Midwest Vegetable Guide - 2022 - 60

Weed Management Strategies
Environmental and Health Hazards
of Herbicides
Herbicides can have nontargeted effects, so it is very
important that you educate yourself about these effects and
consider them when designing weed management systems.
The following section contains discussions of some of the
potential environmental and health hazards of herbicides.
Environmental Hazards
Adverse environmental effects from herbicides can have
long-term consequences that are difficult to correct, and must
be avoided. Some environmental hazards, such as herbicide
drift and carryover, will mainly affect your operation, while
other hazards, such as water contamination, affect all
residents in the area. The following sections discuss some of
the potential hazards and methods to avoid them.
Herbicide carryover. Herbicide carryover from persistent
herbicides has been a particular problem to vegetable crop
growers. Persistence depends on herbicide characteristics
(method of degradation, water solubility, and rate of
application) and site characteristics (soil type, rainfall, and
temperature). Avoid carryover because correcting carryover
problems after they have occurred is virtually impossible.
The most important method to avoid herbicide carryover is to
follow label rotation restrictions. The Corn Herbicide
Rotation Restrictions table and Soybean Herbicide Rotation
Restrictions table summarizes some of the label restrictions.
Always refer to the label for specific information. If there are
differences between the table and herbicide label, always
follow label information.
Herbicide drift. Another frequent hazard to vegetable
growers is crop injury from herbicide drift. Certain
herbicides, if not used correctly, can injure nontarget plants.
Herbicides such as clomazone (Command), dicamba, and 2,4D
can drift up to a mile and seriously damage grapes,
tomatoes, peppers, other vegetables, fruit trees, and
ornamental plants. Before spraying clomazone, dicamba, or
2,4-D, survey the area for desirable plants. Spray only on
calm days, and use drift inhibitors when appropriate.
Minimize drift by applying herbicides with nozzles that
produce large droplets. Use an amine formulation of 2,4-D to
reduce vapor drift. Spray clomazone, dicamba, and 2,4-D
when the temperature is expected to be lower than 80°F to
85°F for several days after treatment. Avoid applying
clomazone to wet soils.
Spray tank residuals. Dicamba or 2,4-D residues in spray
tanks also can injure susceptible vegetable crops. Carefully
follow label directions for cleaning spray equipment after
using dicamba or 2,4-D. If possible, do not use the same
spray equipment to apply 2,4-D or dicamba that you use to
apply other pesticides.
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Herbicide resistance. More than 180 weed species have
developed resistance to one or more herbicides. Herbicideresistant
populations tend to develop when herbicides with
the same mode of action for killing weeds are used every year
in the same field. The Herbicide Resistance Action
Committee (HRAC) groups herbicides according to their
modes of action.
Weeds resistant to herbicides in HRAC Group 2 (ALS
inhibitors) make up 30 percent of the documented resistant
biotypes. Sandea, Permit, Matrix, Raptor, and Pursuit are
vegetable herbicides in this group.
Weeds resistant to herbicides in HRAC Group 5
(Photosystem II inhibitors) make up another 20 percent of the
documented resistant biotypes. Atrazine, Sencor, and Sinbar
are in this group. Widespread glyphosate use in agronomic
crops has led to the development of glyphosate-resistant
weeds, although they still only represent 3.5 percent of
resistant biotypes.
Approaches that aim to prevent herbicide resistance combine
the use of herbicides, mechanical (cultivation), and cultural
(crop rotation) weed management practices. It is important to
avoid relying on herbicides from a single HRAC group year
after year. Rotate between, or use tank-mixes of, herbicides
with different modes of action. For example, in asparagus
rotate between Sencor and Treflan. Use tillage to control
weeds that escape from herbicide applications. To minimize
any weed resistance that does occur, it is especially important
to scout your fields, paying special attention to any patches of
a weed normally controlled by the herbicide. Herbicide labels
may contain additional information about avoiding
Water quality. Residues of some herbicides such as
atrazine, metolachlor, and metribuzin have been found in
surface and ground water. Detected levels have normally
been low, but contamination of water resources is a growing
concern. For example, groundwater contamination from
pesticides and nitrates is a particular concern in areas of the
Midwest with sandy soils and shallow groundwater.
Factors determining the potential for groundwater and surface
water contamination include herbicide solubility in water,
rate of degradation, volatility, and tendency for the herbicide
to attach to soil particles or organic matter. Herbicides that
have high water solubility and long persistence are a
particular concern.
Site characteristics (soil type, soil depth, water table depth,
slope, and weather) also can lead to contamination of water
resources from herbicides. You should be aware of the
potential problem of herbicide contamination and take all
possible steps to avoid contamination of surface and
subsurface water resources.
Midwest Veg Guide 2022
Midwest Vegetable Guide - 2022

Table of Contents for the Digital Edition of Midwest Vegetable Guide - 2022
