The Research Story: Dicamba Resistant Soybean Technology
The trait used to develop dicamba-resistant soybean was discovered at the University of Nebraska-Lincoln by a team of researchers led by Dr. Donald Weeks, and was licensed to Monsanto. The trait confers the ability to rapidly metabolize dicamba and was found in the soil bacterium Pseudomonas maltophilia (Behrens et al., 2007). Then through the process of genetic engineering, the trait was inserted into soybean, enabling soybean to breakdown dicamba to an inactive form that is not harmful to the crop.
Dicamba is a selective herbicide that controls broadleaf weeds, but not grasses. It is most often applied in combination with other herbicides, particularly if there is a need for both broadleaf and grass control. It is widely used in cereal grain crops, in pastures, and in turf environments like lawns and golf courses. Dicamba is primarily applied to control weeds that have already emerged from the soil, and may limit the growth of some weeds that germinate later. The two most common formulations of dicamba are Banvel and Clarity, but there are numerous other products that contain dicamba as an active ingredient, such as Status, Distinct, Weed-B-Gon and Trimec (2018 Guide to Weed Management in Nebraska).
How does a herbicide impose selection pressure on a weed population? If the herbicide application delivers a lethal dose to a weed before the weed has reproduced, the weed encounters the pressure. If the weed is capable of surviving the lethal dose, the weed was selected and it is rewarded by being given the chance to reproduce.
If a farmer with a glyphosate-resistant population of a broadleaf weed(s) only added dicamba to glyphosate as a strategy to manage weeds in a dicamba-resistant soybean field, then the selection pressure for a dicamba-resistant individual in that population would be high. Only individuals in the population that are not affected or minimally affected by dicamba would survive to produce seed. Over time, the number of individuals in the population (seeds in the soil) with a gene that allows them to withstand dicamba would increase. Eventually, dicamba would no longer be effective as a weed management tool in that field. This would be an example of evolution occurring rapidly because of human activities.