The use of feed additives is usually subject to restrictive regulations. In general, they are considered as products applied by the farmer to healthy animals for a nutritional purpose on a permanent basis (i.e., during the entire production period of the respective species and category), in contrast to veterinary drugs (applied for prophylaxis and therapy of diagnosed health problems under veterinarian control for a limited time period,partially associated with a waiting period). In the
European Union, for example, feed additives need to demonstrate the identity and traceability of the entire commercial product, the efficacy of the claimed nutritional effects, including the absence of possible interactions with other feed additives, and the safety to the animal (e.g., tolerance), to the user (e.g., farmer, worker in feed mills), to the consumer of animal-derived products,and to the environment (for further details, refer to European Commission, 2003). Problems with feed additive legacy may therefore arise especially with phytogenic feed additives addressed to explicit health claims or in the case of plant-derived substances suspected to modulate metabolism (e.g., through a phytohormonal mode of action). For these reasons, the following discussion focuses on the use of phytogenic compounds as feed additives in swine and poultry diets in terms of claimed antioxidative and antimicrobial actions,beneficial effects on palatability and gut functions,and growth-promoting efficacy.