Owner`s manual

FINAL DECISIONS
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
THE YEAR AFTER THE FLOOD
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.
A CLOSER LOOK AT HERBICIDES
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
Product or Chemical Family
Anaerobic Conditions
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
Information from: University of Wisconsin Cooperative Extension
University of Wisconsin-Extension • Cooperative Extension