Stopping the Most Prolific Weeds in Soybeans

Uncover the nation’s toughest growing conditions for soybeans and explore, in particular, the most daunting weed management battles in soybean growing. Take a retrospective look at tough weed battles and look ahead toward potential weed challenges. We’ll learn:

  • What strategies work best under the worst conditions
  • Key takeaways for growers as they fine-tune their plans for the year
  • Emerging threats that may be lurking around the corner

Images of weeds strangling a field may darken the nightmares of most soybean producers. But they don’t have to lose sleep - controlling weed resistance is possible.

“Weather, from the beginning of time to the end of time, is going to be the number one variable for a grower, but weed resistance is also at the top of the list of concerns, regardless of geography. Weed resistance may not be dictating every grower’s production plans, but it is on their short list of variables that are difficult to control,” said Monty Malone, who provides soybean agronomic support for Crop Science.

According to Andy Lenssen, a soybean systems agronomist with Iowa State University, continued weed resistance is probably the greatest emerging threat to Midwest soybean producers, particularly with marestail or horseweed, waterhemp and potentially Palmer amaranth.

Weeds that create management headaches are those which prolifically produce seeds and those which need to be controlled in an environment where some herbicides are not as effective, Malone points out. Top that tough challenge with “driver” weeds, those with well-documented resistance to certain herbicides, and the nightmare begins.

Palmer amaranth or pigweed is one of the most prolific species that has documented glyphosate resistance. “They just produce enormous amounts of seed, which build up in the seed bank and increase the pressure faster,” Malone said. “When you have an escape with a weed like pigweed that produces an incredible amount of seed, then you inherit a larger problem in the consecutive year.”

Weeds are demonstrating resistance to multiple modes of action, including glyphosate-resistance, noted Lenssen. “We have populations of tall waterhemp in Iowa with resistance to four or five modes of action. There aren’t a lot of herbicides left that will impact that population of these driver weeds.”

Jason Norsworthy, University of Arkansas professor and endowed chair of weed science, advocates the importance of crop rotation and comprehensive integrated weed management programs to manage tough-to-control weeds and prevent the spread of resistance, particularly through mitigating or eliminating seed banks of herbicide-resistant weeds.

“Part of stewardship is weed seed prevention, and the economic weed thresholds of yesterday have no place in today’s environment,” he shared on the Crop Science website. “You cannot allow weed escapes in your fields or turnrows because driver weeds – kochia, waterhemp, giant ragweed and Palmer amaranth – will evolve resistance. Once you discover herbicide-resistant weeds, you can have a complete loss in as few as three years if growers do not use every tool they have to combat them.”

According to Norsworthy, 26 states and counting are infested with glyphosate-resistant Palmer amaranth and 15 states with glyphosate-resistant waterhemp. “This is becoming a national issue and the reason the expansion of resistant weeds continues is because our herbicide programs lack diversity.”

Malone points to the mid-South and lower Midwest as the toughest areas for growing soybeans due to weed threats, but documented weed resistance is in nearly every state.

New management tools will be needed to meet the threat, experts point out. Nonselective herbicide trait systems remain the backbone of many integrated weed management programs and Liberty® herbicide provides a critical option to soybean growers facing increasing resistance pressures, especially as resistance to glyphosate continues. “Liberty is highly effective against the toughest weeds, including resistant driver weeds, and remains the one of most significant nonselective trait system options that’s commercially available in 2015,” Malone noted.

Crop Science’s LibertyLink® system is built upon a foundation of high-performing genetics and superior control, even on the toughest weeds. The system reduces the costliness of weeds and drives high yields while offering growers long-term operational success.

An application of Liberty herbicide offers nonselective post-emergence control of the tough weeds. “It allows the growers a chance to rotate their chemistries and use tankmix residuals to provide multiple effective modes of action (MOA) to combat weeds. Using tankmix partners and multiple effective herbicide modes of action during both the growing season and from year to year will reduce the selection pressure of a single MOA. The only way they’re going to maintain a sustainable weed management program is to rotate chemistry where they don’t overexpose the weed population on their farm to one type of chemistry,” Malone advised.

In the past, producers may have been able to tackle weed escapes using rescue treatments, even controlling larger weeds. “That day is gone,” Malone said. “You have to be very meticulous in managing weeds and control them at the right time.”

Liberty, for example, is most effective on pigweed 2-3 inches tall. As with any treatment, growers should maximize coverage using proper sprayer calibration specific to the chemical and sprayer speed within the label requirements.

“We’re going to have to start clean and use residual herbicides; there’s not another option, both of those scenarios have to exist,” Malone advised. That requires preparing the seedbed prior to planting to kill germinating weeds.

Starting with a clean field to control impending weed populations includes field-to-field decisions about tillage. Lenssen points out that fall tillage is sometimes highly effective at putting weed seed into the soil, providing a safe environment for weeds over the winter. “In many situations, some tillage practices actually protect weed seeds so that you end up having more weeds in the future,” he said.

“Management practices such as thorough seedbed preparation, adequate soil fertility, choice of a well-adapted variety and use of good-quality seed all contribute to conditions of good competition with weeds,” according to a soybean production field guide from the North Dakota State University (NDSU) Extension Service.

Hans Kandel, NDSU Extension Service agronomist, advises farmers to use an integrated plan for weed control, by controlling broadleaf weeds in wheat, for example, then rotating the field to soybeans. Targeting an early planting date, selecting the right variety and utilizing 15-inch row spacing all create the right conditions for the soybean to quickly cover the ground, which also aids in weed control, Kandel shared.

“Monitor your soybeans like you would monitor your corn. Field scouting is important,” Lenssen said.  “It all ties together.  “Selection pressure always works, whether we are speaking of weeds, pathogens or insects.”

Managing the details will help overcome many of those nightmares in weed control.  And perhaps let you get a good night’s sleep, too.


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