A herbicide-resistant weed that arrived in Indiana two years ago isn't standing still.

Marestail populations immune to glyphosate were first identified in 2002 in the southeast Indiana counties of Jackson, Bartholomew, Clark, Jefferson and Jennings. Recent field inspections by Purdue University researchers found the weeds in another 15 counties to the north and west, said Bill Johnson, Purdue Extension weed specialist.

Glyphosate is the active ingredient in many herbicides, including Roundup. Indiana farmers annually plant millions of acres in crops genetically modified to withstand Roundup applications. This year alone, 88 percent of the state's projected 5.45 million acres of soybeans are expected to be Roundup Ready varieties.

"We had a few isolated fields in southeast Indiana that were showing poor control of marestail with glyphosate in 2001 and 2002," Johnson said. "By late 2002, we'd confirmed glyphosate resistance in four counties, and we highly suspected it in six additional counties.

"We did some extensive field surveying in the fall of 2003 and now believe we've found glyphosate-resistant marestail in about 19 counties, mostly in southeastern Indiana. We've found it as far north as Wells County, as far west as Montgomery County and as far south as Perry County."

Marestail - also known as horseweed - is a thin-leafed, annual weed that can grow to more than 6 feet tall if undisturbed. The weed produces seed in July and August but can emerge at almost any time during the year.

"This weed is problematic for a number of reasons," Johnson said. "First, the weed's biology allows it to behave not only as a winter annual but also as a summer annual. I'm convinced this weed can germinate and grow any time the soil is not frozen.

"A second reason marestail is troublesome is that it has already developed resistance to ALS inhibitors and triazines. We're running out of effective tools to manage the weed."

Aceto-lactase synthase (ALS) inhibitors kill weeds by preventing them from producing essential amino acids necessary for growth. Triazine herbicides work by interrupting a weed's photosynthesis.

Marestail's ability to reproduce poses a third challenge, Johnson said.

"The seed of this weed spreads rapidly," he said. "Because it's so adaptable, the weed easily could become a predominant weed on our landscape, much as giant ragweed, giant foxtail and velvetleaf have done."

Glyphosate-resistant marestail was first reported in Delaware in 2000. Resistant marestail moved into other eastern states and trekked west.

Johnson believes the north-and-west migration of glyphosate-resistant marestail against prevailing west-to-east winds indicates agricultural practices - not environmental forces - are behind its march across Indiana. In short, farmers are relying too much on glyphosate-based herbicides.

"Many of the fields with the greatest number of glyphosate-resistant marestail have been in continuous soybeans for years, and many of those are no-till soybeans," he said. "I don't think the occurrence of this weed is a tillage issue alone. I think in some of these fields producers have gone away from using other herbicide modes-of-action to manage the weed, so they're putting a lot of selection pressure on these fields with glyphosate."

Farmers concerned about marestail in their soybean crops should treat fields before planting, Johnson said. He recommended using an alternative to glyphosate.

"We know that 2,4-D is very effective on these weeds, so farmers need to use it in their burndown if they have marestail in their field, regardless of whether they think it is glyphosate-resistant," he said. "Now would be an ideal time to make those burndown applications, because once marestail gets more than about 6 inches tall it is very difficult to control with glyphosate or 2,4-D."

Additional information about marestail control is available on the Purdue Weed Science home page.