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ISSUE 14   AUGUST 5, 1999



    Applying the lowest herbicide rate for control of target weeds or herbicide application at rates that
will provide the greatest return over herbicide and application costs is the goal of many growers. This
"best" herbicide rate will be different for every combination of herbicide-environment-weed species-
crop-weed growth stage-crop growth stage-adjuvant-weed density-tolerance for risk by user.
Sometimes, the "best" herbicide rate will be lower than the lowest rate on the herbicide label. To
understand why this is true, we need to understand some of the assumptions and considerations of
the companies when they write a label.

    Weed Size and Crop Size. Companies must make an assumption about the normal size of weeds
and crops when the herbicide will be applied. Generally, small weeds are more susceptible to herbicides
than large weeds but small crop plants also may be more susceptible. Herbicide rate reduction may be
possible if the herbicide is applied to weeds that are smaller than the weed size on the herbicide label.
The crop also may be smaller than the suggested crop stage at application so knowledge of safety to
the crop would be needed. So, herbicide rates can sometimes be reduced by early application but
special knowledge is needed.

    Environment. Companies can not write a label that anticipates all environments in which a herbicide
may be used. Environment has a large influence on the efficacy of herbicides. So, herbicide rate reduction
may be possible if the specific environment will favor high phytotoxicity from the herbicide but special
knowledge is needed on the environment-herbicide interaction.

    Adjuvants. The efficacy of some postemergence herbicides is enhanced by adjuvants such as
surfactants, crop oils, methylated seed oils or fertilizer. Adjuvants vary considerably in ability to enhance
herbicide efficacy even among a class of adjuvants. For example, some surfactants may double percent
weed control from a herbicide while other surfactants may have little effect. The number of adjuvants
presently sold is so large that testing all adjuvants with all herbicides is not possible. Companies must set
label rates for herbicides by assuming that adjuvants will or will not be used and that adjuvants used will
be similar to those tested with the herbicides. Herbicide rates can sometimes be reduced by using an
adjuvant that is highly effective with the specific herbicide but special knowledge is needed. The herbicide-
adjuvant combination must be safe on the crop as well as provide good weed control.

    Method of Application. Sprayer pressure-volume-speed-nozzle type-delivery system will influence
herbicide efficacy. Companies must assume an average or common application method when writing a
herbicide label. Special knowledge of the best application method for a specific herbicide and situation
may allow a reduction in herbicide rate.

    Weed Species. Weeds may vary considerably in susceptibility to a herbicide. Companies sometimes
list weed species separately on the label with different rates for different weeds. Some labels do not vary
rates by weed species. Herbicide rates can often be reduced if the field is infested only with weed species
that are highly susceptible to the herbicide. Special knowledge of weed response to the herbicides is needed.

    Performance Complaints. Chemical companies recommend product use to obtain adequate performance
across a wide range of conditions and uses. Herbicide rates on labels are often set to reduce the risk of less
than adequate weed control and to reduce numbers of complaints. A herbicide user who is willing to accept
a higher risk of poor weed control can reduce herbicide rates below the labeled rate. This should only be
done when conditions favor high efficacy from the herbicide and susceptible weeds are present.

    Production Cost of the Product. Companies rarely consider the production cost of the product when
they set the labeled rate of a herbicide. Companies that sell herbicides are not selling a product, they are
selling a service. The companies would prefer to recommend the least amount of herbicide to control the
weeds because the value of the service (the weed control) would be the same regardless of rate of herbicide.
So, companies do not deliberately set rates higher than necessary but refinements in rates are possible with
special knowledge about specific situations.

    Are Low Rates Legal? A herbicide user can legally choose a rate lower than listed on the herbicide
label unless the label specifically prohibits low rates. However, the company has no obligation to support
herbicides when the application rate was less than labeled rates. Herbicide users should not expect a
company representative to provide any comfort or assistance if weed control is less than expected from
a rate of herbicide that is less than the labeled rate.



    Keep an open mind and investigate all possible causes and sources of the observed problem when
assessing crop injury. Do not accept, without question, statements of involved persons about the cause
and the source of the problem. The truth often is not obvious. Remember that crop injury can have
many causes other than herbicides and symptomology does not always provide definitive answers.

    The Plant Diagnostic Laboratory at North Dakota State University will accept samples and provide
an opinion on the cause of the problem. However, the Plant Diagnostic Laboratory does not presently
analyze plant tissues or soil for herbicide residues and opinions on the cause of plant injury will be based
on injury symptoms. Refer to herbicide carryover for list of testing labs.

    Be aware that analysis of plant tissues or soil by a private testing laboratory may not provide a definitive
answer to the cause of the problem. Each herbicide must be tested individually so testing can be very
expensive if numerous herbicides are the possible cause of the problem. A positive detection of a herbicide
can be useful but the detected herbicide may not have caused the symptoms. A negative test for a herbicide
does not prove that the herbicide did not cause the problem since the herbicide may cause injury at levels
less than the detection limit or the herbicide may have been degraded before the samples were taken.

    The pattern of crop injury in a field will help identify the source of the injury. A sprayer skip in a field
will be valuable in diagnosing a herbicide problem especially if the applicator remembers the reason and
the time that the skip occurred. The history of herbicide use on the field for the past 2 to 5 years should
be considered. A uniform damage over the field would suggest herbicide carryover or injury from a
direct application rather than drift.

    Drift is nearly always worse near the source of the drift with damage becoming less as the distance
becomes greater. The lessening of injury with distance may not be evident shortly after the drift has
occurred but the differences should become more visible with time since the recovery of the damaged
plants will be more rapid and more complete as the distance from the drift source increases. Crop injury
that is associated with one or two sprayer tank loads would suggest sprayer contamination or a mistake
in mixing where the wrong herbicide or too much of the correct herbicide was put in the spray tank. An
aerial photograph often is very useful in identifying patterns of crop injury in a field.

    The family of the herbicide that caused the injury often can be identified by the injury symptoms and the
species which are not injured. Look in the affected field, in surrounding fields and between fields. The
approximate date of injury can sometimes be determined by observing or learning the date that the injury
first became evident. The size of plants when affected by a growth regulator herbicide can sometimes be
determined by the height of the stem where malformed leaves first occur. Plants that are affected as soon
as they emerge usually are being damaged by a herbicide in the soil rather than drift. Dates that injury
occurred can be related to dates of herbicide application on and around the damaged field.

    The direction of the source of herbicide drift can sometimes be determined by finding "drift shadows"
by trees, buildings or elevated roads. Anything that intercepts or deflects spray droplets can cause an
area of undamaged plants on the downwind side of the object. The shape and direction of the "drift
shadow" often will identify the direction of the drift source. The damage from spray drift sometimes
moves at an angle across nearby fields with a rather distinct line between damaged and undamaged
plants at the edge of the line. Placing tall stakes at the edge of this line through the damaged field will
often form a line that points at the edge of the field that was the source of the spray drift. Spray droplets
move with the wind. Spray droplets will only move down wind so the wind direction during application
will often indicate which potential drift sources are possible and which are not possible. Some herbicides
like 2,4-D ester, MCPA ester and Banvel are volatile and a wind shift after application may cause vapor
drift in a different direction than the drift of spray droplets. Spray droplets only move in the direction that
the wind is moving.

    Some sources of unintended herbicide exposure are very difficult to identify. For example, drift or an
accidental and unreported spraying of a long residual herbicide on a tolerant crop would have no effect
that year but the residual in the soil the next year could damage a susceptible crop. Another example is
soil movement due to wind or water erosion which causes a damaging level of herbicide to move with
the soil.

    An obvious question is whether to destroy or keep the damaged field. A general rule of thumb is that
damage from drift is not as bad as the initial appearance would suggest and a decision should not be
made within one week of the drift. With growth regulator herbicides, about 10 days is needed before
surviving plants will begin to produce new leaves. Evaluation of the level of injury from growth regulator
herbicides should not be attempted prior to 10 days after exposure. With ALS inhibitor herbicides and
glyphosate, the less damaged plants begin to visibly recover and separate themselves from plants with
more injury about two weeks after exposure. Rapid conclusions can lead to bad decisions with spray drift.

    Everyone involved will want to know how much yield loss will be caused by the herbicide damage.
Accurate visual estimation of yield loss from a non-lethal exposure to herbicide is not possible. Some
means of collecting meaningful yield comparisons is essential in obtaining an accurate estimate of yield loss.
When part of a field is injured and part is not injured, yield in the uninjured portion of the field can be
compared to yield in the injured portion. Hand harvesting at several places, harvesters with yield monitors
or harvesting and weighing yield from strips through the field all could be used. Usually, splitting the field
into six or eight strips or pieces is better than comparing one half of the field to the other half of the field.

    Obtaining accurate yield loss data is very difficult when the entire field is damaged. Comparisons to
nearby fields can be done but variability among fields is great.

    Use of the average yield of several nearby fields also could be considered.

Alan Dexter
NDSU/U of MN Extension Weed Specialist



    If analysis of plant or soil samples are needed to document herbicide residue several laboratories
are available. Page 115 of the 1999North Dakota Weed Control Guide contains a list of possible labs.
You may need to call before you send sample to determine which herbicides they can test and if they
can test plant or soil samples.

Richard Zollinger
NDSU Extension Weed Specialist

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