Project Summary/AbstractAntimicrobial resistance is a significant public health concern, and persistent use ofantimicrobial drugs in food animal production systems has been implicated as an importantfactor contributing to its development. The expanding threat of antimicrobial resistance, whichhas been recognized both domestically (CDC, 2013) and internationally (WHO, 2015), hasprompted regulatory actions aimed at preserving efficacy of antimicrobial drugs. In January of2017, the U.S. Food and Drug Administration completed its implementation of Guidance forIndustry# 213, fully enacting policy that disallows use of medically-important antimicrobials asgrowth promotants. This policy restricts use of certain medically important antibiotics totherapeutic applications under the supervision of a licensed veterinarian. Tylosin phosphate isamong the medically important classes of compounds affected by this policy. Tylosin has beenused extensively by the beef feedlot industry for decades to reduce incidence of liverabscesses, and generally is regarded as the most efficacious method for controlling liverabscesses in cattle. Presence of abscesses at harvest is a primary cause for livercondemnation, and thus has significant economic ramifications. Severe abscesses can impactliver function, thereby decreasing animal performance and jeopardizing animal well-being.Additionally, abscesses can expand to affect adjacent tissues, requiring affected areas to beexcised from the carcass at a significant economic loss, and also decreasing processing speedat a cost to abattoirs. Oversight of tylosin usage in feedlots has shifted to veterinarians inresponse to changes in regulatory policy, but actual usage patterns in commercial feedlots haveremained relatively unchanged. In the absence of alternative measures for control of liverabscesses, drastic changes in usage patterns seem unlikely to occur. Tylosin currently isapproved for continuous feeding only, though there are indications that more targeted durationsof use may have efficacy equal to that of continuous feeding. Whereas replacement may not bea viable option at present, reduction of use through strategic application presents itself as ameaningful alternative. Commercial implementation of alternative feeding strategies for tylosinwill require labeling changes that provide for greater flexibility in its application. With this inmind, our proposed project has several aims, the first of which is to provide additional insightrelative to the timing of liver abscess formation in feedlot cattle through non-invasive, ultrasoundimaging. Secondly, we will evaluate two different feeding strategies aimed at decreasing tylosinuse, while maintaining health outcomes similar to those achieved with continuous exposure.