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Weed control outlook: Soybeans & corn

rag weed in corn
Photo by Mike Krivit
Field agronomists report on 2016 weed issues, control and 2017 strategies.

Think Different

Field reports from Winfield United agronomists cited numerous weed control issues and solutions:

  • Herbicide-resistant weeds becoming more severe in every Midwest state.
  • Resistance to herbicide groups like 2 (ALS), 5 (triazines), 14 (PPOs) and 9 (glyphosate) are most severe.
  • Higher rates of preemergent, residual herbicides along with timely post residual applications were most successful. And rotation among “effective” modes of action is critical.
  • Scouting and attention to field-by-field detail of weed resistance can held reduce the weed seed bank.


Every Midwest state continues to report growing herbicide-resistant (HR) weed problems in soybeans; fewer issues in corn. “Farmers who haven’t changed their weed control programs in the past couple of years had the most problems in 2016,” says Ryan Wolf, Winfield United agronomy services manager from Sheldon, Iowa. “Farmers who used higher rates of preemergent herbicides and added metribuzin to their preemergence tank mixes had the most weed control success. Timely post applications also performed well,” he adds.

 Soybean weed challenges

Such HR weed issues echoed across eight states and beyond, according to Winfield United field agronomist’ reports to our inquiries. Waterhemp, marestail, giant ragweed, kochia and the expansion of the dangerous Palmer Amaranth pigweed continue to evade control by more herbicide groups.

“We are seeing more farmers adopt herbicide resistance management strategies because resistance to several herbicide site of action groups like 2 (ALS), 5 (triazines), 14 (PPOs) and 9 (glyphosate) are present across Illinois, says Glenn Longabaugh, Winfield agronomist service from Hazleton, Ind.

Wolf says that resistance to glyphosate and Group 14 (PPOs) herbicides caused the biggest waterhemp control problems in Iowa and South Dakota in 2016, along with glyphosate-resistant kochia and marestail. “When weather delayed application past the recommended control window, Group 14 herbicides failed to achieve kochia control,” he said.

Across Indiana, farmers were “surprised by the explosion of pigweed populations in their fields, which has been building for several years,” says George Watters, Winfield United agronomist from Noblesville, Ind. He cites herbicide histories that allowed for past weed escapes and the spreading of weed seed by combines as culprits in this all-out attack in many fields during 2016.

“Growers have learned that nearly 100% of the waterhemp and Palmer amaranth populations are now resistant to glyphosate (Group 9) and ALS (2) herbicides. And several populations are becoming resistant to foliar application of PPO (Group 14) herbicides,” Watters adds.

In the upper Midwest, glyphosate-resistant weeds like waterhemp, marestail, pigweed (giant and common) continue to move northward in Minnesota, Wisconsin and Michigan. “It’s been difficult for farmers and salespeople to acknowledge the growing problem of glyphosate- and triazine-resistant weeds in the upper Midwest, which makes it hard to establish an effective weed prevention plan,” says Todd Cardwell, Winfield United senior agronomist from River Falls, Wisc.

 Weather delays

Delayed or missed premergence herbicide applications played a role in weed escapes during 2016. “In other cases, the weather turned dry after early applications and control of waterhemp was reduced, causing farmers to fight glyphosate- and ALS-resistant weed escapes with primarily one mode of action – PPO herbicides” said Al Bertelsen, Winfield agronomist from Red Wing, Minn. “Wet conditions or other factors sometimes delayed PPO herbicide applications until target weeds were too tall for effective control.”

 “Effective” modes of action

Attention to detail and good scouting is key field data for future weed control success. Winfield United recommends three “effective” modes of action each year to fight resistant weeds. “If glyphosate-resistant weeds are present, glyphosate cannot be counted as one of the three modes of action used; and same for other resistant weed classes,” Wolf says.

Agronomist Cardwell takes it a step further, stating that growers must create a holestic, long-term plan that accounts for problematic weeds across all crops in a rotation, by field. “This will help control the weed in all crops,” he says.

Along with this strategy, Bertelsen recommends timely applications of PPO herbicides when weeds are small to greatly improve weed control in soybeans. “Just don’t forget to spray using higher volumes and select spray nozzles that increase weed coverage,” he says.

 Don’t cut costs

Yes, it’s tempting given grain prices, but cut rates or weaker herbicides easily lead to partial control and more HR weeds. An investment in a strong herbicide program may be daunting to some farmers, the alternative of weeds taking over a field is far costlier, given the continued multiplier of HR weed seed added to the soil for future years.

“A typical crop protection budget is usually only 9% to 12% of overall crop input budget. However, cutting the herbicide program by 10% (or 1% of total budget) could have a devastating impact on crop yield for multiple years,” agronomist Longabaugh says.

 Soybean strategies for 2017

Joe Rickard, WinField United agronomist from Alliance, Ohio, says success begins by working with a trusted advisor to help navigate through the steps needed for effective weed control.

The process starts with a clean field, Longabaugh notes, since there is no remedy for planting into an established stand of herbicide resistant weeds like waterhemp and palmer. “For other resistant weed threats such as marestail, farmers are effectively using fall burndown herbicide applications or timely spring tillage.”

Jen Koukol, Soybeans, weeds

Maximizing the effectiveness of herbicide applications through proper timing, coverage and use of quality adjuvants will also be important in keeping aggressive weed species at bay.

“In addition to a good preemergence herbicide program, many farmers are also considering either LibertyLink soybeans or the new dicamba-tolerant soybeans for 2017,” says Allen Pung, WinField United master agronomy advisor from Portland, Mich.

Agronomist Watters cautions that while the Xtend technology with XtendiMax herbicide on Roundup Ready 2 Xtend soybeans allows for the use of special dicamba formulations over the top to help control emerged weeds, farmers should not rely on it as a substitute for overlapping residual herbicides. And farmers must follow new, stringent application guidelines to reduce off-target movement and sensitive crop injury.

Regarding control of the aggressive Palmer amaranth species, a complete herbicide resistance management program is needed that incorporates: crop rotation, multiple effective modes of action, overlapping residuals, timely applications, full labeled rates, a complement of adjuvants, and physical removal where applicable.

 Corn issues in 2016

Across the Midwest, weed resistance in corn isn’t as big an issue as in soybeans. “Corn growers do a good job of keeping their fields clean because there are herbicides they can use in corn that cannot be used in soybeans, says Rickard.

Winfield agronomists cite giant ragweed, waterhemp, cocklebur and morningglory as the biggest herbicide-resistant weed challenges in corn.

Overall, Indiana agronomist Watters found that growers were able to manage resistant species fairly well in corn due to still having several effective herbicide groups available for application. “Premixes and/or other combinations of growth regulators (Group 4), triazines (Group 5), shoot inhibitors (Group 15) and HPPDs (Group 27) can still do a good job of controlling weeds in corn.”

Watters says the outbreak of giant ragweed and several other large seeded broadleaves in Indiana was a result of favorable weather and rainfall conditions that encouraged additional flushes later in the season.

In Wisconsin cornfields, Cardwell saw high populations of common and giant ragweed thanks to an unusually wet season; rain decreased the longevity of soil-applied chemistries and kept farmers out of their fields for long periods of time, allowing the weeds to grow quickly. “There’s also a growing population of triazine-resistant weeds, making atrazines less effective than they have been in previous years,” he adds.

Although weather affected herbicide performance this year, agronomist Longabaugh believes that many corn weed problems were due to using a single-pass herbicide program, stand voids, and continuous use of single site/mode-of-action.

Minnesota farmers battled giant ragweed and tall waterhemp in cornfields this year, reports Bertelsen. In addition to herbicide resistance, he says a large seedbank of these two weeds in both corn and soybean fields are causing widespread issues. “Both weeds can germinate over long periods of time, and the germination period may outlast many soil-applied herbicides, allowing the weeds to escape late in the season after the herbicide application window is past.”

 Corn strategies for 2017

To combat weed issues in corn, Bertelsen urges farmers to scout corn later in the season for weed escapes and control weeds when they are small. Controlling weeds in drowned-out spots will help lower weed seedbanks and decrease weed pressure in future years, notes Bertelsen.

“The best strategy for season-long weed control in corn is to start clean with burndown or tillage, followed by a residual herbicide program close to planting and followed again by a sequential postemergence treatment,” says Watters. “Some growers have also found good results by split-applying their residual herbicides.”

Regarding no-till, Wisconsin agronomist Cardwell says farmers had successful weed control in no-till fields using burndown applications last fall, which made their in-season treatments much more effective. “One dollar spent on weed control in the fall is worth at least seven dollars in the spring. Reduced-till or no-till environments have a different weed spectrum, and in these environments, fall applications are very cost-effective.”

Finally, Winfield agronomists cannot emphasize enough the importance of a scouting program – designed to 1) identify where specific weed issues are taking place, 2) to recognize resistant weeds right away, and 3) to maximize weed control through early herbicide applications. This will help farmers stay on top of weeds throughout the 2017 growing season, notes Bertelsen.

“While some farmers may be hesitant to spend the additional money needed to control resistant weeds now, keeping weeds in check will help avoid even bigger, more costly problems in the future,” Wolf says.


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