Herbicide efficacy and crop rotation intervals for use in corn or soybeans

Why diversifying herbicide sites-of-action is key to herbicide long-term effectiveness

When weeds produce seeds, there is a chance through naturally occurring genetic mutations that some individuals may be better able to survive the label rate of a particular herbicide active ingredient, while individuals without that mutation do not survive. Herbicide-resistant individuals will consequently be the only ones in the emerged population exposed to the herbicide to survive to produce seeds of their own. Over time, if the same herbicide is repeatedly used without further diversifying the number of herbicide sites-of-action to which weeds are exposed, the field's population will shift from one that is primarily sensitive to (and able to be controlled by) the herbicide to one that is primarily herbicide-resistant.   

Tank mixing herbicides from multiple, still effective herbicide sites-of-action increases the chances that those individuals that are resistant to one site-of-action, will be controlled by the other sites-of-action and will not survive to to add to the farm's weed seed bank.

A valuable resource summarizing herbicide efficacy and crop rotation interval

Now that corn and soybean crops have either emerged or begun to emerge in northwest Minnesota, the number of active ingredients that can safely be used for weed control has shrunk considerably. In 2021, UMN and NDSU Extension weed scientists Drs. Tom Peters (UMN/NDSU), Joe Ikley (NDSU) and Debalin Sarangi (UMN) updated a set of tables in which they summarize the efficacy of herbicides used either PRE-emergence, early-POST-emergence or POST against common ragweed, giant ragweed, kochia, lambsquarters and waterhemp in glyphosate-tolerant soybean. They created a similar set of tables for PRE and POST options in glyphosate-tolerant corn. An additional set of tables lists the crop rotation intervals for use of the same herbicides in corn or soybean grown ahead of soybean, corn, pea, dry bean, potato, sugarbeet and wheat.  Keep these tables handy when making any last minute preparations for early-POST or POST herbicide applications.