University of Wisconsin-Extension
Floods
can affect weeds both the year they occur and in subsequent
years. The biggest impact in the flood year will be the reduced
competitive ability of the crop. Weeds will take advantage
of the stunted or killed crops and grow to maturity.
In
the year after a flood, new weed problems will be likely.
Some of the weeds carried into the field by floodwaters may
not have germinated in time to be noticed during the previous
growing season. Mechanical and chemical methods need to be
considered in both the flood year and subsequent years to
manage weeds. A bioassay test-in which seeds are planted in
flooded and non-flooded soil samples-can be helpful to determine
if soils are safe for intended crops.
If the
crop recovers after the flood, make an effort to reduce the
impact of weed competition. This may not be practical if fields
are too wet to enter for mechanical or chemical weeding.
- Check
fields regularly to monitor crop and weed development. Take
note of weed species. Are there any new species? This may
happen if weed seeds were carried into the field by floodwater.
Make a field map of these weed locations and use it to plan
next year's weed management program.
- Consider
whether herbicides can be safely applied. Most labels clearly
specify the maximum growth stage of the crop at which the
product can be used. Applications following a mid-season
flood are very likely beyond this "window" of application
timing. Most labels also caution against using herbicides
if the crop is under any stress. Thus, the feasibility of
herbicide use the same year as a flood occurs is limited.
- If
herbicide use is feasible but conditions are extremely wet,
consider using a commercial sprayer equipped with flotational
tires.
Flooding
usually kills the crop or at least injures it so severely
that it will not be be worth harvesting. If this is the case,
try to prevent weeds from going to seed through the use of
mowing, tillage or chemical application.
- As
mentioned above, take note of any new weed species that
are present. Make a field map of the weeds to plan next
year's weed control program.
- Mowing
will allow some weeds to survive but may hasten drying of
the soil more than using herbicides. Mowing is also an option
if the soil is too wet to be tilled.
- Mechanically
tilling the soil, if it is dry enough, will destroy weeds.
It will also aerate the soil more than either mowing or
spraying.
- Applying
non-selective, non-residual herbicides may be a good option
if the soil is too wet to work mechanically.
- Repeat
either mowing, tillage or chemical application if another
generation of weeds emerges that will have time to produce
seed.
Be alert
for new weed problems the year after the flood. Some weeds
may have germinated after you made an assessment of weeds
during the flood year. Others may have remained dormant until
this season. The flood may also have deposited soil that is
different in texture, pH and organic matter content. These
factors may influence herbicide performance and crop safety.
Take soil samples and base herbicide selection and rates on
current soil characteristics.
The "new
soil" may have herbicide residues from the previous season's
application. These levels are unlikely to affect this year's
crop, but it would be wise to do a simple bioassay test to
determine if planned crops are feasible in the flood-deposited
soil. To carry out a bioassay test:
- Take
several soil samples from the flooded field (1 quart per
sample) and plant three or four seeds of the planned crop
in each one.
- Collect
soil samples from a known herbicide-free site to use as
a standard and likewise plant three or four seeds of the
planned crop.
- Grow
the seedlings for two to four weeks.
- If
plants in the flooded soil are normal and appear to grow
as well as those in the herbicide-free soil, indications
are strong that it is safe to plant your crop.
- If
crop growth in the flooded soil is abnormal, have an agricultural
professional determine if the symptoms are related to possible
herbicide residues in the soil or to other causes, such
as nutrient deficiencies or diseases.
Herbicides
decompose in the soil by microbial action. This breakdown
is slowed under flooded (anaerobic) conditions. Soil temperatures
also are cooler under flooded and wet soil conditions, slowing
both microbial and chemical degradation. Thus, the potential
for herbicide carryover that would injure the subsequent crop
may increase after flooding. A summary of possible effects
of flooding on herbicide breakdown is given below:
|
|
Degradation
Under Anaerobic Conditions
|
Product
or Chemical Family
|
|
Triazines
(atrazine, Bladex, Sencor)
|
|
slower
|
Thicarbamates
(Eradicane, Sutan+)
|
|
slower
|
DNAs
(Treflan, Prowl)
|
|
faster
|
Acetanailides
(Lasso, Dual, Frontier)
|
|
can
degrade anaerobically
|
Substituted
ureas (Lorox)
|
|
unknown
|
Roundup
|
|
can
degrade anaerobically
|
Accent
and Beacon
|
|
unknown
|
Hoelon
|
|
much
slower
|
Poast,
Fusilade, Assure
|
|
unknown
|
Should
you allow even more time than product labels specify before
planting rotation crops? Probably not if you have used DNAs
(as noted in chart), but it's difficult to say for other chemicals.
Consider whether floodwaters brought in untreated soil from
other fields. Also consider whether runoff removed a significant
part of the applied product. When in doubt, use the bioassay
test described at right or send a soil sample to a commercial
lab for chemical analysis. In some cases it may be appropriate
to allow an extra week or two beyond the normal plant-back
interval and deep till the field to dilute any remaining residues.
Once
the field has been planted, monitor it carefully for possible
weed problems. If weed densities approach the economic threshold,
use the appropriate mechanical or chemical measures to control
them.
Additional resources:
Your county agricultural agent
Disclaimer
and Reproduction Information: Information in NASD does not represent
NIOSH policy. Information included in NASD appears by permission
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NASD Review: 04/2002
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