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9 steps to pummel yield-robbing waterhemp

As if corn and soybean farmers didn’t have enough to worry about, there’s this: University of Missouri (MU) weed scientists announced the discovery last summer of a waterhemp population that resists six herbicide sites of action. 

It all started when farmers in the north-central Missouri county of Randolph reported a population of waterhemp that appeared to resist 2,4-D. MU researchers then conducted field experiments that confirmed 2,4-D (herbicide site of action Group 4) resistance. But they also found the same waterhemp population resisted:

  • Atrazine (Group 5)
  • Chlorimuron (Group 2, Classic)
  • Fomesafen (Group 14, Flexstar, Reflex) 
  • Glyphosate (Group 9, Roundup) 
  • Mesotrione (Group 27, Balance Flexx)

Of the eight herbicides applied, only dicamba (Group 4) and glufosinate (Liberty, Group 10) provided acceptable control, say the MU scientists. 

Corteva Agriscience’s Enlist Weed Control System – still awaiting approval from China for a soybean commercial – has two herbicide components. One consists of 2,4-D choline, while another combines 2,4-D choline with glyphosate. 

So What Do You Do? 

Left unchecked, waterhemp can ravage soybean yields. The MU researchers say six-way resistant waterhemp requires a diversified management approach. Rather than relying on glyphosate, 2,4-D, or any other single herbicide, they recommend a variety of appropriate cultural, mechanical, and biological control tactics. Here are nine ideas.

1. Scout, scout, scout

Farmers still need to get in their fields and scout, says Dave Roome, customer technical specialist for Corteva Agriscience. That’s not only the case for the present year but in upcoming years, as well, he says. Documentation of current weeds during the growing season can help with future weed-management decisions, he says.

2. Properly identify weeds

Waterhemp and Palmer amaranth can resemble each other. One way that normally works to tell them apart is petiole length. Palmer amaranth’s petiole is longer and often exceeds the lengths of one of its leaves. As a rule, waterhemp’s petioles are shorter than the leaf length, Roome says.

3. Layer residual herbicides

Mid-South farmers battling Palmer amaranth layer several burndown and preemergence residual treatments followed by timely postemergence treatments. The same approach can be used in fields infested by waterhemp, says Roome.

4. Spray on time

“Four inches from your chest, Pyle, 4 inches!” bellowed a leather-lunged drill sergeant as he instructed a hapless Marine recruit in how to hold his rifle in the 1987 war drama Full Metal Jacket

Well, if 4 inches is good enough for the Marines, it’s good enough for farmers managing waterhemp.

If the label states that you need to spray a postemergence herbicide on weeds at a 4-inch height, spray them at that height or before they reach it. Not only will limited control result at a height like 8 inches, but also the taller weeds will shield smaller weeds and block the spray, Roome says.

5. Narrow rows

“Narrow rows can shade weeds and close the canopy sooner,” says Roome. 

There is a flip side in all this, though, in the white mold country of the Upper Midwest. Row spacings of 15 inches can promote a stuffy air environment in which white mold thrives. Wider rows are more conducive to airflow that can discourage white mold.

A fungicide applied around R1 followed by a subsequent application 14 to 21 days later can enable you to curb white mold in-season. However, that adds costs. A better approach is to select varieties tolerant to white mold on suspect fields.

“The first line of defense is genetics,” says Steve Schnebly, Pioneer senior research manager.

6. Hone application technology

“Some farmers and applicators do a great job of paying attention to gallonage, pressure, and nozzles,” says Roome. “Also, the boom needs to be 20 to 24 inches above the targeted species. Ideally, nozzles should overlap each other. Booms oustide of these metric may apply varying amounts of herbicide. Some weeds can get 175% of the (labeled) herbicide rate, while others get 10%,” he says.

7. Keep communication lines open

“With today’s farms being like they are, there is a lot of rented ground, and it can be tough to track down who is farming it,” says Dave Hillger, a Corteva Agriscience field specialist. Make the effort to find out who is farming it and ask what kind of herbicide-tolerant – or nonherbicide-tolerant technology – is being planted.

“Having open communication can help head off (off-target) situations before they occur,” says Hillger.

8. Remove rogue waterhemp prior to harvest

This can help a weed seed bank from building in subsequent years. 

“Physically removing them from the field is the best way to eliminate weed seeds,” says Dawn Refsell,Valent field market
development manager. 

9. Remember other weeds

Palmer amaranth is also surfacing in Midwestern soybean fields. “Giant ragweed is coming back in full force in Indiana and in the Midwest,” says Roome. “We are also seeing viny weeds like bindweed. Also, green and yellow foxtail are coming back.”


Madison Rinehart, a Corteva Agriscience intern, and Dave Roome show a waterhemp plant that’s past the prime control height. 

Manage Yield-Robbing Diseases

In many areas in 2018, weather conditions fueled a plethora of corn diseases like gray leaf  spot, gibberella, and anthracnose stalk rots (ASR). Keep an eye on those fields this fall as you plan for 2019, says Sue Brakhane, a DeKalb/Asgrow technical agronomist for south-central Iowa. If ASR troubled corn-on-corn fields in 2018, plant a more tolerant hybrid this year, she says. 

In soybeans, frogeye leaf spot has moved northward, surfacing in states as far north as Iowa, says Brakhane. Since it has resisted strobilurin fungicides in all major soybean growing areas, farmers need to use multiple mode of action fungicides, variety selectiona and crop rotation in order to mange frogeye leaf spot, she points out.

Good SCN News 

More than 95% of the soybean varieties that resist soybean cyst nematode (SCN) share the same source of SCN resistance: PI 88788. Anytime the same control measure is repeatedly used (glyphosate-resistant weeds, anyone), resistance may develop. That’s also the case for SCN-resistant varieties. 

Peking is another source of SCN resistance. The knock against SCN-resistant varieties using the Peking source of resistance has been lowered yield potential.

No more. David Thompson, Stine Seeds national sales and marketing manager, notes Stine has several SCN-resistant varieties using Peking resistance that are top yielders. “Farmers have options (for SCN management) that they didn’t have before,” he says.

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