Research was conducted from 2011 to 2014 to determine weed population
dynamics and frequency of glyphosate-resistant (GR) Palmer amaranth with
herbicide programs consisting of glyphosate, dicamba, and residual
herbicides in dicamba-tolerant cotton. Five treatments were maintained in
the same plots over the duration of the experiment: three sequential POST
applications of glyphosate with or without pendimethalin plus diuron PRE;
three sequential POST applications of glyphosate plus dicamba with and
without the PRE herbicides; and a POST application of glyphosate plus
dicamba plus acetochlor followed by one or two POST applications of
glyphosate plus dicamba without PRE herbicides. Additional treatments
included alternating years with three sequential POST applications of
glyphosate only and glyphosate plus dicamba POST with and without PRE
herbicides. The greatest population of Palmer amaranth was observed when
glyphosate was the only POST herbicide throughout the experiment. Although
diuron plus pendimethalin PRE in a program with only glyphosate POST
improved control during the first 2 yr, these herbicides were ineffective by
the final 2 yr on the basis of weed counts from soil cores. The lowest
population of Palmer amaranth was observed when glyphosate plus dicamba were
applied regardless of PRE herbicides or inclusion of acetochlor POST.
Frequency of GR Palmer amaranth was 8% or less when the experiment was
initiated. Frequency of GR Palmer amaranth varied by herbicide program
during 2012 but was similar among all herbicide programs in 2013 and 2014.
Similar frequency of GR Palmer amaranth across all treatments at the end of
the experiment most likely resulted from pollen movement from Palmer
amaranth treated with glyphosate only to any surviving female plants
regardless of PRE or POST treatment. These data suggest that GR Palmer
amaranth can be controlled by dicamba and that dicamba is an effective
alternative mode of action to glyphosate in fields where GR Palmer amaranth
exists.