Man vs. Weeds: 7 ways to win

  • Whether we're losing the fight against glyphosate-resistant weeds is the question Kevin Bradley has been asking this year. The University of Missouri Extension weed specialist and his colleagues in the Weed Science Society of America have compiled this list of seven ways to stop them.

  • 1. Know your enemy. Understand the biology of your weeds, Bradley says. Waterhemp is one of the most troublesome herbicide-resistant weeds, and produces 300,000 seeds per plant. The seeds germinate all season, but tilling helps reduce emergence. Giant ragweed also shows resistance, but emerges early. That's the time to control it.

  • 2. Plant into a clean slate.
    With the glyphosate-only program, you may have planted soybeans into weedy conditions, and sprayed later. The problem is that the bigger the weed, the harder it is to kill. If a few glyphosate-resistant weeds survive and produce seed, more resistant plants grow the next year. Within a few years, the entire weed population may be resistant.

  • Bradley says a clean-slate program may still involve an early burndown herbicide. But you need to include another preplant or preemergence herbicide with a different mechanism of action, possibly one with residual activity. For example, Roundup PowerMax plus Clarity or Sharpen can control 90% or more of weeds. If the field still isn't clean, tilling may be necessary.

  • 3. Be diverse.
    Use every tool in your weed-control toolbox -- chemistry, tillage, timing, and cropping programs -- says Bradley. Crop rotation (such as corn and soybeans) should discourage development of weed resistance because of the different production practices and herbicides. It doesn't work all by itself when it comes to waterhemp, however, says Bradley.

  • Normally, you would expect corn's canopy to control late-emerging waterhemp. "I've been seeing more fields where there are small waterhemp plants in corn with viable seed at harvest," he says. "When it goes through the combine, the seeds get scattered. Then it explodes in the following crop." Scout cornfields, control the late emergers, and keep mature weeds with viable seed out of the combine.

  • 4. Cover the ground. Cultural practices like narrow-row soybeans and fall or spring cover crops can provide free weed control. Narrow-row or drilled soybeans will form a complete soil canopy much quicker than 30-inch rows. Weeds can't grow if sunlight doesn't penetrate through to them. Studies show far lower density of late-season waterhemp in narrow rows, says Bradley.

  • Bradley says we don't know everything cover crops do to suppress weeds, but they do suppress weeds. Rye or some other crop seeded after harvest will cover the ground into spring. It will check weed emergence until you're ready to kill the rye and plant a new crop. In some studies, cover crops reduce weed emergence to less than one third of what exists without a cover crop. This provides a great head start to weed-free planting.

  • 5. Employ multiple weapons. Glyphosate was good until the weeds figured it out. "Use an effective preemergence residual herbicide," says Bradley. "The post-only program is unwinnable for the long term." The goal is to get the crop planted and growing with little weed competition. Then if you need a postemergence application of glyphosate, it is more likely to be effective.

  • For later postglyphosate applications, consider a tankmix of an overlapping residual product that will continue to suppress late-emerging weeds. An overlapping residual will better control glyphosate-resistant weeds. These include Cinch, Outlook, Dual II Magnum, Warrant, and Prefix. Bradley says the LibertyLink system works, but requires preplant residuals with timely applications on weeds of appropriate size.

  • 6. Don't skimp. Bradley says it's usually a mistake to reduce herbicide rates. Incomplete control can add to the resistant-weed seed bank. Apply herbicide on the recommended weed size, typically 4 inches or smaller. You start losing soybean yield if they are any larger. These full rates with multiple weapons won't be as cheap as the glyphosate-only program you liked in the past.

  • "You can't nickel-and-dime this anymore," he says. "If you find a program that works at getting you ahead of the weeds before planting, use it. The problem is, if you have to come back in the growing season to rescue a weed problem resistant to glyphosate, you don't have many other options. I worry that we are just driving ourselves to multiple resistance issues with other herbicides."

  • 7. Reduce weed seeds. "All of our focus should be on reducing the amount of weed seed at harvest," says Bradley. "We can do it; we just have to use all of the tools." Some Southern farmers have reverted to hand-weeding to keep weeds from going through a combine or cotton picker. That may seem outdated, but hiring a weeding crew at $29.43 per acre compares favorably to multiple-herbicide programs.

  • Farmers are holding their own with waterhemp, Bradley says. "In Missouri, more preplant residual products are being used," he says. However, a survey of 1,300 soybean growers in 22 states shows growers who haven't seen resistance on their farms are staying with the total postprogram. "I wish we could get everyone to be proactive," he says.

  • The 2,4-D- and glyphosate-tolerant Enlist Weed Control System from Dow AgroSciences is coming for corn in 2014 and soybeans in 2015. Monsanto and BASF plan to launch dicamba-tolerant soybeans in 2014 under the Roundup Ready Xtend Crop System. "They'll be great to have in our toolbox," Bradley says. "If we don't use good weed-resistance management techniques, we'll be back here in five years with another resistance."

Are we losing the war against weeds that are resistant to herbicides? Here are seven ways to stop them.

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