Good Management Can Control Herbicide-Resistant Weeds
ALEXANDRIA, LA.
Louisiana grain farmers appear to be holding
off the invasion of herbicide-resistant
weeds, with only pockets of resistance in
the state, according to Daniel Stephenson, LSU
AgCenter weed scientist at the Dean Lee Research
Station.
“Overall, from a weed management standpoint,
it has been real quiet this year,” Stephenson
said. “I’ve been well pleased with the efforts
of the growers in this state.”
He said farmers are doing a good job of using
pre-emerge herbicides to stop the problem
weeds before they get a foothold.
Glyphosate-resistant Palmer amaranth, first
confirmed in Louisiana in 2009, has spread
south in Louisiana with confirmation this year
in soybean fields in Avoyelles and Catahoula
parishes, Stephenson said. “It is continuing to
spread and can be found in numerous parishes
in the state.”
Stephenson said he is concerned about herbicide-
resistant weeds spreading rapidly in fields
that are farmed inside the levees of the Mississippi
River and the Atchafalaya Basin because
the seeds are buoyant and can be carried by
water.
Corn fields may be harboring Palmer plants
that will only be found by harvesters, making it
essential that farmers thoroughly clean their
equipment, he said.
If farmers encounter heavy infestations of
Palmer amaranth or johnsongrass, they would
be well advised to drive their harvesters around
the affected area of a field. “To lose a 50-foot-by-
50-foot area of soybeans is a lot better than having
to fight this monster next year because the
harvester will just spread the problem,” he said.
Killing frosts will take care of weeds in the fall
and early winter in central and north Louisiana,
but south Louisiana will remain vulnerable,
Stephenson said. It’s possible that a weed could
go through two life cycles and generate twice as
many seeds. Even with a killing frost expected,
growers should still maintain their fields weedfree
until the first frost occurs.
Farmers should take precautions to make
sure their fields are clean before planting next
year, Stephenson said. And they should consider
using a residual herbicide at planting, followed
by an early post-emergence herbicide
such as Dual Magnum or Zidua. Either can be
mixed with glyphosate.
Data have shown that yields will be maximized
by keeping soybean fields weed-free for the first
five weeks after emergence, he said.
Herbicide-resistant johnsongrass has been
confirmed in Rapides, Avoyelles and Pointe
Coupee parishes, according to Stephenson.
Liberty herbicide is effective on glyphosate-resistant
johnsongrass, although farmers have
not widely adopted Liberty Link soybeans.
Graminicide chemistry in products such as
Select Max, Fusilade and Assure II offer alternatives
to glyphosate for fighting johnsongrass
in both Roundup Ready and Liberty Link soybean,
Stephenson said. However, using only a
graminicide to control johnsongrass could lead
to failures because of resistance development.
Soybean fields where glyphosate-resistant
johnsongrass is suspected should be planted in
corn the following year because there are good
options for control, he said. Liberty Link soybeans
are another option the next year, although
two to three applications of Liberty will
be needed.
Suspected glyphosate-resistant Italian ryegrass
is a problem for Louisiana corn farmers,
he said, and yields can be reduced by as much
as 70 percent. Residual herbicides, such as
Dual Magnum applied in the fall followed by
paraquat in the spring, is an effective program
for controlling ryegrass, but growers should
contact LSU AgCenter weed scientists for more
specifics.
Stephenson said he urges consultants and
farmers to let LSU AgCenter weed scientists
know about problem weeds involving suspected
resistance. “We won’t publicly name the farmers,
but we need to let their neighbors know.” Δ