Reminders of 2009's challenging corn harvest are cropping up in soybean fields across Illinois. High densities of volunteer corn are requiring growers to take action with post-emergence herbicides.

Aaron Hager, University of Illinois Extension weed specialist, says the first step in selecting a herbicide to control volunteer corn is to determine the type of corn planted in 2009.

"Volunteer corn can be controlled with glyphosate or glufosinate unless it carries the traits conferring resistance to these herbicides," Hager says. "If volunteer corn in 2010 originated from a herbicide-resistant hybrid planted in 2009, alternatives to glyphosate or glufosinate will be needed."

Volunteer corn density also determines if additional management options are needed. Previous research shows that volunteer corn, growing in clumps or as individual plants, can reduce soybean yield. The higher the volunteer corn density and the longer the interference duration, the greater the soybean yield loss.

Hager says several post-emergence herbicides provide excellent control of volunteer glyphosate- or glufosinate-resistant corn. ACCase-inhibiting herbicides (such as clethodim, quizalofop, fluazifop and sethoxydim) provide broad-spectrum control of grasses and are frequently tankmixed with glyphosate for control of volunteer glyphosate- or glufosinate-resistant corn. Other post-emergence herbicides that can control or suppress volunteer corn include glufosinate (Ignite), imazaquin (Scepter) or imazamox (Raptor).

"Instead of including a tank-mix partner with glyphosate during the initial post-emergence application, farmers often wait to see if additional volunteer corn emerges before treating," Hager said. "While it's understandable, the longer volunteer corn grows with soybean, the greater the likelihood of soybean yield loss. Growers will also have to use higher rates of ACCase-inhibiting herbicides to control larger volunteer corn."

For more information, check out The Bulletin, an online publication written by U of I Extension specialists in crop science.