Evaluating the effects of antimicrobial drug use on the ecology of antimicrobial resistance and microbial community structure in beef feedlot cattle

Introduction

Use of antimicrobial drugs (AMDs) in food producing animals has received increasing scrutiny because of concerns about antimicrobial resistance (AMR) that might affect consumers. Previously, investigations regarding AMR have focused largely on phenotypes of selected pathogens and indicator bacteria, such as Salmonella enterica or Escherichia coli. However, genes conferring AMR are known to be distributed and shared throughout microbial communities. The objectives of this study were to employ target-enriched metagenomic sequencing and 16S rRNA gene amplicon sequencing to investigate the effects of AMD use, in the context of other management and environmental factors, on the resistome and microbiome in beef feedlot cattle.

Methods

This study leveraged samples collected during a previous longitudinal study of cattle at beef feedlots in Canada. This included fecal samples collected from randomly selected individual cattle, as well as composite-fecal samples from randomly selected pens of cattle. All AMD use was recorded and characterized across different drug classes using animal defined daily dose (ADD) metrics.

Results

Overall, fecal resistome composition was dominated by genes conferring resistance to tetracycline and macrolide-lincosamide-streptogramin (MLS) drug classes. The diversity of bacterial phyla was greater early in the feeding period and decreased over time in the feedlot. This decrease in diversity occurred concurrently as the microbiome represented in different individuals and different pens shifted toward a similar composition dominated by Proteobacteria and Firmicutes. Some antimicrobial drug exposures in individuals and groups were associated with explaining a statistically significant proportion of the variance in the resistome, but the amount of variance explained by these important factors was very small (<0.6% variance each), and smaller than associations with other factors measured in this study such as time and feedlot ID. Time in the feedlot was associated with greater changes in the resistome for both individual animals and composite pen-floor samples, although the proportion of the variance associated with this factor was small (2.4% and 1.2%, respectively).

Discussion

Results of this study are consistent with other investigations showing that, compared to other factors, AMD exposures did not have strong effects on antimicrobial resistance or the fecal microbial ecology of beef cattle.

Genomic Characterization of Enterococcus hirae From Beef Cattle Feedlots and Associated Environmental Continuum

Enterococci are commensal bacteria of the gastrointestinal tract of humans, animals, and insects. They are also found in soil, water, and plant ecosystems. The presence of enterococci in human, animal, and environmental settings makes these bacteria ideal candidates to study antimicrobial resistance in the One-Health continuum. This study focused on Enterococcus hirae isolates (n = 4,601) predominantly isolated from beef production systems including bovine feces (n = 4,117, 89.5%), catch-basin water (n = 306, 66.5%), stockpiled bovine manure (n = 24, 0.5%), and natural water sources near feedlots (n = 145, 32%), and a few isolates from urban wastewater (n = 9, 0.2%) denoted as human-associated environmental samples. Antimicrobial susceptibility profiling of a subset (n = 1,319) of E. hirae isolates originating from beef production systems (n = 1,308) showed high resistance to tetracycline (65%) and erythromycin (57%) with 50.4% isolates harboring multi-drug resistance, whereas urban wastewater isolates (n = 9) were resistant to nitrofurantoin (44.5%) and tigecycline (44.5%) followed by linezolid (33.3%). Genes for tetracycline (tetL, M, S/M, and O/32/O) and macrolide resistance erm(B) were frequently found in beef production isolates. Antimicrobial resistance profiles of E. hirae isolates recovered from different environmental settings appeared to reflect the kind of antimicrobial usage in beef and human sectors. Comparative genomic analysis of E. hirae isolates showed an open pan-genome that consisted of 1,427 core genes, 358 soft core genes, 1701 shell genes, and 7,969 cloud genes. Across species comparative genomic analysis conducted on E. hirae, Enterococcus faecalis and Enterococcus faecium genomes revealed that E. hirae had unique genes associated with vitamin production, cellulose, and pectin degradation, traits which may support its adaptation to the bovine digestive tract. E. faecium and E. faecalis more frequently harbored virulence genes associated with biofilm formation, iron transport, and cell adhesion, suggesting niche specificity within these species.

Environmental performance of commercial beef production systems utilizing conventional productivity-enhancing technologies

The objective of this study was to evaluate the effects of using conventional productivity-enhancing technologies (PETs) with or without other natural PETs on the growth performance, carcass traits and environmental impacts of feedlot cattle. A total of 768 cross-bred yearling steers (499 ± 28.6 kg; n = 384) and heifers (390 ± 34.9 kg; n = 384) were offered a barley grain-based basal diet and divided into implanted or non-implanted groups. Steers were then allocated to diets that contained either: (i) no additive (control); natural feed additives including (ii) fibrolytic enzymes (Enz), (iii) essential oil (Oleo), (iv) direct fed microbial (DFM), (v) DFM + Enz + Oleo combination; conventional feed additives including (vi) Conv (monensin, tylosin, and beta-adrenergic agonists [βAA]); or Conv with the natural feed additives including (vii) Conv + DFM + Enz; (viii) Conv + DFM + Enz + Oleo. Heifers received one of the first three dietary treatments or the following: (iv) probiotic (Citr); (v) Oleo + Citr; (vi) Melengesterol acetate (MGA) + Oleo + βAA; (vii) Conv (monensin, tylosine, βAA, and MGA); or (viii) Conv + Oleo (ConvOleo). Data were used to estimate greenhouse gas (GHG) and ammonia (NH3) emissions, as well as land and water use. Implant and Conv-treated cattle exhibited improvements in growth and carcass traits as compared to the other treatments (P &lt; 0.05). Improvements in the performance of Conv-cattle illustrated that replacing conventional feed additives with natural feed additives would increase both the land and water required to satisfy the feed demand of steers and heifers by 7.9% and 10.5%, respectively. Further, GHG emission intensity for steers and heifers increased by 5.8% and 6.7%, and NH3 emission intensity by 4.3% and 6.7%, respectively. Eliminating the use of implants in cattle increased both land and water use by 14.6% and 19.5%, GHG emission intensity by 10.5% and 15.8%, and NH3 emission intensity by 3.4% and 11.0% for heifers and steers, respectively. These results demonstrate that use of conventional PETs increased animal performance while reducing environmental impacts of beef production. Restricting use would increase the environmental footprint of beef produced for both domestic and international markets.

Prevalence and Risk Factors Associated With Antimicrobial Resistance in Bacteria Related to Bovine Respiratory Disease-A Broad Cross-Sectional Study of Beef Cattle at Entry Into Canadian Feedlots

A broad, cross-sectional study of beef cattle at entry into Canadian feedlots investigated the prevalence and epidemiology of antimicrobial resistance (AMR) in Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis, bacterial members of the bovine respiratory disease (BRD) complex. Upon feedlot arrival and before antimicrobials were administered at the feedlot, deep nasopharyngeal swabs were collected from 2,824 feedlot cattle in southern and central Alberta, Canada. Data on the date of feedlot arrival, cattle type (beef, dairy), sex (heifer, bull, steer), weight (kg), age class (calf, yearling), source (ranch direct, auction barn, backgrounding operations), risk of developing BRD (high, low), and weather conditions at arrival (temperature, precipitation, and estimated wind speed) were obtained. Mannheimia haemolytica, P. multocida, and H. somni isolates with multidrug-resistant (MDR) profiles associated with the presence of integrative and conjugative elements were isolated more often from dairy-type than from beef-type cattle. Our results showed that beef-type cattle from backgrounding operations presented higher odds of AMR bacteria as compared to auction-derived calves. Oxytetracycline resistance was the most frequently observed resistance across all Pasteurellaceae species and cattle types. Mycoplasma bovis exhibited high macrolide minimum inhibitory concentrations in both cattle types. Whether these MDR isolates establish and persist within the feedlot environment, requires further evaluation.

Association between computer-aided lung auscultation and treatment failure risk in calves treated for respiratory disease

The use of computer-aided lung auscultation (CALA, Whisper Veterinary Stethoscope; Merck Animal Health, Madison, New Jersey, USA) is a relatively new approach to assist in confirming the diagnosis of bovine respiratory disease (BRD). For this prospective cohort study at 1 feedlot in the United States, a CALA score was generated for 2726 feeder cattle (calf-fed Holsteins and mixed-breed beef animals) at the time of the first BRD diagnosis and treatment. All cattle were treated according to the same BRD protocol prescribed for that facility and the protocol was not influenced by the CALA score. Data were collected for 120 d after enrollment. In this study, the risk of BRD retreatment and the risk of BRD mortality were each significantly (P < 0.05) associated with the CALA score at the time of first BRD diagnosis and treatment, and those risks increased (numerically and in some cases statistically) as the CALA score increased.

3-Nitrooxypropanol Decreased Enteric Methane Production From Growing Beef Cattle in a Commercial Feedlot: Implications for Sustainable Beef Cattle Production

Effects of the investigational methane (CH4) inhibitor 3-nitrooxypropanol (3-NOP) on animal performance, health and enteric CH4 production of beef cattle were evaluated in a commercial feedlot. Two concurrent studies were conducted: a large pen study (4,048 cattle, eight pen replicates per experimental group) to measure animal performance and health and a small pen study (a subset of 50 cattle from the large pen study, n = 25 per experimental group) to measure enteric CH4 emissions. Within the study, animals (body weight ± SD, 282 ± 8 kg) were assigned in a completely randomized design to one of two groups: control, fed a backgrounding diet (70% corn or barley silage, 30% steam-flaked barley grain concentrate; dry matter (DM) basis) and 3-NOP, fed the backgrounding diet containing 3-NOP. The treatment group in the large pen study was adapted to 3-NOP (12 ± 3 d) before receiving the final target level of 200 mg/kg of DM, which was fed for 108 ± 8 d. Animals in the small pen CH4 emissions study received a basal diet or a basal diet with 3-NOP, with the dose increased every 28 d: low (150 mg/kg DM; 1.27 g/d), medium (175 mg/kg DM; 2.25 g/d), and high (200 mg/kg DM; 2.75 g/d). Intake in the small pens was monitored by electronic feeding bunks and CH4 was measured using the GreenFeed system. In the large pen study, total weight gained, average daily gain, and animal health variables were not affected by 3-NOP, but DM intake (DMI) tended to decrease (P = 0.06) by 2.6% relative to control (8.07 kg/d), while gain:feed ratio tended to be improved (P = 0.06) by 2.5% relative to control (0.161 kg weight gain/kg DMI). In the small pen study, average daily consumption of 3-NOP increased with inclusion rate whereas average DMI was decreased by 5.4% (P = 0.02) compared with control (10.4 kg/d). On average, addition of 3-NOP decreased (P = 0.001) CH4 emissions (g/d) by 25.7% and yield (g CH4/kg DMI) by 21.7%. In conclusion, supplementing a backgrounding diet with 3-NOP decreased CH4 yield and tended to improve feed efficiency of beef cattle fed in a commercial feedlot with no negative impacts on animal health.

Use of 3-nitrooxypropanol in a commercial feedlot to decrease enteric methane emissions from cattle fed a corn-based finishing diet

The present study evaluated enteric CH4 production, dry matter (DM) intake (DMI), and rumen fermentation in feedlot cattle supplemented with increasing concentrations of 3-nitrooxypropanol (3-NOP). A total of 100 crossbred steers (body weight, 421 ± 11 kg) was randomly assigned to one of four treatments (n = 25/treatment): control (no 3-NOP) or low (100 mg/kg DM), medium (125 mg/kg DM), and high (150 mg/kg DM) doses of 3-NOP. The study was comprised of 28 d of adaptation followed by three 28-d periods, with CH4 measured for 7 d per period and cattle remaining on their respective diets throughout the 112-d study. Each treatment group was assigned to a pen, with the cattle and diets rotated among pens weekly to allow the animals to access the GreenFeed emission monitoring (GEM) system stationed in one of the pens for CH4 measurement. Measured concentration (mg/kg DM) of 3-NOP in the total diet consumed (basal diet + GEM pellet) was 85.6 for low, 107.6 for medium, and 124.5 for high doses of 3-NOP. There was a treatment × period interaction (P < 0.001) for DMI; compared with control, the DMI was less for the low and high doses in period 1, with no differences thereafter. Compared with control (10.78 g/kg DMI), CH4 yield (g/kg DMI) was decreased (P < 0.001) by 52%, 76%, and 63% for low, medium, and high doses of 3-NOP, respectively. A treatment × period effect (P = 0.048) for CH4 yield indicated that the low dose decreased in efficacy from 59% decrease in periods 1 and 2 to 37% decrease in period 3, while the efficacy of the medium and high doses remained consistent over time. Irrespective of dose, hydrogen emissions increased by 4.9-fold (P < 0.001), and acetate:propionate ratio in rumen fluid decreased (P = 0.045) with 3-NOP supplementation, confirming that other hydrogen-utilizing pathways become more important in the CH4-inhibited rumen. The study indicates that supplementation of corn-based finishing diets with 3-NOP using a medium dose is an effective CH4 mitigation strategy for commercial beef feedlots with a 76% decrease in CH4 yield. Further research is needed to determine the effects of 3-NOP dose on weight gain, feed conversion efficiency, and carcass characteristics of feedlot cattle at a commercial scale.

Feedlot Cattle Antimicrobial Use Surveillance Network: A Canadian Journey

Antimicrobial drugs are important tools for maintaining human and animal health. Globally, antimicrobial use (AMU) in food-producing animals is under increasing scrutiny due to its potential to promote antimicrobial resistance (AMR). Historically, comprehensive Canadian data related to the types of antimicrobial drugs used, extent of use, common indicators of use and the demographics of the cattle populations receiving antimicrobial drugs have been limited, in part due to segmentation in the cattle industry and fragmentation of the drug distribution system. Appropriate AMU estimates are required to understand AMU practices, to interpret AMR levels and patterns, to meaningfully assess associated public health risks, and to inform stewardship activities. The Canadian beef cattle industry has a long history of collaboration in AMU and AMR research. Prior research projects identified both opportunities and challenges in the collection of AMU data. Cornerstone projects provided insight into the complexity of collecting AMU data in Canada's feedlot sector. This paper will discuss how the lessons learned from past work have contributed to the formation of a Canadian fed-cattle antimicrobial surveillance program that was initiated in 2019. This important surveillance program will allow feedlot cattle AMU to improve management decisions and support AMU best practices in the evolving Canadian AMR landscape.

Multidrug Resistance in Pasteurellaceae Associated With Bovine Respiratory Disease Mortalities in North America From 2011 to 2016

Multidrug-resistant (MDR; resistance to ≥3 antimicrobial classes) members of the Pasteurellaceae family may compromise the efficacy of therapies used to prevent and treat bovine respiratory disease (BRD) in feedlot cattle. This study examined the prevalence of multidrug resistance in strains of Mannheimia haemolytica and Pasteurella multocida collected from BRD cattle mortalities in North America. Isolates of M. haemolytica (n = 147) and P. multocida (n = 70) spanning 69 Alberta feedlots from 2011 to 2016 and two United States feedlots from 2011 to 2012 were examined for antimicrobial resistance (AMR) in association with integrative and conjugative elements (ICEs). Overall, resistance was high in both bacterial species with an increase in the prevalence of MDR isolates between 2011 and 2016. Resistance to >7 antimicrobial drugs occurred in 31% of M. haemolytica and 83% of P. multocida isolates. Resistance to sulfadimethoxine, trimethoprim/sulfamethoxazole, neomycin, clindamycin oxytetracycline, spectinomycin, tylosin, tilmicosin, and tulathromycin was most common. Although >80% of strains harbored three or more ICE-associated genes, only 12% of M. haemolytica and 77% of P. multocida contained all six, reflecting the diversity of ICEs. There was evidence of clonal spread as P. multocida and M. haemolytica isolates with the same pulsed-field gel electrophoresis profile from the United States in 2011 were isolated in Alberta in 2015–2016. This work highlights that MDR strains of Pasteurellaceae containing ICEs are widespread and may be contributing to BRD therapy failure in feedlot cattle. Given the antimicrobial resistance gene profiles identified, these MDR isolates may be selected for by the use of macrolides, tetracyclines, and/or in-feed supplements containing heavy metals.

A One Health Comparative Assessment of Antimicrobial Resistance in Generic and Extended-Spectrum Cephalosporin-Resistant Escherichia coli from Beef Production, Sewage and Clinical Settings

This study aimed to compare antimicrobial resistance (AMR) in extended-spectrum cephalosporin-resistant and generic Escherichia coli from a One Health continuum of the beef production system in Alberta, Canada. A total of 705 extended-spectrum cephalosporin-resistant E. coli (ESC^r) were obtained from: cattle feces (CFeces, n = 382), catch basins (CBasins, n = 137), surrounding streams (SStreams, n = 59), beef processing plants (BProcessing, n = 4), municipal sewage (MSewage; n = 98) and human clinical specimens (CHumans, n = 25). Generic isolates (663) included: CFeces (n = 142), CBasins (n = 185), SStreams (n = 81), BProcessing (n = 159) and MSewage (n = 96). All isolates were screened for antimicrobial susceptibility to 9 antimicrobials and two clavulanic acid combinations. In ESC^r, oxytetracycline (87.7%), ampicillin (84.4%) and streptomycin (73.8%) resistance phenotypes were the most common, with source influencing AMR prevalence (p < 0.001). In generic E. coli, oxytetracycline (51.1%), streptomycin (22.6%), ampicillin (22.5%) and sulfisoxazole (14.3%) resistance were most common. Overall, 88.8% of ESC^r, and 26.7% of generic isolates exhibited multi-drug resistance (MDR). MDR in ESC^r was high from all sources: CFeces (97.1%), MSewage (96.9%), CHumans (96%), BProcessing (100%), CBasins (70.5%) and SStreams (61.4%). MDR in generic E. coli was lower with CFeces (45.1%), CBasins (34.6%), SStreams (23.5%), MSewage (13.6%) and BProcessing (10.7%). ESBL phenotypes were confirmed in 24.7% (n = 174) ESC^r and 0.6% of generic E. coli. Prevalence of bla genes in ESC^r were bla_CTXM (30.1%), bla_CTXM-1 (21.6%), bla_TEM (20%), bla_CTXM-9 (7.9%), bla_OXA (3.0%), bla_CTXM-2 (6.4%), bla_SHV (1.4%) and AmpC β-lactamase bla_CMY (81.3%). The lower AMR in ESC^r from SStreams and BProcessing and higher AMR in CHumans and CFeces likely reflects antimicrobial use in these environments. Although MDR levels were higher in ESC^r as compared to generic E. coli, AMR to the same antimicrobials ranked high in both ESC^r and generic E. coli sub-populations. This suggests that both sub-populations reflect similar AMR trends and are equally useful for AMR surveillance. Considering that MDR ESC^r MSewage isolates were obtained without enrichment, while those from CFeces were obtained with enrichment, MSewage may serve as a hot spot for MDR emergence and dissemination.

Whole Genome Sequencing Differentiates Presumptive Extended Spectrum Beta-Lactamase Producing Escherichia coli along Segments of the One Health Continuum

Antimicrobial resistance (AMR) has important implications for the continued use of antibiotics to control infectious diseases in both beef cattle and humans. AMR along the One Health continuum of the beef production system is largely unknown. Here, whole genomes of presumptive extended-spectrum β-lactamase E. coli (ESBL-EC) from cattle feces (n = 40), feedlot catch basins (n = 42), surrounding streams (n = 21), a beef processing plant (n = 4), municipal sewage (n = 30), and clinical patients (n = 25) are described. ESBL-EC were isolated from ceftriaxone selective plates and subcultured on ampicillin selective plates. Agreement of genotype-phenotype prediction of AMR ranged from 93.2% for ampicillin to 100% for neomycin, trimethoprim/sulfamethoxazole, and enrofloxacin resistance. Overall, β-lactam (100%; bla_EC, bla_TEM-1, bla_SHV, bla_OXA, bla_CTX-M-), tetracycline (90.1%; tet(A), tet(B)) and folate synthesis (sul2) antimicrobial resistance genes (ARGs) were most prevalent. The ARGs tet(C), tet(M), tet(32),bla_CTX-M-1, bla_CTX-M-14, bla_OXA-1, dfrA18, dfrA19, catB3, and catB4 were exclusive to human sources, while bla_TEM-150, bla_SHV-11–12,dfrA12, cmlA1, and cmlA5 were exclusive to beef cattle sources. Frequently encountered virulence factors across all sources included adhesion and type II and III secretion systems, while IncFIB(AP001918) and IncFII plasmids were also common. Specificity and prevalence of ARGs between cattle-sourced and human-sourced presumptive ESBL-EC likely reflect differences in antimicrobial use in cattle and humans. Comparative genomics revealed phylogenetically distinct clusters for isolates from human vs. cattle sources, implying that human infections caused by ESBL-EC in this region might not originate from beef production sources.

Investigation of a Reduction in Tylosin on the Prevalence of Liver Abscesses and Antimicrobial Resistance in Enterococci in Feedlot Cattle

Recent concerns over linkages between antimicrobial resistance in human pathogens and antimicrobial use in livestock have prompted researchers to investigate management strategies that reduce the current reliance on in-feed tylosin to control liver abscesses in feedlot cattle. A total of 7,576 crossbred yearlings were allocated to the study (~253 animals/pen, 10 replicate pens per treatment) and individually randomized to one of three treatments. Tylosin phosphate (11 ppm) was included in-feed (1) for the first 125 days on feed (DOF) (FIRST-78%), (2) for DOF 41 to 161 (LAST-75%), or (3) for the entire feeding period (CON; day 0–161). Fecal composites were collected from the pen floor on days 0, 81, and 160 of the finishing period. Serial dilutions were spread plated for enumeration of enterococci on Bile Esculin Azide (BEA) agar and BEA amended with 8 μg/ml erythromycin. Results indicated that although the proportion of Ery^R enterococci increased with DOF (P < 0.01), neither treatment (P = 0.34) or treatment × DOF (P = 0.37) affected antimicrobial resistance. Of the 538 isolates, 97% were enterococci, with mixed species isolated early in the feeding period and only Enterococcus hirae isolated at the end. Isolates were most frequently resistant to tylosin (86%), erythromycin (84%), and doxycycline (31%). Macrolide and tetracycline resistant isolates harbored erm(B), msrC, and tet(L), tet(M), tet(O) genes, respectively. Overall, the proportion of Ery^R enterococci increased (P < 0.05) in all three treatments over the feeding period. Compared to the control cattle, FIRST-78% cattle had more severe (P < 0.05) liver abscesses, while there was a trend (P < 0.08) for this response in LAST-75% cattle. There was no difference (P > 0.05) in total liver abscesses, growth performance, carcass traits, morbidity, or mortality among treatments. These results support the potential to reduce the duration and therefore quantity of tylosin administered to feedlot cattle during the feeding period without impacting animal productivity.

Antimicrobial Use on 36 Beef Feedlots in Western Canada: 2008-2012

The accurate quantification of antimicrobial use (AMU) in production animals is critical for monitoring trends in exposure to antimicrobial drugs (AMD) over time and examining potential associations with antimicrobial resistance in bacteria. In this study, a census sample of cattle was used to quantify individually-dosed and in-feed AMU as both numbers of animal daily doses (nADD) and total grams of AMD (gAMD) used in cattle placed in 36 western Canadian feedlots between 1-November, 2008 and 31-October, 2012; representing about 21.5% of fed cattle in Canada during that time period. Of the ~2.6 million cattle placed during the 48-month period, 45% were calves, 63% were male, 62% arrived in the fall or winter, and 39% were assessed as high risk for developing bovine respiratory disease (BRD). The proportion of cattle categorized as high risk (HR) for developing BRD was consistent over the 4 years of placement cohorts. Both medically important AMU and ionophore use were summarized but presented separately. A decrease in AMU was observed over the study period, both as nADD and total gAMD, which was primarily driven by a decline in the in-feed administration of tetracyclines. Most in-feed AMU was directed toward prevention and control of liver abscesses. The majority of individually dosed AMU was administered as metaphylaxis to address BRD risks, with category III AMD (medium importance to human medicine as categorized by Health Canada Veterinary Drugs Directorate) used most frequently. Not surprisingly, risk level for developing BRD influenced parenteral AMD exposures, with 95% of cattle categorized as being HR for developing BRD receiving individually dosed AMD compared to 59% of cattle categorized as being low risk (LR) for developing BRD. Cattle categorized as HR for developing BRD were more likely to receive macrolides for BRD metaphylaxis compared to cattle categorized as LR for developing BRD, and cattle categorized as LR for developing BRD were more likely to receive tetracycline for the same purpose. In summary, these data provide an unprecedented representation of AMU in fed cattle in western Canada and direction for future monitoring of AMU in fed cattle.

Plasmid Distribution among Escherichia coli from Livestock and Associated Wastewater: Unraveling Factors That Shape the Presence of Genes Conferring Third-Generation Cephalosporin Resistance

A key concern with agricultural wastewater storage ponds is that they may provide an environment conducive for horizontal exchange of antibiotic resistance genes (ARGs), thereby facilitating the emergence of antibiotic resistant pathogens. Central to this exchange are mobile genetic elements like plasmids; yet, the factors shaping their presence in agricultural environments remain poorly understood. Here, using Escherichia coli as a model bacterium, we examined genetic backgrounds and plasmid profiles of generic fecal and wastewater isolates and those possessing bla_CTX-M and bla_CMY-2 genes (which confer resistance to third-generation cephalosporins) to delineate factors shaping the environmental persistence of plasmid-associated ARGs in beef cattle feedlots. The wastewater environment exerted minimal influence on plasmid repertoires, as the number of plasmids and distribution of different incompatibility groups did not differ between generic fecal and wastewater isolates. The bla_CTX-M and bla_CMY-2 genes were associated with IncF and IncA/C plasmids, respectively, and host isolates possessing these ARGs had fewer plasmids than generic isolates, suggesting ARG-bearing plasmids may associate predominantly with such hosts to compensate for the metabolic burden imposed by these plasmids. Phylogeny also appeared to be a factor for bla_CTX-M genes, as their bacterial hosts were restricted to particular genetic lineages, including the environmentally adapted ET-1 clade, as noted previously for these genes. Ultimately, these findings have important implications for evaluating human health risks of agricultural wastewater with respect to environmental persistence of ARGs and may help identify options for improving wastewater treatment.

Calculation of Antimicrobial Use Indicators in Beef Feedlots-Effects of Choice of Metric and Standardized Values

The potential for antimicrobial use (AMU) to lead to the development of antimicrobial resistant bacteria is an increasingly important priority in human and veterinary medicine. Accurate AMU quantification is essential to assessing the risk of antimicrobial resistance due to AMU. The quantification of AMU in production animals can be difficult, and feedlot beef cattle present a number of unique challenges. This paper presents selected parenteral data from western Canadian beef feedlots to illustrate variations in interpretation of AMU that can arise from the use of different metrics and standards. Specific examples presented compare the number of animal daily doses calculated from a given amount of antimicrobial drug (AMD) using actual and estimated weights of cattle at exposure, dose-based to weight-based indicators representing the same amount of antimicrobial, dose-based AMU indicators using different estimated durations of effect (DOE), and AMU indicators calculated using different standard weights of cattle at exposure. Changing these factors when calculating AMU indicators can have notable influences on the results obtained. Transparency about the methods used to calculate AMU indicators is critical to ensure that comparisons of use among different populations is meaningful and accurate.

Presence and Diversity of Extended-Spectrum Cephalosporin Resistance Among Escherichia coli from Urban Wastewater and Feedlot Cattle in Alberta, Canada

A recent preliminary study from our group found that extended-spectrum cephalosporin-resistance determinants can be detected in the majority of composite fecal samples collected from Alberta feedlot cattle. Most notably, bla_CTX-M genes were detected in 46.5% of samples. Further isolate characterization identified bla_CTX-M-15 and bla_CTX-M-27, which are widespread in bacteria from humans. We hypothesized that Escherichia coli of human and beef cattle origins share the same pool of bla_CTX-M genes. In this study, we aimed to assess and compare the genomic profiles of a larger collection of bla_CTX-M-positive E. coli recovered from fecal composite samples from Canadian beef feedlot cattle and human wastewater through whole-genome sequencing. The variants bla_CTX-M-55, bla_CTX-M-32, bla_CTX-M-27, bla_CTX-M-15, and bla_CTX-M-14 were found in both urban wastewater and cattle fecal isolates. Core genome multilocus sequence typing showed little similarity between the fecal and wastewater isolates. Thus, if the dissemination of genes between urban wastewater and feedlot cattle occurs, it does not appear to be related to the expansion of specific clonal lineages. Further investigations are warranted to assemble and compare plasmids carrying these genes to better understand the modalities and directionality of transfer.

Characterization of the Microbial Resistome in Conventional and "Raised Without Antibiotics" Beef and Dairy Production Systems

Metagenomic investigations have the potential to provide unprecedented insights into microbial ecologies, such as those relating to antimicrobial resistance (AMR). We characterized the microbial resistome in livestock operations raising cattle conventionally (CONV) or without antibiotic exposures (RWA) using shotgun metagenomics. Samples of feces, wastewater from catchment basins, and soil where wastewater was applied were collected from CONV and RWA feedlot and dairy farms. After DNA extraction and sequencing, shotgun metagenomic reads were aligned to reference databases for identification of bacteria (Kraken) and antibiotic resistance genes (ARGs) accessions (MEGARes). Differences in microbial resistomes were found across farms with different production practices (CONV vs. RWA), types of cattle (beef vs. dairy), and types of sample (feces vs. wastewater vs. soil). Feces had the greatest number of ARGs per sample (mean = 118 and 79 in CONV and RWA, respectively), with tetracycline efflux pumps, macrolide phosphotransferases, and aminoglycoside nucleotidyltransferases mechanisms of resistance more abundant in CONV than in RWA feces. Tetracycline and macrolide–lincosamide–streptogramin classes of resistance were more abundant in feedlot cattle than in dairy cow feces, whereas the β-lactam class was more abundant in dairy cow feces. Lack of congruence between ARGs and microbial communities (procrustes analysis) suggested that other factors (e.g., location of farms, cattle source, management practices, diet, horizontal ARGs transfer, and co-selection of resistance), in addition to antimicrobial use, could have impacted resistome profiles. For that reason, we could not establish a cause–effect relationship between antimicrobial use and AMR, although ARGs in feces and effluents were associated with drug classes used to treat animals according to farms’ records (tetracyclines and macrolides in feedlots, β-lactams in dairies), whereas ARGs in soil were dominated by multidrug resistance. Characterization of the “resistance potential” of animal-derived and environmental samples is the first step toward incorporating metagenomic approaches into AMR surveillance in agricultural systems. Further research is needed to assess the public-health risk associated with different microbial resistomes.

Association of lung lesions measured by thoracic ultrasonography at first diagnosis of bronchopneumonia with relapse rate and growth performance in feedlot cattle

Background

Severity of lung lesions quantified by thoracic ultrasonography (TUS) at time of bronchopneumonia (BP) diagnosis predicted death among steers not treated for this condition. Further research is needed to confirm that lung lesions detected by TUS can be associated with negative outcomes in cattle with BP that subsequently were treated.

Objective

To quantify the effects on relapse rate and average daily gain (ADG) of lung lesions detected by TUS at first BP diagnosis in feedlot cattle.

Animals

Prospective cohort of mixed beef‐breed steers (n = 93; 243 ± 36 kg) and heifers (n = 51; 227 ± 42 kg) with BP at 4 feedlots.

Methods

Thoracic ultrasonography was performed by the same clinician and 16‐second TUS videos were evaluated offline for maximal depth and area of lung consolidation, maximum number of comet tails, and maximal depth of pleural fluid. Individual ADG was calculated between 1 and 120 days after arrival. Effects of lesions on relapse rate and ADG were investigated using mixed regression models.

Results

Maximal depth of lung consolidation was associated with a higher risk of relapse (odds ratio [OR], 1.337/cm; 95% confidence interval [CI], 1.042‐1.714) and lower ADG (− 34 g/cm; −64 to −4). Maximal area of lung consolidation also was associated with a higher relapse risk (OR, 1.052/cm²; 1.009‐1.097) but not with ADG. Comet tails and pleural fluid were not associated with risk of relapse or ADG.

Conclusions and Clinical Importance

Quantifying maximal depth and area of lung consolidation by TUS at first BP diagnosis can provide useful prognostic information in feedlot cattle.

Clonal expansion of environmentally-adapted Escherichia coli contributes to propagation of antibiotic resistance genes in beef cattle feedlots

Livestock wastewater lagoons represent important environmental reservoirs of antibiotic resistance genes (ARGs), although factors contributing to their proliferation within these reservoirs remain poorly understood. Here, we characterized Escherichia coli from feedlot cattle feces and associated wastewater lagoons using CRISPR1 subtyping, and demonstrated that while generic E. coli were genetically diverse, populations were dominated by several 'feedlot-adapted' CRISPR types (CTs) that were widely distributed throughout the feedlot. Moreover, E. coli bearing beta-lactamase genes, which confer reduced susceptibility to third-generation cephalosporin's, predominantly belonged to these feedlot-adapted CTs. Remarkably, the genomic region containing the CRISPR1 allele was more frequently subject to genetic exchange among wastewater isolates compared to fecal isolates, implicating this region in environmental adaptation. This allele is proximal to the mutS-rpoS-nlpD region, which is involved in regulating recombination barriers and adaptive stress responses. There were no loss-of-function mutS or rpoS mutations or beneficial accessory genes present within the mutS-rpoS-nlpD region that would account for increased environmental fitness among feedlot-adapted isolates. However, comparative sequence analysis revealed that protein sequences within this region were conserved among most feedlot-adapted CTs, but not transient fecal CTs, and did not reflect phylogenetic relatedness, implying that adaptation to wastewater environments may be associated with genetic variation related to stress resistance. Collectively, our findings suggest adaptation of E. coli to feedlot environments may contribute to propagation of ARGs in wastewater lagoons.

A retrospective analysis of feedlot morbidity and mortality outcomes in calves born to dams with known viral vaccination history

This retrospective analysis aimed to determine the effects of a maternal viral vaccination program (MVVP; Express Verified) on calf health during the feeding period. In low- and high-risk populations, calves born to dams vaccinated pre-breeding with program products had improved morbidity and mortality outcomes compared with non-program animals.

Comparison of antimicrobial resistance genes in feedlots and urban wastewater

The use of antibiotics in livestock production in North America and possible association with elevated abundance of detectable antimicrobial resistance genes (ARG) is a growing concern. Real-time, quantitative PCR (RT-qPCR) was used to determine the relative abundance and diversity of ARG in fecal composite and catch basin samples from 4 beef feedlots in Alberta. Samples from a surrounding waterway and municipal wastewater treatment plants were also included to compare the ARG profile of urban environments and fresh water with that of feedlots. The relative abundance of 18 resistance genes across 5 antibiotic families including sulfonamides, tetracyclines, macrolides, fluoroquinolones, and β-lactams was examined. Sulfonamide, fluoroquinolone, and β-lactam resistance genes predominated in wastewater treatment samples, while tetracycline resistance genes predominated in cattle fecal composite samples. These results reflect the types of antibiotic that are used in cattle versus humans, but other factors such as co-selection of ARG and variation in the composition of bacterial communities associated with these samples may also play a role.

Environmental Growth of Enterococci and Escherichia coli in Feedlot Catch Basins and a Constructed Wetland in the Absence of Fecal Input

Population structures of fecal indicator bacteria (FIB) isolated from catch basins, a constructed wetland, and feces from a beef cattle feedlot were compared over a two-year period. Enterococcus hirae accounted for 92% of the fecal isolates, whereas secondary environments were characterized by greater relative abundance of environmentally adapted species including Enterococcus casseliflavus. While enterococci densities in the catch basins and wetland were similar under wet and drought conditions, E. hirae predominated during rainy periods, while E. casseliflavus predominated during drought conditions. Environmentally adapted species accounted for almost half of the erythromycin resistant enterococci isolated from the wetland. Densities of Escherichia coli were also comparable during wet versus drought conditions, and the relative abundance of strains from environmentally adapted clades was greater in secondary environments compared to feces. Unlike enterococci, fewer environmentally adapted E. coli strains were isolated on selective media containing ceftriaxone from the wetland compared to feces, suggesting resistance to this antibiotic may not be well maintained in the absence of selective pressure. Overall, these findings suggest that secondary environments select for environmentally adapted FIB. While these species and clades tend to be of limited clinical relevance, they could potentially serve as reservoirs of antimicrobial resistance.

Mannheimia haemolytica in feedlot cattle: prevalence of recovery and associations with antimicrobial use, resistance, and health outcomes

BACKGROUND

Mannheimia haemolytica is an important etiological agent in bovine respiratory disease.

OBJECTIVES

Explore risk factors for recovery of susceptible and resistant M. haemolytica in feedlot cattle and explore associations with health outcomes.

ANIMALS

Cattle (n = 5,498) from 4 feedlots sampled at arrival and later in feeding period.

METHODS

Susceptibility of M. haemolytica isolates tested for 21 antimicrobials. Records of antimicrobial use and health events analyzed using multivariable regression.

RESULTS

M. haemolytica recovered from 29% of cattle (1,596/5,498), 13.1% at arrival (95% CI, 12.3-14.1%), and 19.8% at second sampling (95% CI, 18.7-20.9%). Nearly half of study cattle received antimicrobial drugs (AMDs) parenterally, mostly as metaphylactic treatment at arrival. Individual parenteral AMD exposures were associated with decreased recovery of M. haemolytica (OR, 0.2; 95% CI, 0.02-1.2), whereas exposure in penmates was associated with increased recovery (OR, 1.5; 95% CI, 1.05-2.2). Most isolates were pan-susceptible (87.8%; 95% CI, 87.0-89.4%). AMD exposures were not associated with resistance to any single drug. Multiply-resistant isolates were rare (5.9%; 95% CI, 5.1-6.9%), but AMD exposures in pen mates were associated with increased odds of recovering multiply-resistant M. haemolytica (OR, 23.9; 95% CI, 8.4-68.3). Cattle positive for M. haemolytica on arrival were more likely to become ill within 10 days (OR, 1.7; 95% CI, 1.1-2.4).

CONCLUSIONS AND CLINICAL IMPORTANCE

Resistance generally was rare in M. haemolytica. Antimicrobial drug exposures in penmates increased the risk of isolating susceptible and multiply-resistant M. haemolytica, a finding that could be explained by contagious spread.

Antimicrobial Resistance in Escherichia coli Recovered from Feedlot Cattle and Associations with Antimicrobial Use

The objectives of this study were to estimate the prevalence of antimicrobial resistance (AMR) and to investigate the associations between exposures to antimicrobial drugs (AMDs) and AMR in fecal non-type specific Escherichia coli (NTSEC) recovered from a large population of feedlot cattle. Two-stage random sampling was used to select individually identified cattle for enrollment, which were sampled at arrival and then a second time later in the feeding period. Advanced regression techniques were used to estimate resistance prevalences, and to investigate associations between AMD exposures in enrolled cattle and penmates and AMR identified in NTSEC recovered from the second sample set. Resistance was most commonly detected to tetracycline, streptomycin, and sulfisoxazole, and was rarely identified for critically important AMDs. All cattle were exposed to AMDs in feed, and 45% were treated parenterally. While resistance prevalence generally increased during the feeding period, most AMD exposures were not significantly associated with AMR outcomes. Exposures of enrolled cattle to tetracycline were associated with increased resistance to tetracycline and trimethoprim sulfa, while beta-lactam exposures were associated with decreased likelihood of detecting streptomycin resistance. Pen-level AMD exposure measures were not associated with resistance outcomes. These findings suggest that tetracycline treatment of feedlot cattle can be associated with modest increases in risk for recovery of resistant NTSEC, but the numerous treatments with an advanced macrolide (tulathromycin) were not associated with detectable increases in resistance in NTSEC. All cattle were exposed to in-feed treatments of tetracycline and this could limit the ability to identify the full impact of these exposures, but these exposures varied for enrolled cattle varied, providing an opportunity to evaluate a dose response. While AMD exposures were not associated with detectably increased risks for resistance to critically important AMDs, rare resistance outcomes and infrequent exposure to other important AMDs (e.g., cephalosporins) limited our ability to rigorously investigate questions regarding factors that can influence resistance to these important AMDs.

Right-Sided Congestive Heart Failure in North American Feedlot Cattle

Background

Anecdotal reports suggest the incidence of right‐sided congestive heart failure (RHF) in feedlot cattle is increasing; however, the rate of occurrence and risk factors are largely unknown.

Objective

The purposes of this study were to evaluate the risk of RHF over time and among feedlots, to characterize some of the risk factors for RHF, and to investigate how risk factors may affect the timing of RHF occurrence.

Animals

The population at risk consisted of 1.56 million cattle that were placed in 10 Canadian feedlots during the years 2000, 2004, 2008, and 2012, and 5 US feedlots during the year 2012.

Methods

A retrospective observational study was conducted. Variables, including year of feedlot entry, were evaluated for association with RHF using zero‐inflated negative binomial and logistic regression models. Factors affecting time to RHF were evaluated using Cox proportional hazard regression analyzes. Death from digestive disorders (DD) served as a control.

Results

The risk of RHF in Canadian feedlots doubled from the year 2000 to the year 2012 (P = .003). For every 10,000 cattle entering US feedlots in 2012, 11 cattle died from RHF and 45 cattle died from DD. The median time to RHF was 19 weeks. Cattle treated for bovine respiratory disease were 3 times more likely to die from RHF, and they died earlier in the feeding period.

Conclusions

A doubling of the incidence of RHF over a short time period is concerning, particularly for US feedlots situated at moderate altitudes in the High Plains.

Management of Pregnant Heifers in the Feedlot

Pregnant heifers in the feedlot pose many economic and management issues to the producer. Heifers that enter the feedlot pregnant will have increased costs associated with them regardless of the management strategy implemented. It is imperative that practitioners be aware of management concerns associated with pregnant heifers in order to provide sound recommendations for their clients. The purpose of this article is to provide the bovine practitioner with a summary of current literature and present common options for managing pregnant heifers in a feedlot setting.

Evaluation of a Computer-aided Lung Auscultation System for Diagnosis of Bovine Respiratory Disease in Feedlot Cattle

Background

A computer‐aided lung auscultation (CALA) system was recently developed to diagnose bovine respiratory disease (BRD) in feedlot cattle.

Objectives

To determine, in a case–control study, the level of agreement between CALA and veterinary lung auscultation and to evaluate the sensitivity (Se) and specificity (Sp) of CALA to diagnose BRD in feedlot cattle.

Animals

A total of 561 Angus cross‐steers (initial body weight = 246 ± 45 kg) were observed during the first 50 day after entry to a feedlot.

Methods

Case–control study. Steers with visual signs of BRD identified by pen checkers were examined by a veterinarian, including lung auscultation using a conventional stethoscope and CALA that produced a lung score from 1 (normal) to 5 (chronic). For each steer examined for BRD, 1 apparently healthy steer was selected as control and similarly examined. Agreement between CALA and veterinary auscultation was assessed by kappa statistic. CALA's Se and Sp were estimated using Bayesian latent class analysis.

Results

Of the 561 steers, 35 were identified with visual signs of BRD and 35 were selected as controls. Comparison of veterinary auscultation and CALA (using a CALA score ≥2 as a cut off) revealed a substantial agreement (kappa = 0.77). Using latent class analysis, CALA had a relatively high Se (92.9%; 95% credible interval [CI] = 0.71–0.99) and Sp (89.6%; 95% CI = 0.64–0.99) for diagnosing BRD compared with pen checking.

Conclusions

CALA had good diagnostic accuracy (albeit with a relatively wide CI). Its use in feedlots could increase the proportion of cattle accurately diagnosed with BRD.

Variable efficacy of a vaccine and direct-fed microbial for controlling Escherichia coli O157:H7 in feces and on hides of feedlot cattle

To evaluate the efficacy of a type-III secreted proteins vaccine and a Lactobacillus-acidophilus-based direct-fed microbial (DFM) for controlling Escherichia coli O157:H7, cattle (n=864) were allocated to the following groups: DFM, finishing diets containing 10(9) colony-forming units (CFU)/animal/day L. acidophilus and Propionibacterium freudenreichii; VAC, finishing diets and 2 mL intramuscular injection of vaccine at allocation and 28 days later; or CON, finishing diets only. Cattle within replicates were stratified by initial levels of E. coli O157:H7 and randomized to experimental groups, with 30 pens allocated on June 15, 2011 (AS1), 18 pens allocated on June 28, 2011 (AS2), and 18 cattle per pen. Rectal fecal samples and perineal swabs were collected at 28-day intervals until shipment to slaughter (103-145 days on trial). Numbers of cattle with enumerable E. coli O157:H7 (≥1.6 CFU/g feces) were reduced in AS1 and AS2 by VAC (p=0.008), although interventions had no impact on numbers of E. coli O157:H7 shed. For AS1, VAC reduced prevalence of E. coli O157:H7 in feces (p=0.03) and perineal swabs (p=0.04) in the feeding period but not at shipment to slaughter. For AS2, prevalence of E. coli O157:H7 was not reduced in either feces or perineal swabs by VAC at any time. For AS1, DFM reduced prevalence of E. coli O157:H7 in perineal swabs (p=0.01) during the feeding period. For AS2, DFM increased E. coli O157:H7 detection in feces (p=0.03) and perineal swabs (p=0.01) at shipment to slaughter. Seventy-five percent of AS1 E. coli O157:H7 isolates had only stx1, while 87% of AS2 isolates had stx1 and stx2 genes. Of the two interventions, VAC shows the most potential for pre-harvest control of E. coli O157:H7, but due to variable efficacy of both DFM and VAC, additional product development is necessary to ensure more consistent pre-harvest control of E. coli O157:H7.

Methodological comparisons for antimicrobial resistance surveillance in feedlot cattle

BACKGROUND

The purpose of this study was to objectively compare methodological approaches that might be utilized in designing an antimicrobial resistance (AMR) surveillance program in beef feedlot cattle. Specifically, four separate comparisons were made to investigate their potential impact on estimates for prevalence of AMR. These included investigating potential differences between 2 different susceptibility testing methods (broth microdilution and disc diffusion), between 2 different target bacteria (non-type-specific E. coli [NTSEC] and Mannheimia haemolytica), between 2 strategies for sampling feces (individual samples collected per rectum and pooled samples collected from the pen floor), and between 2 strategies for determining which cattle to sample (cattle that were culture-positive for Mannheimia haemolytica and those that were culture-negative).

RESULTS

Comparing two susceptibility testing methods demonstrated differences in the likelihood of detecting resistance between automated disk diffusion (BioMIC®) and broth microdilution (Sensititre®) for both E. coli and M. haemolytica. Differences were also detected when comparing resistance between two bacterial organisms within the same cattle; there was a higher likelihood of detecting resistance in E. coli than in M. haemolytica. Differences in resistance prevalence were not detected when using individual animal or composite pen sampling strategies. No differences in resistance prevalences were detected in E. coli recovered from cattle that were culture-positive for M. haemolytica compared to those that were culture-negative, suggesting that sampling strategies which targeted recovery of E. coli from M. haemolytica-positive cattle would not provide biased results.

CONCLUSIONS

We found that for general purposes, the susceptibility test selected for AMR surveillance must be carefully chosen considering the purpose of the surveillance since the ability to detect resistance appears to vary between these tests depending upon the population where they are applied. Continued surveillance of AMR in M. haemolytica recovered by nasopharyngeal swab is recommended if monitoring an animal health pathogen is an objective of the surveillance program as results of surveillance using fecal E. coli cannot be extrapolated to this important respiratory pathogen. If surveillance of E. coli was pursued in the same population, study populations could target animals that were culture-positive for M. haemolytica without biasing estimates for AMR in E. coli. Composite pen-floor sampling or sampling of individuals per-rectum could possibly be used interchangeably for monitoring resistance in E. coli.

The use of lung biopsy to determine early lung pathology and its association with health and production outcomes in feedlot steers

The objectives of this study were to determine if percutaneous lung biopsy can be used to characterize early pathologic changes in bovine lung associated with bovine respiratory disease (BRD), to determine if specific infectious respiratory pathogens can be identified in association with these changes, and to determine whether pulmonary pathology at arrival and at the time of initial diagnosis are associated with health and production outcomes. One hundred auction-market derived crossbred steer calves from a commercial feedlot in southern Alberta were included in this study. A percutaneous lung biopsy technique was used to obtain lung samples from the right middle lung. Steers were sampled 295 times yielding 283 samples with 210 (74%) containing lung tissue. Overall, histopathological changes were observed in 20 (9.5%) of lung biopsy samples. There were too few samples with pathology to reveal an association between lung pathology and subsequent health events. In general, percutaneous lung biopsy can be done safely on feedlot steers in a commercial feedlot setting with few clinical side effects. This technique did not prove useful as a diagnostic tool or prognostic indicator for early BRD. However, it may be useful for the diagnosis of BRD in targeted populations of commercial feedlot steers.

Metrics for quantifying antimicrobial use in beef feedlots

Accurate antimicrobial drug use data are needed to enlighten discussions regarding the impact of antimicrobial drug use in agriculture. The primary objective of this study was to investigate the perceived accuracy and clarity of different methods for reporting antimicrobial drug use information collected regarding beef feedlots. Producers, veterinarians, industry representatives, public health officials, and other knowledgeable beef industry leaders were invited to complete a web-based survey. A total of 156 participants in 33 US states, 4 Canadian provinces, and 8 other countries completed the survey. No single metric was considered universally optimal for all use circumstances or for all audiences. To effectively communicate antimicrobial drug use data, evaluation of the target audience is critical to presenting the information. Metrics that are most accurate need to be carefully and repeatedly explained to the audience.

Effects of wheat or corn distillers dried grains with solubles on feedlot performance, fecal shedding, and persistence of Escherichia coli O157:H7

Distillers dried grains with solubles (DDGS) are a coproduct of the ethanol industry and are often used as a replacement for grain in livestock production. Feeding corn DDGS to cattle has been linked to increased fecal shedding of Escherichia coli O157:H7, although in Canada, DDGS are often produced from wheat. This study assessed the effects of including 22.5% wheat or corn DDGS (DM basis) into barley-based diets on performance, carcass characteristics, animal health, and fecal E. coli O157:H7 shedding of commercial feedlot cattle. Cattle (n = 6,817) were randomly allocated to 10 pens per treatment group: WDDGS (diets including 22.5% wheat DDGS), CDDGS (diets including 22.5% corn DDGS), or CTRL (barley substituted for DDGS). Freshly voided fecal pats (n = 588) were collected and pooled monthly for fecal pH measurement and screened for naturally occurring E. coli O157:H7 by immunomagnetic separation (IMS) and direct plating (DP). Hide swabs (n = 367) were collected from randomly selected cattle from each pen before slaughter. Pen-floor fecal samples (n = 18) were collected from treatment groups at entry to the feedlot (<14 d on the finishing diet) and after adapting to the finishing diet for ≥ 14 d, inoculated (10(9) cfu of a 5 strain naldixic acid-resistant E. coli O157:H7 mixture), incubated (20°C) and evaluated weekly (IMS and DP) to assess fecal E. coli O157:H7 persistence. The WDDGS group had 3.0% poorer ADG (P = 0.007), 5.3% poorer G:F (P < 0.001), and a decreased proportion of Canada Quality Grade AAA carcasses (P = 0.022) compared with CTRL cattle. The CDDGS group had a similar ADG (P = 0.06), a decreased proportion of Canada Yield Grade (YG) 1 (P < 0.001), and greater proportions of Canada YG 2 (P = 0.003) and YG 3 (P < 0.001) carcasses compared with the CTRL group. There were no differences among groups in any of the animal health parameters assessed. Inclusion of DDGS in cattle finishing diets had no effect on fecal shedding (P = 0.650) or persistence (P = 0.953) of E. coli O157:H7. However, feces from cattle on starter diets <14 d had longer persistence of E. coli O157:H7 (week) than cattle on finishing diets ≥ 14 d (P < 0.003). Inclusion of DDGS in feedlot diets depends on commodity pricing relative to that of barley and for WDDGS must also include the risk of feedlot performance and carcass grading disadvantages. Feeding cattle barley based-diets with 22.5% corn or wheat DDGS did not affect fecal shedding of E. coli O157:H7.

Comparison of enrofloxacin and ceftiofur sodium for the treatment of relapse of undifferentiated fever/bovine respiratory disease in feedlot cattle

This commercial field trial compared the efficacy of enrofloxacin and ceftiofur sodium in beef cattle at high risk of developing undifferentiated fever (UF), also known as bovine respiratory disease (BRD) that received tilmicosin at feedlot arrival, were diagnosed and initially treated for UF with tilmicosin, and subsequently required a second UF treatment (first relapse). Feedlot cattle (n = 463) were randomly assigned to 2 experimental groups: ENRO or CEF. Second UF relapse, 3rd UF relapse, overall case fatality and BRD case fatality rates were lower in the ENRO group than in the CEF group (P < 0.05). There were no differences in average daily gain (allocation to re-implant date), chronicity, histophilosis case fatality or miscellaneous case fatality rates between the groups (P ≥ 0.05). A per-animal economic advantage of Can$57.08 was calculated for the ENRO group versus the CEF group. In feedlot cattle in western Canada at high risk of developing UF, it was more cost effective to administer enrofloxacin than ceftiofur sodium for treatment of UF relapse.

Evaluation of the diagnostic and prognostic utility of ultrasonography at first diagnosis of presumptive bovine respiratory disease

This project investigated the use of ultrasonography at first diagnosis of presumptive early bovine respiratory disease (BRD) in feedlot cattle from western Canada. One hundred seventy-four cattle (116 cases and 58 controls) at high risk of developing BRD were enrolled in a prospective longitudinal study over 2 y (2006-2007). Cattle with clinical signs relating to the respiratory system and assessed as sick at the time of feedlot arrival (arrival fever cases) or assessed as sick in the pen 3 to 30 d post-arrival (post-arrival fever cases, post-arrival no fevers cases) were eligible for enrollment. Control animals were identified at the time of case enrollments. Ultrasonography was done using a 3.5 sector transducer at enrollment and at 2, 4, and 6 wk post-enrollment. Lung lesions were identified at least 1 time over the course of the trial in 32/116 (28%) cases and 9/58 (16%) controls. At enrollment, lung lesions were identified in 20/115 (17%) cases and 2/55 (4%) controls (data unreadable n = 4). Post-arrival fever cases (14/48) were the most likely to have a lesion identified using ultrasound. In arrival fever cases, average daily gain (enrollment to last ultrasound, average 34 d) was improved (P = 0.007) in cattle identified with a lesion at enrollment using ultrasound compared with those not identified with a lesion at that time, potentially demonstrating the effects of gut fill at arrival weighing, as these sicker animals may have eaten less prior to arrival and, therefore, had more room for improvement in weight over time due to restoration of normal gut fill. None of the ultrasound time points explored (enrollment, 2, 4, or 6 wk post-enrollment) were associated with the animal health outcomes of interest (subsequent treatment, chronicity, wastage, or mortality) for cattle enrolled at arrival or post-arrival.Ultrasonography using a 3.5 sector transducer was not particularly effective as a prognostic/diagnostic tool for early detection of BRD, but may be useful in targeted populations of animals with respiratory disease of longer duration (such as chronic pens).

The development of a novel percutaneous lung biopsy procedure for use on feedlot steers

The purpose of this study was to develop a percutaneous lung biopsy technique to be used on steers in a commercial feedlot setting. Thirty-four crossbred steer and heifer calves from a commercial feedlot in southern Alberta were used in this study. The calves originated from the auction market and all were chronically affected with bovine respiratory disease (BRD). A technique was developed to obtain a lung sample from the right cranioventral lung lobe, intercostal space (ICS) 2, using a manual or an automatic biopsy instrument with a 14- or 12-gauge (ga) biopsy needle. Overall, lung parenchyma was successfully harvested in 55.9% of experimental animals and in 55.0% of lung biopsy trials. Compared with postmortem diagnosis, the biopsy resulted in the same pathologic diagnosis for 75% of biopsy samples when evaluated using standardized criteria by the same veterinary pathologist. The success rate was 61.5% and 42.9% in a hospital or field setting, respectively. With an automatic instrument, lung was recovered from 57.9% and 37.5% of samples obtained using a 12- or 14-ga biopsy needle, respectively. One experimental animal or 2.9% of the total had fatal complications from the procedure. In a commercial feedlot setting, the procedure took 20 min for each animal. Percutaneous lung biopsy of the right cranioventral lung lobe may be a viable technique when used on feedlot steers affected with chronic pneumonia. These findings suggest that using an automatic instrument with either a 14- or 12-ga biopsy needle may yield lung samples that are suitable for histopathological evaluation. However, this technique needs to be further evaluated in a field setting.

Efficacy of a florfenicol-flunixin meglumine combination product versus tulathromycin or ceftiofur crystalline free acid for the treatment of undifferentiated fever in feedlot calves

In this field trial, a new combination product containing florfenicol and flunixin meglumine (FLOR-FM) was compared with commercially available products that contained only tulathromycin (TULA) or ceftiofur crystalline free acid (CCFA) for the treatment of undifferentiated fever (UF; rectal temperature >/=105.0 degrees F) in beef calves that received long-acting oxytetracycline at feedlot arrival. The overall mortality rate of the FLOR FM group (2.0%) was significantly (P less than .050) lower than the rates in the TULA and CCFA groups (10.0% and 20.0%, respectively; 50 animals/group), even though the first UF relapse rate of the FLOR FM group was significantly (P less than .050) higher than that of the TULA group. In the FLOR FM group, this resulted in per-animal economic advantages of Can$46.23 (versus TULA) and Can$108.77 (versus CCFA) based on equal costs for initial UF therapy. These results demonstrate that it is more cost-effective to administer FLOR FM than TULA or CCFA for initial UF therapy in feedlot calves at high risk for bovine respiratory disease that receive metaphylactic long-acting oxytetracycline at feedlot arrival.

A comparison of 2 vaccination programs in feedlot calves at ultra-high risk of developing undifferentiated fever/bovine respiratory disease

The aim of this study was to compare 2 vaccination programs in feedlot calves at ultra-high risk of developing undifferentiated fever (UF)/bovine respiratory disease (BRD). At feedlot arrival, 3882 calves were enrolled in the study and randomly allocated to 2 groups, which were housed by group in 12 pens. At the time of allocation, 1 group (MLV3-BT2) received a multivalent, modified-live viral vaccine containing infectious bovine rhinotracheitis virus (IBRV) and types I and II bovine viral diarrhea virus (BVDV), as well as a Mannheimia haemolytica (MH) and Pasteurella multocida bacterin-toxoid. The other group (MLV4-BT1) received a vaccine containing IIBRV, type I BVDV, bovine respiratory syncytial virus, and parainfluenza-3 virus, as well as a MH bacterin-toxoid. At an average of 69 days post arrival, the groups received their respective viral vaccines. The initial UF treatment, overall chronicity, overall wastage, overall mortality, and BRD mortality rates were significantly (P < 0.05) lower in the MLV3-BT2 group than in the MLV4-BT1 group. Average daily gain and the proportions of yield grade Canada 3 and quality grade E carcasses were significantly (P < 0.05) higher in the MLV3-BT2 group than in the MLV4-BT1 group. No significant (P > or = 0.05) difference in the dry matter intake to gain ratio was detected between the 2 groups. In economic terms, there was a net advantage of $20.86 CDN/animal in the MLV3-BT2 group. This study demonstrates that it is more cost effective to use an MLV3-BT2 vaccination program than a MLV4-BT1 vaccination program in feedlot calves at ultra-high risk of developing UF/BRD.

Microbiological and histopathological findings in cases of fatal bovine respiratory disease of feedlot cattle in Western Canada

The aim of this study was to describe the microbiologic agents and pathologic processes in fatal bovine respiratory disease (BRD) of feedlot cattle and to investigate associations between agents and pathologic processes. Ninety feedlot calves diagnosed at necropsy with BRD and 9 control calves without BRD were examined, using immunohistochemical (IHC) staining and histopathologic studies. Mannheimia haemolytica (MH) (peracute, acute, and subacute cases) and Mycoplasma bovis (MB) (subacute, bronchiolar, and chronic cases) were the most common agents identified in fatal BRD cases. Significant associations (P < 0.10) were detected between microbiologic agents and between agents and pathologic processes. When IHC staining was used, 25/26 (96%) of animals that were positive for bovine viral diarrhea virus (BVDV) were also positive for MH; 12/15 (80 %) of animals that were positive for Histophilus somni (HS) were also positive for MB; and all of the animals that were positive for HS were negative for MH and BVDV. This quantitative pathological study demonstrates that several etiologic agents and pathologic processes are involved in fatal BRD of feedlot cattle.

The effect of bovine viral diarrhea virus infections on health and performance of feedlot cattle

The aim of this study was to investigate the effect of bovine viral diarrhea virus (BVDV) infections (unapparent acute infections and persistent infections) on the overall health and performance of feedlot cattle. Calves from 25 pens (7132 calves) were enrolled in the study. Overall and infectious disease mortality rates were significantly higher (P < 0.05) in pens categorized at arrival as positive for type I BVDV and lower in pens that were positive for type II BVDV than in negative pens. Mortality attributed to BVDV infection or enteritis was significantly more common (P < 0.05) in the pens containing persistently infected (PI) calves than in pens not containing PI calves (non-PI pens). There were no statistically detectable (P > or = 0.05) differences in morbidity, overall mortality, average daily gain, or the dry matter intake to gain ratio between PI and non-PI pens. Although type-I BVDV infections in feedlots appear to contribute to higher mortality rates, the presence of PI calves alone does not appear to have a strong impact on pen-level animal health and feedlot performance.

A multiplex polymerase chain reaction assay for the identification of Mannheimia haemolytica, Mannheimia glucosida and Mannheimia ruminalis

The objective of this study was to design a multiplex PCR assay to identify Mannheimia haemolytica, Mannheimia glucosida and Mannheimia ruminalis. The multiplex PCR included primer sets HP, amplifying a DNA region from an unknown hypothetical protein, Lkt and Lkt2, amplifying different regions of the leukotoxinD gene, and 16S to amplify universal bacterial sequences of the 16S rRNA gene. Based on positive amplification, isolates were delineated as M. haemolytica (HP, Lkt, 16S), M. glucosida (HP, Lkt, Lkt2, 16S), or M. ruminalis (HP, 16S). The validity of the assay was examined against 22 reference strains within the family Pasteurellaceae and 17 field isolates (nasal) that had been collected previously from feedlot cattle and tentatively identified as M. haemolytica based on morphology and substrate utilization. Additionally, 200 feedlot cattle were screened for M. haemolytica using multiplex PCR. Forty-four isolates from 25 animals were identified as M. haemolytica. The PCR assay positively identified all M. haemolytica, as confirmed by phenotypic tests and clustering based upon cellular fatty acid methyl ester (FAME) profiles. Selected nasal isolates that exhibited evidence of haemolysis, but were M. haemolytica-negative based on PCR, were also confirmed negative by phenotypic and FAME analyses. The multiplex PCR assay required no additional phenotypic tests for confirmation of M. haemolytica, within the group of bacteria tested.

Evaluation of the efficacy and cost-effectiveness of melengestrol acetate in feedlot heifer calves in western Canada

The purpose of this study was to evaluate the relative efficacy and cost-effectiveness of feeding melengestrol acetate (MGA) to feedlot heifer calves in western Canada. Heifers fed MGA had significantly (P less than .05) improved average daily gain, feed conversion, and carcass quality grade and lower rates of initial undifferentiated fever treatment and bovine respiratory disease mortality. However, heifers fed MGA had less desirable (P less than .05) carcass yield grade. There was a net economic advantage of Can $11.31/animal in favor of heifers fed MGA. Based on these results, it is efficacious and cost-effective to feed MGA to heifer calves raised in standard large-pen commercial feedlots in western Canada.

A comparison of florfenicol and tulathromycine for the treatment of undifferentiated fever in feedlot calves

The purpose of this study was to compare the relative cost-effectiveness of florfenicol with that of tulathromycin for treatment of undifferentiated fever (UF) in feedlot calves at ultra-high risk of developing UF that receive metaphylactic tulathromycin on arrival at the feedlot. Calves that received therapeutic florfenicol had lower overall mortality (P=.045) and bovine respiratory disease mortality (P=.050) compared with calves that received therapeutic tulathromycin, but no significant differences were detected in feedlot performance, carcass characteristics, or other animal health variables. There was a net advantage of Can$41.19/treated animal in the florfenicol group versus the tulathromycin group. This study demonstrates that it is more cost-effective to use florfenicol than tulathromycin for the initial treatment of UF in feedlot calves at ultra-high risk of developing UF that receive on-arrival metaphylactic tulathromycin.

An evaluation of the relative efficacy of tulathromycin for the treatment of undifferentiated fever in feedlot calves in Nebraska

A field trial was performed under commercial feedlot conditions in central Nebraska to assess the relative efficacy of tulathromycin (TULA) to florfenicol (FLOR) for the treatment of undifferentiated fever (UF) in feedlot calves that did not receive a metaphylactic antimicrobial or vaccines/bacterins containing Mannheimia haemolytica or Histophilus somni at feedlot arrival by comparing animal health, feedlot performance, and carcass characteristic variables. Two hundred recently weaned, auction market derived, crossbred beef calves that met the study-specific case definition of UF were randomly allocated in a 1:1 ratio to 1 of 2 experimental groups as follows: TULA, which received tulathromycin administered subcutaneously at the rate of 2.5 mg/kg body weight (BW) once at the time of allocation; or FLOR, which received florfenicol administered subcutaneously at the rate of 40 mg/kg BW once at the time of allocation. In terms of animal health, the first UF relapse (RR = 0.65), overall mortality (RR = 0.33), and BRD mortality (RR = 0.29) rates in the TULA group were significantly (P < 0.05) lower than in the FLOR group. There were no significant (P > or = 0.05) differences between the TULA and FLOR groups for the other animal health variables measured. There was no significant (P > or = 0.05) difference in average daily gain between the TULA and FLOR groups. There were no significant (P > or = 0.05) differences in the overall distributions of quality grade and yield grade between the experimental groups; however, a significantly (P < 0.05) higher proportion of carcasses in the TULA group graded yield grade USDA-4 as compared with the FLOR group. In the economic analysis, the benefits observed resulted in an economic advantage of $52.50 USD/animal in the TULA group due to lower first UF relapse and overall mortality rates, even though the occurrence of yield grade USDA-4 carcasses increased and the initial UF treatment cost was higher.

Evaluation of the efficacy of tulathromycin as a metaphylactic antimicrobial in feedlot calves

The purpose of this study was to determine the efficacy and cost-effectiveness of tulathromycin (DRAX) versus tilmicosin (MIC) or oxytetracycline (TET) as a metaphylactic antimicrobial in feedlot calves. Calves that received DRAX had significantly (P<.05) lower initial undifferentiated fever (UF) treatment and relapse rates; lower overall chronicity, overall mortality, and cause-specific mortality rates; higher average daily gains; and improved quality grades. However, calves that received DRAX also had poorer (P<.05) yield grades compared with calves that received MIC or TET and worse feed conversion compared with calves that received MIC. Net advantages in the DRAX group were 3.79CanDollars/animal and 16.96CanDollars/animal compared with the MIC and TET groups, respectively. Based on these results, DRAX is a more efficacious and cost-effective metaphylactic antimicrobial than MIC or TET in feedlot calves at ultra-high risk of developing UF. In addition, this study presents a comparison between two methods ("deads out" and "deads in") of calculating feedlot performance variables.

An evaluation of the metaphylactic effect of ceftiofur crystalline free Acid in feedlot calves

The relative effect of metaphylactic ceftiofur crystalline free acid (CCFA) versus metaphylactic tilmicosin was evaluated in beef calves under commercial feedlot conditions in Nebraska. At feedlot arrival, 11,605 animals at ultrahigh risk of developing bovine respiratory disease (BRD) were allocated to one of three experimental groups: CCFA-3 (6.6 mg/kg SC), CCFA-7 (6.6 mg/kg), or TILM-3 (tilmicosin, 10 mg/kg SC). Animals were eligible for subsequent BRD treatment 3 (CCFA-3 and TILM-3 groups) or 7 (CCFA-7 group) days later. Compared with the TILM-3 group, overall chronicity, overall mortality, BRD mortality, and metabolic mortality rates were significantly (P < .05) lower in the CCFA-3 and CCFA-7 groups; average daily gain was significantly (P < .05) higher in the CCFA-3 group; the proportion of quality grade No Roll carcasses was significantly (P < .05) lower in the CCFA-3 and CCFA-7 groups; and there were per-animal advantages of 22.05 dollars and 18.98 dollars in the CCFA-3 and CCFA-7 groups, respectively. In beef calves at ultrahigh risk of developing BRD, it is more cost effective to administer metaphylactic CCFA than tilmicosin at feedlot arrival.

Determination of the seroprevalence of Neospora caninum in feedlot steers in Alberta

A study was conducted in Alberta to determine the seroprevalence of Neospora caninum in feedlot calves purchased from various auction markets throughout western Canada. Four feedlots (1 feedlot from each of the Airdrie and High River areas and 2 feedlots from the Strathmore area) were selected for sampling. At each feedlot, a random 10% sample of feedlot steer and bull calves entering the feedlot from September 2001 to December 2001 were enrolled in the study until there were a maximum of 500 animals enrolled per feedlot. Blood samples were collected from 1976 male animals at the time of entry to the 4 study feedlots. The animals represented 375 groups purchased from 70 sale points throughout British Columbia, Alberta, Saskatchewan, and Manitoba. Of the 1976 animals tested, 128 animals tested positive for antibodies to N. caninum. The prevalence and adjusted 95% confidence limits for N. caninum in beef calves on entrance to the feedlot in western Canada were 6.5% (95% CI, 5.1 to 8.2). There were no significant (P > or = 0.05) associations between the risk of treatment, the risk of designation as "chronic," and the risk of death and antibodies to N. caninum either before or after adjusting for feedlot, entry weight, entry date, and clustering of disease within lots at each feedlot. In addition, there was no significant (P > or = 0.05) association between serological status and feedlot entry weight or average daily gain. Note that there was no information available on feed conversion because the calves were mixed within existing commercial feedlot pens and the actual feed intake of each animal could not be determined. Adjustment for the concentration of antibodies to bovine viral diarrhea virus on arrival did not change any of the examined associations between N. caninum status and calf health or performance. The results of this study demonstrated that the prevalence of N. caninum in feedlot calves in western Canada was less than the prevalence reported in the United States. Additional studies are required to determine whether the inferior rate of gain and feed efficiency observed in the southern United States with animals testing positive for antibodies to N. caninum also occurs under the management conditions used in western Canada.

Comparison of a multivalent viral vaccine program versus a univalent viral vaccine program on animal health, feedlot performance, and carcass characteristics of feedlot calves

A field study was conducted under commercial feedlot conditions at 2 sites in western Canada to determine the relative effects of a univalent viral vaccine (MLV 1) program versus a multivalent viral vaccine (MLV 4) program on animal health; feedlot performance; and carcass characteristic variables of fall-placed, auction market derived, feedlot calves. Five thousand one hundred and sixty-three calves were processed and randomly allocated to 1 of 2 experimental groups as follows: MLV 1, which received a modified live infectious bovine rhinotracheitis (IBR) virus vaccine upon arrival at the feedlot and again at approximately 70 days on feed (DOF); or MLV 4, which received a modified live IBR virus, parainfluenza-3 virus, bovine viral diarrhea virus, and bovine respiratory syncytial virus vaccine upon arrival at the feedlot and again at approximately 70 DOF. A total of 20 pens (10 pens at the site located near High River, Alberta and 10 pens at the site located near Vegreville, Alberta) were allocated to the study. On both a live and carcass weight basis, final weight, weight gain, and average daily gain (ADG) were significantly (P < 0.05) improved in the MLV 4 group as compared with the MLV 1 group. However, there were no significant (P > or = 0.05) differences in DOF, daily dry matter intake, dry matter intake to gain ratio (DM:G) live, or DM:G carcass between the experimental groups. In addition, there were no significant (P > or = 0.05) differences between the experimental groups in any of the carcass characteristic variables measured. The initial undifferentiated fever (UF) treatment rate was significantly (P < 0.05) lower in the MLV 4 group as compared with the MLV 1 group. There were no significant (P > or = 0.05) differences in the other measures of health between the experimental groups. In the economic analysis, there was a net advantage of $0.74 CDN per animal in the MLV 4 group as compared with the MLV 1 group due to lower initial UF treatment and improved ADG, even though the cost of the vaccine program was higher in the MLV 4 group.

An evaluation of the relative efficacy of a new formulation of oxytetracycline for the treatment of undifferentiated fever in feedlot calves in western Canada

A field trial was performed under commercial feedlot conditions in western Canada to compare the efficacy of a new formulation of long-acting oxytetracycline (LA 30) to a standard long-acting oxytetracycline formulation (LA 20) and florfenicol (FLOR) for the treatment of undifferentiated fever (UF) in calves that received metaphylactic tilmicosin upon arrival at the feed-lot. Seven hundred and ninety-seven recently weaned, auction market derived, crossbred, beef calves suffering from UF were allocated to 1 of 3 experimental groups as follows: LA 30, which received intramuscular long-acting oxytetracycline (300 mg/mL formulation) at the rate of 30 mg/kg body weight (BW) at the time of allocation; LA 20, which received intramuscular long-acting oxytetracycline (200 mg/mL formulation) at the rate of 20 mg/kg BW at the time of allocation; or FLOR, which received intramuscular florfenicol administered at the rate of 20 mg/kg BW at the time of allocation and again 48 hours later. Two hundred and sixty-six animals were allocated to the LA 30 group, 265 animals were allocated to the LA 20 group, and 266 animals were allocated to the FLOR group. The relative efficacy of the LA 30 group, as compared with the LA 20 and FLOR groups, was assessed by comparing relapse, chronicity, wastage, and mortality rates. The overall mortality (RR = 0.50) rate in the LA 30 group was significantly (P < 0.05) lower than in the LA 20 group. However, the overall chronicity (RR = 2.56) and overall wastage (RR = 6.97) rates of the LA 30 group were significantly (P < 0.05) higher than in the LA 20 group. There were no significant (P > or = 0.05) differences in UF relapse rates or cause specific mortality rates between the LA 30 and LA 20 groups. In the economic analysis, there was an advantage of $28.59 CDN per animal in the LA 30 group compared with the LA 20 group. The overall chronicity (RR = 2.25) and overall wastage (RR = 2.80) rates of the LA 30 group were significantly (P < 0.05) higher than the FLOR group. There were no significant (P > or = 0.05) differences in UF relapse rates, overall mortality rates, or cause specific mortality rates between the LA 30 and FLOR groups. In the economic analysis, there was an advantage of $12.90 CDN per animal in the LA 30 group compared with the FLOR group. In summary, the results of this study indicate that it is more cost-effective to use a new formulation of long-acting oxytetracycline (300 mg/mL formulation administered at a rate of 30 mg/kg BW) than a standard long-acting oxytetracycline formulation (200 mg/mL formulation administered at a rate of 20 mg/kg BW) or florfenicol for the treatment of UF in feedlot calves that have previously received metaphylactic tilmicosin upon arrival at the feedlot.