Finding effective alternatives to antibiotics and zinc oxide in livestock feed remains challenging, but phytogenic compounds show promising potential. In the first part of the present study, the in vitro antimicrobial activities of carvacrol, eugenol, garlic oil, star anise oil, and tea tree oil as well as their effects on the biofilm formation of two Escherichia coli field isolates, quorum sensing of Chromobacterium violaceum, and the adhesion of an E. coli field isolate to piglets’ small intestinal mucus were determined. Based on these results, two prototypes were formulated. Phytogenic feed additive (PFA) Core 2, containing carvacrol, eugenol, and star anise oil, showed stronger in vitro antimicrobial activity, inhibition of biofilm formation, and quorum sensing than PFA Core 1, which was mainly composed of garlic oil and tea tree oil. In the second part of the present study, 1000 post-weaning piglets were divided into four groups receiving a control or diets with either PFA Core 1, PFA Core 2, or zinc oxide. Only PFA Core 2 and zinc oxide significantly improved body weight, daily gain, feed efficiency, and fecal scores compared with the control, while PFA Core 1 increased the feed efficiency and fecal scores. The results show that feed additives based on carvacrol and eugenol can improve the growth performance of post-weaning piglets and reduce the incidence of diarrhea, possibly by influencing detrimental bacteria. Furthermore, the present study demonstrates the potential of combinations of in vitro assays to support the development of effective feed additives.
