Maintenance of healthy animals requires prevention of infection by pathogenic organisms. In addition, specific alteration of a host's microflora may have beneficial effects on animal production by alteration of ruminal flora, resulting in changes in the proportions of volatile fatty acids produced during ruminal digestion. Thus, antimicrobial compounds may improve production efficiency of healthy animals fed optimal nutritional regimens. Production-enhancing antimicrobial compounds can be classified as ionophore or nonionophore antibiotics. Antimicrobial compounds are administered in the feed at low dose rates relative to high doses required for therapeutic effects. Feed additives can be given once the rumen is functioning, although some antibiotic compounds can be fed to calves prior to this point.

Antimicrobial growth promotants commonly used in livestock are detailed in see Growth Promotants and Production Enhancers: Antibacterial Growth Promoters for Potential Use in Livestock Production. Antimicrobials are used in male and female animals without adverse effects on ovarian and testicular development or function because they are poorly absorbed. Unlike anabolic steroids, they do not affect carcass composition. Antimicrobials are commonly used in conjunction with estradiol, zeranol, or TBA, and generally their combined effects are additive.

Table 3
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Antibacterial Growth Promoters for Potential Use in Livestock Production Compound Class Absorption Effects Bambermycins Phosphoglycolipid Not absorbed Increase FCEa, growth promotion in poultry, cattle Lasalocid sodium Ionophore Increase FCE in cattle Monensin sodium Ionophore Poorly absorbed Increase FCE, increase DLWGb in cattle and lambs Salinomycin Ionophore Increase DLWG and FCE Virginiamycin Peptide Not absorbed Growth promotion in poultry Zinc bacitracin Peptide Not absorbed Growth promotion in poultry a Feed conversion efficiency b Daily liveweight gain

Ionophore Antibiotics

Ionophores (eg, monensin and lasal-ocid) modify the movement of monovalent (sodium and potassium) and divalent (calcium) ions across biologic membranes, modify the rumen microflora, decrease acetate and methane production, increase propionate, may improve nitrogen utilization, and can increase dry matter digestibility in ruminants. Their main effect is to increase feed efficiency, but they may also improve growth rates of ruminants on high-roughage diets. Administration of monensin to cattle results in 2–10% improvement in liveweight gain (in animals on a high-roughage diet), 3–7% increase in feed conversion efficiency, and up to a 6% decrease in food consumption. Initially, monensin was used only as a feed additive for ruminants fed in confinement, but its use has been extended to grazing animals. Other ionophores generally have similar effects. Doses range from 6–30 ppm in the diet. Ionophores are absorbed from the gut, rapidly metabolized by the liver, and re-enter the gut from bile. Some ionophores also have a therapeutic use (eg, for prevention of coccidiosis in ruminants and poultry).

Nonionophore Antibiotics

These compounds are used to selectively modify microbial populations within animals to improve production efficiency and to maintain health by combating low-level infections, particularly in intensive systems. Phosphoglycolipid antibiotics (eg, flavophospholipol) alter ruminal flora by inhibiting the action of some gram-positive gut microorganisms and peptoglycan formation, yielding similar production responses to those produced by ionophores. The means by which specific compounds exert their antimicrobial effect differ. Antibiotics may have a nitrogen-sparing effect, thereby increasing the availability of amino acids to the animal.

Most feeds for broiler and pig production in some countries contain antimicrobial growth promoters. These compounds can also be administered to calves, yearlings, and finishing cattle either in milk replacer or in supplementary concentrates. Antibiotic compounds, in general, increase growth rate by 2–10% and feed conversion efficiency by 3–9%. Their effects are greater in young animals, and production responses are reduced when production conditions are optimized (good housing, optimal health, and hygiene). They have minimal effects on carcass composition other than that due to better growth rate.

The development of microbial resistance to antibiotics in treated animals, which can then be spread to humans, is an important concern regarding the widespread use of antimicrobial feed additives in food production. There is circumstantial evidence that use of subtherapeutic doses of antimicrobials creates selective pressure for the emergence of antimicrobial resistance, which may be transmitted to the consumer from food or through contact with treated animals or animal manure. A ban on the use of antibiotics as feed additives decreased drug-resistant bacteria in a Danish study. While overall mortality rates of chickens were not affected, more feed was consumed per kg of weight. Therapeutic use of antibiotics was increased, but the total volume of antibiotic use was significantly decreased. The EU has banned bacitracin, carbodox, olaquin-dox, tylosin, virginiamycin, avilamycin, flavophospholipol, lasalocid sodium, monensin sodium, and salinomycin as of 2009.

Last full review/revision March 2012 by Christopher D. Reinhardt, MS, PhD