Volunteer soybeans in corn is a concern for some growers this season. In some cases we saw shattering during soybean harvest last fall and those seeds are germinating in the newly planted corn this spring. This really makes that soybean a weed in that corn field, and because in most cases it was glyphosate resistant soybeans the glyphosate treatments this fall were not effective on the soybean seedlings. This is a similar situation when we see volunteer corn in soybeans except we need to look a different group of herbicide chemistries to solve the problem.
Like most weed herbicide applications, 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 for best results.
Using 2,4-D to control volunteer soybeans is an option. Work done at SDSU shows that soybeans are not as sensitive to 2,4-D as they are to the other plant growth regulator herbicides such as dicamba or clopyralid.
The 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 would provide effective control over a wider range of volunteer soybean growth stages.
source: igrow.org