Grass Weed Control Options When PREs Fail In Grain Sorghum

DR. LARRY STECKEL

JACKSON, TENN.
   Over the last several years activation of PRE applied herbicides with timely rainfall has been more the rule than the exception in Tennessee.  That is not the case for most of the state this year. The most common call in this regard the last several days have been on grain sorghum fields where the crop and the weeds have emerged together due to lack of an activating rain on the PRE applied herbicides.
   We have good POST applied herbicide options for broadleaf weeds like Palmer amaranth in grain sorghum. Herbicides like dicamba, 2,4-D, Huskie and atrazine applied in some combination will control even large Palmer amaranth.
   Unfortunately, the options to control grass weeds in grain sorghum is very limited. Atrazine at 2 lbs/A plus a qt/A of COC can fairly consistently control small, less than 3 lf, annual grasses like crabgrass, signalgrass, etc. The problem is that atrazine applied in this fashion will injure grain sorghum if it is less than 3 leaf.
   The reason for this is there is an enzyme in grass plants called GST that detoxifies atrazine. Corn has a lot of it at emergence which is why atrazine can be applied to it at spiking. However, most other grass species do not build up much GST until they reach the 3 leaf stage. That is the reason the atrazine label specifies that grain sorghum must have at least 3 leaves before atrazine can be applied POST and why most annual grass weeds are immune to atrazine applications after they reach 3 leaves.
   The only other option to control annual grass weeds in grain sorghum is Facet L. The common name for Facet is quinclorac, and you may know it by its former name Paramount. Facet L can be applied to grain sorghum from spiking until 12” tall.  The rate is 22 to 32 oz/A and it should be applied with a qt/A of COC. After the grain sorghum reaches 3 lf then atrazine can be tankmixed with it.  The typical atrazine rate tankmixed with Facet L is one qt/A. My understanding is that Facet L is in fairly limited supply so it may be difficult to get. ∆
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee