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Aims: Our objective was to determine how injectable antimicrobials affected populations of Salmonella enterica, Escherichia coli and Enterococcus spp. in feedlot cattle., Methods and Results: Two arrival date blocks of high-risk crossbred beef cattle (n = 249; mean BW = 244 kg) were randomly assigned one of four antimicrobial treatments administered on day 0: sterile saline control (CON), tulathromycin (TUL), ceftiofur (CEF) or florfenicol (FLR). Faecal samples were collected on days 0, 28, 56, 112, 182 and study end (day 252 for block 1 and day 242 for block 2). Hide swabs and subiliac lymph nodes were collected the day before and the day of harvest. Samples were cultured for antimicrobial-resistant Salmonella, Escherichia coli and Enterococcus spp. The effect of treatment varied by day across all targeted bacterial populations (p ≤ 0.01) except total E. coli. Total E. coli counts were greatest on days 112, 182 and study end (p ≤ 0.01). Tulathromycin resulted in greater counts and prevalence of Salmonella from faeces than CON at study end (p ≤ 0.01). Tulathromycin and CEF yielded greater Salmonella hide prevalence and greater counts of 128ERY R E. coli at study end than CON (p ≤ 0.01). No faecal Salmonella resistant to tetracyclines or third-generation cephalosporins were detected. Ceftiofur was associated with greater counts of 8ERY R Enterococcus spp. at study end (p ≤ 0.03). By the day before harvest, antimicrobial use did not increase prevalence or counts for all other bacterial populations compared with CON (p ≥ 0.13)., Conclusions: Antimicrobial resistance (AMR) in feedlot cattle is not caused solely by using a metaphylactic antimicrobial on arrival, but more likely a multitude of environmental and management factors., (© 2022 The Authors. Journal of Applied Microbiology published by John Wiley & Sons Ltd on behalf of Society for Applied Microbiology.)

Our objective was to longitudinally assess the presence and quantity of Salmonella enterica throughout the gastrointestinal tract of finishing beef steers with and without liver abscesses (LA). Crossbred steers [n = 225; initial body weight (BW) = 353 ± 39 kg] were used in an observational case-control design with individual steer designated as the experimental unit. Nasal, ruminal fluid, and fecal samples were collected at feedlot arrival, 1 wk after adaptation to the finishing diet, and the day before harvest at a commercial abattoir. Healthy and abscessed liver tissue samples were collected at harvest. Steers were fed a steam-flaked corn-based diet without tylosin phosphate. After harvest, LA prevalence was determined, and steers were sorted into those with and without LA. Data were analyzed using the GLIMMIX procedure of SAS. Overall, LA prevalence was 18.7% (n = 42). Nasal Salmonella prevalence was 34.4% but did not differ between steers with or without LA (P = 0.73), and nasal Salmonella concentrations did not differ between treatments (P = 0.85) or collection day (P = 0.50). Ruminal fluid Salmonella prevalence was 73.7% but did not differ between treatments (P = 0.83). From feedlot arrival to harvest, ruminal fluid Salmonella concentration and prevalence decreased (P < 0.01). Fecal Salmonella prevalence tended (P = 0.09) to be 6.4% greater in steers with LA. Conversely, fecal Salmonella concentration tended (P = 0.07) to be 5.9% greater in steers without LA. Fecal Salmonella prevalence was greatest after transition to the finishing diet (P < 0.01), whereas fecal Salmonella concentration was greatest before harvest (P < 0.01). Liver Salmonella prevalence was 9.8% and 6.5% for steers with and without LA, respectively, but did not differ between treatments (P = 0.47). Similarly, liver Salmonella concentrations did not differ in steers with or without LA (P = 0.18). Overall, these data suggest Salmonella concentration and prevalence were affected by time on feed, but not indicative of LA formation. [ABSTRACT FROM AUTHOR]