Rotate herbicides to help control triazine resistant weeds
CHICAGO A farmer applies
atrazine preemergence, comes
back with an early atrazine and
crop oil treatment, and still has
pigweed come through in his corn.
Chances are very good that he’s
dealing with a weed which has
developed a resistance to triazine
herbicides.
More than 15 weeds have been
identified by weed scientists
across the country as having
developed resistance to triazines,
according to Ron Ritter,
University of Maryland Extension
weed control specialist. The
triazine family includes atrazine,
Princep (simazine), Bladex
(cyanazine), and the assymetrical
triazines Sencor and Lexone
(metribusin). When a weed
develops resistance to one of these
compounds, it’s apparently
resistant to all of the triazines,
Ritter says.
Ritter points out that problems
so far are not considered
widespread. Pigweed and lamb
squarters are the two weeds most
often mvolved so far in the Nor
theast, he says, and work in 1983
suggests that velvetleaf may also
be developing resistance.
Washington state nurserymen
were the first to suspect that weeds
were gaining resistance to
triazines, Ritter says. Nurserymen
were continually using Princep for
controlling weeds in their conifer
plantations, and eventually, the
broadleaf weed groundsel was not
being controlled by the herbicide.
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It was in the mid to latter 1970’s
that pockets of triazine resistant
weeds became recognized in the
Northeast, Ritter says. Several
factors are involved in their
development and spread.
“We’ve found that resistance
develops primarily in no-till areas
where growers are not using crop
rotations and are continually using
triazine herbicides,” Ritter says.
“The growers often are not con
trolling their escapes with
cultivation or postemergence
herbicides.”
Ritter says the farms are usually
dairy operations where com is
continually grown and manure is
spread on the land. Resistant
weeds often first appear in end
rows and along the edges of a field,
the specialist says, and from there
are spread to other areas of the
field and the farm by harvest
equipment and through spreading
manure.
“It’s difficult to verify the
existence of triazine resistant
weeds on your farm,” Ritter says.
“However, if the triazine her
bicides are giving poor weed
control you should be aware that
the potential exists.”
Ritter feels that the triazine
resistance situation demonstrates
the need for rotating both crops
and herbicides. Herbicides from
the same chemical family should
not be used year after year, and
escape weeds should be controlled
with cultiviation and/or
PLACE
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In end rows
(Date)
postemergence herbicides,
How to handle
Once there is a problem with a
resistant weed, however, it can be
handled. Ritter says both Banvel
herbicide and 2,4-D will control
pigweed or lambsquarters and are
usually the most costeffective.
“Our success in handling these
problems has been to come back
with Banvel,” Ritter says.
“Farmers are looking for an
economical herbicide program,
particularly this year, and one
option is to use an inexpensive
ST. LOUIS, Mo. Soybean oil
continues to be a good and legal
choice for use as a pesticide ad
juvant, according to Larry
Beauregard, Associate Director of
Soy Oil Programs for the
American Soybean Association.
“A recent ruling by the En
vironment Protection Agency
(EPA) which restricts the use of
non-labeled materials as pesticide
diluents has caused some con
fusion,” says Beauregard. “But
this ruling doesn’t apply to
vegetable oils used as crop ad
juvants.”
He explains that adjuvants and
diluents are not used the same way
in pesticides
“An adjuvant, often referred to
as a crop oil concentrate, is a
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triazine preemergence and then
follow with Banvel.”
Banvet is labeled at a full pint
per acre rale as an overlay to a
grass herbicide when com is from
spike to five inches tall. Or it can
be used at 1/2 pint per acre after
com exceeds five inches in height.
Triazine resistance is
genetically passed on from
resistant plants through seed,
Ritter says. While the resistance
concept has been proven in the
triazines, Ritter finds it interesting
that genetic resistance is not
Soybean oil is crop adjuvant
material added to a pesticide
mixture to improve the pesticide’s
performance. It modifies
characteristics such as
spreadability or droplet size.
“A diluent, often referred to as a
carrier, is a material used to dilute
a concentrated pesticide to field
use strength.”
The EPA policy ruled that
diluents must be labeled for a
specific pesticide to be used
legally. If a pesticide does not
specify a diluent, water must be
used. The policy clearly states that
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showing up with some other her
bicide groups.
“The oldest herbicide on the
market which has been so widely
used is 2,4-D,” he says, “but we see
little evidence of resistance
problems with it.” However,
scientists are also now working on
problems with some herbicide
groups being broken down rapidly
in the soil by microbial action.
Herbicide rotation - not using
compounds from the same
chemical family year after year -
may be the best prevention at the
present time. v
the ruling does not apply to ad
ditives, adjuvants or surfactants.
Beauregard says that if fanners
understand the policy’s restric
tions, there should be only limited
effect on soybean oil’s use as a
crop spray, since most farmers use
soybean oil as a crop oil or ad
juvant.
Beauregard predicts that in
response to the EPA ruling,
manufacturers of pesticides for
ULV applications will apply for
labeling to legally clear soybeans
oil’s use as a diluent.
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