While historically it has not been a concern for Kentucky grain crops producers, volunteer corn problems have increased in the past several years and could potentially reduce the benefit of crop rotation if left untreated.

University of Kentucky College of Agriculture professors James Martin and Ric Bessin discussed potential problems caused by volunteer corn in soybean and corn fields and control methods during the recent UK IPM Training School.

Volunteer corn is a weed that grows from grain left in fields after a harvest. It competes with newly planted crops for nutrients, which can result in yield losses in those crops. It also attracts corn rootworms, which are the most common group of insect pests in corn throughout the Corn Belt.

The corn rootworm is typically not as big of a concern in fields that annually rotate between corn and soybeans than in fields that grow continuous corn. This is because soybeans normally starve out rootworms that hatch in the spring.  However, volunteer corn left in soybean fields allows rootworm larvae to complete their feeding cycle. Excessive volunteer corn can result in building higher pest populations for the next growing season. The increased pest populations can result in significant yield losses in fields that rotate to corn the following year.

"Corn rootworm is considered to be a billion-dollar pest, meaning it costs farmers a billion dollars in yield losses and control costs," said Bessin, an entomologist.

In addition to being a food source for rootworm larvae, if volunteer corn is not controlled by tasseling, it attracts corn rootworm beetles for egg laying. The beetles lay their eggs in the soil around the base of volunteer corn. The eggs will remain dormant until they hatch next spring.

The best method to control volunteer corn and the corn rootworms it attracts is to kill the volunteer corn early in the growing season.

Controlling volunteer corn will depend on a number of factors, said Martin, a weed scientist. Preplant tillage will control volunteer corn if the plants are emerged. Since most growers use no-till, herbicides are the main strategy for controlling volunteer plants.

If the volunteer corn plants do not have the glyphosate-resistant trait, then glyphosate will be a good option, either as a burndown treatment before planting or as an in-crop application in fields with glyphosate-resistant corn or soybeans. 

If volunteer plants are glyphosate resistant, then the strategy may be more complex and will depend on whether soybean or corn is planted.

Several post-emergence grass herbicides will control emerged volunteer corn in soybeans, either before planting or after the crop has emerged. Many of these can be tank-mixed with glyphosate to broaden the spectrum of weeds controlled; however, consult the label for recommended additives. Growers should be timely in their applications in order to limit competition to soybeans. Delaying applications may lead to yield loss and take longer to control plants.

Managing glyphosate-resistant volunteer corn plants in corn will be a challenge. A burndown application of paraquat plus a photosystem II inhibitor herbicide such as atrazine or Linex, can control volunteer plants before planting corn. Select Max is another burndown option for managing emerged plants; however, growers must wait a minimum of six days before planting corn.

Additional details for managing volunteer corn are included in the UK extension publication AGR-6: Weed Control Recommendations for Kentucky Grain Crops.