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 three likely causes.
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 snapshot is 2 years old. It is very likely that the GR junglerice populations have increased in infestation level in many fields over the past 2 years.
Another possible contributor has been the dry conditions most experienced. Those grass species under drought stress could show less response to any herbicide.
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 in tankmix with glyphosate or clethodim but can also antagonize grass control for several days after application 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 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 ozs of a 2 lb clethodim product and plan to increase the rate from there. The rates I often hear quoted are 8 to 12 ozs and they are not working 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.