• By Larry Steckel and Clay Perkins •
There have been several reports last week of multiple glyphosate and/or clethodim applications not controlling barnyardgrass or jungle rice. We really do not know the precise reasons for the lack of control in any given field, but previous research and past experience suggest that there are three likely causes.
1. The first is an increasing prevalence of glyphosate-resistance (GR) in those grass species. A survey conducted in 2018 and 2019 estimated that about 15% of our junglerice populations was resistant to that herbicide. However, that snap shot is two years old. It is very likely that the GR jungle rice populations have increased in infestation levels in many fields over the past two years.
2. Another possible contributor has been the dry conditions most experienced. Those grass species under drought stress could show less response to any herbicide.
3. Finally, the frequent use of dicamba is reducing the effectiveness of glyphosate and/or clethodim on both those grasses. Please keep in mind dicamba will not only antagonize the grass control when tankmixed with glyphosate and clethodim but can also antagonize grass control for several days after application. As such glyphosate and/or clethodim grass control can still be hindered by dicamba applied three to five days prior.
The dicamba antagonism of glyphosate and/or clethodim is also rate dependent. Research by our former graduate student Clay Perkins showed that for each 1/4-pound increase in dicamba use rate resulted in a subsequent 10% reduction in jungle rice control from glyphosate.
All our research would indicate the best management practices to gain good control of barnyardgrass or jungle rice is to first use a good crop oil surfactant with any clethodim product particularly under drought stress.
Second, do not to plan to get good grass control with glyphosate tankmixed with dicamba. Instead, apply your intended grass control post application a good week after a dicamba application.
Also use at least 16 ounces of a 2-pound clethodim product and plan to increase the rate from there. The rates I often hear quoted are 8 to 12 ounces, and they are not working with any consistency.
Finally, follow the Engenia, Tavium and XtendiMax labels of using nor more than 0.5 pound of dicamba per acre. Higher rates can lead to less grass control with glyphosate or clethodim.
Dr. Larry Steckel is Extension weed specialist; Dr. Clay Perkins is a post-doctoral research associate. Both are with the University of Tennessee.