Implementation of better management practices that promote antimicrobial stewardship on farms is needed due to increased concerns with the impact of antimicrobial usage (AMU) on development and transmission of antimicrobial resistance from animals to humans. Interventions to reduce AMU can be applied by monitoring antimicrobial treatments at farm-level and identifying diseases that contribute to AMU. Mastitis is the most frequent bacterial disease occurring on dairy farms (USDA–APHIS–VS–CEAH, 2008), and it is well known that the occurrence of mastitis results in major economic losses for dairy farmers (Ruegg, 2005). Intramammary administration is the major route of AMU on dairy farms when using a dose-based metric to calculate AMU. (Pol and Ruegg, 2007; Saini et al., 2012a; Stevens et al., 2016) Dry cow therapy typically accounts for 31 to 70% of intramammary treatments (Pol and Ruegg, 2007; Saini et al., 2012a; Stevens et al., 2016; Schrag et al., 2020b). The overall hypotheses of this dissertation is: 1)the greatest quantity of AMU on large dairy farms is for treatment and control of mastitis, and 2) that AMU and costs on farm can be reduced by reducing the days of treatment for clinical mastitis during lactation or implementing selective treatments at dry-off. The aims of this dissertation are to: 1) quantify AMU on large dairy farms and contrast total AMU by route and active ingredients using both dose-based and mass-based metrics. With these results, we will demonstrate how antimicrobial treatments related to udder health impact on total AMU on large dairy farms. 2) Estimate direct costs of treated and non-treated clinical mastitis using data obtained from commercial dairy farms and contrast variation in treatment costs among herds. 3) Estimate direct costs at dry-off and potential saving if selective dry cow therapy was used. And 4), evaluate a method to potentially decrease AMU at dry-off by performing a clinical trial to determine if use of an alternative dosing schedule for an immune stimulant (pegbovigrastim) reduces the need for administration of antimicrobials at dry-off.
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