Speakers outline how low-dose antibiotics alter microbiomes to promote growth and why alternatives are sought

The panel revisited mechanisms behind antibiotic growth promotion in animals and the implications for developing alternatives. Dr. Marcio Costa said farmers observed that low-dose antibiotics increase growth rates and that the prevailing explanation is selection of a microbiome better able to extract energy from feed: "If they give low dose antibiotics, the animals will grow faster," Costa said, describing experimental transfers where microbiomes from treated animals conferred weight gain when colonized into mice.

Costa and Dr. Phillip Myer discussed trade-offs: growth promotion has been associated with decreased microbial diversity and the selection of taxa that enhance digestion; the benefit differs by species and production cycle (e.g., short-lived poultry vs. longer-lived cattle). Both speakers emphasized the need to translate mechanistic insights into practical non-antibiotic interventions—nutrition, targeted native microbes, or management changes—while monitoring risks of co-selection for antimicrobial-resistance genes when using high-dose supplements such as zinc oxide.

The council Q&A highlighted that industry, regulatory and species-specific considerations matter: for poultry and swine, where slaughter occurs early, the practical outcomes and desired carcass traits differ from beef systems where animals live longer and chronic outcomes matter more.