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Under normal moisture conditions, herbicides are absorbed by the leaves
or roots of hydrated, healthy plants. Herbicide uptake by weed or crop
plants represent a significant portion of the herbicide removed from a
system in a normal growing season. The herbicide is degraded by a tolerant
crop while susceptible weeds die. In a drought, less moisture is available
to promote healthy plant growth. Drought-stressed plants are often less
responsive to herbicides, or require higher herbicide rates for effectiveness.
A common plant response to drought is the formation of a thicker cuticle
to reduce transpiration; this, however, makes foliar uptake of herbicides
more difficult. Plants under water stress take up less herbicide and are
less able to degrade an herbicide. In addition, low soil water potential
means that more of the herbicide is bound to the soil and less is present
in the soil-water solution for uptake by plants. The consequences of these
interactions can be less activity on weeds and increased injury to the
crop.
Absorption by Soil Organic Matter and Clay
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| Figure 2. Plants absorb herbicide in solution more easily than on a soil particle. |
Many herbicides interact in a process that is heavily influenced by moisture content. Herbicides are absorbed by plants from the soil water solution. The amount of herbicide available for uptake is regulated by the herbicide soil-binding characteristics and the water solubility of the herbicide. Herbicides that bind strongly to the soil are slowly released into the soil water and may persist longer than loosely bound herbicides. Under drought conditions, herbicides that bind tightly to soil particles remain bound and have relatively little chance of desorbing back into the soil water solution. When moisture becomes more abundant, large-scale release of bound residual herbicide can injure sensitive follow crops. Thus, prolonged drought increases the chance for herbicide carryover in the subsequent year.
Leaching refers to the downward movement of herbicides with water that
percolates through the soil. Some of this downward movement occurs naturally
and helps distribute the herbicide to different depths in the soil where
weed seeds are found. New herbicides exhibit relatively little leaching.
Leaching does not account for much of the herbicide lost in a field. Under
drought conditions, downward movement and mixing of herbicides in the
soil profile is reduced, which may leave a more concentrated layer of
herbicide near the soil surface. This increased herbicide concentration
may also affect carryover to sensitive crops.
Active herbicide breakdown by soil microbes is the primary way herbicides
dissipate from a field. Microbes use the herbicide molecules as an energy
source in their life cycle, but they only have access to herbicides that
are in the soil solution. During a drought, herbicides bind tightly to
the soil particles and there is less in the soil solution. This reduces
the rate of herbicide dissipation and increases the chance of carryover
to sensitive crops in the following year. Normal precipitation and good
soil water promotes favorable microbial populations, which in turn help
degrade herbicides. Under drought conditions, microbial populations are
greatly depressed resulting in less herbicide degradation.
Hydrolysis and other natural chemical breakdowns can occur with herbicides
in soil. However, these events require the presence of water and a moist
environment. Sometimes these processes are pH dependent, but in all cases
they require moisture for the breakdown to occur. During a severe drought,
such chemical breakdowns cease or are greatly reduced. Again, this can
contribute to the carryover of an herbicide from one season to the next.
Certain herbicides are more likely to carry over in a drought. Herbicides
such as atrazine or some members of the imidazolinone or sulfonylurea
families can persist under these conditions. Following label plant back
restrictions for such herbicides is even more important during drought.
Using field collected soil samples to examine potential plant back crops
can be informative, but it requires adequate sampling of fields and time
for the crops to grow. Such studies are best conducted with your local
Colorado State University Extension county agent. Some small
grain crops (e.g., wheat) are generally more tolerant to herbicides than
oats or barley. Such information can help a producer select a crop least
likely to suffer adverse effects from persistent herbicides.
| 1 P. Westra, Colorado State University Extension
weed science specialist and professor, bioagricultural sciences and pest
management; S. Nissen, Extension weed science specialist and
associate professor bioagricultural sciences and pest management. 5/02.
Special thanks to Dr. Patrick Miller for the figures used in this fact sheet.
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