This study evaluated the longitudinal effect of enrofloxacin or tulathromycin use in calves at high risk of bovine respiratory disease (BRD) on antimicrobial resistance genes and mutations in quinolone resistance-determining regions (QRDR) in fecal *E. coli*. Calves were randomly assigned to enrofloxacin, tulathromycin, or no treatment groups. Fecal samples were collected at enrollment and at 7, 28, and 56 days post-treatment. *aac(6')Ib-cr*, *bla-CTX-M*, *bla-TEM*, *tetA*, and *tetB* resistance genes were detected. The enrofloxacin group showed significantly higher detection of *gyrA* mutated at position 248 at 7 and 28 days compared to the control group. Findings highlight the potential impact of enrofloxacin use on resistance selection and persistence.

R. V. Pereira, C. Foditsch, J. D. Siler, S. C. Dulièpre, C. Altier, A. Garzon, L. D. Warnick