Nine weed species in the U.S. have been confirmed to be resistant to glyphosate, the most widely used herbicide in the nation, according to the Weed Science Society of America (WSSA).

“Unfortunately, it is too late to prevent glyphosate resistance from developing,” says David Shaw, WSSA president. “It’s a problem that is already with us. The challenge now is to adopt effective management techniques that can keep resistance from spreading.”

Strains of the following weeds show resistance: Common ragweed, common waterhemp, giant ragweed, hairy fleabane, horseweed, Italian ryegrass, johnsongrass, Palmer amaranth and rigid ryegrass.

Resistance can be a problem for growers who rely almost exclusively on glyphosate for weed control, particularly those who grow soybeans, corn, cotton and sugar beet crops genetically engineered to tolerate glyphosate. Using a single herbicide increases the odds that the weed population will shift to resistant plants able to escape treatment.

One way to prevent or manage resistance, suggest university scientists, is to rotate the types of herbicides used for weed control to make it tough for weeds to adapt. Initially, some farmers were slow to adopt the practice.

“One issue may have been the mistaken perception that adopting resistance management practices will cost more, since glyphosate tends to be very affordable,” Shaw says. “But studies show just the opposite is true.”

Researchers are comparing the economics of university-recommended herbicide-resistance management programs using glyphosate exclusively for weed control in a four-year research project now underway in Illinois, Indiana, Iowa, Mississippi, Nebraska and North Carolina. Net returns on fields managed according to recommended best practices are equal to or greater than the returns on those where glyphosate is used alone, according to results from the first three years of study. Increased yields appear to offset any increase in herbicide costs.

“When glyphosate was first introduced for weed control, its unique way of inhibiting protein synthesis and growth in plants led many to believe that resistance would not be an issue,” Shaw says. “Obviously, that prediction was wrong. However, best management practices can slow the development of resistant weeds, and one effective approach is to rotate glyphosate with herbicides that work very differently.”