Source: University of Minnesota Crop News
Last week Jeff Gunsolus, University of Minnesota received several inquires about the presence and potential impact of volunteer soybean in corn and cost-effective control procedures. The scenario of volunteer soybean in corn is a fairly recent phenomenon due to the wide-spread use of the glyphosate-resistant technology in corn and soybean. As a result, data on corn yield loss potential as a function of volunteer soybean density is not available. However, one NDSU study conducted by Dr. Richard Zollinger evaluates several herbicide options to control volunteer soybean in corn. You can find a general summary titled Control of Volunteer Roundup Ready Crops here and click on Weed Control Ratings.
Overall, soybean size at time of herbicide application will determine the degree of control with small, V2 to V3 soybeans more effectively controlled than soybeans in the V4 to V6 growth stage. Application to small plants is especially important if you want to keep your herbicide application rates low.