WEED CONTROL

Herbicide Residue

Herbicide Persistence

The persistence of phytotoxic levels of a herbicide for more than
one year can be a problem with some of the herbicides used in
North Dakota. Herbicide residues are most likely to occur
following years with unusually low rainfall because chemical and
microbial activity needed to degrade herbicides are limited in
dry soil. Crop damage from herbicide residues can be minimized by
applying the lowest herbicide rate required for good weed
control, by using band rather than broadcast applications, and by
moldboard plowing before planting the next crop. Moldboard
plowing reduces phytotoxicity of some herbicides by diluting the
herbicide residue in a large volume of soil. Moldboard plowing is
effective in reducing the residual effects of trifluralin,
Sonalan, Prowl, Nortron SC, atrazine, and Lexone/Sencor.

Herbicide residues often can be detected by bioassay. A soil
sample representative of the whole field must be obtained by
sampling at many places to the depth of the tillage layer. Also,
a sample of soil known to be free of herbicide residues must be
obtained from near the treated field to serve as the untreated
check. The samples should be dried and the clods broken so that
the largest particles are no larger than a wheat kernel. Prepare
at least two samples each of the untreated check soil and the
test soil in pots or other containers with holes in the bottom
for water drainage. 

The crop to be grown in the field should be used as one bioassay
species. Preparing extra pots and testing a more susceptible
species may be helpful in detecting residues. Plant in each pot
12 seeds of large-seeded crops like corn or soybean, or 20 seeds
of small-seeded crops like cereals or flax. Water the soil for
germination and plant growth as needed, but do not over-water.
When the plants are about 2 inches tall, thin to about six
large-seeded or 12 small-seeded uniform seedlings in each
container. The containers should be kept at about 70 to 75 F, and
in direct sunlight. 

Observe the plants in the untreated check and test samples for
two to three weeks after emergence. Some tangible measurements
such as plant height and leaf length can be taken for evaluation,
along with visual observation of abnormalities. 

Symptoms of some herbicides, like atrazine and Lexone/Sencor do
not develop until several days after emergence. The soil should
be washed from the roots to observe root growth, especially for
dinitroaniline herbicides such as Prowl, trifluralin and Sonalan.
Window bioassay does not provide accurate information on Ally or
Amber carryover.