Challenges imposed by unfavorable environmental conditions during and after last fall's harvest continue to plague farmers. In many areas, fall herbicide applications were delayed or precluded after the 2009 harvest. As a result, fall-emerging weeds became well established before the onset of winter conditions.

Aaron Hager, University of Illinois Extension weed specialist, said, "It's important to control all existing vegetation prior to planting the 2010 crop. Last week's warm temperatures, coupled with adequate soil moisture, accelerated the growth of existing vegetation. Several winter annuals are flowering and early season summer annuals growth is robust."

The first step to controlling existing vegetation is to scout fields and determine what species are present and at what densities. Then a control plan can be set into motion.

Hager believes tillage before planting will be common this year. While tillage operations can control existing weeds, large clods can form if tillage is done when the soil is too wet. These clods can prevent good distribution of soil-residual herbicides that might be applied before or after planting.

"Tillage done while the soil is too wet may not provide complete control of existing weed vegetation, especially larger weeds," Hager says. "Large weeds that escape tillage are often damaged by the physical disturbance and can be difficult to control with a subsequent herbicide application. This becomes especially important if preplant tillage is used to control herbicide-resistant weeds, such as glyphosate-resistant horseweed."

If the best option to control existing vegetation is through herbicides, choose herbicides based on weeds present. Adequate control of glyphosate-resistant horseweed can only be achieved by not relying exclusively on glyphosate.

"Tankmix partners or alternative herbicides are needed to provide adequate burndown control of glyphosate-resistant horseweed," Hager says. "Products containing saflufenacil, 2,4-D, dicamba, glufosinate or paraquat are other herbicides that can be used to control horseweed prior to corn or soybean planting. Research shows improved control of emerged horseweed from applying two- or three-way herbicide tankmixtures as compared with single-herbicide burndown treatments."

Cool temperatures can slow the activity of many burndown herbicides, and translocated herbicides are often slower acting than contact herbicides under these conditions, Hager adds. When the forecast calls for several days or nights of cool air temperatures, symptoms of activity on existing vegetation may develop sooner with a contact herbicide compared with a translocated herbicide.

If both tillage and herbicides are used, Hager advises farmers to consider their situation when determining which method should be used first.

"If weeds are large before any management operation is implemented, spraying a burndown herbicide a few days before the tillage operation may work best," he says. "If aggressive preplant tillage is planned to alleviate field ruts, it's probably better to till before applying a soil-residual herbicide."

He recommends placing the soil-residual herbicide into the top inch of the soil profile. Deeper placement might improve control of certain species such as large-seeded broadleaf species, but it can also dilute the herbicide and reduce its effectiveness.

For more information, read The Bulletin online at http://ipm.illinois.edu/bulletin.