It Takes a System to Kill Weeds

December 5, 2015 02:37 AM

Getting rid of yield-robbers takes careful planning and multiple modes of action

Weed control isn’t about killing weeds; it’s about protecting yield—and the stakes are high. The challenge is not only gaining control of weeds this year, but every year following while herbicide resistance is building. One Essex, Mo., farmer, Chris Porter, knows the battle well and has managed to increase yields while decreasing weed infestation.

Porter started noticing weed resistance to glyphosate in the early 2000s and was forced to change his management techniques. “Palmer pigweed has been a disaster,” he says. “We just couldn’t ever do anything to kill it. Marestail can sometimes be even harder to control. We’ve found if we get residual we’ll catch them.”

Growing a successful crop is more complex than just spraying glyphosate, or any other single mode of action. Weeds are harder to control, are resistant to more herbicide groups and can steal bushels—a lot of them. If you aren’t mindful of your herbicide system, you might render every herbicide class useless. It’s smart to follow these weed management best practices:

  • Apply multiple, effective modes of action.
  • Consider new traits or herbicides.
  • Use both pre-emergent and residual herbicides.
  • Scout to apply herbicides at the appropriate time and rate.
  • Try mechanical weed control.

If you rely on only one mode of action, you will increase the rate and frequency of weed resistance. “What we see is nature fighting back,” says Aaron Hager, University of Illinois associate professor of weed science. When farmers overuse a single herbicide mode of action, it imposes selection pressure, which means more resistant weeds are produced, he says.

Identify the difference between a herbicide active ingredient and a herbicide group. Sometimes a simple switch isn’t as easy as it sounds.

“Look at your label for the herbicide group number,” says Joyce Tredaway Ducar, Auburn University Extension weed scientist. 

You’ll need to scout to determine which weeds are the problems and which groups will work. When you identify the herbicide groups that work on your farm, plan a method of attack. Make sure the herbicides have at least two different and effective sites of action so you don’t wear out a single site from overuse.

“There are obviously alternatives, but it’s not as simple as you think,” Hager says. 

Among new alternatives are technologies from Bayer CropScience, Dow AgroSciences and Monsanto Company.

Bayer anticipates launching HPPD-tolerant Balance GT soybeans in 2018, pending regulatory approvals. These soybeans will be paired with Balance Bean herbicide, which contains HPPD (group 27) and glyphosate (group 9).

“The initial offering will have gytol, glyphosate tolerance and the next season we’ll add glufosinate (group 10) tolerance,” says Brent Philbrook, Bayer regional manager of field operations research and development in the Midwest. “Not only does [Balance Bean herbicide] have residual, it has reactivation.”

The residual is said to last eight to 10 weeks. During that time, it can be reactivated with rainfall to control later-emerging weeds, Philbrook says.

After more than a decade of research, regulatory hurdles and testing, Bayer is in the final stages of approval for this product. As with any single modes of action, failure to use this product as recommended could lead to reduced effectiveness in less time than it took to bring it to market.

Philbrook encourages farmers to use pre-emergent herbicides with residual; use Balance Bean or Liberty (glufosinate, when the trait is available) for post-emergence applications and scout to make sure your herbicides are effective. The addition of glufosinate tolerance allows farmers to rotate modes of action every year to help decrease the frequency of resistant weeds, he says. 

In 2015, Dow AgroSciences introduced Enlist corn and seed production of Enlist soybeans. The company hopes to have a full launch of the Enlist platform in 2016, pending regulatory approval. The platform includes corn, cotton and soybeans. All Enlist crops will provide 2,4-D (group 4) and glyphosate tolerance. Soybeans and cotton also include glufosinate tolerance.

“We created Enlist because there was a need to control weeds and slow resistance,” says John Chase, Enlist commercial leader for the U.S. The system uses Enlist Duo herbicide with Colex-D Technology, which has a new 2,4-D and glyphosate formulation.

Dow says the new formulation diminishes risk to farmers and applicators. “Enlist Duo with Colex-D Technology decreases risk to a near zero volatility formulation,” Chase says. “It’s 96% less volatile than 2, 4-D ester with 90% reduction in drift.”

Chase says this dramatic decrease is caused by the new formulation and by using low-drift nozzles that create bigger droplet sizes to reduce drift. In addition to protecting farmers’ liability from drift, Dow has a detailed stewardship program to minimize the risk of misuse.

Monsanto is gearing up Xtend soybeans for an expected 2016 launch, pending regulatory approvals. Soybeans with the trait will be tolerant to dicamba (group 4) and glyphosate. The soybeans join Xtendflex, introduced in 2015, for cotton with tolerance to dicamba, glyphosate and glufosinate.

Resistance Already Built In New Technology
Remember, these new technologies employ herbicides that have been around since the 1960s for group 4 and the 1980s for group 27. That time allowed weeds to build resistance. Here are examples of problem weeds and where they have resistance.

Hype about Xtend is recent, but testing is not. “We had Roundup Ready Xtend soybeans on our research farm as early as 2007,” says Dan Childs, Monsanto weed management technology development representative.

The Roundup Ready Xtend soybeans will be paired with Roundup Ready Xtend herbicide with dicamba-glyphosate premix or XtendiMax herbicide. A new formulation of dicamba has been made to reduce off-target movement and volatility.

“Banvel is a DMA salt, with a reputation of volatilization. We are using a different salt, a DGA salt, which along with VaporGrip will lower dicamba’s volatility,” Childs says. 

“Roundup is there to control grasses, and what’s left is the dicamba to control the Roundup-resistant weeds,” Childs says. 

Monsanto hopes overlapping residuals will avoid quickly building a tolerance to this program. 

BASF Crop Protection offers an additional dicamba option for dicamba-tolerant soybeans and cotton. “Engenia will be an effective new tool with BAPMA salt on the core dicamba molecule to reduce volatility,” says Chad Brommer, BASF technical marketing manager for herbicides. “Engenia herbicide is awaiting EPA registration.”

New technology can be helpful, but it’s not a silver bullet—use it with pre-emergent and residual. “The silver bullet has been tarnished for years and is never coming back,” Hager says. “[You need to] use technology in a stewarded systems approach.”

“Spray weeds when they are less than 4" tall,” Ducar says. “Starting clean at planting, however, is also very important. Make sure you have a good pre-emergent down.”

Some weeds, such as waterhemp and palmer amaranth, can grow 2" per day in certain conditions. Waiting 21 to 28 days after planting for a post-emergent application can cause severe yield loss and build the weed seed bank faster. Pre-emergent herbicides with residual can hold off weed problems until canopy.

Scout following each herbicide application, including post, to make sure the herbicide is effective and the timing is correct.

Check to see if there was a weather event, sprayer malfunction or other application error before jumping to the conclusion that pre-emergent didn’t work. “If you notice a herbicide stops working, record that and let a weed specialist know,” Ducar says. 

It doesn’t take long for waterhemp to take over fields and hurt yields since it grows quickly. Consider pre-emergent and residual herbicides to catch weeds.

Timing, rate and efficacy are essential when it comes to post-emergent weed control. This is for multiple reasons: You don’t want to damage your crop, lower-than-labeled rates can increase the likelihood of resistance and ineffective herbicides allow resistant weeds to thrive.

When it comes to timing, check the product label to make sure you don’t spray before or after the optimal window. Establish what weeds the product is labeled to kill, along with weed and crop height restrictions. 

Lower-than-recommended rates are part of the reason resistance has spread across the U.S. “You may think, ‘I can save money by reducing rates,’ but don’t,” Hager says. “The expenditure column is directly related to the revenue column when it comes to weeds.”

Don’t buy a herbicide with greater resistance just because it has a lower price tag. It could make your fields a breeding ground for resistant weeds. 

“The most critical message is don’t let weeds go to seed,” says Jeff Carpenter, regional corn and soybean portfolio manager for DuPont Crop Protection. “Incorporate products that bring multiple modes of action that are effective against your weed spectrum.”

Scouting will show you if your timing and rates were right and if the herbicide was effective. The priority is to maximize weed control and minimize the spread of resistant weed seed.

“You cannot replace scouting,” Hager says. “Know your fields and weed populations.”

If you have weed escapes, consider using tillage or hiring a chopping crew to reduce weed seed spread during harvest. “If I see enough escapes, we send in a chopping crew,” Porter says. He does this every year in his cotton fields and as needed in corn and soybean fields.

While the thought of dusting off the cultivator or disk or hiring a crew might sound like a time, money and resource suck, it could save your fields. In some fields, especially no-till or extremely weedy fields, it could cost less than using ineffective chemicals. 

“Any weed that goes to seed is a yield robber for future years,” Hager says. “Don’t you want to maximize your returns?”

If you don’t want to cultivate, disk or hire a crew, simply get out of the combine when you see weeds that have gone to seed. Chop those weeds or avoid running over them with the combine to reduce their spread.

“People are aware of the resistant weed problem when it’s way too late,” Ducar says. “If you don’t have it now and you’re still using just one mode of action, you will build resistance—it’s just a matter of time. You’ve got to be proactive.”

Keep an eye on the news. Watch your neighbors’ fields for weed pressure. If you notice they’re beginning to have issues with resistance, understand weed seeds are one strong wind away from your farm. 

“If you cut back on yield-reducing pests and weeds, you increase your returns,” Hager says. Isn’t that the ultimate goal? 

This story is part of a series on herbicide resistance, which puts many farmers in a “billion dollar bind.” Review previous stories that detail more ways resistance hurts yield, how much profit it steals and control methods at

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