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INTRODUCTION Antibiotics have been used in livestock production for over 50 years (Kim et al., 2005). When used at subtherapeutic levels in swine feeds, antibiotics improve growth rate and feed efficiency, reduce mortality and morbidity and improve reproductive performance (Cromwell, 2002). Antibiotics are also widely used at intermediate levels to prevent disease and at therapeutic levels to treat disease in various species of livestock (Cromwell, 2002). However, due to concerns about residues in animal foods and the potential for the development of microbial resistance to antibiotics, the possibility exists for the implementation of a complete ban in the use of antibiotics in animal feed (Adjiri-Awere and Van Lunen, 2005). As a consequence, the development of alternatives to antibiotics is receiving considerable attention (Turner et al., 2001).