July 08, 2010

Herbicide-resistant weeds affecting soybean fields

by K.C. Jaehnig

CARBONDALE, Ill. -- There’s a horror film playing down at your local soybean field. It’s called, “The Return of the Weeds That Wouldn’t Die,” and like the prequel, it’s not a pretty picture.

“In many fields, our primary herbicides are no longer working to control herbicide-resistant weed species,” says Bryan G. Young, the weedman of Southern Illinois University Carbondale. Young will talk with farmers about strategies for dealing with such weeds during the College of Agricultural Sciences’ free annual field day, set for 9 a.m. Thursday, July 15, at its Belleville Research Center on Illinois Route 161.

Waterhemp, marestail, giant ragweed -- all weeds once controlled by glyphosate, the power behind the popular herbicide Roundup -- are digging in for the long haul in Southern Illinois, and many farmers don’t know how to deal with them.

“We’ve been a Roundup Ready Soybean world for the last 14 years,” Young says. “We haven’t had to add other herbicides, but now we need them. For young farmers, it’s a new experience. Even experienced farmers are a little fuzzy on the use of pre-emergence and post-emergence herbicide options beyond just glyphosate. But this is where we are now, so we have to use these herbicides as well as possible to gain effective control of some of these glyphosate-resistant weeds.”

Young says he expects some farmers won’t want to hear this.

“It’s a much more complex system than just applying glyphosate, and no one wants more complexity,” he says.

But the simple response -- applying more glyphosate -- simply won’t kill the glyphosate-resistant weeds and in the long run will cost more, too.

“Last year, a grower contacted me after he didn’t get control of his waterhemp with glyphosate,” Young notes. “It was late in the season by the time he determined his herbicide program wasn’t as effective as he’d hoped. By that point, he had one option -- besides hand weeding, which he wasn’t going to do. That herbicide option was going to cost $30 an acre for a single application, on top of what he’d already paid for the glyphosate. “

Young says farmers should assume a single herbicide will no longer do the job and plan their strategies accordingly. For most, that likely will mean applying herbicides before their soybeans come up.

“That reduces the amount of weeds that emerge when the soybeans are growing,” he says. “If you then have to apply a post-emergence herbicide, there will be fewer weeds out there, and they might be smaller, too.”

Which herbicides? At what rates? When do you apply them?

“Everybody hates this answer, but it depends,” Young says.

It depends on factors such as the growers’ tillage systems, when they like to plant, whether they want to manage weeds in the fall or the spring, which weeds are most troublesome, whether rain delays planting.

“All affect herbicide choices,” Young says. “I can say that if you have normal field conditions, you might be able to achieve success with a particular way of managing weeds, but it won’t fit all your acres. Ideally, each field should be managed individually, but no farmer wants to do that.”

The worst management choice is doing things the way you’ve always done them.

“That approach might work for a little while, but it’s not sustainable,” Young says. “This is a genetic problem in the resistant weeds -- it’s not going away. Once the weeds don’t respond, you’re down that slippery slope of spending more money on herbicide and more of your valuable time to manage weeds than previously.”