By: Sarah Lancaster
We can debate whether or not volunteer corn is truly a “weed,” but it is definitely a problem for soybean 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.
One of the factors that makes volunteer corn management difficult in soybeans is that this corn is typically resistant to glyphosate and/or glufosinate. In addition, tank mixes with dicamba or 2,4-D 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 in soybean crops.
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.
At planting options
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 (Gangster), or fomesafen + metolachlor (Prefix) or saflufenacil + imazethapyr (Optill) did not reduce volunteer corn growth.
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 improve control.