Research Highlights

Integrated Weed Management Effectively Fights Resistance

By Laura Temple

Recent introductions of herbicide-tolerant soybean systems have given farmers more options to control tough weeds like Palmer amaranth and other glyphosate-resistant species. These new tools have raised questions about how best to manage different herbicides and tank mixes.

“With five traits available, plus a variety of residual products, farmers want to know the right combination of herbicides to control their problem weeds,” says Ben Beale, University of Maryland Extension educator. “Farmers need the options offered by soybeans with tolerance to multiple herbicides, but choices create more complex decisions. For example, when glyphosate, glufosinate and 2,4-D are all options, should farmers ask if they should use one, two or all three herbicides, and if they should be applied together or separately.”

Beale is leading field trials comparing 15 different herbicide treatments to help answer these questions, funded by the Maryland Soybean Board. A key component of the research project was sharing the results with farmers. Typically Beale and others share this information through a series of local workshops, but in 2020, they shared integrated weed management strategies through a virtual training.

“We shared recommendations for integrated weed management of herbicide-resistant weeds, which includes cultural and chemical practices,” Beale says. “These practices work together to allow farmers to successfully manage problem weeds.”

According to Brian Johnson, who farms near Westover and serves as a director for Maryland Soybean Board, grants for research like this directly support Maryland soybean farmers. “We select projects to receive grants based on research priority, cost-effectiveness and positive impact to farmers.”

Multiple Herbicide Modes of Action

The herbicide programs in Beale’s field trials combined post-emergence herbicides for broadleaf and grass control. In Maryland and the Mid-Atlantic region, he says grasses like fall panicum and foxtail tend to be problems along with glyphosate-resistant Palmer amaranth and common ragweed. 

“We found that most treatments provided excellent control when applied to 3- to 4-inch weeds,” he shares. “We saw a little antagonism between glyphosate and glufosinate in tank mixes, but that treatment still provided at least 90 percent weed control.”

The results reinforce Beale’s herbicide program recommendations for soybean weed control.

• Apply pre-plant residual herbicides with at least two effective modes of action to weed-free fields.
• Time a post-emergence herbicide application when the next flush of weeds are 3- to 4-inches tall. Choose an effective post-emergence option that works with the soybean system and add a delayed residual product to extend weed control until canopy.
• Make a second post-emergence application if needed, again when weeds are 3- to 4-inches tall.

“Farmers should tailor their post-emergence tank mix to fit their herbicide system, the weeds in the field and drift concerns,” Beale adds. “Then check the post-emergent herbicide product label for approved residual tank mix options.”

He notes that his 2021 trials will include additional tank mixes based on product labels for glufosinate-, dicamba- and 2-4,D-tolerant soybean systems. 

Cultural Weed Control

Beale’s recommendations and the information shared in the virtual training also emphasize the value of cultural practices that improve weed control and limit the spread of weed resistance. 

“We’ve found that equipment is the most common spreader of invasive weeds,” he says. “We have fields that don’t have resistant weeds, and cleaning equipment well before entering those fields can keep them out. Farmers should be especially diligent cleaning equipment bought from other parts of the country.”

He notes that hay, waterfowl and other wildlife can also spread invasive, herbicide-resistant weeds. Learning to recognize and identify these weeds quickly when scouting can prevent long-term problems. 

“It’s much easier to scout, catch weeds early and pull a few by hand before they produce seed one season than to ignore them and have hundreds or thousands of resistant weeds the next season,” Beale says.

Integrated weed management encompasses many other cultural practices that help control problems like Palmer amaranth.

• Narrow rows help soybeans reach canopy closure more quickly. “That shading limits Palmer amaranth later in the season,” Beale explains. “Narrow rows help address a common challenge in hot, dry weather, when soybeans grow more slowly. Palmer amaranth will find and grow in any open area.”
• Crop rotations that avoid continuous soybeans allow for different herbicide modes of action to be used in different crops, preventing development of resistance. 
• Allow cover crops to produce as much biomass in the spring as possible before terminating suppresses weeds.
• Strategic deep tillage may make sense in some fields, and it only needs to be done once. Palmer amaranth seeds are very small, and deep tillage buries them so the majority won’t germinate. 
• Techniques to destroy weed seeds at harvest, such as seed impact mills on combines, can reduce weed seedbank populations, alleviate future weed pressure.

“Combining the cultural practices that fit a farming operation with the herbicide-tolerant soybean system that best addresses the weeds in a field allows farmers to successfully manage problem weeds,” Beale says. “And these efforts help prevent development of additional resistance.”

Published: Jul 19, 2021