Strategies Can Prevent Herbicide-Resistant Weeds
UNIVERSITY PARK (Centre
Co.) By taking some precau
tions, you can keep resistant
weeds from sabotaging your her
bicide program, according to a
Penn State weed scientist.
“Weed resistance has been a
problem with several classes of
herbicides,” said Dr. William Cur
ran, assistant professor of weed
science in Penn State’s College of
Agricultu'al Sciences. “In the
northeastern United States, tria
zine resistance is by far the most
serious of these.”
Since the 19605, more than SO
species of weeds throughout the
world have developed biotypes
that are resistant to different herbi
cide families. The Northeast has at
least four weed species that
require additional time or resour
ces to manage because they’re
triazine-resistant.
Herbicide-resistance has been
caused by monoculture cropping
patterns such as growing continu
ous com; using herbicides that
persist in soil; using less tillage
and thus reducing nonchemical
weed-control options; and spread
ing resistant weed seeds along
with cattle manure.
“When choosing an herbicide
program, base your decision
mainly on anticipated weed prob
lems and potential herbicide resis
tance,” said Curran. “Also consid
er crop and herbicide rotations,
potential herbicide injury to crops,
your tillage system, your herbi
cide application equipment, soil
characteristics, potential environ
mental hazards and cost.”
Crop management strategies
you will get higher production with lower cost and less maintenance.
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that can help deter herbicide resis
tant weeds include:
• Use herbicides only when
necessary.
• Rotate herbicides with diffe
rent sites of action in weeds, such
as amino acids, enzymes or pro
teins. Do not make more than two
consecutive applications of herbi
cides with the same site of action
to the same field unless you
include other weed-control prac
tices. Two consecutive applica
tions means single annual applica
tions for two years or two split
applications in one year.
• Apply herbicides in tank
mixed, prepackaged or sequential
mixtures that include multiple
sites of action. The herbicides
must be effective against poten
tially resistant weeds for this
strategy to work.
• Rotate crops, particularly
those with different life cycles,
such as winter wheat, barley and
other winter annuals; alfalfa and
other perennials; and com, soy
beans and other summer annuals.
Do not use herbicides with the
same site of action against the
same weed in these different crops
unless other effective control
practices are also included.
• When growing new
herbicide-resistant crop varieties,
do not use more than two consecu
tive applications of herbicides
with the same site of action
against the same weed unless
other weed control practices are
included.
• When feasible, combine her
bicide use with mechanical weed
control.
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• Where there is little potential
for soil erosion, include primary
tillage as a component of your
weed management program.
• Scout fields regularly and
identify the weeds present.
Respond quickly to changes in
weed populations to restrict the
spread of weeds that may have
developed resistance.
• Clean tillage and harvest
equipment before moving it from
Table 1. Herbicide modes and sites of action and representative examples within each
group.
Growth regulators
Amino acid synthesis
Lipid inhibitors
Seedling shoot inhibitors
Photosynthesis inhibitors
Cell membrane disrupters
iment inhibitors
* Herbicides with multiple sites of action are less likely to select for herbicide resistant
weeds.
swine
feeding
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fields infested with resistant
weeds.
• Encourage local organizations
such as railroads, public utility
companies and highway depart
ments to use vegetation manage
ment systems that do not lead to
selection of resistant weeds. If
these organizations use total veg
etation control (TVC), resistant
weeds from TVC areas can spread
multiple sites *
ALS synthase
EPSP synthase
Acetyl-CoA carboxylase
(ACCase)
multiple sites *
D-l quinone protein
Photosystem I
Protoporhyrinogen oxidase
rnthesis
Carotenoid bi<
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cage
systems
to cropland.
For more information on weed
control and herbicides, consult the
“1993-94 Penn State Agronomy
Guide,” available for $6 from the
Penn State Cooperative Extension
office in your county or from the
Publications Distribution Center,
112 Agricultural Administration
Building, University Park, PA
16802, (814) 865-6713.
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