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The Syngenta Field Report features information and experts from Syngenta sharing observations about issues growers are dealing with in the fields.

“Overlapping Residuals” Key to Problem Weed Control

Dec 14, 2010

Chuck Foresman

Weeds are getting wilier. With changes in cultural practices and weed control technology, they adapt so they can continue to thrive. 

University researchers have noted during the past decade or so that some problem weeds have developed extended germination periods. For example, giant ragweed used to be a problem early in the spring, but now the weed germinates through mid-summer. Palmer pigweed germinates throughout the growing season, and can produce viable seeds even late in the year because it grows so quickly. Plus, these species have also proven their ability to develop resistance to herbicides and significantly cut yield.
I think the best way to control such weeds is to develop an “overlapping residual” strategy. Residual herbicides are critical to control weeds that emerge over a long period of time. The herbicide begins working on weeds as they germinate, which is when they are most vulnerable. The herbicide stays in the soil for a period of time to protect crops for several weeks. 
But, with really competitive weeds like Palmer pigweed, you want to get a different residual herbicide mode of action out there before the first one runs out. You don’t want to overuse any chemistry to encourage resistance, and you want to prevent weeds from competing with the crop.
You can find recommendations for overlapping residual programs at
Are you using a program with overlapping residuals? Take no prisoners. These weeds wait for you to get comfortable, and then they pounce.
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