No volunteers needed here

Back in June 2019, farmers attending the University of Minnesota’s (U of M) Southern Research and Outreach Center field day discussed the year’s biggest weed problem with Tom Hoverstad, a U of M weed scientist.  

Waterhemp? Marestail? Velvetleaf? 

Nope, nope, and nope.

The answer instead was something that farmers plant each year. “Volunteer corn is the weed of the year,” says Hoverstad. “There is so much of it.”

The culprit driving 2019 volunteer corn infestations was downed corn, spurred by windy weather during the 2018 harvest. The following year, the resulting ears and kernels germinated in the subsequent crop.

Derecho = Downed Corn 

In 2020, this scene repeated itself with more ferocity on August 10. A derecho with triple-digit wind speeds ripped through Nebraska, Iowa, Illinois, Indiana, Wisconsin, and Michigan. The damaged stalks that dropped ears and kernels may lead to volunteer corn problems in 2021.

Yield consequences of volunteer corn are severe. A 2007-2008 South Dakota State University study found a density of 5,000 volunteer plants per acre (one plant every 3.5 feet of row) sliced soybean yields about 20%.  

Volunteer corn clumps, seven to 10 plants emerging from dropped ears, slice yields more than individual plants. University of Nebraska (NU) researchers found 3,500 clumps of corn per acre reduced soybean yields 40%. Meanwhile, the same population of individual plants reduced yields by 10%. 

Grain quality also suffers. “With soybeans, you’re going to potentially have corn comingling with the soybean grain,” says Joe Bruce, a technical services manager for AMVAC. 

Volunteer corn also has an insect twist. “Volunteer corn can attract corn rootworm beetles,” adds Bruce. Thus, following corn with soybeans – which still works well to deter corn rootworm absent extended diapause or the western corn rootworm variant – can backfire. Corn rootworm beetles can lay their eggs in volunteer corn. The next year when a farmer plants corn, eggs can hatch into root-chomping larvae. Thus, tools like rootworm-resistant traits or soil-applied insecticides may need to be used, says Bruce. 

Volunteer corn in a young corn field
Photo credit: Gil Gullickson

Volunteer Corn In Corn 

Things aren’t any better in corn. “If you have all this volunteer corn competing with the current corn crop, you will ultimately lose yield because your population is too high,” says Bruce. “It could also lead to more corn lodging as the crop competes with itself.”

NU researchers found that a volunteer corn population of 3,500 plants per acre – one-tenth of a normal 35,000 plants per acre planting population – sliced yields 2% in corn. Doubling the density to 7,000 plants per acre caused a 5% yield reduction. 

The NU researchers found that, as with soybeans,  clumps of volunteer corn clipped yields more than individual plants: 7,000 clumps of corn per acre sliced yields 14% compared with 5% for individual plants.

What to Do?

Volunteer corn is easier to control in soybeans than in corn. Volunteer corn soybean herbicides include: 

  • Assure II (quizalofop, Group 1) 
  • Fusilade DX (fluazifop, Group 1)
  • Select Max (clethodim, Group 1) 
  • Poast Plus (sethoxydim, Group 1) 

Liberty can control volunteer corn in LibertyLink soybeans, but only if the hybrid planted the preceding year was not LibertyLink, say Amit Jhala, University of Nebraska Extension weed management specialist, and Jenny Rees, University of Nebraska Extension educator.

Farmers and applicators also need to check herbicide labels for compatibility with adjuvants and herbicides. Some tank mixes restrict the use of certain adjuvants. 

“You can’t use crop oil as an adjuvant with Select Max and glyphosate,” says Hoverstad. 

Antagonism between herbicides also exists. For example, Fusilade antagonism exists with dicamba, says Dean Grossnickle, a Syngenta agronomic services representative. 

“You may have to increase your rate [of Fusilade] or add another surfactant when mixing it with dicamba,” he says. He says Syngenta trials he conducted showed increased volunteer control resulted when he added crop oil to the tank mix. 

Chemical control is more difficult in corn. Bruce says one way to do it is to plant Enlist corn in 2021 in fields not planted to Enlist corn in 2020. 

Besides tolerance to 2,4-D choline, corn in the Enlist weed control system also tolerates the “fop” herbicide Assure II – the only such herbicide labeled for volunteer corn control in Enlist corn, says Bruce. 

“If there’s any volunteer corn in that field that does not contain the Enlist gene, it will die,” says Bruce. 

Assure II doesn’t work for every herbicide-tolerant system, though. It can’t be applied to Roundup/LibertyLink corn, say Jhala and Rees. 

Liberty herbicide can also be applied to a Roundup Ready and LibertyLink hybrid, but only if emerging volunteers are from a Roundup Ready hybrid planted the previous year, say Jhala and Rees. Liberty won’t be effective if a Roundup Ready and LibertyLink hybrid was planted the previous year, they point out.

A separate herbicide application may be needed if volunteer corn overwhelms a field. “Even 90% to 95% volunteer corn control may not be enough in those cases,” says Grossnickle. “You can cultivate to take out volunteer corn between rows, but there still may be too much volunteer corn growing within the row.” 

In those cases, farmers may need to plant a different crop in 2021. 

“It will be catastrophic to do corn-on-corn if you have 70 to 80 bushels [per acre] of corn lying on the ground,” says John Long, a Brevant retail product agronomist.

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