Impact Of Pigweed Size On Dicamba Activity
DR. REID J. SMEDA
AND DR. JASONWEIRICH
PORTAGEVILLE, MO.
Palmer amaranth has become one of the
most competitive and troublesome weeds in
broadleaf agronomic crops throughout the
Mid-south. Factors that contribute to difficulty
in managing Palmer amaranth include herbicide
resistance (dinitroanilines, acetolactate synthase
inhibitors, Protox inhibitors, glyphosate),
continual emergence throughout the growing
season, and rapid growth (short window for
proper timing with postemergence herbicides).
New technology on the horizon, specifically
dicamba- and 2,4-D-resistant broadleaf crops,
offers additional options for managing Palmer
amaranth. An experiment was established at the
Delta Research Center to determine the response
of glyphosate-resistant Palmer amaranth
at different growth stages to variable rates of
dicamba. Land was tilled in early May to stimulate
emergence of Palmer amaranth. In late May,
plastic cups were used to cover Palmer amaranth
plants at different stages of growth. The
experimental area was then treated with a mixture
of gramoxone, glyphosate, and fomesafen to
remove all uncovered vegetation. Using a randomized
complete block design with 5 replications,
10 by 45 foot plots were treated
approximately two weeks later with one of the
following rates of dicamba: 0, 0.25, 0.38, 0.5,
0.75, and 1 lb ae per acre. At the time of treatment,
a colored flag was placed adjacent to 6
Palmer amaranth plants at the following growth
stages: 2-4, 5-7, 8-10, and 11-13 inches.
Palmer amaranth was visually assessed for herbicide
injury at 2 weeks after treatment; a scale
of 0 (no control) to 100 (complete control) was
used on individual plants. For agronomically acceptable
control, a minimum rating of 90 is
needed. In general, 2-4 and 5-7 inch Palmer
amaranth exhibited more injury than larger
plants. To achieve 70 percent control or higher
on smaller plants, 0.75 or 1 lb/A of dicamba was
needed. For larger plants, the highest level of
control was 60 percent with 1 lb of dicamba. For
each Palmer amaranth growth stage, there was
a step-wise increase in control as dicamba rates
increased from 0.25 to 1 lb/A. The experiment
will be carried out for 5 weeks and plant control
and biomass estimated to determine the extent
of dicamba activity on Palmer amaranth. Δ
ASHLEY A. SCHLICHENMAYER: Graduate Student,
University of Missouri
DR. REID J. SMEDA: Associate Professor, University
of Missouri
DR. JASON WEIRICH: Extension Assistant Professor,
University of Missouri