Densities of volunteer corn in this year’s soybean crop  — at least in Illinois — are perhaps as high as any time in recent memory.

This is the result of the difficult harvest last year in the Midwest, which saw thousands of acres of corn left standing during the winter. The infestations of volunteer corn in soybeans show that many ears dropped to the ground in those unharvested fields.

One observer recently estimated a stand of volunteer corn at approximately 500,000 plants per acre! These very dense stands suggest that special consideration will be needed to control volunteer corn with post-emergence herbicides.

Determine 2009 Hybrids

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 the corn carries the traits conferring resistance to these herbicides.

Glyphosate-resistant corn hybrids have been planted throughout the Midwest, and the trait conferring resistance to glufosinate is found in many hybrids as well.

Also, certain “stacked” hybrids carry both traits. If volunteer corn in 2010 originated from a herbicide-resistant hybrid planted in 2009, alternatives to glyphosate and glufosinate will be needed.

Determine Density

The density of volunteer corn also determines whether additional management will be needed. Research has demonstrated that volunteer corn — whether growing in clumps or as individual plants — can reduce soybean yield. Generally, the higher the volunteer corn density and the longer the interference duration, the greater the soybean yield loss.

Herbicide Options

Several post-emergence herbicides provide excellent control of volunteer glyphosate-resistant or glufosinate-resistant corn. The ACCase-inhibiting herbicides (clethodim, quizalofop, fluazifop and sethoxydim) provide broad-spectrum control of grasses. They are frequently tank-mixed with glyphosate to control volunteer glyphosate- or glufosinate-resistant corn.

Labels of ACCase inhibitor herbicides sometimes caution about reduced effectiveness when applied in combination with certain other post-emergence broadleaf herbicides.

Spray additive recommendations for ACCase inhibitors can vary depending on how the product is used — alone or in a tankmix — and on the type of glyphosate formulation it is tank-mixed with.

For example, additive recommendations can vary depending on if a product is tank-mixed with a glyphosate formulation containing a “built-in” adjuvant system or that itself requires additional surfactant.

Other post-emergence herbicides that can control or suppress volunteer corn include glufosinate (Ignite), imazaquin (Scepter), and imazamox (Raptor). The activity of glufosinate on volunteer corn is often best when applications are made during periods of warm air temperatures and sunny conditions.

Instead of including a tank-mix partner with glyphosate during the initial post-emergence application, farmers sometimes opt to wait to see if additional volunteer corn emerges before treating it.

This approach is understandable, but it causes two big problems. The longer and larger volunteer corn grows with soybeans, the greater the likelihood of soybean yield-loss and the higher the rate of an ACCase-inhibiting herbicide that will be needed to control the volunteer corn.

(Editor’s note: This article first appeared in The Bulletin, the crop development and pest management newsletter of the University of Illinois. It was edited for clarity.)