• By Larry Steckel and Clay Perkins •
The results of some University of Tennessee Extension studies to determine why junglerice has become a major weed pest in Tennessee Xtend cotton and soybean acres have provided some insights. This research was in part supported by the Tennessee Soybean Promotion Board and Cotton Incorporated.
The first part of the research was a survey of weed escapes the past two falls in over 100 Xtend crop fields. We found that junglerice was found in better than 7 out of every 10 fields.
This was by far the most prevalent weed found in Xtend crops. Other weeds of note were Palmer amaranth (present in 50% of fields), barnyardgrass (present in 40% of fields), fall panicum (present in 11% of fields) and goosegrass (present in 3% of fields).
Reasons for escapes
The second part of the research tried to determine the reason for the junglerice escapes. Progeny from junglerice collected during the survey were screened for glyphosate resistance. In all, 13% of the junglerice populations could no longer be effectively controlled with even a high rate of glyphosate.
Since many have tried to supplement junglerice control with a tankmix of clethodim and glyphosate, we screened it as well. All sampled junglerice populations could still be controlled with clethodim.
The data would indicate that a reason for the junglerice escapes in Xtend cotton and soybean fields is in part due to an evolution of glyphosate resistance. This begs the question: “If none of the populations is resistant to clethodim and only 13% is glyphosate-resistant, then why are those two herbicides failing to control this weed so often?”
What happened in the field?
In order to answer this question, field studies evaluated non-glyphosate-resistant junglerice control comparing an application of dicamba plus glyphosate with glyphosate alone and dicamba plus clethodim with clethodim alone. In these studies, the addition of dicamba to glyphosate reduced junglerice control 30% compared with glyphosate alone. The same result also occurred where clethodim plus dicamba provided 30% less junglerice control than clethodim alone.
The lack of control does not end there. The research also showed that using the TTI spray tips to apply glyphosate or clethodim reduced junglerice control about 7% compared to using an AIXR spray tip.
In summary, this data would suggest that the overall reason for the poor junglerice control in about 70% of the Xtend fields surveyed the past two years is largely due to dicamba antagonism of glyphosate and clethodim, in part due to reduced coverage from the mandated TTI spray tips and in part due to glyphosate-resistance.
The take home message is first try to keep junglerice from ever coming up by using a good pre-applied grass herbicide. The second point is to avoid trying to control this weed post with a dicamba tankmix.
Dr. Larry Steckel is a University of Tennessee Extension weed specialist. He may be reached at firstname.lastname@example.org. Clay Perkins is a graduate research assistant.