Pesticide-resistant weeds closing in on Pennsylvania

Credit: USDA Early growth stage of water-hemp, a weed that is becoming resistant to certain herbicides.

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UNIVERSITY PARK, Pa. -- Weeds that have become resistant to glyphosate -- the active ingredient in Round-Up herbicide -- are an evolving national threat. At least three glyphosate-resistant species are approaching Pennsylvania, necessitating new strategies for weed control, according to a Penn State College of Agricultural Sciences specialist.

Dwight Lingenfelter, Penn State Extension weed scientist, said. Glyphosate-based herbicide programs routinely controlled these weeds, but now the effectiveness of those programs is dwindling.

"There’s a species called Palmer pigweed or Palmer amaranth, which is a huge problem -- especially in cotton-growing regions," said Lingenfelter. "In the past, farmers were spending only maybe $20 to $30 an acre to control pigweed; now they’re up over $90 to $100 an acre, because of its resistance to a number of herbicide modes of action."

Currently, no major outbreaks of this weed exist in Pennsylvania, but reports from Delaware and Maryland indicate it probably will creep into the Commonwealth’s cropping systems, especially in southern Pennsylvania.

Lingenfelter said a second resistant species slowly invading the state, water-hemp, already creates big problems in the Midwest and South and is resistant to numerous herbicides.

"We had a person bring in a sample of water-hemp this summer, so we know there are some populations in our state currently," he said. "We’re also seeing glyphosate-resistant species of horseweed or marestail spreading throughout the state -- it’s very common in the mid-Atlantic region and Midwestern states."

While it might sound like it’s losing its effectiveness, glyphosate is still vital in "burn-down" weed-control programs, which work by killing any vegetation on a treated field.

"It’s still a very effective herbicide for a number of species in our area," he said. "It controls a number of weeds in the burn-down period and still is a foundation or backbone for many weed-control programs. We recommend using other herbicides in combination with it to control weeds that aren’t being controlled by glyphosate alone.

"We also encourage tank-mixing herbicides or using pre-packaged products so multiple modes of action are in the weed-control program."

The mode of action is the way an herbicide affects the weed to kill it, Lingenfelter explained.

Newer herbicide products introduced in the last five years can help control resistant species in burn-down programs. But Lingenfelter pointed out that, while "new" products are being introduced on the market, the industry hasn’t produced a formulation that employs a new mode of action in more than 15 years.

"The reality is that many companies are repackaging products and giving them different trade names so it looks like we have a lot of new herbicides, when in reality we do not. And if they were to discover a new mode of action in some lab today, we wouldn’t reap the benefits of it for at least 10 years, because it takes that long to get through all of the testing phases and field trials before it would hit the market."

Lingenfelter said the diversity and rotation of crops grown in Pennsylvania provides an advantage over states in the Midwest and South when it comes to fighting resistant weeds. Corn, cotton and soybeans are the primary field crops in the Midwest and South, and more than 90 percent of the acres are sprayed with glyphosate, so weeds are pushed to develop resistance.

"Here in Pennsylvania, we typically rotate between corn, soybeans, alfalfa, small grains and sometimes various vegetable crops, depending on the area of the state," he said. "Because of this, we use a variety of weed-control methods. Not only does this allow for different herbicides and a rotation of herbicide modes of action, but it allows for other weed-management techniques -- such as mowing forage crops or the addition of cover crops -- and other cultural tactics such as variations in planting date, seeding rate or row spacing.