Expert tips for preplant control of volunteer corn

Meaghan Anderson
Iowa State University Extension

Due to extensive damage to corn fields last year from the derecho, volunteer corn is a looming threat for many fields this spring.

Farmers intending to plant corn in these derecho-affected fields have limited options for managing volunteer corn, thus thoughtful planning is critical.

I asked a collective of weed scientists their thoughts on best management practices for controlling volunteer corn prior to planting corn. I received responses from University colleagues in 8 states and Canada. The following is a summary of their thoughts and some resources that you may find helpful.

Tillage

Tillage is the first option many think of for preplant control of volunteer corn. Several respondents mentioned that this may be the best option for farmers, particularly those who have an effective in-season herbicide option to control any corn that emerges after planting. Tillage needs to be aggressive enough to dislodge plants, including their root systems, in order to prevent re-rooting. A challenge with tillage is that it can stimulate more germination events.

Herbicides for burndown of volunteer corn going to corn

Most advice centered around best choices for herbicides and managing these options most effectively. The traits of the corn planted in 2020 dictate what options are available. I am assuming the volunteer corn will be glyphosate-tolerant, thus eliminating the most effective herbicide option.

Keep in mind that burndown applications typically occur early in the season. Even though volunteer corn is likely to be present, temperatures at this time generally are not optimum for corn growth and thus may reduce the effectiveness of herbicide treatments.

Clethodim

Several clethodim products are labeled for killing corn prior to planting corn, but at least a 6-day waiting period is required between clethodim application and corn planting. Based on my survey, this appears to be widely-considered the most effective option to kill volunteer corn prior to corn planting. Clethodim should be more consistent on small volunteer corn plants than other options because it is a systemic herbicide.

Several potential drawbacks were noted to using clethodim for this purpose. Applicators should be aware that there is potential for crop injury in replant situations following clethodim application, but respondents noted the issue is rare.

Crop oil concentrate, the most effective adjuvant with clethodim, is not allowed for this use. Only one application per season is allowed, and antagonism can be a problem when tank-mixing clethodim with broadleaf herbicides. While we don’t know whether antagonism is a concern when mixed with soil-applied products, the most effective application of clethodim will probably be one that occurs with only the specific adjuvants advised by the labels (2.5 to 4 pound ammonium sulfate per acre and non-ionic surfactant at 0.25% v/v).

Paraquat + Triazine

Several respondents reported good results controlling corn with paraquat plus a triazine (atrazine or metribuzin) but that it could be more inconsistent than clethodim.

A benefit of this herbicide mixture would be that applications are approved for either preplant or preemergence use in corn and doesn’t need a delay between application and planting. A concern is the size of corn being treated and the density of corn. Since paraquat is a contact herbicide, applications are going to be more consistent when plants are larger, perhaps in the 5-12” range. When plants are very small, the growing point is below ground and a contact herbicide application can act like a frost that corn is able to recover from. In fields with high densities of corn, spray coverage could be an issue.

Glufosinate

If 2020 corn hybrids did not contain the glufosinate tolerance trait, glufosinate is an option. Like paraquat, glufosinate is a contact herbicide and is unlikely to kill small corn plants less than 3 to 4” in height. Glufosinate works best in warm, sunny conditions.

More than one respondent noted that regardless of product choice, fields may need multiple applications to account for inconsistency in control and prolonged emergence of volunteer corn. Perhaps the best advice when considering how to manage the large populations of volunteer corn and still plant corn was one from respondent: “The best answer is plant soybean, since they are over $13.”

The take home message from all this advice, is that we have limited options available this spring to control volunteer corn both before and after corn planting, so take time now to make some plans to assure you choose the control method that works best for your operation.

Volunteer corn is a looming threat for many farmers, especially those in Iowa after 2020’s derecho. Tillage and a variety of herbicides may help some farmers fight again this unwanted plant.