Why top herbicide can’t kill superweeds

PURDUE (US) — Soil microbes may explain why some “superweeds” are resistant to glyphosate, the active ingredient in Roundup.

Glyphosate is the most widely used herbicide in the United States, but some plants have grown resistant to it. This has caused farmers to turn to additional herbicides to kill the resistant weeds.

While the mechanisms that have led to resistance are not fully known, Bill Johnson, a professor of weed science at Purdue University, and colleagues think soil microbes may play a role.

Most laboratory tests done to understand glyphosate resistance are done in sterile soil, void of microbes.


“The soil you’re growing the plants in is important to the results,” says Jessica Schafer, a graduate student in botany and plant pathology. “If we’re growing in a sterile media, we could get some false positive results because the plants are more tolerant to glyphosate in those conditions.”

Steve Hallett, an associate professor of weed science, and Schafer grew giant ragweed, horseweed, and common lambsquarter in both sterile soil and field soil and subjected them to glyphosate. In each soil, strains of weeds both susceptible and resistant to glyphosate were tested.

Both versions of giant ragweed were damaged more from the glyphosate in field soil. The susceptible version of common lambsquarter was also more heavily damaged in field soil. Horseweed fared the same no matter which soil or strain—susceptible or resistant.

The results, published in the journal Weed Science, show that microbes can play an important role in the activity of glyphosate, presumably by invading the glyphosate-weakened plants. The results also suggest that glyphosate-resistant weeds may be more resistant to disease pressure as well.

“Soil microbes can be minor to major contributors to how glyphosate is able to affect plants,” Hallett says. “We may be selecting not only for glyphosate resistance, but inadvertently selecting for weeds that have disease resistance as well.”

A weed’s ability to withstand glyphosate was based on dry shoot and root weight after testing. The sterile soil used in the study came from field soil exposed to gamma radiation to kill microbes and bacteria. The irradiated soil was tested to ensure that its nutrients were not diminished.

Hallett, Johnson, and Schafer say further studies would look at how fungi in the soil affect root development, both with and without glyphosate.

“Dirt is a living organism,” Johnson says. “It’s important to know how all the pieces interact.”

The research was conducted with internal funding from Purdue’s Department of Botany and Plant Pathology.

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