Kanwar, NeenaScott, Harvey MorganNorby, BoLoneragan, Guy H.Vinasco-Torres, JavierCottell, Jennifer L.Chalmers, GabhanChengappa, Muckatira M.Bai, JianfaBoerlin, Patrick2014-07-232014-07-232014-05-29http://hdl.handle.net/2097/18130The study objective was to determine the effects of two treatment regimens on quantities of ceftiofur and tetracycline resistance genes in feedlot cattle. The two regimens were ceftiofur crystalline-free acid (CCFA) administered to either one or all steers within a pen and subsequent feeding/not feeding of therapeutic doses of chlortetracycline. A 26-day randomized controlled field trial was conducted on 176 steers. Real-time PCR was used to quantify bla[subscript CMY-2], bla[subscript CTX-M], tet(A), tet(B), and 16S rRNA gene copies/gram of feces from community DNA. A significant increase in ceftiofur resistance and a decrease in tetracycline resistance elements were observed among the treatment groups in which all steers received CCFA treatment, expressed as gene copies/gram of feces. Subsequent chlortetracycline administration led to rapid expansion of both ceftiofur and tetracycline resistance gene copies/gram of feces. Our data suggest that chlortetracycline is contraindicated when attempting to avoid expansion of resistance to critically important third-generation cephalosporins.en-USChlortetracyclineCeftiofur resistanceTetracycline resistanceArticle (publisher version)