Changes in Dicamba Use are Ahead
Authors: Mueller Thomas C. , Steckel Lawrence E. 
Published: May 2022

The first steps in the development of dicamba tolerant transgenic plants were conducted by Sandoz Crop Protection in Palo Alto, CA and then contractually followed up at the University of Nebraska. The subsequent development by Monsanto of soybean and cotton varieties that would tolerate
post-emergent application of dicamba substantially changed the use patterns of dicamba in the United States. Transgenic dicamba-tolerant (DT) seeds were first approved in 2016 in the United States, although the post-emergent use of dicamba was not legal that year. In 2017 and 2018, there was
substantial market penetration of DT soybean and cotton seeds into the market and the occurrence of dicamba off-target movement (OTM) was highly variable across the United States. The driving force behind these new seed traits was the widespread failure of glyphosate to control broadleaf weeds
effectively, especially those from the Conyza and Amaranthus genera. Herbicide research and development in the United States has historically involved both industry and academic weed scientists usually operating in a symbiotic and mutually respectful relationship, although there may have been
disagreements at times about some aspects of herbicides and their development. The relatively recent introduction of DT varieties and legal post-emergent dicamba in the United States was a dynamic time for weed control, and adoption of DT crops and subsequent OTM of dicamba greatly changed
the working relationships between academic scientists and representatives from Monsanto and some other private companies. Perhaps the greatest change was the lack of access of research materials that would be available to academic scientists for evaluation prior to the retail sale of those
materials. Historically, academic scientists would have the ability to evaluate various new technologies and provide objective, independent comments on their potential utility prior to commercialisation. Monsanto largely restricted access to the DT seeds or new herbicide formulations. There
are some states that have long-established policies that they will not recommend a new herbicide technology unless they have examined it under their specific field conditions. For example, University of Arkansas researchers would not recommend the use of post-emergent dicamba on DT crops when
it first became legal to use. Monsanto responded by filing legal challenges of various types against the University of Arkansas faculty, including 64 exhibits of various legal aspects. Many other states had varying degrees of restrictions placed upon their research efforts, and most scientists
had to sign various forms of confidentiality agreements to obtain access to the research material from Monsanto. There were two major differences in the dicamba labels when first introduced in 2017 and then again two years later. The first major difference was an entire section on herbicide
resistance confirmation validation and management that was clearly stated on the label. The authors of this paper remind the readers that the reason we have dicamba being used is because of the wholesale failure of glyphosate resistance management systems. The golden era of weed control, essentially
100% weed control with no crop injury, was lost due to the complete lack of any resistance management strategies. The US EPA added label language hopefully to avoid evolution of resistance in weeds from happening again with essentially any new herbicide. Many herbicide users expect all new
labels to include resistance management language. The second obvious change to the label was the duration of validity, which was normally many years in previous pesticide registrations. The first dicamba label, which came out in 2017, was valid for only two years. The next label which came
out in 2019, was only valid for five years, although there is widespread speculation on the future of that label.