NUTRITION PIH-86
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Feed Management to Prevent Drug Residue Problems in Pork
Authors
Gary L. Cromwell, University of Kentucky
LeRoy G. Biehl, University of Illinois
James McKean, Iowa State University
Kenneth B. Meyer, Purdue University
Robert A. Wilcox, Kansas State University
Reviewers
Glenn Brown, Delphi, Indiana
Dean and Jane Pretzer, Diller, Nebraska
Dale Keesecker, Washington, Kansas
Gary Weber, USDA, Extension, Washington, D.C.
Antibiotics and chemotherapeutics are widely used in swine
feeds. They are effective in improving the rate and efficiency of
growth and in reducing mortality and morbidity associated with
respiratory and intestinal diseases in pigs. Certain feed addi-
tives require a withdrawal period prior to slaughter in order to
insure that residues do not occur in the carcass. The additives
that require withdrawal and their withdrawal times are given in
Table 1. In addition, Table 1 includes those feed additives that
do not require any withdrawal.
The feed additives that have caused the greatest residue
problem and received the most attention in recent years are the
sulfonamides. The term, sulfonamide, includes sulfamethazine,
sulfathiazole, and other sulfonamide drugs. Although this fact
sheet will concentrate primarily on methods of preventing sul-
fonamide residues, many of the techniques that are discussed also
apply to the prevention of residues that could result from other
drugs.
Forms of Sulfonamides Used in Feeds
The only sulfonamides that can be legally used in feeds are
sulfamethazine and sulfathiazole. They are approved only in com-
bination with certain other antibiotics and only at one level of
inclusion (100 grams per ton). The feed additive combinations
that include sulfonamides are Aureo SP-250O and PfiChlor SP-250O
(chlortetracycline, penicillin and sulfamethazine), Tylan-SulfaO
(tylosin and sulfamethazine) and CSP-250O (chlortetracycline,
penicillin and sulfathiazole). In addition, these and other sul-
fonamides are sometimes used as water medications for controlling
pneumonia, scours and other bacterial infections.
Efficacy of Sulfonamides
Sulfonamides are primarily used for young pigs during the
early growth stages. Most pig starter feeds and about 75% of
grower feeds are medicated. Approximately one-third of these
medicated feeds contain sulfamethazine or sulfathiazole. One rea-
son for the popularity of the sulfonamide-antibiotic combinations
is that they are very effective growth promoters, as shown in
Table 2. A summary of 453 experiments involving 13,632 pigs indi-
cates that pigs fed sulfa-antibiotic combinations from 19 to 56
lb. gained 20.5% faster and required 7.8% less feed per pound of
gain than control pigs that received no antibiotics. For 10 other
antibiotics, the average improvements in daily gain and effi-
ciency of feed utilization were less than the sulfa-antibiotic
combinations, 13.8% and 6.5%, respectively. Similar trends were
found in a summary of 280 experiments involving slightly heavier
pigs, fed from 37 to 109 lb. (Table 2).
The sulfa-containing feed additives also have been shown to
help maintain acceptable performance in herds having acute or
chronic respiratory infections, such as atrophic rhinitis.
Sulfonamide Residues
The tolerance level for sulfamethazine and sulfathiazole in
pork tissue (liver, kidney or muscle) is set by the Food and Drug
Administration at 0.1 ppm. Regulations require that sulfametha-
zine be withdrawn from the feed for 15 days and sulfathiazole for
7 days prior to slaughter in order to insure that tissues do not
exceed the tolerance level.
Shortly after a national monitoring program was initiated by
the USDA in the 1970s, it was discovered that about 15% of hog
carcasses had violative sulfonamide residues. In almost all
cases, the sulfonamide found in the carcass tissues was sul-
famethazine. A major effort was initiated in 1977 by the USDA,
the Cooperative Extension Service and the National Pork Producers
Council to solve this problem by means of research and educa-
tional programs. Additional testing of carcasses for residues at
packing plants was implemented in 1987 with stiff penalties for
producers marketing hogs with violative residues. Although the
problem has not been completely solved, the violation rate is now
quite low, 1.12% in 1989 (43 violations of 3,855 samples) accord-
ing to USDA surveillance data (Figure 1). The incidence of sulfa
residues based on the Sulfa-on-Site (SOS) testing program was
0.27% in 1989 (316 of 116,726 samples), but this is based on mus-
cle rather than liver samples. Sulfamethazine residues in liver
are generally about four times higher than they are in muscle.
Causes of Sulfonamide Residues
What was the reason for the high incidence of sulfamethazine
residues, and why has it been so difficult to eliminate the prob-
lem? Initially, producers were blamed for not complying with the
withdrawal period. However, it was soon realized that some of the
violations were from farms where producers were making a real
effort to follow proper withdrawal times. In some cases, viola-
tions were even being reported on farms where sulfonamides were
not being used in feed or water.
Finally, results of research conducted at Iowa State Univer-
sity, the University of Illinois and the University of Kentucky
shed new light on the problem. Their studies showed that only a
very small amount of sulfamethazine in the feed would cause a
residue in the tissue. An early study at the University of Ken-
tucky indicated that as little as 1 gram of sulfamethazine per
ton of feed could result in a high incidence of violative resi-
dues. Table 3 illustrates data from a later study in which 2
grams of sulfamethazine per ton of feed was found to cause a
violative residue in liver tissue. A higher level of sulfametha-
zine (8 grams per ton) was required before a violative level of
sulfamethazine occurred in the muscle.
Sulfathiazole is excreted more rapidly than sulfamethazine
and, therefore, is less likely to cause residue problems. Table 3
shows that feed can be contaminated with up to 16 grams of sul-
fathiazole per ton, on a continuous basis, before a violative
residue occurs.
A major cause of the high incidence of sulfonamide residues
was, and still is, due to the cross-mixing of clean feed with
sulfonamide-containing feed. Drug carry-over can occur in commer-
cial feed mills and on the farm. It can also result from the
inadvertent purchase of sulfonamide-containing premixes and sup-
plements. As little as 40 lb. of a sulfamethazine-medicated feed
(containing 100 grams per ton), if mixed into a ton of "clean"
feed, will result in a feed containing 2 grams of sulfamethazine
per ton -- a carry-over level that can leave a violative residue
of sulfamethazine in liver tissue.
Preventing Drug Carry-Over in Feeds
Drug carry-over in feeds can occur in a number of ways. Feed
manufacturing equipment such as mixers, pellet mills, augers,
elevator legs, dust control devices and storage bins can harbor
dust and residual feed, which can carry-over into clean feed
(Figures 2, 3, 4 and 5). A vertical screw mixer may contain 40 to
50 lb. of residual feed in the boot after the feed is discharged.
Failure to remove this residual feed will cause the next batch to
be contaminated. In some farm mixers, such as portable grinder-
mixers, even more residual feed can remain; in some cases over
100 lb. per batch. A thorough clean-out or flush of all mixing
equipment, conveyors, augers, elevator legs, and similar equip-
ment is imperative in order to reduce the chance of drug carry-
over. Some producers use a second set of equipment for mixing
sulfonamide-free finishing feeds in order to solve the drug resi-
due problem.
A proper feed mixing sequence will reduce the degree of drug
carry-over. For example, a finishing feed should never immedi-
ately follow a sulfonamide-medicated feed. Instead, a
sulfonamide-medicated feed should be followed with a feed that is
less likely to cause residue problems, such as a grower feed.
The powdered form of the sulfonamides tends to be electros-
tatic and will cling to metal surfaces. Grounding of equipment
will reduce this characteristic, but will not completely elim-
inate it. Fortunately, the granulated form of sulfamethazine (the
form that is present in all commercial antibiotic-sulfamethazine
mixes) has helped to reduce this problem. In a study at the
University of Kentucky, the sulfa level in feed dust taken from
the inside surface of a mixer was 276 ppm when powdered sul-
famethazine was used as compared with only 59 ppm when granulated
sulfamethazine was used.
Feed should never be medicated with powdered sulfamethazine.
This is an illegal practice, and it is likely to cause severe
residue problems. Excessive dust and waste feed should not be
allowed to accumulate around feed mixing and handling equipment,
as they can be a source of drug carry-over. Accumulated dust
should be removed at regular intervals and discarded; it should
not be included in mixed finishing feed.
Bulk delivery trucks also can be responsible for drug
carry-over in feeds if medicated and nonmedicated feeds are
hauled at the same time or if the conveying system on these
trucks is not cleaned out well between delivery of sulfonamide-
medicated feed and delivery of nonmedicated feed. (Figure 6).
Bulk storage bins on the farm should never be used for both
sulfonamide-medicated feed and nonmedicated feed unless they are
thoroughly cleaned between batches. Feed tends to cling to the
sides and corners of the bins (see Figures 7 and 8) and in the
discharge augers. Drug carry-over can occur in these structures
if they are not completely emptied and properly cleaned between
batches of feed. Hog feeders should be emptied and cleaned after
sulfonamide-medicated feeds are used, if the same feeders are
used to finish out hogs. If feeders are not cleaned out com-
pletely, medicated feed can build up in certain parts of feeders
(see Figure 9) and can contaminate several batches of nonmedi-
cated feed. If a thorough clean-out and flushing of the feed
delivery system in a building is not possible, then separate
delivery systems are recommended for sulfonamide-medicated and
nonmedicated feeds. Another alternative is to completely avoid
the use of any sulfonamide-medicated feed in every building that
houses finishing pigs.
The same contamination principles hold true for water medi-
cators. Care should be taken to prevent contamination of clean
water with sulfonamide-medicated water. Also, one should not
medicate the feed and the water with sulfonamides at the same
time. This practice could cause high intakes of sulfonamides and
could result in a residue, even with proper withdrawal times.
Proper Mixing of Feeds
Producers who mix their own feed on the farm must follow
good feed mixing practices to insure uniform dispersal of drugs
and other microingredients in feed. Adequate mixing time is a
must. Both undermixing and overmixing should be avoided. Recom-
mended mixing time for vertical mixers is 15 to 20 minutes and
for horizontal mixers is 6 to 8 minutes per batch. Accurate
scales must be used. Volumetric mix mills should be calibrated
often (at least once a week) to insure proper inclusion rates of
ingredients. Producers must be certain that only approved levels
of drugs and approved combinations of drugs are used in feeds.
Levels and combinations of drugs are regulated by the FDA and are
published in the Feed Additive Compendium (Miller Publishing Co.,
Minnetonka, MN).
Producers should use a record system to keep track of their
medicated feeds. An example of one is shown in Figure 10. A good
record system also will help to avoid mixing errors.
Preventing Access to
Sulfonamide-Containing Manure
Studies at the University of Illinois and Iowa State Univer-
sity indicate that sulfonamide residues in pork can be caused by
pigs having access to sulfamethazine-containing manure. Pigs
housed on solid floors that allow accumulation of manure and
urine are more likely to pick up sulfonamide from the floors than
those housed on slotted floors. Lagoons that receive wastes from
buildings where sulfonamide is being used can be a source of con-
tamination when lagoon water is used in finishing house flush
systems.
Following sulfonamide withdrawal, pigs should be moved to a
clean pen or the pen should be thoroughly cleaned at the time of
withdrawal. These pens should be cleaned 3 to 7 days following
sulfonamide withdrawal. Pigs should not be allowed to have access
to manure in trucks, holding pens, etc., where other hogs that
may have had sulfonamide in their feed were kept. Holding pens
that allow pooling of urine should be avoided before and during
marketing.
Adherence to Withdrawal
Producers must be certain that they comply with the proper
withdrawal periods; 15 days for sulfamethazine and 7 days for
sulfathiazole in feed. Water medications may require longer with-
drawal periods. To be on the safe side, it is best to include
sulfonamides only in the starter feed. If sulfonamides are used
in grower feed, they should not be used beyond 125 lb. body
weight. Sulfonamides should be left out of the finisher feed.
Some producers finish their hogs in a separate building and avoid
the use of sulfonamides in the finishing building. This practice
also solves the recycling problem caused by sulfonamide-
contaminated manure. Sows and gilts that are sent to slaughter
also can contribute to residue problems if withdrawal times are
not adhered to.
Some have suggested that finishing hogs be fed only corn for
several days prior to slaughter. While this practice might help
insure that sulfonamide will not be present in the preslaughter
feed, it may be a costly practice. Corn is extremely low in
lysine and other amino acids, so growth rate and feed conversion
will be markedly reduced by feeding shelled corn for any extended
period of time, even to finishing hogs. However, this practice
might be feasible as a last resort for producers having a serious
residue problem.
Testing Live Hogs for Sulfonamide Residues
Kits are now available for on-the-farm testing of live hogs
for potential residues. In these tests, urine is collected and
tested. Producers who anticipate a potential problem with sul-
fonamide residues or who are interested in their sulfa-residue
status can test a few hogs before they go to slaughter to insure
that they are free of residues. When help is needed, swine prac-
titioners can perform these tests or have access to persons who
can perform the tests. Sulfamethazine test kits are available
from the following companies:
Environmental Diagnostics IDEXX Co. Idetek, Inc.
Box 908 100 Fore St. 1057 Sneath Ln.
Burlington, NC Portland, ME San Bruno, CA
27215 04101 94066
Pork Quality Assurance
The National Pork Producers Council has developed a three-
stage Pork Quality Assurance program which is designed to assist
pork producers in eliminating sulfonamide and other drug resi-
dues. The basis for the program is education and management
changes followed by voluntary testing for residues. Information
on the program is available from the National Pork Producers
Council, Box 10383, Des Moines, IA 50306.
Summary
Drug residues in pork carcasses are a deterrent to consumer
acceptability of pork and to international sales of pork. Drug
residues can be greatly reduced and even be eliminated by adher-
ence to the following practices.
1. Use only approved levels and combinations of drugs.
2. Follow good feed mixing practices (especially adequate mix-
ing time) to insure that feed is mixed properly.
3. Maintain a record system to keep track of drug premixes and
medicated feed usage.
4. Mix batch feeds in proper sequence to reduce the chance of
carry-over of drugs into finishing feed.
5. Clean out or flush feed mixing, conveying and feeding equip-
ment to reduce drug carry-over into finishing feeds.
6. Adhere to proper withdrawal periods for drugs.
7. Prevent recycling of drugs via manure and urine.
8. Use on-farm testing program to insure freedom from drug
residues.
9. Read and follow the guidelines outlined in the Pork Quality
Assurance Program of the National Pork Producers Council.
10. Inform other pork producers how to check for residues and
the problems associated with residues.
Reference to products in this publication is not intended to
be an endorsement to the exclusion of others which may be simi-
lar. Persons using such products assume responsibility for their
use in accordance with current directions of the manufacturer.
REV 6/90 (5M)
Figure 1. Incidence of sulfonamide residues in pork liver from 1977 to 1989.
Figure 2. This type of grinder-mixer is commonly used on hog farms. Mixers
can harbor excessive residual feed and dust, and should be cleaned after mixing
sulfonamide-medicated feed.
Figure 3. Vertical screw mixers are commonly found in small feed mills and in
some feed mixing centers on hog farms. Because the discharge opening is above
the lower end of the mixing auger, considerable amounts of feed can remain
after feed no longer comes out of the discharge opening. This type of mixer
also can harbor sulfonamide-laden dust.
Figure 4. Elevator legs can hold sizable amounts of residual feeds or
ingredients. Some of this material can be incorporated in the next batch of
feed.
Figure 5. Augers often leave residual feed in the housing because the screws
must have clearance. Drag-type conveyors are preferred because they are self-
cleaning.
Figure 6. A feed delivery truck can be a source of drug carry-over. Feed can
remain in the lower horizontal conveyor and in the vertical conveyor. Although
the amount of residual feed depends on the design of a particular system,
residual feed can range up to 100 pounds.
Figure 7. Movement of feed out of a bin occurs directly above the discharge
opening. The remaining material then cascades down the slope of the crater
that is subsequently formed. Failure to completely empty bins before refilling
will result in residual feed being left in the bin.
Figure 8. Typical feed flow in a bin with the dark areas illustrating where
the feed is most likely to remain and contaminate the next batch.
Figure 9. Typical feed flow in a hog feeder. Residual feed areas are
indicated by dark areas. Failure to completely empty the feeder before
refilling will result in residual feed being left in the feeder.
Figure 10. A feed mixing record sheet will help to eliminate mixing errors and
help producers keep track of medicated feeds.
% Figures are available in hard copy.
______________________________________________
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