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Planting early? Watch those weeds

Jeff Caldwell 03/23/2012 @ 11:49am Multimedia Editor for Agriculture.com and Successful Farming magazine.

If you plant corn now, you may be setting yourself up for some increased pest pressure later on in the growing season. And, the unseasonably warm weather isn't just enjoyable for you. The weeds that made it through the winter could strike your fields with a vengeance later in the year.

If the warm spring was preceded by a warmer and drier-than-normal winter, as it was in many parts of the Midwest, the weeds could be popping, says University of Illinois Extension weed scientist Aaron Hager.

"An obvious implication is the abundance of winter annual weed vegetation growing in many fields across the state. Survivorship over the winter of weeds that emerged last fall was undoubtedly higher than usual, and the recent stretch of warm air temperatures has prompted these weeds to resume growth several weeks ahead of average," he says. "Windshield observations found that henbit began to flower over two weeks ago in many areas of central Illinois."

Marestail, or horseweed, could be a big one this spring. It's an annual, but in some cases can emerge quickly in the spring after a mild winter.

"Unlike many other annual species, it may exist as a winter or summer annual. Populations of winter annual horseweed typically emerge in the fall, within a few days or weeks after seed is dispersed from the parent plant," Hager says. "Summer annual populations can emerge in early or late spring -- as late as early summer in some instances."

If you do have a weed crop already started, Hager advises adding a burndown herbicide on top of a typical soil-residual herbicide to knock down early summer annual seed germination. Because of the longer-than-normal timeframe for weed control this year, it's not a good year to rely only on preplant soil-applied residual herbicides.

"If tillage is done in mid-March and no soil-residual herbicides are applied within a few days of the last tillage operation, be sure to scout for emergence of summer annual weed species prior to planting. If summer annual species have emerged before corn is planted, consider including a herbicide that has burndown activity when applying a soil-residual herbicide," he says."Reduced rates of soil-residual herbicides applied in mid-March are unlikely to provide much residual control of summer annual weed species following crop emergence. If a soil-residual herbicide is applied this early to fields where corn will be planted, consider applying the full recommended rate for the particular soil type."

Finally, consider splitting that full rate into 2 separate applications, one before and one after planting. "Apply perhaps 50% or 60% now or sometime before planting and the remainder before summer annual weed species emerge," Hager adds. "It is beneficial to control winter annual weeds in fields to be planted to soybean as soon as possible, but extended residual control of summer annual weeds can be achieved when soil-residual herbicides are applied closer to planting."

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