Beating Them Back

Panel: Herbicide-resistant weeds threaten U.S. farms

Published in the February 2014 Issue Published online: Feb 20, 2014
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International weed-resistance experts warn U.S. crop producers that herbicide-resistant weeds are aggressively taking hold in many parts of the country and pose a significant threat to U.S. crop productivity and profitability. As farmers in other countries have demonstrated, however, these so-called “monster weeds” can be controlled with a combination of conventional and nonconventional farming practices.

Four leading experts shared weed management observations and expertise during the “Weed Resistance in the Americas” panel at the 2013 Farm Progress Show in October. The event in Decatur, Ill., followed a week-long weed resistance tour of the U.S. and Canada hosted by Bayer CropScience. Panel participants included Stephen Powles, director of the Australian Herbicide Resistance Initiative at the University of Western Australia; Aaron Hager, associate professor of weed science at the University of Illinois; Pedro J. Christoffoleti, head of the Department of Crop Science at the University of São Paulo in Brazil; and Harry Strek, head of profiling and market support in the Weed Control Biology Research group at Bayer CropScience in Frankfurt, Germany.

Moderator Arlene Cotie, product development manager with Bayer, said the panelists wanted to educate and urge growers to take action to protect their fields and yields.

“One of the greatest threats to yield loss and food security—on a local and global basis—is the development of herbicide-resistant weeds,” Cotie said. “It is a growing global problem that is changing agronomic practices and threatens the long-term viability of economical weed control and food production.”


Global problem hits home

Resistance by weeds to the widely used herbicide glyphosate has grown over the past two decades. Earlier this year, the Stratus Glyphosate Resistance Tracking study found that nearly half of U.S. farmers surveyed in 2012 reported glyphosate-resistant weeds, up from 34 percent the prior year. Hager confirmed that Illinois farmland has experienced the continued evolution of herbicide-resistant weeds, including waterhemp and Palmer amaranth, which rapidly proliferate due to their biological characteristics and the lack of diversity in tactics used for their control.

“In the Midwest, we see new populations every season that demonstrate resistance to numerous herbicide classes,” Hager said. “We find resistance is not necessarily limited to one herbicide, but in certain species we find resistance to multiple herbicides, effectively eliminating many of the options farmers would have to try to control these populations in their crops.”

The Illinois agronomist and fellow panelists suggested that producers need to think about nonchemical methods to control weeds. “All of the solutions do not come out of a (herbicide spray) nozzle,” Powles said.

The panelists cited diverse weed-management tactics including the following:

In Australia, growers are adopting nonchemical means of managing weeds, such as using a seed destructor that is pulled behind a combine like a chaff cart at harvest time, destroying weed seeds as they exit with the chaff.

Brazilian growers rely on double- and sequential-cropping to help manage weeds, something not possible in most areas of the U.S. due to the shorter growing season. They also use multiple herbicide modes of action.

U.S. agronomists recommend controlling all weeds before planting a new crop, even if this slows the grower’s planting schedule. In addition, Hager advocates managing weeds later in the season before they go to seed to prevent replenishment of the soil seed bank.


New thinking required

Bayer CropScience advocates similar practices through Respect the Rotation, an initiative that elevates the importance of grower adoption of herbicide diversity to reinforce the principles of Integrated Weed Management through rotation of crops, herbicide-tolerant traits and modes of action.

“Herbicide-resistant weeds are an unconventional threat to food production that we must fight with conventional and unconventional methods,” said Bayer’s Strek. “Accumulated resistance to multiple herbicides complicates the matter.”

Bayer is also working on additional herbicide systems and seed traits.

“These are new solutions that most of the weeds haven’t seen before,” Strek said. “But we don’t want these solutions to burn out and become ineffective.”

“Bayer wants to be a global leader not only in chemical solutions, but also in stewardship and education,” he said. “We recognize that chemicals are often only a short-term solution. It’s important to educate and train growers about future chemical and nonchemical tools for long-term sustainability of weed control technologies.”