-
Antimicrobial Feed Additives
for Swine: Past,
Present and Future
Trends
Livestock Update,
February 2004
Allen Harper,
Extension Animal Scientist
?
Swine, Tidewater AREC and
Mark
Estienne, Swine Research Physiologist, Tidewater
AREC
Background
Sub-therapeutic levels of antimicrobial
feed additives have been used in
swine
feeds since the 1950's for improved growth rate
and feed efficiency
and to maintain pig
performance in the presence of sub-clinical
disease. The
most effective use is in
the diets of weanling and young growing pigs but
responses are also obtained in finisher
pigs and breeding swine. Over 15
years
ago Zimmerman (1986) summarized the data from 239
separate
experiments and reported that
average improvement response to
antimicrobial feed additives in starter
pigs was 15% for growth rate and 6%
for
feed efficiency. In older growing-finishing pigs
the improvement was 4%
for growth rate
and 2% for feed efficiency. It is believed that
weanling and
starter pigs are more
susceptible to stress and sub-clinical disease and
consequently show a greater response to
growth promoting antimicrobial
products. Studies have also indicated
that both starter and finisher pigs have
a greater response to antimicrobials
under farm conditions than at swine
research facilities, possibly because
the disease, sanitation and housing
stresses are typically greater at
commercial farms than in research facilities.
The consistent effectiveness of
antimicrobial feed additives has led to
extensive use in the swine feeding
industry. Cromwell (2001) estimated that
80 to 90% of all starter pig feeds, 70
to 80% of all grower pig feeds, 50 to
60% of all finisher pig feeds and 40 to
50% of all sow feeds are fortified
with
antimicrobial feed additives. There are 18
antimicrobial feed additive
products
currently approved by the U.S. Food and Drug
Administration for
use in swine diets
(Table 1). General reviews on the use and effects
of
antimicrobial feed additives for
swine are available from Cromwell (2001)
and in Pork Industry Handbook fact
sheet 31, Feed Additives for Swine
(Parker and co-workers, 1994).
Despite their effectiveness, continued
use of antimicrobial growth promoters
faces a very uncertain future. In 1999,
the European Union of agricultural
ministers banned the use of
virginiamycin, spiramycin, tylosin phosphate
and zinc bacitracin (Smith, 1999). In
addition, avoparcin, olaquindox and
carbadox are also not allowed for use
as growth promoters in Europe. In this
country the U.S. Food and Drug
Administration (FDA) and Centers for
Disease Control (CDC) have called for
an extensive reevaluation of
continued
use of antimicrobial feed additives (Smith, 1999,
Elliott, 2001).
One concern associated
with use of antimicrobial feed additives is the
potential for antimicrobial residues in
meat products of treated animals. For
this reason certain antimicrobial feed
additives have a legally required
pre-
slaughter withdrawal period during which time the
animals must not be
fed the product
before shipping for slaughter (Table 2). Table 1.
Antimicrobial Agents Approved for use
in Swine Feeds
a
Generic Name
Apramycin
Arsanilic acid
Bacitracin
methylene
disalicylate
Bacitracin zinc
Bambermycins
Carbadox
Chlortetracycline
Lincomycin
Neomycin
Oxytetracycline
Penicillin
Roxarsone
Sulfamethazine
Sulfathiozole
Tiamulin
Tilmicosin
Tylosin
Virginiamycin
a
Classification
Example Trade Names
Antibiotic
Apralan
Chemotherapeutic
Pro-Gen 20%
Antibiotic
Antibiotic
Antibiotic
Antibiotic
Antibiotic
Antibiotic
Antibiotic
Antibiotic
BMD
Baciferm
Flavomycin, Gainpro
CTC,
Pennchlor
Lincomix
Neomix,
Neo-terramycin
OXTC, Pennox
CSP-250, CSP-500
Chemotherapeutic
Mecadox
Chemotherapeutic
3-Nitro
Chemotherapeutic
Tylan 40
Sulfa-G
Chemotherapeutic
CSP-250, CSP-500
Antibiotic
Antibiotic
Antibiotic
Antibiotic
Denagard
Pulmotil 90
Tylan 40, Tylan
100
Stafac
Source: Feed
Additive Compendium, 2004.
Table 2.
Antimicrobial Feed Additives Requiring a Pre-
slaughter
Withdrawal
a
Generic name
Apramycin
Carbadox
Neomycin
Example trade name
Pre-
slaughter withdrawal (days)
Apralan
Mecadox
Neomix Ag 325
28
42
3
Terramycin
Oxytetracycline
10 - 50
grams/ton
0
10 mg/kg body
weight
5
Oxytetracycline +
Neomycin
sulfate
Neo-Terramycin 50/50
5
Roxarsone
Sulfamethazine
Sulfathiozole
3-Nitro 20
Tylan 40 Sulfa-G
CSP-250, CSP-500
Denagard
10 grams/ton
35 grams/ton
200 grams/ton
Pilmotil 18
5
15
7
0
2
7
7
Tiamulin
Tilmicosin
a
Source: Feed Additive Compendium, 2004.
An even greater concern among
regulatory agencies and consumers is the
potential that regular use of
antimicrobial feed additives may lead to the
development of resistant microbes that
may compromise the effectiveness of
antibiotics in treating animal and
human disease. But, there is considerable
debate as to whether the use of
antimicrobial feed additives will lead to a
significant risk of inability to treat
disease. For example, long-term studies
have shown that using feed grade
antimicrobials on swine farms for
extended periods of time does lead to
the development of resistant
organisms
but that the growth promoting effects of the
products continue to
be realized.
Furthermore, within a herd in which antimicrobial
use was
suspended for over 13 years,
significant levels of resistant microbe strains
were still identified (Langlois and co-
workers, 1986). Nevertheless the
impetus for banning antimicrobial feed
additives in food animal production
is
very strong. Such forces as consumer acceptance,
export markets and
medical community
directives are likely to force loss of some, if
not all
antimicrobial feed additives
for performance enhancement in food animals.
Can We Produce Pigs without
Antimicrobial Feed Additives?
The simple (and absolutely correct)
answer to this question is a definite yes.
There are currently a limited but
growing number of small producers that
grow pigs without antimicrobial feed
additives for
specialty markets. And
more conventional producers are reducing the
quantity of antimicrobials being used.
However, what is less apparent is
what
producers have to give up in terms of pig
productivity and efficiency if
antimicrobial feed additives are not
used.
The issue has stimulated interest
in alternatives to antimicrobial feed
additives. Included among potential
alternatives are complex mannose
carbohydrates termed
mannanoligosaccharides derived from the cell wall
of
yeasts. There is evidence that
dietary inclusion of mannanoligosaccharides
has immunomodulatory properties and may
improve growth performance of
weanling
pigs (Spring and Privulescu, 1999). So-called
probiotics are a
second category of
potential replacements for antimicrobial growth
promoters. Probiotics generally refer
to viable microbial cultures that are
intended to increase the
gastrointestinal population of beneficial microbes
while competitively excluding bacteria
that may depress health or growth
performance (Cromwell, 2001).
At the Virginia Tech Tidewater AREC we
assessed two commercially
available
feed additives in 5-week nursery pig trials as
potential alternatives
to traditional
antimicrobial feed additives (Harper and Estienne,
2002). In
trial 1 a
mannanoligosaccharide (Bio-Mos, Alltech
Incorporated) was
evaluated with the
antimicrobial feed additive carbadox (Mecadox,
Phibro
Animal Health). Treatments
included: 1) a control diet with no supplemental
feed additive; 2) a diet containing
Bio-Mos (0.3% during wk 1 and 0.2% for
wk 2-5); 3) a diet containing carbadox
(50 grams/ton); and, 4) a diet
containing both additives.
Results are summarized in Table 3.
There were no main effects of Bio-Mos
supplementation and no interaction
effects of Bio-Mos and carbadox in
combination (P > 0.40). The main
effects of carbadox supplementation were
an 8 % increase in feed consumption and
a 9 % improvement in growth rate
(P <
0.01).
In trial 2, a probiotic product
(BioMate-2B, Chris Hansen Biosystems)
consisting of Bacillus licheniformis
and subtilis was assessed. Treatments
included: 1) a control diet with no
feed additive; 2) a diet containing
Bacillus additive (0.1%); 3) a diet
containing carbadox (50 grams/ton); and,
4) a diet containing both additives.
Results are presented in Table 4. For
the overall 5-wk trial there was no
Bacillus by carbadox interaction (P >
0.44) and no Bacillus main effect