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Clean Soybean Fields Could Protect Future of Bt Corn Hybrids

Editor’s note: This article provided by Gowen Company. YUMA, ARIZONA, (AgPR), July 1, 2009 -- For many soybean growers, little is more aggravating than watching volunteer corn plants pop up above the soybean canopy. However, receint research indicates that volunteer Bt corn can be much more than an eyesore; it may post a serious threat to the long-term viability of insect-resistant corn hybrids.

According to a 2009 Purdue University research report titled “Volunteer Corn Presents New Challenges for Insect Resistance Management” (Krupke, Johnson, et al), the prevalence of volunteer corn in Indiana soybean fields increased from a mean of 3 percent of all fields sampled in 2003 to 12 percent of fields sampled in 2005. The study found this four-fold increase strongly correlated to the increased adoption of glyphosate-resistant corn hybrids during that period.

Purdue University entomologist Dr. Christian Krupke notes that this trend continued during the 2006 and 2007 growing seasons. “What we found was that in areas where triple-stacked corn hybrids were planted in 2006 followed by soybeans in 2007, we had a great deal of volunteer corn in some of those fields,” Krupke says. “Most of the volunteer corn showed up as being both Roundup Ready and having the Bt gene for rootworm.”

Volunteer Corn Allows for Rootworm Survival

Since soybean roots alone do not support corn rootworm larvae, a typical corn-soybean rotation program has typically helped to keep rootworm populations in check. However, the presence of volunteer corn plants in soybean fields allow some larvae produced from overwintering eggs to survive.

Entomologists like Dr. Krupke worry that a relatively high percentage of volunteer Bt corn plants do not express enough of the Bt toxins to effectively control root-feeding larvae. As a result, surviving larvae could accelerate the evolution of Bt-resistant rootworm populations.

Early-season Weed Control Is Imperative

While volunteer corn’s potential to rob soybeans of water, nutrients, sunlight and yield potential has been documented for years, the threat of insect resistance heightens the need for effective, early-season volunteer corn control.

Growers rotating from Roundup Ready corn to Roundup Ready soybeans need to find another herbicide besides glyphosate to control glyphosate-resistant volunteer corn. “The goal of producers should be to control as much volunteer corn as possible as early in the season as possible to protect soybean yield,” states Dr. Michael Owen, professor of agronomy at Iowa State University. “The best management practice involves tank-mixing a graminicide with glyphosate in the first post-emergence treatment to the field.”

Nick Vandervort, Midwest field development manager for Gowan Company, echoes Dr. Owen’s thoughts. “Growers must control this volunteer Bt corn early in the season. If growers wait to control them until the second glyphosate pass is made, they’ve given the insects much more exposure time to sub-lethal doses of the Bt proteins.”

Quizalofop-p-ethyl, the active ingredient in Targa post-emergence herbicide from Gowan Company, has been proven effective against volunteer corn for more than three decades. “Quizalofop-p-ethyl has been proven in our research to be a very effective molecule offering effective and consistent control of volunteer corn in soybeans,” Dr. Owen adds.

Along with controlling both glyphosate-tolerant and conventional volunteer corn in soybeans, Targa also controls perennial grasses such as Johnsongrass, quackgrass and bermudagrass. Additionally, it can be tank-mixed easily with glyphosate for one-pass control of most grasses and broadleaf weeds.

The future might hold a change in how soybean growers perceive the threat posed by volunteer corn. “The findings of Dr. Christian Krupke and Dr. Bill Johnson at Purdue move volunteer corn out of a predominantly weed competition and aesthetics realm and significantly increase its importance,” Vandervort says. “Our industry may be heading toward a ‘zero-tolerance” mindset regarding volunteer corn due to these new findings regarding insect resistance.”

Gowan Company is a family-owned provider of crop protection products for specialty and broad-acre markets of agriculture. Established in 1963, Gowan Company is dedicated to supporting agriculture through regulatory defense, market experience and product development.

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