While crop scientists continue to track Asian soybean rust with sentinel plots this year, there's another threat they're eyeing with a different type of sentinel. This new sentry is marestail that has developed resistance to glyphosate.

Resistant weeds are popping up not just in Roundup Ready cropping systems, but also along roads and waterways and in crops that do not use glyphosate-resistance technology. Marestail could be a good predictor of where we'll be likely to see other weeds that develop resistance, says Purdue University weed scientist Bill Johnson.

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Johnson and two leading experts on resistant weeds last Thursday took time to explain their work in this field to members of the farm media, during a telephone news conference organized by Syngenta Crop Protection.

Marestail tends to grow in fields where other weeds, including lambsquarter, giant ragweed, pigweed, waterhemp and other escapes are also present. Of the 1,300 fields tested in Indiana in 2005, about 50% of them also had lambsquarter and ragweed.

"We feel that marestail is a good sentinel weed for us," Johnson said. It's a "warning shot that the technology is starting to develop some cracks." He and other researchers in the Midwest have already found lambsquarter, giant ragweed and common waterhemp with levels of resistance.

Replacements are a ways off

"The low levels of resistance are really a blessing," he noted. They mean we probably won't see widespread incidences of rapid progressions - where a field shows no resistant weeds one year and complete resistance the next, he said. But, he's discouraged that the "herbicide discovery efforts from major chemical companies aren't great in terms of identifying new options."

Chuck Foresman, technical brand manager for non-selective herbicides with Syngenta, said it's true there have been "woefully few" new modes of action. "We have already found the easy ones," he said, adding "the prospects are not real good - even if we did find one it might be ten years before we can get it to the market."

"We need to diversify our methods of weed control," Foresman said. Some options include using various modes of action, tank mixing, using alternate burn downs, diversifying herbicides and rotating crops. "When we do this, I believe that everyone does benefit," he said.