Near total reliance on one herbicide and one mode of action was a significant factor in building herbicide resistant stands of Palmer amaranth—and other weed species. “Lack of diversity hurt us,” he said. “We can’t depend on one mode of action.”

North Carolina row crop farmers have had “a tremendous problem with resistance,” York said. “It made us change, and it’s expensive. The Southwest is beginning to see some Palmer amaranth resistance. They should start control efforts early, before it overwhelms them. Early detection is important. Watch for live (Palmer amaranth) plants intermingled with dead ones. That’s a red flag.”

Farmers in the Mid-South states have faced similar problems with first ALS and then glyphosate resistance in cotton, soybeans and corn. Glyphosate resistance in marestail first showed up in west Tennessee in 2001 and then spread to Palmer amaranth and Italian ryegrass.

Producers are finding they can still farm in the face of such resistance, says Tom Eubanks, weed scientist with the Mississippi Agricultural and Forestry Experiment Station based in Stoneville, Miss.

“One of the keys is the use of a residual herbicide,” Eubanks told farmers attending the Iowa Soybean Association’s OnFarm Network Conference in Ames, Iowa. “A second is making sure you apply a postemergence herbicide to Palmer amaranth (pigweed) when the plants are small. If you wait until the pigweeds are more than 2 inches tall, it’s almost impossible to control them.

“Palmer amaranth has become the driver weed for us in the South,” he said. “One plant that escapes can grow seven- to eight-feet tall and produce 1 million to 1.5 million seed.”

Weed resistance can come as an unwelcome surprise to unsuspecting farmers in Texas, as well, says Paul Baumann, Texas AgriLife Extension weed specialist out of College Station. Baumann told participants in the grain session of the recent Blackland Income Growth (BIG) conference in Waco that they can have resistant weeds and not know it. “It sneaks up on you.”

Using a highly effective herbicide (such as glyphosate) over several years in a row increases the potential to select “those weeds that are not susceptible to the herbicide,” he said. “That eliminates the competition and the resistant weeds flourish.”