ST. LOUIS, Mo. — The United Soybean Board (USB) has issued a message to American soybean producers: Ignore herbicide-resistant weeds at your own risk.

“We can’t rely on one input or one mode of action to effectively treat these weeds; we’re way past that point,” said Todd Gibson, a USB director and soybean farmer from northwestern Missouri. “Managing this issue will require farmers to adapt to new methods in the same way these weeds are adapting to survive our old methods.”

To that end, the USB is collaborating with 15 land-grant universities and several agribusinesses to develop educational materials and improve farmer awareness of herbicide resistance. Many materials are already available on the group’s website at www.plantmanage

mentnetwork.org

Growers in the Midwest have had to deal with herbicide resistance later than Southern producers. According to the Iowa Soybean Assoc., resistant waterhemp, marestail (horseweed) and giant ragweed are becoming more problematic for the state’s soybean growers.

Glyphosate-resistant horseweed and giant and common ragweed have been identified in Indiana and Ohio. Illinois has the highest number of herbicide-resistant weed varieties of any state, according to the Southern Illinois University Weed Science program. Weeds are developing resistance to multiple modes of action, making herbicide management more intensive.

“Glyphosate-resistant Palmer amaranth has changed the way we manage our weeds in the South,” said Wesley Everman, extension weed specialist with North Carolina State University. “It has forced us to adopt ‘old’ herbicide technologies – PPO inhibitors, root and shoot inhibitors and photosystem II inhibitors.”

He detailed management strategies for Palmer in a presentation made available on the Plant Management Network website (read more below).

The USB soy checkoff found, in a recent grower survey, that many did not consider resistant weeds a threat to profitability. But they can result in early-season weed competition, and that “may be one of the biggest contributors to unseen yield losses in soybeans,” stated a Michigan State University extension update for producers earlier this year.

Proper application timing of pre-emergent residuals is also crucial for combating resistant weeds. The USB’s Take Action program provides educational materials to encourage producers to use residual herbicides, herbicides with multiple modes of action and herbicide rotations. Details on all these practices may be found on the Production page of the USB website at www.unitedsoybean.org/category/topics/production

Crop rotations are also recommended for combating herbicide-resistant weeds. Rotations have other benefits, too – an April profitability analysis of continuous corn and corn-soybean rotations conducted by the University of Illinois showed spring commodity price shifts increased profitability from a rotated corn-soybean system, relative to continuous corn.

A recent highlight on the USB website is a presentation about controlling Palmer amaranth in soybeans with pre-emergent non-PPO inhibitors. “These are going to contain metribuzin, in most cases,” said Everman.

Products containing metribuzin (such as Sencor/Lexone/Tricor) are most effective when used in combination with other modes of action; metribuzin is often combined with Authority MTZ, Boundary and Canopy. A complicating factor in controlling Palmer with metribuzin is a narrow margin of tolerance for metribuzin on soybeans; differences occur by variety.

“We need to be sure to pick a variety that is tolerant and not sensitive to metribuzin,” said Everman. “There are (also) injury concerns where we have light or variable soils.”

Producers can find lists of soybean variety metribuzin tolerance in company and university recommendations. Sandy soils tend to cause more injury; less than 0.5 percent organic matter and a pH of more than 7.5 can result in more injury.

“There’s a lot of soybean varieties that have variable responses to metribuzin,” he said; planting the crop deeper than the herbicide layer can also help.

“We have to reduce that selection pressure on our PPO inhibitors to avoid developing resistance in Palmer amaranth to these PPO inhibiting herbicides,” said Everman. “We need alternative modes of action, and metribuzin fits that gap on many of our acres.”