Phytogenics are plant-derived compounds that include a vast variety of compounds, such as herbs, spices, oleoresins, and essential oils. The composition and concentration of active substances vary widely depending on plant, plant part, geographical origin, harvesting season, storage conditions, and processing techniques (Windisch et al., 2008). The extraction of essential oils is the most predominant plant processing technique. Essential oils contain a mixture of various compounds in different concentrations. The main constituents of essential oils used in swine diets are phenols and terpenes, including anethol, capsaicin, carvacrol, cinnalmaldehyde, curcumin, eugenol, and thymol (Zeng et al., 2015).
The exact mode of action and properties of phytogenics have not been fully understood, but are believed to be mostly related to the antimicrobial action, anti-inflammatory activity, and antioxidant effect of phytogenics. Additionally, phytogenics are often claimed to improve the feed flavor and palatability, which could lead to an increase in voluntary feed intake and growth performance (Windisch et al., 2008), although not well stablished.
The addition of phytogenics to swine diets have sometimes been associated with improvements in performance (Windisch et al., 2008; Zeng et al., 2015). However, the effects of phytogenics on performance have not been consistent (Liu et al., 2018; Soto et al., 2018). There is need for a systematic approach to determine the composition, understand the mode of action, and evaluate the efficiency of phytogenic products. Additionally, safety of phytogenic compounds and potential interactions with feed ingredients and other feed additives warrants further consideration (Jacela et al., 2010a).