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