Glyphosate, the active ingredient in the weed killer sold under the name RoundUp, 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. While the mechanisms that have led to resistance are not fully known, Bill Johnson, a professor of weed science; Steve Hallett, an associate professor of weed science; and Jessica Schafer, a graduate student in botany and plant pathology, believe that soil microbes may play a role.

Most laboratory tests done to understand glyphosate resistance are done in sterile soil, void of those microbes. Schafer said Purdue's findings, published online early in the journal Weed Science, show that those microbes may play a significant role in how glyphosate affects plants.

"The soil you're growing the plants in is important to the results," Schafer said. "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."

Hallett 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.