Weeds to Watch - 6

The first confirmed case of glyphosate-resistant horseweed (marestail) bolted out of the gate in Delaware in 2000. U.S. farmers have been struggling to get it under control since, said Bill Curran, Pennsylvania State University weed scientist.

Glyphosate-resistant horseweed was subsequently found in Maryland and New Jersey in 2002, and Pennsylvania in 2003. There are now 21 states with confirmed populations of herbicide-resistant horseweed. So far, horseweed has exhibited resistance to glyphosate, Photosystem Il-inhibitors, ALS-inhibitors, Bipyridiliums and ureas and amides. Resistance to multiple sites of action has been identified in Delaware and several other states.

Curran said the diverse crop rotations of the Northeast have helped farmers of the region control weed resistance. However, horseweed is a communal weed.

"The way it (horseweed) disperses is by wind -- lots of tiny seeds that move on the wind -- so if your neighbor or your neighbor's neighbor has it, you end up getting it," Curran said. Glyphosate-resistant biotypes of horseweed first cropped up in the southeastern counties of his state, but populations have now picked up in western regions, too. "It will be all over (Pennsylvania) within three or four years," Curran predicted.

WHY IT THRIVES

Urbanization played a partial role in the development of glyphosate-resistant horseweed.

In the mid- to late-1990s and early 2000s, many farmers in southeastern Pennsylvania and the Delmarva Peninsula rented land that was under constant commercial development pressure, Curran said. As a result, farmers started each season with short-term management plans.

"Landowners will rent the ground, and farmers will plant soybeans because it's an easy crop to grow -- it doesn't require that many inputs. It doesn't require nitrogen, and you can keep a field clean with Roundup Ready (technology)," Curran said.

The result was many fields were planted to continuous soybeans with glyphosate as the only mode of weed control.

Glyphosate-resistant horseweed has evolved an interesting resistance mechanism. The weed is armed with a way to squirrel away glyphosate, most likely in a cell vacuole, where it can't hurt the plant.

Although glyphosate can be effective on horseweed, it's never been especially strong against the species at rates historically used in burndown applications. Like the Hydra, a serpent in Greek mythology that sprouted two heads every time one was chopped off, resistant horseweed can branch after a glyphosate treatment.

"It puts all these lateral branches out, so instead of having one stalk with multiple flowers on it, you might have five or six stalks with multiple flowers on them," Curran explained. "So in the end, you might end up with a lot more seed production from the resistant plant that was sprayed." The plants are also extremely fertile. Horseweed can both self-pollinate and outcross with each plant producing 100,000 to 500,000 seeds, depending on its size.

CONTROL OPTIONS

Curran said Northeast farmers have so far been able to manage horseweed by varying herbicide programs without turning to additional tillage. The use of residual herbicides has picked up. He urges farmers to start weed control earlier than ever in the spring, when horseweed is still very small.

Still, farmers in this region are keeping a wary eye on neighboring states. Waterhemp populations are on the rise in Pennsylvania. Glyphosate-resistant Palmer amaranth populations were confirmed in 2013.

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NORTHEAST REGION

Northeastern Weed Science Society = Connecticut, Delaware, Kentucky, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, Tennessee, Vermont, Virginia, West Virginia

WEEDS TO WATCH:

-- Horseweed/marestail

-- Common lambsquarters

-- Burcucumber

-- Common ragweed

-- Dandelion

(Source: Dwight Lingenfelter, Penn State University Agronomist/President of the Northeastern Weed Science Society)

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

(PS/ES/AG/CZ)

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