Be Aware Of The Potential For Herbicide Carryover
URBANA, ILL.
Dry soil conditions like
those experienced in
most of Illinois this
growing season are not always
conducive for achieving
good weed control with soilresidual
herbicides. Dry soil
also slows the rate of herbicide
degradation, which can
increase the potential for damage to subsequent
crops from herbicide carryover. Many factors interact
to determine how long a herbicide remains
active in the soil environment, including
factors related to the herbicide, the soil, and climactic
conditions.
Herbicides vary in their persistence in the soil;
some, such as thifensulfuron, have very little
soil activity, while others, such as picloram, can
persist for several months. Variation can even
exist among herbicides in the same chemical
family. For example, within the imidazolinone
herbicide family, soil persistence of imazamox
is much shorter than that of imazethapyr. The
characteristics of the herbicide molecule that
contribute to long soil persistence are inherent
properties, and little can be done to shorten
their persistence once they have been applied.
Soil properties that influence herbicide persistence
can be broadly categorized into chemical,
physical, and microbial. One of the more
important chemical factors that influences persistence
of some herbicides is pH. Soil pH impacts
how much herbicide is available for plant
uptake and how quickly the herbicide is degraded
by the chemical process known as hydrolysis.
For example, atrazine is more available
for plant uptake when soil pH is high (say, 7.0
or higher), but atrazine also persists longer because
the rate of hydrolysis slows at higher soil
pH values. The degradation rate of many sulfonylurea
herbicides also slows with high soil
pH values, whereas imidazolinone herbicides
persist longer in soils with low pH values.
Physical properties of soil relate to the relative
amounts of sand, silt, clay, and organic matter
a soil contains. Soils with higher amounts of
clay and organic matter have more potential for
herbicide carryover than coarse-textured soils
or those with less organic matter. Higher
amounts of soil organic matter and certain
types of clay particles increase the potential for
herbicide carryover by adsorbing more herbicide
onto these soil colloids. Herbicide bound to soil
colloids is not available for plant uptake, movement
downward through the soil profile, or microbial
degradation. Injury to rotational crops is
possible if these bound residues are displaced
by water molecules either late in the season
they were applied or the following spring.
Soil microorganisms are responsible for the
degradation of many herbicides. The composition
of microorganisms (species and abundance)
in a particular soil influences how quickly herbicide
degradation occurs. The activity of soil
microorganisms is influenced by several factors,
including soil moisture. Herbicide degradation
by soil microorganisms usually occurs most
rapidly when adequate soil moisture is present;
under extremely dry conditions the rate can
slow enough to allow the herbicide to persist
into the next growing season.
How can producers minimize the risk of injury
to rotational crops from herbicides applied this
season? Apart from a return to normal or abovenormal
precipitation, little can be done
to accelerate herbicide degradation
at this point in the season; the process simply
takes time (and moisture). Shallow tillage can
help distribute herbicide more evenly across a
field, but it will do little itself to accelerate herbicide
degradation when soils are excessively
moisture-deficient. Early planting next season
(assuming field conditions are conducive) might
further increase the likelihood of crop injury
from herbicide carryover.
Planting the same crop next season as in 2012
would effectively eliminate the potential for crop
injury from herbicide residues. This solution
may not be feasible for every situation where
herbicide carryover is possible, but it is an option
that warrants some consideration. If crop
rotation must occur where there is concern for
herbicide carryover, delaying planting as long as
possible could allow additional herbicide degradation
to occur.
Is it feasible to plant a cover crop, fall forage
crop, or small-grain crop after the 2012 corn
crop is harvested or destroyed? Keep in mind
that the labels of almost all corn herbicides include
crop rotation intervals specifying the time
that must elapse between application and
planting the rotational crop. In many instances,
the interval between application and planting
winter wheat will not be satisfied this calendar
year. Small-grain crops can be sensitive to even
small amounts of herbicide residue (such as
atrazine) that remain in the soil. Herbicide labels
might not list every possible rotational
crop; rather, they often fall into the most restrictive
interval on the label.
Finally, labels of most corn herbicides also indicate
an interval that must elapse between application
and harvest for grain, silage, or
grazing. Intervals for a number of herbicides are
listed in Table 1. Δ
DR. AARON HAGER: Extension Specialist,
Weed Science/IPM, University of Illinois