How Much Herbicide Is Left?

JACKSON, TENN.
   The answer in most cases is very little to none. With most of Tennessee getting anywhere between 4 to 8” of rain over the last several days most herbicides are simply gone. The first to go would have been the very water soluble herbicides like atrazine, Cotoran, Direx, Sharpen and Prowl. Herbicides like Dual Magnum and Caparol which are a little less water soluble are probably all but gone. Herbicides like Callisto, Reflex and Valor will probably still provide some limited control. Callisto at this point may provide a week of residual control. We typically figure that Reflex and Valor will provide 4 to 5 weeks residual control. I would expect no more than 2 weeks now in areas the water got off very quickly. If water stood on them for 4 hours or so they are gone as well.
   The only herbicides that are likely to provide control even close to what we would typically expect would be the SUs. Herbicides like Classic or Resolve/Basis could still give some residual control. This could be an issue where Basis or Resolve were used on land that was intended for corn but will now go to soybeans. The recrop back to soybeans after a Resolve Q/Basis application is 30 days. Envive, a premix of Valor, Classic and Harmony will probably still have some activity from the Classic. However, most of our Palmer is ALS resistant so have eyes on the field to see if the Valor has washed away.
   Expect a huge flush of weeds now, particularly Palmer amaranth, with all this heat, water and loss of residual herbicides. I would manage fields like no herbicide has been applied. What I mean by that is, regardless of crop, plan to reapply a herbicide that can provide some residual control. Please keep in mind there are season use limits on some of these products. An example would be that by the label a field can have 2.5 qts of atrazine applied if split between two applications. Therefore a field that already has 1.5 qts of atrazine could have another 1 qt if a new flush of weeds shows up this next week. Δ
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee