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Yes, Herbicides Can Control 4-Foot-High Marestail, But…

Successful Control Consists of Several Effective Herbicide Sites of Action and Spending Sufficient Money.

Farmers backed into a weed-control corner by 4-foot-high marestail can take heart. Although not ideal, chemical combinations exist that successfully control monster marestail. 

“If you do it right, you can (successfully) spray that big of marestail,” says Kevin Bradley, a University of Missouri (MU) Extension weeds specialist. 

There’s a price to pay, though. “I will tell farmers who call me up on the phone not to skimp when they are talking about 50-inch-high marestail,” Bradley says. “They have to be willing to pay the money.” 

Costs for multiherbicide programs to control marestail this tall can range between $40 to $50 per acre. 

What Farmers Face

On paper, effective soybean weed management looks easy. Apply a preemergence residual herbicide treatment followed by overlapping postemergence residual products until soybeans canopy. Roguing out any escapes before harvest can nix any weeds that may go to seed.

In reality, it’s much tougher, as this sopping wet spring proved. Some farmers encountered soggy soils that were physically impossible to enter with a sprayer prior to planting. When weather cleared, it was plant or bust, with weed control a secondary thought. After planting, further rains prevented farmers from applying a postemergence herbicide. Meanwhile, marestail grew and grew. 

Such a patch of 4-foot-high marestail exists this summer at MU’s Bradford Field Center, where this year’s MU Pest Management Field Day was held this week. Marestail growth was massive in this demo, Bradley says. Still, MU weed scientists found several successful herbicide combinations, even at this height.

“We sprayed it at 20 gallons (water carrier) per acre,” he says. “We did not skip on adjuvants. And once marestail gets that big, you have to have some burning products in it (the herbicide mix). Whether it's Liberty or Sharpen or metribuzin (Dual Magnum), you're going to see a lot of combinations that maybe you're not thinking of. We’re also mixing things like Gramoxone and Sharpen and 2,4-D together. Don’t skimp (on effective herbicide sites of action),” he says. 

What won’t work are new dicamba products featuring dicamba and glyphosate for dicamba-tolerant soybeans used alone. 

“I like dicamba on marestail,” says Bradley. “I’ve always said one of the benefits of the trait is that it burns down marestail. But (products) like Xtendimax and Engenia are not meant for 50-inch-high marestail. They weren’t designed for that. You need burning products, with two or three effective active ingredients.” 

New Products Coming

One bright spot for weed management is there are new chemistry mixes coming. In mid- to late spring 2020, Bayer Crop Science plans to launch XtendFlex on soybeans, pending regulatory approval. It’s a triple soybean stack that tolerates glyphosate, dicamba, and glufosinate (Liberty) herbicides.  

Meanwhile, the Enlist E3 soybeans will be broadly available in 2020 after garnering Chinese import approval earlier this year, according to Corteva officials. 

The Enlist system confers herbicide tolerance to a new 2,4-D formulation – 2,4-D choline – and glyphosate in corn, soybeans, and cotton and fop herbicides in corn. Herbicide options include Enlist Duo, a mix of glyphosate, and 2,4-D choline. Enlist One is straight 2,4-D choline that can be tank-mixed with approved label herbicides.

Bradley say volatility issues should be lower with the 2,4-D choline than with dicamba labeled for dicamba-tolerant soybeans. 

“Everything I have seen in either plot research or in published literature about the 2,4-D choline is that there is not the issue so far with volatility that some of the dicamba products (labeled for dicamba-tolerant soybeans) have had,” he says. “Now, if the wind is blowing (toward the crop) and you make a bad decision to spray 2,4D, it will hurt (neighboring nontolerant vegetation) just like dicamba. There still is a risk there. But if an applicator does everything right, so far, I have not seen those problems,” he says. 

Xtend Flex will aid farmers in making weed-management decisions, says Bradley. For example, there may be cases where weather nixes dicamba applications (due to label restrictions like wind, etc). However, growers may be able to spray Liberty on Enlist E3 soybeans that day, as it does not have as stringent of label restrictions. 

Inversions

Inversions that lead off-target movement of herbicides may be more common than farmers may think. 

“When I call something an inversion, I am very conservative,” says Mandy Bish, an MU research specialist. “I take into the accuracy of factors like temperature probes, wind, and a lot of other tools.”

Still, she’s confident that last year, inversions occurred 50% of June evenings at Missouri sites. 

Inversions are sporadic, occurring in some areas and not others just a few miles away. Field surroundings matter, says Bish. 

“If you have a tree line blocking a wind from the south, an inversion can form in that field more quickly than in neighboring fields that do not have the wind obstructed,” she says. 

Seed Bank Management

Joe Ege, an MU graduate student, is working with an Australian company that makes the Seed Terminator.This tool attaches to a combine and pulverizes and grinds weed seeds at harvest. This prevents seeds from germinating and spurring future infestations. Several hundred units like this one and another, the Harrington Seed Destructor, are being used by Australian farmers. 

Since waterhemp now resists seven herbicide sites of action, weed scientists are focusing on preventing weeds from germinating in the first place through such tools like the Seed Terminator. Grinding some weeds like giant ragweed or giant foxtail at harvest won’t work that well, since only 30% of those weed seeds remain with R8 (full maturity soybeans).

Such units cost money, though. Prices for the Harrington Seed Destructor, a competing machine, may range between $85,000 to $117,000, says Bob Hartzler, Iowa State University Extension weeds specialist.

Long term, farmers also won’t catch a break when it comes to resistance, even with this tool. 

“You will select for early shattering weeds,” says Bish. She says the only surefire way to manage weeds is through an integrated approach.

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