Table 2.
Risk factors and outcomes investigated by multivariable logistic regression for beef and dairy-type cattle upon feedlot arrival.
| Outcome | ||||
|---|---|---|---|---|
| Risk factor | Isolation of BRD-related bacteriaa | Isolation of AMR BRD bacteriab | BRD-related morbidity | BRD-related mortality |
| Country (Canada, US) | • | • | • | • |
| Sampling year (1st, 2nd) | • | • | • | • |
| Monthly intervalc | • | • | • | • |
| Source (A, B, RD)d | • | • | • | • |
| BRD risk categorye (high, low) | • | • | • | • |
| Weight (kg) | • | • | • | • |
| Sex (female, male)f | • | • | • | • |
| Age class (calf, yearling)g | • | • | • | • |
| Weatherh | • | • | NAj | NAj |
| Co-isolation of other BRD-related bacteria | • | NA | NA | NA |
| Isolation of BRD-related bacteria | NA | NA | • | • |
| Mannheimia haemolytica serotype (A1+A6 vs. A2)i | NA | • | NA | NA |
| At once feedlot capacity (>10 K, <10 K) | NA | NA | • | • |
| Suffered a previous BRD episode | NA | NA | NA | • |
One model per bacterial species i.e., Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis.
One model per bacterial species (i.e., Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni)/antimicrobial combination.
Sampling months were grouped from Aug-Nov, Dec-Feb, and Mar-May as an approximation of the seasons of the year.
Auction (A) calves were predominant among beef-type cattle when compared to backgrounding operations (B) and ranch direct (RD) calves. In those AMR models in which source was a significant explanatory variable and its SE and β-coefficients were substantially high because of a low number of samples within 1 or more stratus, backgrounding operations and ranch direct samples were grouped and modeled against auction cattle. Likewise, auction samples were eliminated from those dairy AMR models in which <5 auction cattle samples were observed.
Risk of suffering a BRD episode during the feeding period.
Bulls accounted for a very small proportion of male population i.e., 2.2%.
Calf, <1-year-old; yearling, over 1-year old.
Weather conditions at feedlot entry included ambient temperature (°C), precipitation - yes (light and heavy rain/snow) or no (none, foggy), and wind speed - high (>20 km/h) or low (<20 km/h).
Due to the uneven distribution of the samples across serotype levels, samples with unknown M. haemolytica serotype isolates were removed from the analysis (n = 3 among beef-type cattle and n = 19 among dairy-type cattle) and serotype A1+A6 isolates were grouped separately from commensal A2 isolates due to their well-documented role in BRD.
Weather-related risk factors were not included in morbidity and mortality analysis since the published literature does not provide evidence of their relevance on this matter (41). Additionally, weather measures were only related to the day that cattle arrived at the feedlots whereas the reported morbidity and mortality took place during the first 120 days on feed.
BRD, bovine respiratory disease; NA, not applicable.