(Crystal Lake) Northwest Herald via Associated Press

HARVARD, Ill. (AP) -- Blue Star Vineyard grape grower Jeff Pankow is careful about what herbicides and chemicals are used on his acres near Harvard.

He is cautious to protect his vines and the wine they eventually will produce. And he is cautious to prevent superweeds.

So-called superweeds haven't cropped up in McHenry County, but herbicide-resistant weeds have been growing in Illinois for several decades, and farmers are taking steps to prevent the spread of resistant strains.

Pankow changes his weed control methods to keep superweeds from sprouting here.

"The rotation helps to avoid resistance," he said.

Those in the agriculture industry are well aware of the threat of tricky weeds that rob nutrients from corn and soybeans and reduce crop yields. Weeds that have developed resistance are harder to kill and often require the use of one or more herbicide or other expensive methods the cost of which can stomp profits.

Plants can develop resistance to heavily used chemicals such as glyphosate, sold under the trade name Roundup, through rapid selection, said Aaron Hager, associate professor of crop sciences at the University of Illinois.

"If we only use a limited number of tools to control weeds, only the resistant ones survive and reproduce," Hager said. "That is the selection process."

Changing weed control methods can help.

"We can't prevent selection nature doesn't want to give up but we can slow down the process," Hager said.

Illinois is one of 22 states in the United States that have documented herbicide-resistant weeds. The state is home to 18 varieties that are resistant to one or more kinds of herbicide. Waterhemp is the most common herbicide-resistant weed in Illinois and the focus of much of the crop science research, Hager said.

Sizing up the effect of these weeds on agriculture is not easy.

"It is very difficult to measure the scope of the problem," Hager said.

Locally, no cases of herbicide-resistant weeds have been reported.

"We just haven't had anything come up here," McHenry County Farm Bureau manager Dan Volkers said.

Resistance has been documented in areas south of the county, said Jeff Kimmel, the Marengo service center manager of Conserv FS Inc., an agricultural cooperative that serves northern Illinois and southern Wisconsin.

The spread of such weeds is always a concern, Kimmel said. He estimated that re-treating crops with a different herbicide after discovering resistance could cost $20 to $30 an acre. By rotating chemicals, Kimmel believes the problem can be largely avoided.

Roundup or glyphosate resistance stems in large part from overuse of the chemical. Glyphosate is such an attractive herbicide because it is toxic to a wide range of weeds, safe for mammals, and degrades rapidly. Most of the soybeans, corn and cotton planted today are Roundup Ready crops, which are grown from seeds genetically modified to tolerate Roundup, according to a recent article in the New York Times.

Although major resistance issues haven't popped up in McHenry County yet, Hager said, the first year that resistance becomes a problem is usually the worst.

"That first year is the most costly," he said. "Once you know you have a problem, you can address it proactively."

Copyright 2010 The AP.

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