Remain Aware Of The Potential For Herbicide Carryover In 2013
URBANA, ILL.
Ideally, soil-residual herbicides
should provide several
weeks of weed control
but not persist long enough in
the soil environment to cause
damage to rotational crops.
Dry soil conditions, similar to
what most of Illinois experienced
during the 2012 growing
season, often slow the rate of herbicide
degradation and increase the potential for damage
to rotational crops from herbicide carryover.
Some remember the dry growing season of 1998
and the problems encountered in 1989 due to
the persistence of one or more soil-residual herbicides
that did not adequately degrade in the
dry soil conditions of 1988. Will the dry soil conditions
during much of the 2012 growing season
lead to similar problems with herbicide
persistence in 2013? Will the precipitation that
was received later in the growing season and
throughout the fall eliminate the potential for
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 have very little soil activity (such as thifensulfuron)
while others can persist for several
months (such as picloram). Variation in soil persistence
even can exist among herbicides within
a particular chemical family. For example,
within the imidazolinone herbicide family, soil
persistence of imazamox (Raptor) is much
shorter than persistence of imazethapyr (Pursuit).
An indication of a herbicide’s soil persistence
can sometimes be inferred from the crop
rotation intervals listed on the respective product
label. Herbicides that tend to persist longer
in the soil generally have longer crop rotational
intervals compared with herbicides that don’t
persist long. For example, labeled crops may be
planted anytime following the application of carfentrazone
(Aim), while 10 months must elapse
between application of fomesafen (Flexstar) and
planting corn. Rotational intervals for a particular
herbicide sometimes vary by rotational
crop, which also provides an indication of which
rotational crops are more sensitive to herbicide
residues remaining in the soil. The chemical
characteristics of the herbicide molecule that
contribute to long soil persistence are inherent
properties of the molecule, and there is little
that can be done to shorten their persistence
once they have been applied. However, variations
in certain soil physical and chemical properties
can influence the persistence of a
particular herbicide apart from its chemical
composition.
Soil properties that influence herbicide persistence
can be broadly categorized into chemical,
physical and microbial. One of the more
important soil chemical factors that influences
persistence of some herbicides is pH. Soil pH, a
measure of the concentration of hydrogen ions
present in the soil solution, determines whether
a soil is considered acidic, neutral or basic. At
higher soil pH values, weakly acidic herbicides,
such as many sulfonylurea herbicides, exist in
the negatively charged or anionic form. The anionic
form is only weakly adsorbed to the clay
and organic matter components of the soil, and
therefore more of the herbicide remains in the
soil solution and available for plant uptake. At
lower soil pH values, weakly acidic herbicides
exist in the neutral form or, in some instances,
the positively charged (or cationic) form. These
neutral and cationic forms of the herbicide are
more strongly adsorbed to the negative surfaces
of clay and organic matter, and therefore are
less available for plant uptake.
Soil pH also can impact how much and how
quickly certain herbicides are degraded by a
chemical process known as hydrolysis. Hydrolysis
is a process by which the herbicide molecule
reacts with water (“hydro” means water) to
cleave (“lysis” means to split or degrade) certain
chemical bonds of the herbicide molecule and
render it either much less active or herbicidally
inactive. The rate of chemical hydrolysis is influenced
by the pH of the soil; hydrolysis occurs
more rapidly under acidic conditions than
under neutral or basic conditions since the
weakly acidic herbicide molecule exists in the
anionic form under basic conditions and is
much less susceptible to hydrolysis. The rate of
chemical hydrolysis also can be influenced by
soil moisture and temperature, with hydrolysis
slowing under dry and cold soil conditions.
Physical properties of soils are related to the
relative amounts of sand, silt, clay, and organic
matter a soil contains. Soils with higher
amounts of clay and organic matter have a
greater 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. Adsorption
is the adhesion of ions or molecules to the
surface of soil colloids. Usually these types of
attractions are weak such that herbicide molecules
move from the colloids into the soil solution
in a reversible manner. However,
adsorption forces can become stronger over
time given certain soil conditions (such as lack
of soil moisture). 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 represent the primary
method of degradation for many herbicides.
These microorganisms include various species
of fungi, bacteria, actinomycetes, and algae. The
composition of microorganisms (species and
abundance) present in a particular soil is influenced
by soil type, pH, organic matter content,
moisture, etc. Most species of microorganisms
are active under aerobic conditions while others
thrive under anerobic conditions. Herbicide
degradation by microorganisms most often occurs
when the microorganisms consume the
herbicide molecules as a source of energy. Certain
herbicides are more easily degraded by microorganisms
than others.
The rate of herbicide degradation by soil microorganisms
is often directly related to prevailing
environmental and soil conditions. Any
condition that impacts the microbial species
composition and population can impact the rate
of herbicide degradation. Temperature and
moisture are two factors that can greatly influence
soil microbial populations. The activity
level of most soil microorganisms is higher
under warmer soil temperatures than under
cooler soil temperatures. This partially explains
why herbicide degradation tends to slow as soil
temperatures drop during the weeks after crop
harvest. Minimal herbicide degradation by soil
microorganisms occurs when soil temperatures
reach approximately 40 degrees Fahrenheit.
Herbicide degradation by soil microorganisms
usually occurs most rapidly when adequate soil
moisture is present. Under extremely dry conditions
the rate of herbicide degradation by soil
microorganisms can slow enough to allow the
herbicide to persist into the next growing season.
What can be done to minimize the risk of injury
to rotational crops from residues applied
during the previous growing season? In the simplest
terms, herbicide degradation simply takes
time and moisture. Currently, soil moisture
across much of Illinois is more abundant than
at this time last year, but the activity of soil microbial
populations remains limited at current
soil temperatures. Shallow tillage can help distribute
herbicide more evenly across a field, and
is more likely to help enhance herbicide degradation
when soil temperatures are warm and
adequate soil moisture is present. Early planting
or planting a rotational crop that is very sensitive
to the herbicide applied last season might
further increase the likelihood of crop injury
from herbicide carryover. Ultimately, the susceptibility
of the rotational crop determines
whether or not persisting herbicide residues will
cause any problems. Planting the same crop
next season as was planted 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 for additional herbicide degradation
to occur. Δ
DR. AARON HAGER: Associate Professor, University
of Illinois