avoid wasting money on products that won’t work on a given population.

Target 50 weeds

The IWGC aims to develop chromosome-level reference genome assemblies for at least 50 critical weedy species worldwide. So far, this strategy has yielded more than 30 weed species’ reference genomes, in partnership with the Corteva Agriscience Genome Center of Excellence. These resources are available to all researchers for a greater understanding of weed evolution and herbicide resistance.

“For example, we know that Cytochrome P450 enzymes are key contributors to non-target-site herbicide resistance, and we can now identify those enzymes in waterhemp and Palmer amaranth,” Tranel said. “With this starting point, we can begin to determine which of these actually metabolize herbicides, which metabolize different herbicides, and which are expressed in the plant in the same way.”

Learning gene sequences of diverse weed species will eventually help model new herbicide target site proteins and design novel, effective herbicides with minimal off-target effects.

Not only will weed genomics provide a wealth of insight into herbicide resistance, but advancing artificial intelligence could help revolutionize integrated weed management strategies to counter the evolution of herbicide resistance. This breakthrough could significantly improve global food security.