Capital Press

American Falls farmer Burt Fehringer adds 2,4-D to his tank of glyphosate when he sprays herbicide on patches of Canada thistle.

"I'd hate to have it become resistant to RoundUp," Fehringer said, explaining his reason for using a second mode of action against his weeds.

Experts warn over-reliance on glyphosate -- the active ingredient in Monsanto's RoundUp herbicide -- is already responsible for resistant weeds in the South and Midwest. They advise growers in the Pacific Northwest can avoid the same mistake by following sound agronomic practices, especially using other chemicals that attack weeds in different ways whenever possible.

This spring, Fehringer intends to try direct seeding -- planting a crop in the previous year's residue with little or no tillage -- on 20 acres with sandy soils. The method can reduce labor and gasoline use, improve soil health, limit erosion and improve moisture retention. Without tillage, however, the only effective way to control weeds is through the use of glyphosate, said Utah State University Extension agronomist Earl Creech.

"If we lose glyphosate, we're going to lose the ability to grow crops on a direct-seeded system," Creech said. "Any time we rely on a single tool, we're just asking for it to break."

Complicating matters, Creech said, is that glyphosate has worked so well chemical companies have been less active in developing alternate herbicides.

Glyphosate entered the market in 1974. It wasn't until 1996 that the first resistant weed, rigid ryegrass, emerged. Resistance was discovered in goose grass the following year. Creech believes the introduction of genetically modified crops designed to withstand glyphosate has broadened the product's use and hastened weed resistance.

Resistance tends to develop fastest in annual plants with large numbers of persistent seeds, especially when a single gene controls resistance, Creech said.

In the Midwest, where growers rotate Roundup Ready corn and soybeans, Creech said resistant weeds have led some farmers to abandon direct seeding and others to try increasingly strong tank mixes.

"We had growers relying 100 percent on glyphosate," Creech said, adding glyphosate has become completely ineffective on certain weeds in the cotton fields of the South.

Once it occurs, resistance persists for decades after a product is removed from the market, Creech said.

Cathy Wilson, director of research collaboration with the Idaho Wheat Commission, said glyphosate resistance was a hot topic at a no-till conference she attended in January. She noted resistant weeds have rapidly arrived since the patent on glyphosate expired about 10 years ago, resulting in lower prices and increased usage.

Though she reasons tank mixing with glyphosate or replacing it with another mode of action when possible should be considered a best practice in the industry, she suspects many growers don't do it due to the hassle. Several days must elapse before planting after applying a tank mix.

"The solution is just good agronomic management practices anyone would have learned in a beginning agronomy course, which is you shouldn't use the same thing over and over again," Wilson said.

Creech recommends tank mixing with a growth regulator herbicide, such as 2,4-d or Dicamba.

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