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Feature   |  Spring 2006

Resistance fighters

Tough weeds withstand attempts to knock them down

Bill Johnson runs a torture chamber. In two facilities tucked away on the Purdue University campus, Johnson subjects more than 800 weeds to unimaginable horrors. He wants information, and he'll stop at nothing to get it.

For starters, Johnson buries the weeds alive. He then manipulates them with water and food. He turns the temperature up or down. Eventually, he administers chemical punishment. Some of the test subjects receive twice the amount of toxic compound necessary to kill them. That final act provides Johnson the answer to the question he keeps asking: “Are you herbicide resistant?”

Bill Johnson

Weed scientist Bill Johnson researches the growing problem of herbicide resistance. His findings will help develop new control measures to rein in resistance. (Photo by Tom Campbell)

Weed whackers

A weed scientist in the Department of Botany and Plant Pathology, Johnson conducts herbicide experiments on some of Indiana 's most problematic weeds, including marestail, Eastern black nightshade, Johnsongrass and lambsquarters. The weeds come from counties all across the Hoosier state.

Johnson and his assistants plant weed seeds in gardening flats and, for about a month, nurture the budding plants with water and fertilizer in a climate-controlled setting. When the weeds reach a specified height, the research team douses them with deadly doses of glyphosate- and aceto-lactase synthase (ALS) inhibitor-based herbicides—two widely used weed-killing substances.

In the weeks that follow, Johnson and company wait for the plants to die. Or not. In many cases, the weeds shake off the lethal brew and keep right on growing. A few tower over Johnson, who stands 6 feet 6 inches tall.

“We grow weeds, and then we torture them,” Johnson says above the steady hum of a greenhouse ventilation system. “We're not unlike a lot of other researchers who are trying to develop plants that can withstand stress. The difference is, they do it to crop plants, and we do it to weeds.”

A growing problem

Johnson hopes his research will shed light on how far weeds with confirmed herbicide resistance have spread in Indiana. The data he collects will be used in public presentations and Purdue Extension publications and become the basis for future weed control recommendations.

Weeds are an economic threat to crop farmers and an aesthetic nuisance to homeowners. To control the unwanted vegetation, Americans spend billions of dollars each year on herbicides. Farmers alone shell out more than $6.5 billion annually, according to the National Center for Food and Agricultural Policy.

Until about 30 years ago, weeds were a headache, mostly because of the extra work and expense it took to get rid of them. But, by the late 1970s, weed scientists began noticing an alarming trend: Some weeds were no longer keeling over when sprayed with herbicides.

By the year 2000, the number of weed species worldwide exhibiting herbicide resistance had risen to nearly 250, and populations of those weeds were increasing. Indiana's first case of glyphosate resistance was documented in 2002 and involved marestail. Glyphosate-resistant marestail has since been found in 28 Indiana counties, mostly in the state's southeast region.

Glyphosate is the active ingredient in Roundup ® brand herbicide. Roundup ® is so effective at controlling weeds that many corn and soybean hybrids are genetically engineered to withstand the herbicide. About 90 percent of the soybean acres and around 25 percent of the corn acres in Indiana are planted with Roundup Ready ® hybrids.

The success rates of Roundup ® and ALS-inhibitor herbicides—often used to control grassy weeds—caused farmers and others to become overly dependent on those products. Overuse, in turn, led to the development of herbicide resistance, Johnson says. The phenomenon is known as “selection pressure.”



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