Halt resistant weeds
Herbicides are a cornerstone
of weed control. Sometimes, though, they’re too good. In some cases, repeated
use of a successful herbicide selects for resistant weed biotypes.
There are ways to prevent or
forestall this from happening, says Jason Norsworthy, University of Arkansas
weed scientist. These eight steps can help reduce problems with
herbicide-resistant or herbicide-tolerant weed biotypes.
1. Know Your Weeds.
Properly identifying weed
species will help pinpoint which herbicide program works best for each acre. Be
sure to closely monitor problem areas with difficult-to-control weeds or dense
2. Rotate For Diversity.
Rotating crops and herbicide
modes of action reduces the selection pressure of a single mode of action. It
keeps weed populations off balance, much as a baseball pitcher does to baffle
batters with a diverse pitch selection.
3. Rotate Crops Right.
Crop rotation helps
forestall herbicide-resistant weeds. But rotate right.
Norsworthy gives the example
of a farmer who planted rice following soybeans. “The field flooded, but he
failed to control the Palmer amaranth on the levees,” he says. “When he rotated
back to soybeans the next year, the Palmer amaranth was worse than ever.”
4. Start Clean.
Proper tillage or using a
burndown herbicide prevents early-season weed competition. Early-emerging weeds
can rob yields even when it’s not apparent.
5. Use Full Herbicide Rates.
“We used to cut rates of glyphosate when it was expensive,”
says Norsworthy. “Farmers not only cut rates, but also delayed applications to
save on herbicide rates. So, they got tall weeds controlled by fewer
applications and lower-than-recommended rates. Eventually, weeds developed
resistance to glyphosate.”
6. Don’t Delay Application.
herbicides early according to the label. This is particularly true for Ignite,
used in LibertyLink systems.
“Ignite works great on small
pigweed, but not on 12-inch-high weeds,” he says.
7. Think Small.
That’s when weeds are at
their most vulnerable. Weed kills are excellent when weeds are tiny, at 2-inch
To get an idea of how small
that is, measure off 2 inches with a ruler. Now transfer that to weed height.
“Farmers need to know how what 2-inch weeds look like,” says Norsworthy.
8. Control Weed Escapes.
If weeds occur, consider
spot herbicide applications, row wicking, cultivation, or hand removal to boost
weed management for future seasons.
“Seed from just letting one
Palmer pigweed getting away from you spreads quickly over 4 to 5 acres the
first year. It’s then 50 to 60 acres the next year,” says Norsworthy.
That’s a high price for
letting just one Palmer amaranth escape, he says.