| Summary: | Prevalence of antimicrobial resistance in fecal Escherichia coli isolates from feedlot cattle was characterized. Tetracycline resistance in isolates from newly weaned, auction market derived calves on arrival at the feedlot in a clinical trial was 9.8% and resistance to three or more antimicrobials was 2.1% compared to 17.6 % and 5.9% in a cohort study. The prevalence of tetracycline resistance at 78.3% and resistance to three or more antimicrobials at 52.5 % in isolates from spring calves submitted to a regional diagnostic laboratory were higher than those found on arrival at the feedlot. Of isolates from composite feedyard pen samples late in the feeding period, 39.4% were tetracycline resistant and 7.6% were resistant to three or more antimicrobials, somewhat higher than on arrival. Use of oxytetracycline in the feed for disease prophylaxis and the metaphylactic use of long-acting injectable oxytetracycline were associated with increased proportions of cattle with one or more resistant E. coli isolates early in the feeding period, while the use of individual animal treatments was not. The proportion of animals with one or more tetracycline resistant E. coli isolates was not different between the control, metaphylactic treatment and prophylactic treatment groups preslaughter; however, there were significantly more resistant animals in all groups preslaughter than at arrival. There were also no associations found between the total volume of parenteral antimicrobials used for disease treatment in individual animals and antimicrobial resistance in the cohort study. In addition, no strong associations were found between pen-level prevalence of antimicrobial resistance antimicrobial use or other variables. There was no significant difference between the proportion of isolates per pen resistant to tetracycline, one or more, two or more antimicrobials, or three or more antimicrobials when using 20, 15, 10 or 5 isolates from composite pen-level fecal samples. Variance for isolates resistant to three or more antimicrobials was partitioned as 12.7% at the feedyard-level and 28.7% at the pen-level. The use of diagnostic laboratory data for AMR surveillance was also discussed, and alternatives to antimicrobial treatment in the feedlot were also investigated. Overall a significant contribution to our understanding of antimicrobial resistance in feedlot cattle was achieved.
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