This article summarizes the experimental
knowledge on efficacy, possible modes of action,
and aspects of application of phytogenic products as
feed additives for swine and poultry. Phytogenic feed
additives comprise a wide variety of herbs, spices, and
products derived thereof, and are mainly essential oils.
The assumption that phytogenic compounds might improve
the palatability of feed has not yet been confirmed
by choice-feeding studies. Although numerous studies
have demonstrated antioxidative and antimicrobial efficacy
in vitro, respective experimental in vivo evidence
is still quite limited. The same applies to the supposition
that phytogenic compounds may specifically enhance
activities of digestive enzymes and nutrient absorption.
Nevertheless, a limited number of experimental
comparisons of phytogenic feed additives with
antibiotics and organic acids have suggested similar
effects on the gut, such as reduced bacterial colony
counts, fewer fermentation products (including ammonia
and biogenic amines), less activity of the gut-associated
lymphatic system, and a greater prececal nutrient
digestion, probably reflecting an overall improved gut
equilibrium. In addition, some phytogenic compounds
seem to promote intestinal mucus production. Such effects
may explain a considerable number of practical
studies with swine and poultry reporting improved production
performance after providing phytogenic feed
additives. In total, available evidence indicates that
phytogenic feed additives may add to the set of nonantibiotic
growth promoters for use in livestock, such as
organic acids and probiotics. However, a systematic approach
toward the efficacy and safety of phytogenic compounds
used as feed additives for swine and poultry is
still missing.