Metabolic Resistance on the Rise in Some Weeds

Metabolic resistance confers unpredictable cross resistance in weeds and should be a concern for all growers and consultants, according to a recent webinar on the subject (photo courtesy B. Hanson.)

Dealing with herbicide resistance in weeds means understanding not only target site resistance, but also metabolic resistance, a more recent discovery by weed scientists they say can present a major challenge in the future.

In a Take Action webinar, University of Illinois Crop Science professor Pat Tranel explained the importance of understanding metabolic resistance in weeds.

Take Action is an education platform designed to help farmers manage herbicide, fungicide and insect resistance. The goal is to encourage farmers to adopt management practices that lessen the impacts of resistant pests and preserve current and future crop protection technology.

Herbicide resistance is a decrease or loss in effectiveness of an herbicide on a weed population. It occurs due to evolution of the weed population. Resistance is not new, Tranel said. Resistance is here to stay and is already limiting herbicide options for weed control.

Target site resistance is when a weed evolves to prevent the active ingredient in an herbicide from binding enzymes. This type of resistance can be managed, Tranel explained, by mixing and rotating effective herbicide groups.

Metabolic resistance is another way weeds can evade herbicide control. This is a multi-step process where plant enzymes convert the parent molecule in the herbicide into metabolites which have decreasing toxicity. Metabolic resistance occurs when a weed population evolves the ability to metabolize an herbicide.

Tranel noted four important reasons why growers and crop advisors should care about metabolic resistance in weeds and possible ways to slow it down.

Metabolic resistance confers unpredictable cross-resistance in weeds. It is increasing, he said, and weed populations are steadily accumulating different herbicide metabolism genes. Because of metabolic resistance, some weed populations are already resistant to yet-to-be-discovered herbicides.

Weed scientists are still encouraging mixing and rotating effective herbicides with different modes of action. This strategy alone, Tranel explained, will not prevent metabolic resistance. It may even increase it, especially when reduced rates of herbicides are used.

“We need to expose fewer weeds to herbicides with non-chemical strategies,” Tranel said. Those strategies include crop rotation, cover cropping and tillage when necessary. Reducing the soil seedbank, keeping field or orchard borders clean and scouting for weed infestations are also recommended.

Continued stacking of different herbicide metabolism genes in weeds could be the beginning of the end of the chemical era of weed control, Tranel warned.

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Cecilia Parsons
Cecilia Parsons has spent the past 30 years covering agriculture in California for a variety of newspapers, magazines and organizations. During that time she has been fortunate to witness some of the important events that have shaped this diverse industry and worked hard to examine and explain these events for readers. When Cecilia first moved to the San Joaquin Valley in 1976, her first journalism job was at a small daily newspaper where she covered “farm news.” From there she branched out to writing for a dairy magazine and a regional weekly agriculture publication. Cecilia is part of a farming family from the rural community of Ducor where she also raises purebred sheep and is attempting to master versatility ranch horse riding.