Speaking at a recent field day at the Stiles Farm, a research center near the central Texas town of Thrall, Morgan said continuous use of the same herbicide or herbicides with the same mode of action in a field, ultimately will create weeds that are resistant to the materials.

“We only have eight modes of action in available herbicides,” Morgan said. “If overuse allows resistance to develop, we lose some valuable weed management options.”

He explained that resistance comes in several guises. Single resistance indicates a weed is resistant to a single herbicide. Cross-resistance means weeds are resistant to herbicides with the same mode of action. Multiple resistance occurs when a weed is resistant to two or more modes of action.

“We want to avoid multiple resistance,” he said.

Morgan said ryegrass species have been identified in wheat that are resistant to herbicides such as Amber, Glean, and Finese – all ALS herbicides.

Palmer amaranth has shown resistance to atrazine in both corn and grain sorghum. Johnsongrass has also shown resistance to ALS herbicides.

“We see herbicide tolerant weeds in row crops,” Morgan said. “Certain herbicides will not harm them. These weeds have never been susceptible and that’s different from resistance that develops after repeated use of a product on a specific target weed.”

He also noted potential for a “species shift,” new weed species taking over when population of another diminishes as a result of an effective herbicide or other control practice.

Resistance, however, poses a more serious problem. He says resistance depends on the weed species, the efficacy of the herbicide, and the frequency of herbicide use.

“Farmers can control only the frequency of use,” he said. “But we know that continuous use of a particular herbicide will contribute to resistance. Consequently, we should rotate two or three herbicides.”

He also cautions that using herbicides with the same mode of action also creates an environment for resistance development.

Prevention depends on product stewardship. Morgan recommends:

  • Alternate non-chemical with chemical control methods.
  • Rotate herbicides, including mode of action.
  • Tank mix different modes of action to apply different types of materials.
  • Rotate crops. Certain weeds are easier to clean up in some crop, because of different herbicides available, than in others.
  • Control “suspect” herbicide resistant weeds as if they were an invasive weed species. Hand rouging, cultivation and other methods may be necessary.
“Farmers have a limited number of weed control tools,” Morgan said, “so it is necessary to keep them available. We know that some 250 different weed species have developed resistance to herbicides.”

e-mail: rsmith@primediabusiness.com