NUTRITION                                         PIH-86


    Feed Management to Prevent Drug Residue Problems in Pork

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

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

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

     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

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.


     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

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

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-

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.

Cooperative Extension Work in  Agriculture  and  Home  Economics,
State  of Indiana, Purdue University and U.S. Department of Agri-
culture Cooperating. H.A. Wadsworth,  Director,  West  Lafayette,
IN. Issued in furtherance of the Acts of May 8 and June 30, 1914.
It is the policy of the Cooperative Extension Service  of  Purdue
University  that  all  persons  shall  have equal opportunity and
             access to our programs and facilities.