These soy and cotton growing regions have seen a huge increase in dicamba use over the last couple of years since dicamba-resistant varieties of these crops were introduced. An estimated 60 million acres of dicamba resistant soybeans and cotton were planted this year, up from just 2 million acres in 2016.
All that dicamba gives weeds that have already been exposed to this old-school herbicide for decades lots of opportunity to develop resistance.
Larry Steckel, the Tennessee Extension weed scientist often featured in the dicamba stories we have reported, notes that while there were a just a few signs of resistance last year, “in 2019 we’re seeing seeing some weed escapes everywhere. It looks just like it did back when Roundup was starting to fail.”
The big problem continues to be Palmer amaranth and waterhemp, both of which have already been documented with resistance to up to six different herbicides. But, as if this isn’t already enough to worry about, weed scientists are also seeing less efficacy on grassier weeds like goosegrass and junglerice. These types of weeds are often less affected by a mixture of two or more herbicides, like the glyphosate and dicamba combo being sold to farmers these days as Engenia and Roundup Xtendimax.
Cotton farmers have been hit especially hard since they are often plagued by these grass weeds. Steckel notes in the DTN piece that “Cotton guys’ herbicides expenses have doubled or tripled this year” as they are forced to use multiple different mixes of individual herbicides as well as the next-gen combo formulas to try and get a handle on their weed problem.
Emily Unglesbee; “Dicamba Weed Control Concerns;” DTN Progressive Farmer; Date published: 8/30/2019, Date accessed: 8/30/2019.