Practice tipPeer reviewed Feed additives for swine : Fact sheets – acidifi ers and antibiotics

Introduction Feed additives are non-nutritive products used in swine diets to improve production effi ciency and performance. If chosen carefully and used properly, feed additives can be effective and can help increase the profi tability of pig production. Not all feed additives are the same or provide a benefi cial response and, therefore, choosing a product will depend on the farm’s specifi c situation and needs.


Introduction
Feed additives are non-nutritive products used in swine diets to improve production effi ciency and performance.If chosen carefully and used properly, feed additives can be effective and can help increase the profi tability of pig production.Not all feed additives are the same or provide a benefi cial response and, therefore, choosing a product will depend on the farm's specifi c situation and needs.This series of fact sheets includes some of the major classifi cations of products used as feed additives.Every effort has been made to ensure that all the information in every fact sheet is current and based on the latest scientifi c publications available at the time of writing.The objective of these fact sheets is to discuss some of the basic concepts to help producers improve their understanding of these products.They also aim to promote more responsible and judicious use of feed additives.
Feed-additive products used in swine diets include natural and synthetic substances and have been grouped in this series of fact sheets according to the classifi cations shown in the text box.
Each group of feed additives is discussed in a separate fact sheet, with special emphasis on some of the common questions that producers might have for each product.Feed additives offer a variety of potential benefi ts.However, they add to total production cost and should be evaluated carefully.Because their use in pig diets is to improve performance and profi tability, an effective feed-additive product must be able to pay for itself.It must be able to provide an improvement in productivity that is, at minimum, equivalent to the added cost of the feed-additive product.This highlights the value of scientifi c data from well-designed experiments as the basis for evaluating such products.Having access to such information is critical in determining if one product's claims are actually possible and repeatable in commercial settings.Producers must always try to verify that the data for a particular product came from controlled, unbiased experiments with supporting statistical data.When choosing between feed-additive products, priority for using a specifi c product should be given to those that have been shown to provide consistent results in research trials.Benefi cial claims from dietary inclusions of acidifi ers include control of bacterial growth in feed, increased growth performance, improvement in nutrient digestibility, and control of harmful bacteria in the gut.

What are acidifi ers?
Acidifi ers are compounds that have acidic properties: they may be organic or inorganic acids.Organic acids that have shown positive effects on growth performance in weaned pigs include citric, formic, fumaric, and propionic acids.In studies involving inorganic acids, 1-3 positive growth responses have been reported with the use of phosphoric acid.However, research evaluating other inorganic acids, such as sulfuric acid, reported negative growth performance. 4Thus, phosphoric acid is the most commonly utilized inorganic acid in swine diets.
Inorganic acids are usually less costly than organic acids.Organic and inorganic acid combinations are often used in commercially available acidifi ers.The response to mixed acids is generally better than to single acids, 5 apparently due to dissociation properties of these acids at various locations in the pig's digestive tract.

What are the benefi ts of using acidifi ers?
A recent report 2 summarizing several studies on acidifi ers indicated that, in general, they appear to improve pig growth performance.However, the magnitude and consistency of the response may vary, depending on inclusion rate and other dietary factors.The exact mode of action of acidifi ers has not been fully elucidated.However, acidifi ers are commonly marketed as growth-promoting products and as alternatives for in-feed antibiotics.Unfortunately, due to the lack of consistent results, use of acidifi ers to replace antibiotic growth promoters is still not justifi ed.Acidifi ers are believed to enhance growth by improving gut health through reduction of pH and buffering capacity of diets, improvement of pancreatic secretions that increase nutrient digestibility, or promotion of benefi cial bacterial growth while inhibiting growth of pathogenic microbes. 2,6There is limited data indicating that acidifi ers can act synergistically with phytase to improve phosphorus and magnesium digestibility. 7,8

What factors affect the response to acidifi ers?
Research suggests that age of pigs can affect the response to acidifi ers, with newly weaned pigs showing the greatest response. 4,9cidifi ers are most benefi cial during the fi rst few days after weaning.The stomach of a weaned pig is not yet physiologically mature and may not be able to secrete a suffi cient amount of acid to aid in digestion of solid food or inhibit proliferation of detrimental bacteria.However, the exact mechanism of the response to acidifi ers is not clear.
Diet composition also may affect the response to acidifi ers.It appears that greater responses are seen when simple diets are fed rather than complex diets containing milk products. 10This is presumably due to conversion of lactose from the milk products to lactic acid by Lactobacillus species in the stomach, thus creating an acidic environment and reducing the need for dietary supplementation with acidifi ers.

Fast facts
Acidifi ers used in pig diets may be in organic or inorganic forms.
Acidifi ers appear to be most effective in newly weaned pigs and in less complex nursery diets.
Growth-promoting effects of acidifi ers in pig diets need to be further investigated to be justifi ed as suitable replacements for antimicrobials.

Disadvantages of acidifi ers
Corrosiveness, one disadvantage of using some acidifi ers, may pose handling and equipment issues to the feed manufacturer.Salts of organic acids are generally odourless and less corrosive than their acid forms, making them easier to handle in the feed manufacturing process.Acidifi ers may negatively affect diet palatability when added at excessive levels, resulting in lower feed intake.There may also be legal restrictions with the use of some acids.For example, pure formic acid is not legal for use in the United States, but salts of formic acid are available for use in feeds.

Summary
Acidifi ers added to pig diets may potentially help improve growth performance by improving digestive processes through several mechanisms.However, a clear mode of action has yet to be described.The use of acidifi ers appears to be most benefi cial in the early period after weaning.Thus, acidifi er use is typically limited to diets for pigs weighing less than 6.75 kg (15 lb).

Fast facts
Use of in-feed antibiotics in pigs is regulated by the Food and Drug Administration and they must be used only as approved.
No extra-label usage is allowed for in-feed antibiotics.
The best responses in growth performance are seen in nursery pigs.
Magnitude of responses may differ depending on herdhealth status and sanitation.
Concerns are increasing about the negative consequences of antibiotic use in food animals.
Antimicrobial agents, such as antibiotics, have been used in pig production for over 50 years.Early studies indicated signifi cant improvements in pig growth performance when antibiotics were fed.With the improvements in production practices and health status of pig herds, positive responses to in-feed antibiotics may not be as large in today's modern facilities.Additionally, the magnitude of response differs with the stage of pig growth.Use of antibiotics as feed additives is subject to regulatory policies to prevent residues and enhance public health.It is therefore important to be aware of the current information available concerning the effects of commonly used in-feed antibiotics in pig production.

How do antibiotics enhance growth?
Antibiotics are non-nutritive feed additives, which means that they do not provide further nourishment to the pig, and their absence in a well-balanced diet will not result in nutritional defi ciency.Antibiotics are included in swine feed for their therapeutic potential as well as their ability to promote growth.Some of the proposed possible mechanisms by which antibiotics improve growth include inhibition of subclinical pathogenic bacterial infections; reduction of microbial metabolism products that may negatively affect pig growth; inhibition of microbial growth, thereby increasing nutrients available to the pig; and an increase in uptake and utilization of nutrients through the intestinal wall. 1

Effi cacy of in-feed antibiotics
Studies 2 on the effects of antibiotic feed additives have indicated signifi cant improvements in growth rate and feed effi ciency (Table 1).These studies, however, were conducted more than two decades ago, when disease pressures in pig farms were relatively greater than in today's facilities.With numerous improvements, such as multi-site pig production, nutrition, biosecurity, and overall pig husbandry practices in the last two decades, responses may not be as great.A more recent study 3 on the use of in-feed antibiotics in modern production systems showed that such additives are still effective in improving growth in nursery pigs, although the magnitude of the response is less (Table 2).However, in fi nishing pigs, no improvement is noted.Many factors can affect the efficacy of antibiotic feed additives, including nutrition, management practices, and health status.When these factors are optimal, less or almost no response to antibiotics can be expected, especially with excellent sanitation practices and lack of bacterial disease pressure.The data on feeding antimicrobials in sow diets, however, is much more limited than that in growing pigs.Antibiotics in sow diets may improve reproductive performance in herds with a high incidence of reproductive problems due to greater disease challenge. 4,5Thus, herds experiencing problems with conception rates and litter size associated with bacterial infections may benefi t from the addition of antibiotics to sow diets.Chlortetracycline and oxytetracycline, the two in-feed antibiotics approved for use in sow diets, are indicated to reduce the incidence of abortion due to Leptospira interrogans serovars and reduce shedding of these organisms.However, routine feeding of antibiotics to the breeding herd is discouraged.

Choosing the proper antibiotic
When the antibiotic appropriate for a specifi c herd is selected, a number of important things must be considered, for example, the disease organisms present in the herd.Certain antibiotics may be more effi cacious in treating respiratory problems, while others may be more effective against enteric pathogens.Stage of production and withdrawal period also will determine the specifi c antibiotic of choice.While in-feed antibiotic use is most prevalent in nursery diets, it is sometimes necessary to use antibiotics in grow-fi nish diets, eg, during outbreaks of bacterial disease.Observing the proper withdrawal time for an in-feed antibiotic is important to avoid residues in the meat.Improper consideration of withdrawal time may result in delays in marketing pigs.The product also must be approved for use in swine, as no extra-label usage is allowed for in-feed antimicrobials.Ultimately, choosing the proper in-feed antibiotic depends on the benefit in production efficiency compared to cost and risk of residue.

Proper use of in-feed antibiotics
While most in-feed antibiotics are available without veterinary supervision, they should not be used indiscriminately.They should be used only for purposes specified on the labels.A good reference for the list of drugs that can be used as feed additives is the Feed Additive Compendium, 6 which is updated regularly to provide upto-date information and provides guidelines on the proper use of antibiotics in feed.Each country has its own regulatory policies regarding use of feed additives in pigs.Thus, the recommendations in this fact sheet may not apply outside of the United States.It is, therefore, important for US producers to be aware of which antibiotics are forbidden in countries that import pork from the United States.

Which antibiotics are approved for use as feed additives in pig diets in the United States?
Antibiotics and combinations approved for use in swine diets, including withdrawal times, are listed in Table 3. Florfenicol and tilmicosin, which are classified as Veterinary Feed Directive (VFD) drugs, are also included in the list.Veterinary Feed Directive drugs can be used only under the order and professional supervision of an appropriately licensed veterinarian. 7Before a VFD drug can be used, the producer must first contact the veterinarian to diagnose and treat the existing health problem.A VFD order can be written only by a veterinarian for drugs that are approved for that swine category and under a valid client-patient relationship. 7This is accomplished by filling out a form in a format approved by the Food and Drug Administration Center for Veterinary Medicine.All pertinent information must be provided by the veterinarian.The veterinarian, producer, and feed miller must all follow the responsibilities outlined by the Food and Drug Administration Center for Veterinary Medicine when using VFD drugs.Issued VFDs for florfenicol and tilmicosin have an expiration period of 90 days.

Summary
Increased productivity, efficiency, and profitability are the goals of every swine-production business.Antibiotics have been used in swine diets for several decades to improve growth performance, as well as to control and treat diseases.Because of the improvements made in housing, nutrition, production, and health-management practices over the years, the impact of antibiotics on growth performance may not be as large or as consistent in response as those observed during the early years of antibiotic use.In-feed antibiotics remain an effective tool in improving production efficiency, but are not a substitute for good production management.These products must be used properly and responsibly.
of copper and zinc for growing pigs Mold inhibitors, mycotoxin binders, and antioxidants Phytase Phytogenic feed additives (phytobiotics-botanicals) Probiotics and prebiotics This begins a series of peer-reviewed Practice tip articles, each including two or three fact sheets.

Table 1 :
Effectiveness of in-feed antibiotics on production responses in pigs*

Table 2 :
Effectiveness of in-feed antibiotics in nursery and grow-finish pigs reared in modern production systems*

Table 3 :
Withdrawal periods for FDA-approved in-feed antibiotics and combinations*