The use of a poultry feed additive composition which is a flowable mixture of phytogenic compounds including at least an oil component which is microencapsulated essential thyme oil, and a saponin component which is the saponin contained in particulate dried quillaja bark powder. The mixture contains at least 0.5% saponin component (w/w), and the oil component in an effective ratio of at least 0.2:1 (w/w, oil component per saponin component), for improving the feed conversion efficiency in antibiotic-free poultry production.