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Residue Avoidance Program: Feed Handling Systems
RAP-M2 Reprinted November 1985
Electronic version July 2001
Prepared by Robert Wilcox, extension
specialist, formula feeds quality control,
Kansas State University; Lee Kilmer, extension
dairy specialist, Iowa State University; Bernard
Curran, veterinary practitioner, Eldridge, Iowa.
Reviewed by Jay Firth, veterinarian, Feed
Specialties, Des Moines, Iowa; Lee Boyd,
executive secretary, American Feed
Manufacturers Association.
Residue Avoidance Program
Feed Handling Systems
There are many combinations of
equipment used to store, convey, and
feed livestock feedstuffs. All of them
include, as basic components, feed
storage, feed conveyance, and
feeders where feed is made acces-
sible to the livestock. Each of these
components represents an opportu-
nity for the feed to become contami-
nated, possibly causing residues.
Drug carryover in feed products can
occur in a number of ways. Feed
manufacturing equipment such as
mixers, pellet mills, conveying augers,
elevator legs, dust control devices,
and storage bins can contain dust and
residual feed capable of contaminat-
ing clean feed that follows.
Feed Storage
The most common type of on-farm
feed storage is the upright vertical bin.
Most are metal but can be wooden,
concrete, or of other materials. Figure
1 shows the normal pattern of feed
flow from an upright bin. Note that the
material directly above the discharge
opening is the first to leave the bin.
Then, as a vee is formed, materials
at the top cascade toward the low
point move-out. The result is an
emptying of the bin from the top down
with some materials in the lower parts
incorporated into the discharge flow.
Thus, feed put into a bin first is not
necessarily the first feed out.
Slope of the hopper bottom will affect
this flow of feed. Figures 2 and 3
illustrate the minimum hopper slopes
for whole grains and for most ground
materials. Some feed materials resist
lateral movement and can form a
tunnel, which is called rat holing. An
extreme example is shown in figure 4.
Feed hangup, such as rat holing, is a
form of segregation and can be
caused by several factors. These
include feeds containing relatively
high levels of molasses or added fat,
or those that are moldy, coarsely
ground, or abnormally moist. Feed
hangup can be overcome by bin
design (live bottom bin), by vibrators,
by bursts from air jets, or direct
agitation of the materials (poles,
stirrers, etc.).
Feed carryover can occur when feed
attaches to the sidewalls (figure 5). If
such bins are refilled before com-
pletely empty, feed in the dark areas
will remain for a considerable length
of time and possibly break off small
quantities into the new feed stream.
Because of this effect, such bins
should be completely emptied and
then visually inspected for residual
feed whenever a change is made in
kinds or amount of animal drugs used
or to a nonmedicated feed. If residual
feed contains animal drugs, potential
for drug carryover exists.
Figure 1. Normal feed flow. Figure 2. Minimum hopper slopes.
45
�
min.
Grain hopper
slope
Figure 3. Minimum hopper slopes
for corn.
A frequently overlooked, but simple
measure to help reduce feed contami-
nation is to identify each bin on a
farmstead. Painting a number, feed
name, or letter on the side of the bin
near the filling door or ladder will
serve to identify each bin and avoid
confusion by feed delivery truck
drivers, hired employees, and others.
Bin management techniques can
greatly reduce carryover and improve
feed quality. Periodically, all feed bins
should be emptied completely and
inspected. Feed not removed by
normal means should be manually
removed. This must be done when
changing feed medications or switch-
ing to withdrawal feeds because it
reduces the chance for mixing medi-
cated and withdrawal feeds, and will
also reduce buildup of moldy feed in
bins. Moldy feeds are particularly a
problem during hot, humid months
and winter months when moisture
condenses inside bins and runs down
the sides onto feeds. Where several
bins share a common wall, periodic
inspection for holes that allow feed to
flow between bins should be made.
Similar inspections of augers that
pass through bins may detect dump-
ing of medicated feeds into withdrawal
feed bins. Dusts from conveying
medicated premixes and feed into
overhead bins may leave a layer of
medication on unmedicated feeds.
Feed Conveyors
Screw augers are a common means
of feed conveyance between storage
facilities and to feeders. Figure 6
gives a cross section of a typical
screw auger. Note the gap between
the screw flights and the housing. This
is necessary to prevent undue wear
on the edges of the flights and on the
auger housing. This gap allows a
layer of feed to remain in the conveyor
even though no more feed comes out
the discharge end. This residual feed
is usually removed when the next
conveyance of feed is made. Depend-
ing on the length of the screw auger,
considerable amounts of medicated
feed can be carried over and dis-
charged into the next bins or feeders
that are filled.
Drag-type conveyors have much less
residual feed carryover but are more
expensive to purchase and maintain.
The same precautions as for screw
conveyors must be exercised.
Pneumatic Conveyor
Systems
Pneumatic conveyance systems are
increasing in popularity because of
their capability to move large quanti-
ties of feed over extended distances.
Segregation of ingredients can occur
and, unless specific management
techniques are implemented, residual
feed left in the tubes may be a source
of contamination of following feeds.
Dust collectors (cyclones) may be
needed in bins to ensure uniform feed
distribution. Designation of which bins
are being filled should be clearly
visible at any coupling locations. This
can help avoid a mistake in sending
feed to the wrong location.
Any feed delivery systems may have
low spots where feed can collect. For
example, spaces between augers and
diversion slides in feeder chutes will
fill with feed as it passes over the
chute. Therefore, it is recommended
that withdrawal feeders be placed at
the far end of auger runs or, better
yet, on separate systems to avoid
potential contamination. Levels of as
little as 1 pound of medicated feed per
100 pounds of non-medicated feed
may contribute to drug carryover,
which may result in drug residues in
animal tissues.
60
�
min.
Feed hopper slope
62
�
Ground
corn
26
�
Whole
corn
Angle of repose
Figure 4. Extreme case of feed
hang-up.
Packed feed Figure 5. Feed flow in bulk bin.
Feeders
Feeders come in a variety of shapes
and sizes. Most provide a storage
area from which the feed flows by
gravity to the animal access area.
Typical feeders are shown in figures 7
and 8. Note the feed flow pattern and
the areas where feed might continue
to reside after the gravity flow ceases.
Depending on the shape of the feeder,
considerable amounts of a medicated
feed could remain in a feeder that
appears to be empty. Addition of new
feed may dislodge part of the old feed,
making it available to the livestock
using the feeder.
Because of this characteristic, feeders
should be completely emptied, then
visually inspected for residual feed
whenever changes are made in kinds
or levels of animal drugs or to non-
medicated feed.
Drop or floor feeding systems are less
likely to cause contamination because
residual feed is minimized. Feed
allowed to build up in corners of pens
due to overfeeding may be a potential
source of residues.
What is Segregation?
Segregation can occur in ingredients
and in mixed feeds. In ingredients, it
is the separation of certain fractions or
particles from the remainder of the
material. In mixed feeds, segregation
is separation of one or more ingredi-
ents, or fraction of an ingredient, from
the remainder of the mix.
Segregation may occur at a number of
sites in the handling and processing
operations used to manufacture and
deliver feeds (figures 9 and 10).
Partial remixing may occur during
handling and processing operations,
reducing the overall segregation effect
on the finished feed.
Mixed feeds are subject to segrega-
tion because of differences in particle
size, particle shape, and density of
the various ingredients. The feed
ingredients, for example, can be
satisfactorily mixed in the mixer
become slightly segregated as they
drop into the bin, be somewhat
remixed in the bin auger and elevator
leg, become markedly segregated in
the free-air fall into the feeder, and be
remixed (partially) in the outflow from
the feeder.
Where Does Segregation Occur?
Table 1 lists some of the more com-
mon sites for segregation of mixed
feed. Note