You couldn't blame farmers for taking a certain amount of pleasure in being able to kill tall weeds with glyphosate when Roundup Ready crops first came on the market more than 10 years ago.

Spraying a big weed with a labeled rate of the product and watching it shrivel up, turn brown and die was one of the best shows in town until growers began finding they couldn't kill some weeds with any rate of the herbicide.

“You might say our experience with Roundup Ready crops has been a tale of irony,” says Southern Illinois University Professor Bryan Young. “We killed a lot of tall weeds because we could. But we should have felt sick when we realized those weeds had been with our crop all that time.”

In another one of those cases of irony, Young spoke at two company-sponsored media events at Commodity Classic in Grapevine, Texas. His words were different at each event but the message was the same: Farmers should no longer rely on a single herbicide to control weeds in their crops.

“No matter what the herbicide, we've always been able to track the development or evolution of herbicide-resistant weed species by systems that relied solely on one particular herbicide,” he said at the Bayer CropSciences-sponsored Ag Issues Forum at Commodity Classic. “When we used cultivation, too much cultivation led to weeds in the crop row.

(Young also spoke at a media event held by BASF for its new Kixor herbicide. Kixor, which is not yet registered for use in the United States, is from a new class of chemistry that will offer both rapid burndown and residual control of broadleaf weeds.)

“In this case, we used Roundup or glyphosate, and we used it to the extent we broke it in some areas. Now we're on a downhill slide as far as being able to rely on that single herbicide. The clear message is we need a diversified approach to preserve the efficacy of these products.”

Young, professor of weed science and agronomy at Southern Illinois, acknowledged that glyphosate continues to work well in some areas of the country where herbicide resistance has not reared its ugly head.

“If you talk about weed resistance to farmers in places like northeast Iowa, they don't know what you're talking about,” he said. “But if you look at the United States as a whole, we've lost the simplicity and flexibility of using that herbicide.”

U.S. farmers have gone from using only one herbicide on a clear majority of their crop acres to applying two, he said. The second, 2-4-D, has been applied on an increasing number of acres to control glyphosate-resistant marestail, according to figures compiled by USDA.

Young and other weed scientists - and the major chemical manufacturers - have begun encouraging farmers to apply a residual herbicide at or near planting to prevent weeds from emerging and competing with the crop. Some companies are now providing rebates to encourage farmers to apply residual herbicides.

Many growers in the area south of Bloomington in Illinois are now dealing with glyphosate-resistant marestail, Young said. Glyphosate-resistant water hemp, a close cousin to Palmer amaranth or Palmer pigweed in the South, is also becoming a problem.

“I'm getting about as many calls on waterhemp as I was getting with glyphosate-resistant marestail three years ago,” he said at the BASF media event. “The big problem is where you have fields with resistant waterhemp and marestail.”

Thus far, weed scientists have uncovered no incidents of herbicide resistance to Ignite, the herbicide used in Bayer CropScience's LibertyLink system or the HPPD herbicides such as Balance, Callisto or Impact.

Bayer will be recommending the application of a residual herbicide in its LibertyLink soybeans, which it expects to introduce in the 2009 growing season. Monsanto is offering to provide growers from $5 to $13 per acre if a second in-crop treatment of Roundup WeatherMax or PowerMax is needed in corn or soybeans after they've applied a company-approved residual herbicide.

BASF outlined some of the basics of its new Kixor herbicide during its third annual “Science Behind” Symposium at the Commodity Classic. Kixor, which controls more than 90 broadleaf weeds, including weeds that have become more tolerant of glyphosate, could be registered in third quarter of 2009.

“It's critical to get new chemistry to growers for weed management,” said Young said. “I am very interested in Kixor and new chemistries to help manage weeds and decrease the risk of growing herbicide tolerance.”