We can debate whether or not volunteer corn is truly a “weed,” but it is definitely a problem for some farmers (Figure 1). According to research conducted in South Dakota, soybean yield loss was 8 to 9% when volunteer corn density was about one plant per ten square feet. Yield loss increased to 71% at volunteer corn densities of about one plant per square foot. Conversely, other scientists concluded that corn grain yield is not reduced by volunteer corn – so long as the volunteer corn was harvested along with the hybrid corn. However, the authors also noted negative impacts such as harvest inefficiency, disease occurrences, and poor stewardship of insect-resistant traits.
One of the factors that makes volunteer corn management difficult is the prevalence of glyphosate- and/or glufosinate-resistant varieties and hybrids. In addition, tank mixes with dicamba or 2,4-D to control broadleaf weeds may reduce the effectiveness of glyphosate and Group 2 herbicides like clethodim (Select Max, others) or quizalofop (Assure II, others). However, there are some steps farmers can take early in the growing season to manage volunteer corn.
Figure 1. Volunteer corn emerging with soybeans. Photo by Sarah Lancaster, K-State Research and Extension.
As mentioned above, glyphosate will not control glyphosate-resistant volunteer corn. However, paraquat (Gramoxone, others) will control volunteer corn that has emerged prior to soybean planting. Glufosinate (Liberty, others) will also control volunteer corn -- as long as the corn is not glufosinate-resistant (LibertyLink).
One thing to remember with burndown herbicide applications is that they must come in contact with the growing point to ensure the corn plant will not regrow, which means contact herbicides will be ineffective if applied to volunteer corn smaller than V6. In some cases, tillage may be the most effective option to avoid regrowth.
At planting options - soybeans
In research conducted at the University of Nebraska, pre-emergence applications of sulfentrazone in combination with imazethapyr, cloransulam, metribuzin, or chlorimuron (Authority Assist, Authority First, Authority MTZ, or Authority XL) reduced volunteer corn growth compared to non-treated controls. Other treatments, including flumioxazin (Valor, others) alone or in combination with chlorimuron (Valor XLT) or cloransulam (FirstRate), or fomesafen + metolachlor (Prefix) or saflufenacil + imazethapyr (Optill) did not reduce volunteer corn growth. There are no residual herbicide options to control volunteer corn at the time of field corn planting.
Group 2 herbicides (Select Max, Assure II, Fusilade, Poast, and others) are typically very effective over-the-top options for volunteer corn control in soybean. However, research from Indiana and Canada suggests that volunteer corn control by clethodim formulations without “fully loaded” surfactants can be reduced up to about 60% when applied with glyphosate or glyphosate plus 2,4-D and up to about 75% when applied with glyphosate plus dicamba. The reduction in control can be minimized by increasing the rate of the Group 2 herbicide to the maximum labeled rate or by using a more aggressive adjuvant. Research from North Dakota suggests that adding a high surfactant oil concentrate (HSOC) can improve volunteer corn control by tank mixtures of clethodim plus glyphosate, but neither NIS nor AMS improves control.
One potential option to control volunteer corn in emerged corn is to use an Enlist® corn hybrid. Enlist corn hybrids can be sprayed with Assure II herbicide, which would control glyphosate and/or glufosinate-resistant volunteers. However, there are very few varieties that are well-suited for Kansas.
For more detailed information, the “2023 Chemical Weed Control for Field Crops, Pastures, and Noncropland” guide is available online at https://www.bookstore.ksre.ksu.edu/pubs/CHEMWEEDGUIDE.pdf or check with your local K-State Research and Extension office for a paper copy.
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Sarah Lancaster, Weed Management Specialist