Biological Science: Research Developments and Innovations Vol. 3

Antimicrobial stewardship in dairy production must reconcile two legitimate objectives: timely treatment of bacterial disease to protect animal health and welfare, and reduction of avoidable antimicrobial exposure that can select for resistance within animals, herds and connected environments. This critical narrative review evaluates selective therapy, antimicrobial-resistance dynamics and One Health strategies across the dairy production system. Literature published from 1 January 2000 through 4 July 2026 was identified through live searches of scholarly databases and indexes, supplemented by citation searching and authoritative institutional sources. Evidence is strongest for selective dry-cow therapy in appropriately selected herds and for culture-guided selective treatment of non-severe clinical mastitis. Randomised trials and meta-analyses show that substantial reductions in intramammary antimicrobial use can be achieved without materially compromising udder-health outcomes when eligibility criteria are robust and, in most evidence bases, internal teat sealants protect untreated healthy quarters. The resistance consequences are less linear. Herd-level intramammary use is not consistently associated with resistance in milk-associated commensals, whereas systemic exposure has shown clearer associations with multidrug resistance in non-aureus staphylococci and faecal Escherichia coli. Preweaned calves represent a distinct amplification compartment because direct treatment, age-related microbiome dynamics and feeding of milk containing antimicrobial residues can increase carriage or shedding of resistant bacteria. Manure and farm-environment studies demonstrate persistence and movement of resistance determinants, but direct attribution of human infections to dairy-origin resistance remains difficult. Effective stewardship therefore requires more than reducing treatment counts: it depends on disease prevention, rapid diagnostics, route- and class-sensitive prescribing, calf and waste-milk management, environmental controls, comparable use and resistance metrics, veterinary oversight, worker competence and policy structures that preserve therapeutic access. The most defensible One Health strategy is consequently a risk-based system that reduces unnecessary exposure while maintaining prompt, evidence-based treatment of animals that are likely to benefit.