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120 million reasons why you should care about herbicide-resistant weeds

If you
don’t think herbicide-resistant weeds will ever threaten your fields, Tennessee
farmers will tell you 120 million reasons why you should care.

Steckel, University of Tennessee Extension weeds specialist, pegged the impact
that glyphosate-resistant weeds have upon Tennessee soybean production at $120
million annually. He reported these calculations at presentations he made at
Bayer CropScience and BASF media events at this week’s Commodity Classic in
Nashville, Tennessee.

the breakdown:

Herbicide costs have increased
$72,000,000. That’s based on $45 per acre across 1.6 million soybean acres.

Application costs have increased
$8,000,000. That is figured using $5 per acre costs across 1.6 million soybean

Weed competition losses have
risen $40,000,000. That is figured using 30% of the affected soybean acres
having a 17% loss of yields of 35 bushel per acre soybeans using a $14 per
bushel soybean price.

Ugly, Ugly Weed

If it
was a crop, Palmer amaranth would be a farmer’s dream. This pigweed family
member thrives under stress. Initially originating in the desert Southwest,
Palmer amaranth has a huge taproot that can extend downward five feet. Its high
photosynthetic rate enables it to grow 2 to 3 inches daily in temperatures like
98-degree heat. Steckel tells of one Tennessee field where Palmer amaranth grew
five feet in 20 days.

postemergence applications of glyphosate set up Palmer amaranth resistance
first in cotton and then in soybeans in Mid-South states like Tennessee.

total postemergence era is over, and I don’t think it’s ever coming back,” says

you’re a Midwestern or Northern Plains farmer with no resistance problems,
you’re in a great position to head off problems. “Resistance management is more
proactive than reactive,” says Steckel. “If we look back 10 years in Tennessee,
using a combination of a preemergence product followed by a post product would
have delayed resistance and we would not have had this huge seed bank. We are
losing (acres) to this weed.”

Mid-South farmers are layering several burndown and preemergence residual
treatments followed by timely postemergence treatments to prevent Palmer
amaranth from getting out of control. One bright spot is preemergence
herbicides with a mode of action other than glyphosate can curb Palmer amaranth
if you control it at heights around 3 inches or so. Postemergence herbicides
other than glyphsoate—such as Flexstar, a PPO inhibitor—can then be applied.
Again, they must be applied when Palmer amaranth plants are small.

showed a slide showing a difference between Flexstar applied postemergence to
3-inch high Palmer amaranth vs. an application made two days later, when Palmer
amaranth had grown to a six-inch height.

Flexstar two days earlier made all the difference in the world,” he says. Weeds
choked out the field where Flexstar was sprayed at a 6-inch weed height.

notes Mid-South farmers are also rotating out of soybeans with corn to break up
the Palmer amaranth cycle. “There are a lot of good herbicides in corn that can
control it,” he says. “The problem is if corn is harvested in August, lots of
Palmer pigweed can grow after that and will produce a good seedbank.” Thus,
control measures must be in place after harvest, such as tillage or residual
products or mowing.


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