There have been several reports last week of multiple glyphosate and/or clethodim applications not controlling barnyardgrass or junglerice. We really do not know the precise reasons for the lack of control in any given field but based on previous research and past experience there are a couple likely causes.
The first is an increasing prevalence of glyphosate resistance in those grass species. Surveys conducted in 2018 and 2019 estimated that about 15% of our junglerice populations was resistant to that herbicide. However, that snapshot is 3 years old. It is very likely that the glyphosate-resistant junglerice populations have increased in infestation levels in many fields over the past 3 years.
Second, and likely the most prevalent reason for lack of grass control, is the frequent use of dicamba. There is a good bit of published research, including recent studies conducted in west Tennessee on our junglerice, that clearly shows that dicamba reduces the effectiveness of glyphosate and/or clethodim. Please keep in mind dicamba will not only antagonize the grass control when in tankmix with glyphosate or clethodim but can also antagonize grass control for several days after a dicamba application. As such glyphosate and/or clethodim grass control can still be hindered by dicamba applied 3 to 5 days prior.
The dicamba antagonism of glyphosate is also rate dependent. Research by our former graduate student Clay Perkins showed that for each 1/4 lb increase in dicamba rate, a subsequent 10% reduction in junglerice control was found.
All our research would indicate the best management practices to gain good control of barnyardgrass or junglerice is to first use a good crop oil surfactant with any clethodim product, particularly when grass is under drought stress. Second, do not plan on getting 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 ozs of a 2 lb clethodim product and plan to increase the rate from there. The rates some say they are using are too low to work with any consistency. Finally, follow the Engenia, Tavium and XtendiMax labels of not using more than 0.5 lb of dicamba/acre. Higher rates can lead to less grass control when tank mixed with glyphosate or clethodim, though it is best to not tankmix these products when a target is either barnyardgrass or junglerice.