Minimizing Aflatoxin in Corn
By Mississippi State University - Aflatoxin is a naturally occurring toxic chemical by-product from the growth of the fungus Aspergillus flavus on corn and other crops such as peanuts and cottonseed.
Grain
containing aflatoxin is toxic to animals,
especially young animals and
poultry; therefore, facilities that
handle grain routinely test loads
before accepting delivery.
Aflatoxin problems are more
likely in Mississippi than in the
Corn Belt, because the state’s hot,
humid climate is ideal for fungal
growth. Also, little hybrid resistance
exists and few if any decontamination
methods have proven
successful and been granted federal
approval.
Stressful Conditions
The Midsouth’s climatic conditions
dictate that aflatoxin potential
will continue to threaten corn producers
until control measures are
identified. Aflatoxin problems
have historically developed during
years with severe high-temperature
stress, particularly when coupled
with water deficiency and insect
ear and stalk damage. In 1977 and
1998, Mississippi had severe problems
with aflatoxin-contaminated
corn.
You can minimize the likelihood
of developing a problem by using
sound agronomic practices, properly
storing and drying grain,
maintaining grain quality, and sanitizing
grain-handling equipment.
Aflatoxin can infect corn by airborne
spores in the field during
grain filling or during storage and
handling. Kernel infection may
occur through the silk, cob, or
direct contact. Fungus spores overwinter
on plant residue on the soil.
However, management practices
intending to reduce the inoculum
level have little impact on aflatoxin
development in subsequent
years, because the fungus is abundant
in the Midsouth nearly every
year.
Management Practices
Aflatoxin develops in the field
when corn is exposed to severe
environmental conditions known
to stress kernel development and
promote fungal infection within
the ear. Management practices that
improve plant health strongly discourage
aflatoxin development.
Timely planting, adequate fertility,
good weed and insect control,
supplemental irrigation, suitable
plant population, and hybrid selection
should help reduce aflatoxin
potential. Although hybrid evaluations
conducted in Mississippi in
1998 indicated little aflatoxin
resistance in commercially available
hybrids, hybrids that perform
well in drought conditions generally
have lower aflatoxin concentration
than hybrids that yield poorly
in drought conditions.
Harvest Timing
Producers may reduce the likelihood
of aflatoxin buildup in the
field by harvesting corn before it
reaches the industry standard of
15.5 percent moisture.This system
reduces duration when ears may be
exposed to unfavorable drying
conditions that promote aflatoxin
development in the field. Corn
reaches physiological maturity at
about 30 percent moisture and can
be harvested any time thereafter.
Mississippi research indicates corn
will normally lose around 0.6 percent
moisture per day during the
dry-down period. This rate is not
influenced much by hybrid maturity.
Thus, you can reduce field
exposure by at least 1 to 2 1/2 weeks
by harvesting corn at 20 to 25 percent
moisture, compared to letting
the corn dry in the field to 15 percent
moisture.
The disadvantage of early harvest
is that wet, warm grain is an
ideal environment for rapid aflatoxin
escalation if it is not handled
properly. Dry your high-moisture
grain (16-30 percent) to below 15
percent moisture within 24 hours
after harvest or immediately haul
the grain to an elevator (that will
dry the grain).
Storage
Do not store grain in trucks,
combines, bins, or any nonaerated
site for more than 4 to 6 hours.
These conditions quickly escalate
aflatoxin levels and deteriorate
grain quality, because fungal
growth and grain respiration will
rise quickly in high-moisture grain,
particularly with normal Mississippi
August and early September
air temperatures.
Conversely, aflatoxin approaches
dormant levels when grain
moisture drops to about 12 percent,
especially when air temperatures
decline to around 55 ºF. If you plan
to dry the grain yourself, do not
harvest more corn than you can dry
within these constraints.
Aflatoxin problems often develop
in grain bins being used to dry
corn. You must minimize grain
depth (commonly 3-6 feet deep) to
quickly dry high-moisture corn
using in-bin drying system. Stirring
devices may assist drying but
cannot overcome aeration problems
that limit the drying rate in
deep-layered grain. Other drying
systems, such as continuous flow
and portable batch driers, normally
dry grain within these constraints,
if harvest capacity does not exceed
volume of the drying system.
Harvest and Handling Practices
You may improve grain quality
by altering harvest and handling
procedures. Fungi readily invade
kernels with cracked or damaged
seed coats. If you suspect a problem,
keep obviously stressed,
stunted, or damaged areas and field
edges from healthy corn.
Increasing fan speed, opening
sieves, and reducing ground speed
help enhance grain quality collected
by a combine. Postharvest
screen cleaners and gravity separators
help reduce moderate aflatoxin
levels (50 to 100 ppb) below the
FDA standard (20 ppb).
Daily clean out corn and debris
left in combines, trucks, pits, grain
carts, and augers; clean bins before
use because these are potential
contamination sources. Spores
from fungi on infected grain may
readily disperse during handling,
contaminating subsequent grain. A
chlorine cleaning solution (3/4 cup
bleach/gallon of water) kills fungal
growth on handling facilities.
Detection
Methods historically used for
aflatoxin detection range from
visual observations to complex lab
analyses. A “black light” test uses
long-wave ultraviolet light to illuminate
a bright yellow-green fluorescence
indicative of a fungal
metabolism product that often preludes
aflatoxin. This product is
called Kojic acid and should not be
confused with aflatoxin. The
“black light” test has limited use;
use only as a preliminary test to a
more accurate chemical analysis.
Elevators or grain markets
should use chemical analyses to
determine aflatoxin content. You
may also submit samples for
analysis to the State Chemical Lab
or buy an aflatoxin test kit from
chemical supply companies.
Sampling and Testing
Substantial aflatoxin testing
variability is common because few
kernels are normally contaminated
with aflatoxin (less than 0.1 percent),
but concentration in individual
kernels is often very high.
You may improve sampling by
increasing the sample size and
using proper sampling techniques.
Chemical extraction of aflatoxin
requires grinding the grain sample.
Testers should grind the original
sample before a subsample is removed.
This improves distribution
of contaminated particles to the
subsample. Testers may also grind
particles finer, increase the size of
the subsample, and increase number
of analyses per sample to
reduce variability. The latter two
recommendations, however, will
increase the time and expense
involved with the analytical procedure
and may be impractical in
some situations.
Action Levels
The United States Food and
Drug Administration action levels
for corn contaminated with aflatoxin
establish guidelines for specific
uses. Research indicates that
aflatoxin-contaminated corn within
these action levels will not injure
the health of specific animals listed
or humans consuming foods derived
from these animals.
Source: Mississippi State University Extension Service - November 2004