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.