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, to the knowledge of Jeff Gunsolus, University of Minnesota Extension specialist, data on corn yield loss potential as a function of volunteer soybean density is not available. However, there is one North Dakota State University study conducted by Richard Zollinger that evaluates several herbicide options to control volunteer soybean in corn. You can find a general summary titled Control of Volunteer Roundup Ready Crops at http://z.umn.edu/ndweedguide andclick on Weed Control Ratings.
Overall, soybean size at time of herbicide application will determine the degree of control with small, V2-V3 soybeans more effectively controlled than soybeans in the V4-V6 growth stage. Application to small plants is especially important if you want to keep your herbicide application rates low.
Due to its low cost, several growers have been interested in using 2,4-D to control volunteer soybean, however, based on Zollinger's work and personal experience, soybeans are not as sensitive to 2,4-D as they are to the other plant growth regulator herbicides such as dicamba or clopyralid. Another low-cost alternative being discussed by producers is atrazine and would be a better option than 2,4-D due to it's crop safety and level of soybean control but is less effective as soybeans enter the V4-V6 growth stage.
Plant growth regulator herbicides, such as Hornet, contain clopyralid and should provide effective control to smaller soybeans. Products that contain dicamba – such as Status, Distinct and numerous generics – should provide effective control over a wider range of volunteer soybean crop sizes.