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Don't simplify; diversify

William of Ockham might have been a great theologian when he developed Ockham's Razor back in the fourteenth century. Still, his belief that the simplest solution normally is the best wouldn't work for devising weed-control systems.

“Anything simple is destined to fail quickly and cost farmers money,” says Mike Owen, Iowa State University Extension weed specialist.

That's because resistant-weed biotypes can quickly multiply under simple systems using herbicides applied year after year.

“There's an old saying, ‘If it's not broke, don't fix it,’ ” says Owen. “In this case, if it's not broken, that's exactly the time you should fix it. If you keep doing the same thing, you end up with herbicide-resistance problems. Diversity must be practiced in crop production and, in particular, weed management.”

Another reason to diversify weed control surfaced in 2010. University of Illinois (U of I) weed scientists identified the world's first waterhemp biotype resistant to HPPD-inhibitor herbicides. (HPPD inhibitors include Impact, Callisto, and Laudis.) Biotypes resistant to HPPD inhibitors join waterhemp biotypes resistant to triazines, PPO inhibitors, ALS inhibitors, and glyphosate.

“This discovery illustrates how water-hemp can adapt to modern production practices,” says Chuck Foresman, Syngenta manager of weed resistance strategies. “It doesn't necessarily mean you have to stop using an HPPD inhibitor. But what it does mean is you need to rotate or diversify the types of herbicides used.”

One way to diversify is to mix up different modes of action in herbicide applications. A way to do this and also get a jump start on weeds is to apply preemergence residual herbicides.

“Soil residual herbicides will be an integrated part of weed management,” says Aaron Hager, U of I Extension weed specialist.

Still, successful use of preemergence residual herbicides hinges on three criteria:

• Placement. “Whether you incorporate mechanically or by precipitation, you target the top 1 to 2 inches of the soil profile,” says Hager.

• Full rates. “Full rates have a greater likelihood of longer residual control than lower rates,” says Hager.

• Soil moisture and application time. “Most soil residual herbicides must be in the soil solution to be effective,” Hager says. “If you put a herbicide on top of dry soil for three weeks, it won't work as well as if you get precipitation just a few days afterward.”

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