RESIDUE AVOIDANCE PROGRAM - FEED HANDLING SYSTEMS
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 Fifth, veterinarian for Feed Specialties, Des Moines,
Iowa; Lee Boyd, executive secretary, American Feed Manufacturers Association.
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 accessible to the livestock. Each of these components represents
an opportunity for the feed to become contaminated, 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 contaminating
clean feed that follows.
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. Figure 2 and figure
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]. lf such
bins are refilled before completely 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.
A frequently overlooked, but simple
measure to help reduce feed contamination 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 switching to withdrawal feeds because it reduces the chance
for mixing medicated 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
dumping of medicated feeds into withdrawal feed bins. Dusts from conveying
medicated premixes and feeds into overhead bins may leave a layer of medication
on unmedicated feeds.
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. Depending on the length of the screw auger,
considerable amounts of medicated feed can be carried over and discharged
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 quantities 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.
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 figure
7 and figure 8. Note the feed flow pattern
and the areas where feed might continue to reside after the gravity flow
Depending on the shape of the feeder,
considerable amounts of a medicated feed could remain ln 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 nonmedicated
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. ln mixed feeds, segregation
is separation of one or more ingredients, or fraction of an ingredient,
from the remainder of the mix.
Segregation may occur at a number of
sites ln the handling and processing operations used to manufacture and
deliver feeds [figure 9 and figure
10]. Partial remixing may occur during handling and processing operations,
reducing the overall segregation effect on the finished feed.
Mixed feeds are subject to segregation
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 fail into the feeder, and be remixed (partially]
in the outflow from the feeder.
Where Does Segregation Occur?
Table 1 lists some of the more common
sites for segregation of mixed feed. Note that these are all subsequent
to the mixing equipment. Theoretically, segregation can occur in mixers
but the result is the same as incomplete mixing. Table 2 lists some possible
Table 1. Sources of segregation problems.
|Mixer surge bin
||Free fall from mixer segregates particles
Air pressure relief segregates particles
Mill or equipment vibration
||Segregation of particles at elevator discharge
||Segregation of cyclone collector
Free fall from collector to bin
Angle of repose of feed segregates particles by size
|Holding bin, bagging bin
||Segregation occurs as feed falls through air
|Bulk bin, bulk truck, customer bin
||Funneling during discharge accentuates segregation
Vibrations of mill or equipment segregate particles
|Dust collecting systems
||Very fine particles tend to be aspirated off, particularly when air
flow rates are high
Returned dust is not thoroughly mixed back into feed
Table 2. Overcoming segregation tendencies of mixed feed.
||Grind ingredients to a uniform particle size range Use liquids to agglomerate
||Process to more uniform particle shape range If shape is a desired
characteristic, use fat or molasses to agglomerate
||Agglomeration is the most common way to overcome density differences
Finer particle size reduces tendencies to segregate because of density
What Causes Segregation?
The major factors involved in segregation
are particle size, shape, and density. Very large particles and very fine
particles tend to segregate when combined. Particle shape is a definite
factor both when particles are ln free fall through air and as they pile
up ln storage. Flat particles will tend to fail slower and remain where
they fail. Round or near round particles will fall faster and will tend
to roll towards the storage walls. Particles with high density will be
less affected by the free fail air resistance than will those of low density.
The less dense and smaller particles will tend to be carried toward the
walls by the air currents created in the bin.
These factors interact in many ways.
For example, a formula that combines large, flat, low-density particles
with small, cuboidal, high density particles (such as a mix of rolled oats
and salt] results in a mixture that segregates markedly when subjected
to vibrations or dropped into a storage bin. Addition of other ingredients
having physical properties between these extremes will greatly improve
the stability [reduce the tendency to segregate] of the mixed feed. Additions
of fats, molasses, or other liquids also will reduce particle segregation
by agglomerating smaller particles into larger ones.
Electrostatic charge buildup in mixing
equipment may cause small particles to attach to the mixer or metal handling
equipment. Some drug compounds are more susceptible to this phenomenon
than others. Enough drug may become attached during production of medicated
feeds to produce residue in subsequent batches. Electrical grounding of
mixing and delivery equipment will minimize this carryover. Selection of
drug premixes with larger particle sizes will reduce electrostatic carry