Giant ragweed joins list of glyphosate-resistant weeds

Agronomists are close to making official what many Illinois farmers already know: Glyphosate-resistant giant ragweed is here.

The troublesome weed is showing definite signs in university plots of resistance to the popular herbicide, according to Bryan Young.

Young, a weed scientist at Southern Illinois University, discussed the problem during a tour of plots sponsored by Valent USA. The company also used the tour as an opportunity to discuss Fierce, a new herbicide developed by Valent.

Young said Fierce could be another tool to battle the growing number of weeds showing resistance to the glyphosate, the chemical making up Roundup.

"We’ve got some populations we can’t control with glyphosate in Illinois and Purdue,” he said. “We haven’t documented or confirmed anything yet. We haven’t reported it as resistant, but we can’t control it in some fields.”

Young said he has personal experience with glyphosate-resistant giant ragweed.

"On my own farm this year I put a gallon of glyphosate on it, and I need to address it again,” he said. “And I think it’ll be with a mower.”

Indeed, check plots at SIU’s Belleville Research Center are overrun with giant ragweed.

"If a grower really wants the cheapest way to get rid of waterhemp, let the giant ragweed grow,” Young joked. “You won’t find any waterhemp. Because (ragweed) comes up early and it’s going to shade out a lot of waterhemp.”

Waterhemp already has been established as developing resistance to glyphosate. Fierce, a preplant herbicide that will be available to growers in 2011, promises long residual control on many of the glyphosate-resistant weeds.

Eric Ott, field marketing development specialist for Valent, said Fierce will provide residual control of up to eight weeks on weeds including marestail, lambsquarters, ragweed and annual grasses.

The new herbicide will contain flumioxazin and pyroxasulfone. Flumioxazin is the active ingredient in Valent’s Valor herbicide, which has become the most-often used active ingredient in soybean pre-emergence applications.

Fierce can be applied in the spring up to seven days before planting corn and up to three days after planting soybeans.

Young said Fierce should help farmers who have had problems with glyphosate-resistant weeds.

"It will have some suppression of it, yes,” he said. “It’s not totally useless against giant ragweed. It has activity on most large-seeded broadleafs as well as small-seeded broadleaf weeds like waterhemp.

"In corn they’re looking at Valor and (the active ingredient in Fierce). In soybeans you can add Classic to that.”

Young said that in Illinois this year growers aren’t seeing as much marestail as usual.

"I don’t know if it’s because of all the tillage that we’ve had because of the ruts at harvest,” he said. “But waterhemp’s coming on, giant ragweed’s been an issue for us. And I’m hearing a lot about some of the grasses, too.”

Glyphosate-resistant weed pressure is like to continue, according to Young.

"It’s something we expect to increase, especially if growers don’t start managing things differently than just glyphosate by itself,” he said.

"And with the cheap price of glyphosate, I don’t see that changing a whole lot this year in some areas where glyphosate’s been effective.”

The spread of resistant weeds can be dramatic.

"Failure can be pretty quick,” Young said. “Last year you thought you had better than 95 percent control, no major problem.

"Next year you might have 50 to 70 percent control, and that’s as good as you can get with the herbicide. You respond to it; in other words you’re not proactive, you’re reacting.”

Source: Agri news