When
soils are moist during the growing season, herbicides break
down through microbial and chemical processes. These reactions
may be slowed greatly in drought conditions. If herbicide
residues are significant, they may injure rotational crops
in the following season. For this reason, growers need to
be aware of herbicide residues and take steps to decrease
risk of injury.
Herbicides
vary greatly in soil persistence and carryover to next year's
crops.
- Essentially
no risk. Herbicides presenting essentially no risk of
carryover for next year's crops include: 2, 4-D, Roundup,
Gramoxone, Basagran, Poast, Assure, Fusilade, Sutan, Select,
Banvel, Clarity, Blazer, Eptam, Eradicane, Lorox, Buctril,
Reflex, Cobra, Butyrac, and MCPA.
- Moderate
risk. Herbicides presenting a moderate risk of carryover
to next year's crops include: Sencor, Lexone, Bladex, Treflan,
Prowl, Accent, Beacon, Broadstrike, Velpar, Balan, Stinger,
Classic, Pinnacle, Lasso, Dual, Frontier, Surpass and Harness.
- High
risk. Herbicides presenting a high risk of carryover
to next year's crops include: atrazine, Pursuit, Scepter,
Command and Princep.
- Check
the label of herbicides used during the drought season.
It will tell you the normal interval between application
and planting for a specific rotational crop. Footnotes frequently
show if the risk of carryover is greater under certain conditions
(such as soil pH or dry soils).
- Select
this year's herbicides carefully. Do not choose herbicides
or use rates that have significant injury potential by themselves.
Do not use products that may interact with carryover levels
of last year's products. For example, do not use metribuzin
(Sencor, Lexone) in soybeans this year if atrazine was used
in corn planted during the drought year.
- Use
tillage. Tillage will dilute the herbicide, especially
if it is concentrated near the surface or in bands over
the row.
- Look
for herbicide tolerance. Select crop varieties or hybrids
with greater tolerance to the herbicide used during the
drought year. This information is not available for all
varieties. Ask your seed supplier for assistance.
- Use
good management practices. Good seedbeds, proper seeding
depth and rate, adequate soil fertility, and insect and
disease protection will minimize the effect of herbicide
carryover. Many crops can tolerate a single stress relatively
well, but two or more stresses can result in significant
loss of crop vigor and yield.
If you
choose to test for herbicide carryover, the best time to do
so is between late October and mid-November for most of Wisconsin.
By this time, soil temperatures reach and remain below 50
degrees F., a point at which herbicide breakdown is minimal.
Do not take soil samples for residues before this time; they
may indicate levels greater than actually present when you
plant next year.
- A
bioassay test may be helpful if doubts remain about planting
because of possible herbicide residues. The test will
alert you to residue problems by comparing the productivity
of your intended crop variety in both affected and unaffected
soils. (Follow the guidelines in the UW-Extension publication
"A Simple Test for Atrazine Residues.") Begin
the test at least three weeks prior to planting so that
sufficient plant growth is available to assess carryover
potential. The herbicide label may also contain suggestions
on running a bioassay test, as well as information on crop
rotations and carryover potential.
- A
chemical test for herbicide residues can also be done by
private laboratories. These tests are expensive and
the results may not be easy to interpret. However, they
may be appropriate in cases where bioassays cannot be done
or where high value crops are concerned.
Additional resources:
Your county agricultural agent
Related publications:
UW-Extension publications-
"A Simple Test for Atrazine Residues," (A2882);
"Reduced Herbicide Rates: Aspects to Consider," (A3563);
"Row Crop Cultivators," (A3483).
Disclaimer and Reproduction Information: Information in
NASD does not represent NIOSH policy. Information included in
NASD appears by permission of the author and/or copyright holder.
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