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 tetracy-clines 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.

Bovine respiratory disease is the primary animal health concern facing feedlot producers. Many antimicrobial mitigation strategies are available, but few studies have compared feedlot performance during both the receiving and finishing periods following application of different antimicrobials used as metaphylaxis at arrival. The objective of this study was to compare antimicrobial metaphylaxis methods on clinical health and growth performance across both the receiving and finishing periods. A total of 238 multiple-sourced steers in 2 source blocks were used in a generalized complete block design. The 4 treatments included: 1) a negative control, 5 mL of sterile saline injected subcutaneously (CON); 2) subcutaneous administration of florfenicol at 40 mg/kg of BW (NUF); 3) subcutaneous administration of ceftiofur in the posterior aspect of the ear at 6.6 mg/kg of BW (EXC); and 4) subcutaneous administration of tulathromycin at 2.5 mg/kg of BW (DRA). The morbidity rate for the first treatment of BRD was decreased for the DRA and EXC treatments compared to CON and NUF (P < 0.01). Additionally, average daily gain (ADG), dry matter intake (DMI), and gain-to-feed (G:F) were greater (P ≤ 0.02) in the DRA treatment during the receiving period compared to all other treatments. The ADG was also greater (P < 0.05) for EXC than the CON treatment throughout the finishing period. Nonetheless, other growth performance variables did not differ among metaphylactic treatments during the finishing period (P ≥ 0.14). Likewise, no differences in carcass characteristics or liver abscess score were observed (P ≥ 0.18). All complete blood count (CBC) variables were affected by day (P ≤0.01) except mean corpuscular hemoglobin concentration (P = 0.29). Treatment ×time interactions were observed for platelet count, white blood cell (WBC) count, monocyte count and percentage, and lymphocyte percentage (P ≤0.03). However, there were no observed hematological variables that differed among treatment (P ≥0.10). The results indicate that some commercially available antimicrobials labeled for metaphylactic use are more efficacious than others in decreasing morbidity rate.

Growing concern over breeding soundness examination failures has led to increased interest in identifying factors that may influence semen quality of developing bulls. An experiment was conducted to evaluate relationships between semen quality and scrotal circumference, adiposity, and rumen conditions of Angus bulls (n = 48; 332 ± 47 d of age) that were developed over an 84-d period. Bulls underwent breeding soundness examinations at each of four 21-d intervals. Backfat thickness was measured on d 0 and 84, and rumen pH and temperature were continuously monitored throughout the experiment via rumen boluses (eBolus; eCow, Devon, UK). Simple linear regression was conducted (JMP 13.0; SAS Institute Inc., Cary, NC) to determine independent relationships between semen characteristics and respective explanatory variables. Change in scrotal circumference between d 0 and 84 was directly related to change in percent morphologically-normal sperm (R2 = 0.12; P < 0.05), but no other semen quality characteristics (P ≥ 0.17). Change in backfat thickness throughout the experiment tended to be inversely related to change in percent morphologically-normal sperm (R2 = 0.06; P < 0.10), but was directly related to changes in semen volume (R2 = 0.10; P < 0.05) and total sperm per ejaculate (R2 = 0.12; P < 0.05), but not change in motility (P = 0.24). Mean rumen pH tended to be directly related to changes in semen volume (R2 = 0.07; P < 0.10) and total sperm per ejaculate (R2 = 0.07; P < 0.10), but not changes in motility (P = 0.28) or percent morphologically-normal sperm (P = 0.94). Mean rumen temperature was not related to any semen quality characteristics (P ≥ 0.15). Collectively, these results suggest that rumen pH, as well as changes in scrotal circumference and adiposity are related to and independently explain variation in semen quality of developing bulls.

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