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Resistant marestail, waterhemp require producers to adopt new control strategies, says MU Extension agronomist


Resistant marestail, waterhemp require producers to adopt new control strategies, says MU Extension agronomist 
CORNING, Mo. – The crop pests marestail and waterhemp are difficult to control. Both are prolific seed-producers that grow rapidly, reduce yields and are prone to herbicide resistance. Controlling these weeds will require producers to shift to more complex herbicide systems, said a University of Missouri Extension agronomist.

“We’re entering a new era with our cropping systems, where we have to move away from the easy two-pass, post-emergence program and move to including a pre-emergence program,” said Julie Abendroth, MU Extension agronomy specialist in Ray County. “We need to look at controlling problematic weeds before planting and protecting yield season-long.”

Heavy reliance on glyphosate for weed control has allowed resistant weed populations to build in Missouri and other major soybean-producing states. Glyphosate-resistant marestail in the state was first found in southeast Missouri in 2002, Abendroth said. Marestail is typically one of the first weeds in a field to develop glyphosate resistance, she said.

“Even without the resistance, marestail is very difficult to control,” Abendroth said. “But it has become much more of a problem because of herbicide resistance, reduced tillage and lack of diversity in crop rotations.”

Missouri is predominantly a soybean-producing state, with two acres of soybeans grown for every one acre of corn, she said. “Even alternating corn and soybean, that’s not much of a rotation.”

Marestail’s abundant seed production and unpredictable life cycle are major culprits in the spread of glyphosate resistance. The weed can germinate in spring or fall, making treatment difficult. Each plant produces about 200,000 seeds, which are easily carried by wind, Abendroth said.

“Marestail seeds are airborne, like dandelion seeds. Dispersal of glyphosate-resistant marestail could be very rapid,” she said.
Producers need to treat marestail before it reaches 6 inches in height or herbicide treatments will quickly become ineffective. “Once marestail gets taller than this, it starts to develop a woody stem and there’s not a lot of leaf area to catch the herbicide,” Abendroth said. “You really want to be proactive.”

Producers who do not have a fall marestail problem may still benefit from a spring application, she said.

Waterhemp is another serious weed problem in the state. The first glyphosate-resistant waterhemp in the United States was found in northwest Missouri in 2005. Waterhemp was one of the first U.S. weed species identified with resistance to multiple herbicide classes and was the first U.S. weed to develop three-way resistance in the same population, Abendroth said. “Once you start getting that much resistance built in, the options for control are quite limited.”

Like marestail, waterhemp has a wide germination window and is a prolific seed producer, with a single plant producing as many as a million seeds, she said. It can grow an inch per day.
Unlike marestail, male and female waterhemp flowers are on separate plants, Abendroth said. “This is important because waterhemp inherently has a lot more genetic diversity. We see a lot more hybridization, which increases the potential for herbicide resistance to develop.”

Resistance is spread through the pollen, which is 10 times smaller than corn pollen and can travel a half-mile, she said.

“This is a big factor in the spread of resistance,” Abendroth said. “If you are managing it well, but your neighbor isn’t, the pollen movement is a problem.”

The new Liberty Link system will be effective for fields developing glyphosate-resistant weed populations, though not as effective as the original Roundup system, she said. Liberty Link is an alternative to the Roundup system employing glufosinate instead of glyphosate.

Being proactive with weed control is most important to avoid problems and slow the spread of resistance, Abendroth said.
“No matter what technology we develop, waterhemp will continue to adapt and find holes in the system,” she said. “A management program that uses a soil-applied herbicide followed by a post-emergence herbicide may have less variability in weed control, crop yield and net return.”