Recently, phytogenic feed additives have gained much attention as an alternative to antibiotic growth promoters. This appears to be strongly driven by the ban on most of the antibiotic feed additives within the European Union in 1999, a complete ban enforced in 2006, and ongoing discussions to restrict their use outside the European Union owing to the concern about bacterial resistance to antibiotics in humans (Windisch et al., 2008). Accordingly, various herbs or their extracts are now being utilized as feed additives due to their widespread antioxidative (Wei and Shibamoto, 2007) and antimicrobial actions (Özer et al., 2007), beneficial effects on palatability and gut functions (Windisch et al., 2008). Nonetheless, their modes of action are not yet fully understood, and further scientific investigation is needed