Early preplant
Early preplant (EPP) herbicides are applied several weeks to several months
prior to crop planting. Advantages of EPP programs include prevention of weed
establishment, elimination of burndown treatments at planting, reduced potential
for herbicide carryover from one crop season to the next, and spreading out the
workload. EPP also discourages insects that are attracted to weedy
fields, such as black cutworm moths. For best results, such herbicides
should be applied before germination of summer annual weeds.
 Technologies like this hooded sprayer (in cotton) can reduce crop injury
and spray drift. |
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Refrain from using EPP herbicides on sloping or highly erodible land, especially if
planning to apply up to 45 days before planting. Several weeks of occasional heavy rains
on sloping fields can cause soil erosion and high rates of surface runoff, even in no-till
systems. This could wash chemicals down slope (attached to soil particles and dissolved
in surface runoff water) and into waterways. Use of EPP herbicides on sandy soils or other
soil types with high leaching potential should be avoided at all times.
Burndown
Burndown herbicides control emerged weeds before or just after planting, but
prior to crop emergence (the exception is new herbicide-tolerant systems which
allow post emergence application). These burndown herbicides can be mixed
with preplant or preemergence treatments. The most commonly used
burndowns are contact and translocators. Both are nonselective and have no soil
activity. The weed species present, size, life cycle, and herbicide efficacy determine
what and how much burndown to apply.
Note: Postemergent products frequently have short or no residual soil effects. They
have less impact on the environment if excessive rains occur.
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Preplant/Preemergence
The success of this program depends on rainfall to activate the herbicide soon after
application. Without moisture, its performance and subsequent weed control
suffer. In moisture-insufficient areas, herbicides that are applied after planting
are often preferred over soil-applied products. This is particularly true for no-tillers
who use row cleaners or "spider wheels" on their planters. These types of
attachments can move herbicide-treated soil from the row to the row-middles,
creating weed problems in the untreated zone (for example, a foxtail flush in the
corn row!). The result of this can be an expensive postemergence treatment.
Generally, no-tillers can use either of the following preemergence programs:
1. A full-season program where
herbicides
control weeds
throughout most of the
growing season, or
2. A short-season program where
the crop
canopy shades out
later-germinating
weeds (such
as no-till drilled soybeans).
The choice of which program to use should always be based on scouting
results. Either one can be followed by a postemergence program if necessary.
Postemergence
Postemergence programs have gained in popularity over the last five years. Its
primary advantage is that problem weeds or escapes can be controlled well into the
growing season without damaging the crop or reducing yield potential.
Commonly, postemergence programs are successful in systems that use
burndown herbicides only-no soil-applied residual herbicides are used.
Generally, soil type or amount of crop residue on the surface does not affect
postemergence treatments. An exception may be continuous small grain production
where extremely high levels of residue may prompt higher application rates or a
greater volume of carrier. However, postemergent application does depend on
proper timing and correct identification of the target weeds.
No-tillers rarely use a postemergent program alone. The best chance of success
for this approach is in fields that are weed-free when planted -- rare for no-till.
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