Rationale: The majority of antibiotic use in the dairy industry is for the treatment and prevention of intramammary infections (IMI); in the Netherlands, approximately 60 % of all antimicrobial use in dairy is for this purpose, with two-thirds being dry cow therapy (Lam et al., 2012). In the United States, over 90 % of dairy cows receive dry cow therapy after every lactation (USDA-APHIS, 2016), with the goal of treating or preventing IMI during the dry period. These infections are strongly associated with risk of development of clinical mastitis in the first two weeks post-calving, which represents the highest risk period for this disease (Green et al., 2002). To combat this, blanket dry cow therapy (intramammary antimicrobial treatment of all quarters of all cows after the last milking of the lactation) was recommended for decades as part of a comprehensive strategy to reduce IMI in the dry period (Neave et al., 1969), and has been widely adopted in North America and the United Kingdom (Ruegg, 2017). Although cow2 level selective dry cow therapy has been in use in some regions for several decades (Schultze, 1983), interest has more recently increased worldwide, in part driven by concern for antimicrobial use and its relationship with the development of antimicrobial resistance between species (WHO, 2015), including nation-specific regulations (Santman-Berends et al., 2016). Selective dry cow therapy has been employed because it is a means to rapidly reduce the amount of antimicrobials used in dairy cattle (Vanhoudt et al., 2018), rather than because it is known to contribute importantly to antimicrobial resistance (Oliver et al., 2011).

With a greater concern for prudent antibiotic use in the dairy industry, it is important that decision making with regards to dry cow therapy at both the cow and herd levels be evidencebased. Choosing ineffective antibiotics, or using antibiotic when not warranted, unnecessarily contributes to use while having little impact on controlling disease, which has substantial bearing to both profitability and animal welfare (Leslie & Petersson-Wolfe, 2012). Systematic reviews of randomized controlled trials yield the highest level of evidence for efficacy of treatment under field conditions (Sargeant and O’Connor, 2014), and comparative efficacy can be examined using network meta-analysis for multiple comparisons. Establishing the efficacy of both cow-level antibiotic therapy and herd-level dry cow antibiotic protocols for the prevention of IMI will serve to improve decision makers’ ability to engage in effective stewardship of antibiotics.