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Winter annual weeds? Time to treat soon

Jeff Caldwell 10/04/2012 @ 9:05am Multimedia Editor for Agriculture.com and Successful Farming magazine.

Depending on what weed pressures hit your fields hardest, you may want to get out, scout and consider putting down some post-harvest herbicide before winter rolls in.

Specialists have already advised scouting and conducting soil tests in your fields for fertilizer needs. It's also a good idea to add to that a scouting trip to get a feel for the most problematic weeds this year, says University of Illinois weed scientist Aaron Hager.

"Be sure to scout fields before making any application to determine what weeds are present and if their densities are high enough to warrant treatment," Hager says in a university report. "Many herbicides used prior to or after crop planting/emergence can be used in the fall, but not all herbicides are labeled for fall application. Also, some herbicides approved for fall application have application timing restrictions."

Application timing is important; if you're putting down 2,4-D or glyphosate, both of which have little soil-residual activity, make sure you wait until after winter annual weeds have emerged, usually around mid- to late-October, Hager says. If you're putting down a product with more residual activity in the soil, an earlier application won't hamper efficacy.

"Horseweed/marestail populations are increasing in minimum and no-tillage cropping systems across much of the southern two-thirds of Illinois. Horseweed completes its life cycle in one year, but unlike many other annual species, it may exist as a winter or summer annual. Winter annual horseweed populations typically emerge in the fall, within a few days or weeks after seed is dispersed from the parent plant," according to a university report. "With the increasing prevalence of horseweed, including glyphosate-resistant populations, fall herbicide applications may work better than spring applications. Glyphosate alone may not provide adequate control no matter when applied, but a higher application rate of products (such as 2,4-D) can be used in fall than in spring."

But, if you're hit harder by summer annual weeds, don't rely on residual control from a fall application, Hager says. Instead, plan on timing a spring application as closely as you can to planting next year's crop as you can.

"We do not recommend fall herbicide applications as a method to provide residual control of summer annual weed species," he says. "We recommend against increasing the application rate with the idea of controlling summer annual species next spring."

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